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HE MORAL EQUIVALENT OF WAR. This summons to respond to the energy _! crisis, uttered by President Jimmy Carter less than 10 years ago, today seems so remote that it might be the subject of a good trivia question. The mid-1980s are witness to a vast demobilization of resources and personnel once committed to energy research. Armies of re- searchers have been redirected, retired, or laid off. Pilot and demonstration facilities have been mothballed. Spending on alternative fuels re- search has dwindled. The cumulative effect of this rapid negative change is to discourage academic investigators and young graduates from seriously considering the formidable en- ergy research problems that remain unsolved. Yet there is nothing trivial about energy. The cost of energy over the last decade has had a significant influence on the rate of inflation in the United States. The energy processing in- dustries constitute one of the country's largest industrial segments. In 1985, shipments of pe- troleum and coal products amounted to $194 billion, and the value of natural gas produced in the United States exceeded $42 billion.2 The availability of secure fuel supplies is vital to national defense. Cheap and abundant energy supplies are just as important to national eco- nomic competitiveness in peacetime. No indus- trial segment has a greater impact on national well-being, jobs, defense, and economic com- petitiveness. No industrial segment is more dependent on chemical engineers for its health and progress. Research, development, and com- mercial operations in the energy processing industries all draw heavily on the knowledge and techniques of chemical engineers. There is nothing trivial about our nation's primary metals industries either in 1985 they accounted for $126 billion in shipments. This industrial sector and the energy industries have a number of characteristics in common. Both face an uncertain future shaped by the declining quality of domestic raw materials reserves and substantially higher quality reserves in countries that need to obtain foreign exchange through commodity sales, regardless of market price or profit considerations. The United States de- pends heavily on foreign imports for both energy and metals resources (Figure 6.14. The natural FRONTIERS IN CH£MICAL ENGI1VEERING resources industries have experienced cutbacks in support for industrial and academic research that are similar to those in the energy industries. Energy processing and natural resources processing share numerous fundamental tech- nical problems, many of which fall squarely in the domain of the chemical engineer. The cur- rent retrenchment in research in both fields does not imply that the problems have largely been solved. Indeed, the problems are as challenging as ever, and they are not going to be solved overnight. Now is the time to conduct the fundamental research needed for their solution. How do we take a future-oriented approach to research on energy and metals? What criteria do we use to set research priorities? Short-term projections of prices and availability of re- sources are poor guides to a national policy for research. It is virtually impossible to predict the course that energy prices will take over the next few years or to prognosticate political events that might affect the supply of key minerals to the United States. The most anyone can say is that oil prices will rise and that a real threat exists to the stability of our supply of several key minerals. What is predictable is that the cost of pro- ducing oil and minerals in the United States will be influenced by the rate of depletion of re- coverable domestic sources. This rate will in turn be influenced by the economics of recover- ing usable materials from these domestic sources. It is certain that there will be a need for engineers to develop and manage technologies that can slow the depletion rate by permitting recovery of a greater fraction of the resources that are there. The United States is the recognized world leader in energy and minerals technology; chem- ical engineering has been and must continue to be the key discipline in maintaining that posi- tion. The primary thrust of chemical engineering research in these areas should be to provide the basic knowledge that would give the nation the capability to react to future shifts in the prices and availability of energy and minerals. This capability can be developed by pursuing fun- damental research in the priority areas laid out in this chapter. Our ultimate objectives should be to extract and process current resources
PROCESSING OF ENERGY AND NATURAL RESOURCES 20 15 m 10 o I.: :::: - ~ I.: :~:~:~:~:~:~:~ I::-:- :-:-:-:-:-:-:-:-:-:-` ,.~:~:~:~:~:~:: .~:~:~: :::::::::: :` % Share 1979 1985 20 - Net Impose 43 28 ~ ...... O ~ 1970 1975 1980 1985 1990 1995 2000 Percentage Niobium Manganese Mica (sheet) Strontium Bauxite & alumina Cobalt Platinum group Tantalum Potash Chromium Tin Asbestos Barite Zinc Nickel Tungsten Silver Mercu y Cadmium Selenium 100 100 100 100 97 95 92 92 77 73 72 71 69 69 68 68 64 57 55 54 ................... ............. ...................... , , . ,,, ,. = :::::::::::::::::::::::::::::::::: . ~ , ....... . .... ................................. .............................. 2-' -'t'' '' ' ' '' ''. ~ "a"' ' - 2 ' 2 2 2 - .~ ~ - ' ' - '. _ l ~ ' ~ _ :::::::::::::::::::::::::::::::::::::::::::::::::::' - ' ''- -'''' ''- '''" ......... _ ' 2 ' ''2~ '' '' ' ' - ', ~ .2 2, ' ' ' 2 ' ' 2 ' ~ - '. _ ' ' ' ' ' ' ' _ ' '- ""'t"'.~-~"""""""""""" ,, 2 ' ' 2 2 ' - ' - ' ' ' - · ' ' _ ::::::::::::-:::::-::1 - ~ 1 . : 4: 3 .:.:.:.:.:.:. .:. :: : : 1 .......................... ~ ...................... ::::::::: :::::::::::::::: ::: : :: Brazil, Canada, Thailand South Africa, France, Brazil, Gabon India, Belgium, France Mexico, Spain Australia, Jamaica, Guinea, Surinam alre, amble, Canada, Norway South Africa, Great Britain, Soviet Union Thailand, Brazil, Malaysia, Australia Canada, Israel South Africa, Zimbabwe, Yugoslavia, Turkey Thailand, Malaysia, Bolivia, Indonesia Canada, South Africa China, Morocco, Chile, Peru Canada, Peru, Mexico, Australia Canada, Australia, Botswana, Norway Canada, China, Bolivia, Por ugal Canada, Mexico, Peru, Great Britain Spain, Algeria, Japan, Turkey Canada, Australia, Peru, Mexico Canada, Great Britain, Japan FIGURE 6.1 The United States is becoming ever more dependent on foreign sources of oil and minerals. The top graph displays trends in U.S. production and consumption of petroleum feedstocks from 1970 to 2000. It shows the growing contribution of imported oil to U.S. consumption, a contribution that is projected to increase rapidly in the 1990s. The bottom table shows that the United States depended in 1985 on foreign suppliers for 20 minerals and metals, some of which are critical to national security. Courtesy, Chevron Oil Company (top) and the U.S. Bureau of Mines (bottom). more efficiently and to develop ways of exploit- ing alternative resources. TECHNOLOGIES FOR EXPLOITING ENERGY SOURCES In the early part of this century, technologies were developed for exploiting apparently lim- itless reservoirs of gaseous and liquid fossil fuels. Because these fuels are easier to handle and cleaner than coal, they made great inroads on the use of coal as a primary energy source. However, in recent years, it has become apparent that easily ac- cessible, high-quality reservoirs of gaseous and liquid fossil fuels are not inexhaustible. Moreover, the demand for energy continues to increase as the population in- creases and labor-sparing ma- chines are developed. It is a basic premise that this challenge can best be met by developing technologies for more efficient use of existing resources and for utilizing previously un- tapped sources of energy. A number of approaches to these goals appear to hold promise for success in the next few decades. Enhanced Oil Recovery Technologies for oil produc- tion can be divided into three classes: primary recovery, sec- ondary recovery, and enhanced oil recovery. In primary recov- ery, the oil and gas flow naturally through the reservoir rock to the production well, impelled b subterranean pressure. For typ- ical light oils, only lS-20 percent of the oil in the formation is extracted in primary recovery. Secondary recovery processes extend primary recovery by in- jecting water or gas to maintain reservoir pressure as the oil is removed. These processes are well established and generally recover an additional lS-20 per- cent of the original oil. Thus, for conventional medium and light crude oils, which have rela- tively low viscosity at reservoir conditions about one-third of the original oil can be re- covered by primary and secondary methods. Enhanced oil recovery (EOR) processes (also called tertiary recovery processes) are used to recover a portion of the remaining two-thirds
82 of the original oil. Adverse reservoir properties and conditions limit both the applicability of EOR processes and the extent of recovery from those reservoirs to which the processes can be applied effectively. In addition, there are entire oil deposits so viscous "ultraheavy" crudes, bitumens, and tars that primary recovery is not possible and secondary recovery processes are generally ineffective. Such deposits in the United States approach the potential of con- ventional oil reserves, and on a worldwide basis they are several-fold greater than total reserves of the lower viscosity conventional crude oils. Much of the total resource of these extremely viscous oils is in Canada and Venezuela. The ability to recover as much as possible of the remaining two-thirds of conventional oils in known formations and to utilize ultraheavy crude deposits will become increasingly impor- tant as U.S., and ultimately worldwide, reserves of conventional crude oils are depleted. The three classes of EOR technologies that have been studied extensively are thermal re- covery, miscible flooding, and chemical flood- ing. For each of these methods, the following two basic problems must be overcome if we are to recover a significant part of the remaining oil. · Oil deposits are found in porous sedimen- tary rocks with limited pathways (permeability) for flow of oil through the reservoir formation to a producing well. The first problem is to achieve microscopic recovery efficiency to displace the oil from the rock matrix and cause it to How through the formation along a specific pathway. Figure 6.2 depicts an oil droplet in porous sand. Because of interracial tension, the oil cannot be moved by water pumped into the formation. If the interracial tension of the oil is lowered whether by increased temperature, by an oil-miscible sweeping fluid, or by chemical additives-the oil can move from its original location through the porous sand. · The second problem is to achieve macro- scopic sweep efficiency recovery of oil from a significant fraction of the reservoir formation. This problem is generally more intractable than the first. The reservoir formation is not homo- geneous in porosity or permeability, which causes FRONTIERS IN CHEMICAL E.VGINEERING FIGURE 6.2 Interfacial tension imprisons residual oil in rock, preventing its displacement by water. Without inter- facial tension, oil flows freely, leaving no residual portion in the rock. Courtesy, Amoco Production Company. phenomena such as fingering and allows pockets of oil to be bypassed, dramatically lowering the sweep efficiency of all current EOR methods (Figure 6.31. A basic challenge for chemical engineers is how to detect and/or model the movement of oil, water, gas, and injected chem- icals in the heterogeneous environment of the reservoir. This problem is discussed in detail in Chapter 8, which is devoted to computer- assisted process and control engineering. Thermal recovery methods involve the use of steam and in-situ combustion. Thermal EOR processes add heat to the reservoir to reduce the viscosity of the oil or to vaporize it. In addition, these processes use steam or oil com
PROCESSING OF E^~Y AND NATURAL RESOURCES FIGURE 6.3 Oil can be recovered from reservoirs by pumping in steam, gas, or specialty chemicals. All methods face common problems posed by inhomogeneities in the rock containing the oil. Some major problems include poor vertical coverage, inefficient sweeping that bypasses pock- ets of oil, and severe channelling of fluids along fissures or highly permeable layers of rock. Chemical methods of compensating for these inhomogeneities would boost the yields and cut the operating costs of enhanced oil recovery. Courtesy, Amoco Production Company. bustion products as a drive fluid to move oil to producing wells. Thermal processes are most often used in reservoirs of viscous oils and tars on which tests have established that primary production will be small and waterflooding will be largely ineffectual. Thus, thermal methods are usually used in place of, rather than after, secondary or primary methods. The first EOR method to achieve widespread commercial acceptance, steam injection (Figure 6.4), has been used commercially in California for more than 20 years and now accounts for more than 80 percent of U.S. EOR production. A less well developed thermal method, in- situ combustion, holds much promise but also poses a tremendous challenge to the theory and practice of chemical engineering. The in-situ combustion process involves many processes occurring simultaneously. Heat is generated within the reservoir by injecting air or oxygen to burn part of the reservoir oil (Figure 6.51. It is common practice to co-inject water with or above the oxidant to scavenge energy from hot rock lying behind the burn front. In-situ com- bustion often achieves considerably higher tem- peratures than steam flooding, and the heat not only physically and chemically reduces the oil viscosity but also partially vaporizes the oil, 83 which is driven forward by a combination of steam, hot water, and gas. The earth itself is the reaction vessel and chemical plant. The complicated reaction chem- istry and thermodynamics involve mixers, re- actors, heat exchangers, separators, and fluid flow pathways that are a scrambled design by nature. Only the sketchiest of flowsheets can be drawn. The chemical reactor has complex and ill-defined geometry and must be operated in intrinsically transient modes by remote con- trol. Overcoming these difficulties is a true frontier for chemical engineering research. Another EOR approach to reducing the vis- cosity of oil in the reservoir is miscible flood- ing-the injection of fluids that mix with the oil under reservoir conditions. Such fluids include carbon dioxide, light hydrocarbons, and nitro- gen. Supply and cost of carbon dioxide are often more favorable than for other injectants. Ex- tensive research and field testing have estab- lished the technical viability of miscible flood- ing, and a number of commercial carbon dioxide miscible flooding projects are in operation. Chemical EOR methods are based on the injection of chemicals to develop fluid or inter- facial properties that favor oil production. The three most common of these methods are poly- mer flooding, alkaline flooding, and surfactant flooding. Commercial implementation of polymer flooding and alkaline flooding is in progress, and there is confidence that research can make these processes more cost-effective and extend their applicability to a greater fraction of the known reservoirs. The research focus is on improving the thermal, chemical, and biological stability of polymers and making them more cost-effec- tive. Research targets in alkaline flooding in- clude better definition of the reactions of alkaline materials with the rock formation and the use of ancillary chemicals to improve performance. As with polymer flooding, a primary objective is to extend the applicability to more severe conditions. Of the chemical EOR technologies, surfactant flooding is the most complex, the farthest from commercial feasibility, and the most challenging in terms of research needs, yet it has the greatest ultimate potential. It involves injecting surfac
~4 il. I I I I I I I 1` ,, I, i, L, ~ I, FRONTIERS IN CHEMICAL ENGINEERING STACK GAS SCRUBBER PRODUCTION FLUIDS (OIL, GAS, WATER) SEPARATION AND PRODUCTION WELL (a) OIL AND WATER ZONE NEAR ORIGINAL RESERVOIR TEMPERATURE (a) HOT WATER ZONE (a) HEATED OIL ZONE tents, such as sulfonated crude oil, to mobilize the oil for subsequent recovery by a waterflood. The most recent National Petroleum Council (NPC) study of EOR3 estimates that chemical EOR technologies constitute more than 60 per- cent of the additional EOR potential for ad- vanced technology and that surfactant flooding represents more than 90 percent of this poten- tial. Research objectives are to extend surfac- tant flooding to more severe conditions and to make it more cost-effective. The stakes in continued research and devel- opment of EOR technologies are enormous. They involve decreasing U.S. dependence on imported oil and extending the useful lifetime of the world's exhaustible supply of petroleum. The NPC study of EOR estimates that with currently implemented EOR technologies the _' ~ STEAM AND CONDENSED WATER ZONE FIGURE 6.4 Steam flooding is one of two principal thermal methods for oil recovery and has been commercially applied since the early 1960s. A mixture of steam and hot water is continuously injected into the oil-bearing formation to displace mobilized oil to adjacent production wells. Reprinted with per- mission from Enhanced Oil Recovery. Copyright 1984 by the National Petroleum Council. total ultimate EOR potential for the United States is 14.5 billion barrels (Figure 6.6), which is more than 50 percent of the U. S. total estimated future recovery by primary and sec- ondary processes. The NPC study also esti- mates that successful development of projected advanced EOR technologies could make pos- sible the recovery of 27.5 billion barrels of domestic oil (Figure 6.61. This is more than 10 years of U.S. production at current rates and could provide an important augmentation of domestic supplies well into the next century. Shale Oil Production Oil shales are a large, virtually untapped source of hydrocarbons. U.S. reserves repre- sent several hundred billion barrels of oil and
INJECTION WELL WATER PUMP ~ ~ , ~ ~ rim_= ~SSI.\G OF ENERGY AND NATURAL RESOURCES AIR COMPRESSOR PRODUCTION WELI COMBUSTION GASES T - 1 - l- (it) COLD COMBUSTION GASES (0 COKING REGION I I , I J ~ - ~ 1 _r~ (at) OIL BANK (NEAR INITIAL TEMPERATURE) (is) BURNING FRONT AND COMBUSTION ZONE am, I I I I L ~ I ~ r =- - ~-\ CONDENSING OR HOT WATER ZONE A_ , ' - ~ ' , - 1- . --' - - J- ~J (50° - 200°F ABOVE INITIAL TEMPERATURE) W AIR AND VAPORIZED WATER ZONE ~L (is) STEAM OR VAPORIZING ZONE . (is) INJECTED AIR AND WATER ZONE (APPROXIMATELY 400°F) (BURNED our FIGURE 6.5 In-situ combustion is a major thermal means of oil recovery. Heat is generated in the reservoir by injecting air and burning part of the oil. This partially vaporizes the remaining oil, which is then driven forward by a combination of steam, hot water, and gas. Any oil left behind becomes fuel for the in-situ process. Water is also injected into the well; it improves the efficiency of the process by transferring heat from the rock behind the combustion zone (7) to the rock immediately ahead of the combustion zone (4). Reprinted with permission from Enhanced Oil Recovery. Copyright 1984 by the National Petroleum Council. are located in both the western and eastern states. Eastern oil shales are intimate mixtures of inorganic silts and insoluble organic material that have been consolidated into rock. In west ern oil shales, the matrix is a carbonate-based marlstone. The organic content of both shales is typically 5-30 percent, and most of it is a polymeric, insoluble petroleum precursor called kerogen. Shale oil can be recovered by heating the rock through the range from 250 to 500°C, where the kerogen is thermally decomposed to liquid and gaseous products, leaving 20-35 per cent of the organic matter as coke (Figure 6.7~. Because of the insolubility of kerogen and the difficulty of physically separating it from the shale, this retorting method is the only recovery process that has been developed. Retorting can be carried out above ground or in situ. The former process involves mining the shale and heating it in a vessel. Process devel- opment has been concentrated on three impor- tant engineering problems. The first is how to handle the large quantities of solids that must be processed. The second involves how to transfer heat to those solids. And the third concerns the effect of shale particle size on the efficiency of oil recovery. In-situ retorting is an alternative to mining the shale, but only if the shale bed can be made sufficiently porous to allow injection of air to burn part of the kerogen
~6 and the resulting coke and to permit outflow of the retorting products. In shallow beds, blast ing can lift the overburden and fracture the shale to permit these necessary flows. In deeper de posits, partial mining followed by blasting shale into the result ing space is used to create a porous rubble bed underground. Engineering research has fo cused on ways of producing po rosity, on gas How and combus- ADVANCEDTECHNOLOGY tion in porous beds, and on As BlLLlON BARRELS recovery of products from the large quantities of off-gas. Much of this research and development must be done in the field on a large and costly scale. It is possible that greater po rosity in shale beds could be achieved by chemical commi nution of the shale. For example, the treatment of western oil shales with acid solutions might result in comminution by inducing cor rosive stress fracture of the car bonate rock. Chemical engineering research in this area, as well in the elucidation of oil-rock interactions, might provide insights for new strategies for oil shale production. Conversion of Coal to Gaseous and Liquid Fuels Coal is the giant of fossil fuel resources. World reserves are many times those of petro- leum, and the United States is one of the major resource holders. Coal can be used directly in combustion or converted to gas or liquid. Only combustion consumes significant amounts of coal today. Coal is currently economically useful only in plants that are equipped for large-scale handling of solids, and it is used only indirectly as a raw material for chemical synthesis. Accordingly, there has been considerable research on pro- cesses for converting coal into gaseous or liquid fuels and chemicals. Only gasification has ad- vanced to commercial status. FRONTIERS IN CHE^~lCAL ENGINEERI1YG Miscible Thermal ~ Chemical IMPLEMENTED TECHNOLOGY 14.5 BILLION BARRELS FIGURE 6.6 Prospects for enhanced oil recovery using implemented and advanced technologies are shown above. The ultimate amount of U.S. oil that can be recovered by ''implemented technology," technology that presently exists in at least the proven field test stage, is estimated to be 14.5 billion barrels. Using "advanced technology," technology that might be conceivably developed before 2013, adds another 13 billion barrels of oil to the estimate, for a total of 27.5 billion barrels. A comparison of the distribution of ultimate recoveries by method is also shown. Most of the increase in the estimate from applying advanced technology comes from improvements in chemical flooding methods. The projections assume that crude oil has a nominal price of $30 per barrel and that the minimum rate of return on capital is 10 percent. Reprinted with permission from Enhanced Oil Recovery. Copyright 1984 by the National Petroleum Council. Coal is gasified by heating it in the presence of steam to make synthesis gas (syngas), a mixture of carbon monoxide and hydrogen. A variety of processes for coal gasification have evolved, and several U.S. pilot facilities have been built on scales of 100 to 1,000 tons per day during the last 10 years. The Great Plains coal gasification plant in North Dakota is op- erating at 10,000 tons per day. Coal syngas has lower energy content than natural gas for fuel use, but is widely used for the synthesis of liquid fuels and other chemicals. For example, Tennessee Eastman is operating a commercial plant that converts 900 tons of coal per day into syngas that is in turn converted into acetic an- hydride and other chemicals by a series of cata- lytic reactions. The Tennessee Eastman process is an excellent example of innovative chemical engineering in the design and construction of an efficient plant to synthesize organic chemi- cals from nonpetroleum raw materials. (See "Acetic Anhydride from Coal" on pp. 88-89.)
PROCESSING OF E1VERGY A^~D VA TURAL RESOURCES CRUSHED OIL & FINES SHALE RETORT IT FINES COMBUSTION . 1 SPENT SHALE UNDERGRADE _ MATERIAL , GAS TO CLEANUP SOLID LIQUID SEPARATION FIGURE 6.7 Steps in oil shale retorting are shown. Oil shale is crushed and then heated in a retort to drive off the oil that is trapped in the rock. Any oil left behind, as well as particulates returned to the process as the recovered oil is processed, is burned to provide heat for the retorting. The oil that is recovered from the shale is chemically treated to produce synthetic crude for further processing in conventional refineries. Courtesy, Amoco Oil Company. Coal syngas can be converted into liquid hydrocarbon fuels by catalytic reactions. One process for this conversion, the Fischer-Tropsch process, was developed in Germany during World War II and is being operated on a large scale in South Africa. Today's pilot facilities and pioneering uses of syngas are establishing a technical and economic basis for the genera- tion of commercial coal gasification projects that is expected to emerge in the 1990s. Coal gasification research and development have concentrated on handling of solids, prob- lems with ash, and dealing with the sulfur and nitrogen compounds present in coal. The newest pilot plants are investigating catalytic gasifica- tion. The integration of coal gasifiers with elec- tric power generators in a combined-cycle mode (Figure 6.8) is an emerging field for design studies and economic evaluation. Much of the combined-cycle equipment is being perfected in natural-gas-burning plants that will come on stream in the next few years. In-situ coal gasification has been demon- strated in small-scale field tests. Compared with aboveground gasification, in-situ gasification has the potential advantage of mitigating many of 87 l OILHYDRO- TREATING ~ SYNCRUDE _ SOUDS DISPOSAL the problems associated with materials corro- sion and mechanical solids handling; the main environmental problem is groundwater contam- ination. A further benefit of in-situ coal gasifi- cation is the potential for exploiting coal re- serves that cannot be mined economically. However, it is no less complex than in-situ recovery of heavy tars or shale oil, and the engineering challenges are comparable. The conversion of coal into liquid materials can be accomplished by pyrolysis or by direct liquefaction heating coal in the presence of a hydrogen source. Neither of these routes is yet economically feasible. In pyrolysis, coal is split into a hydrogen-rich liquid and a hydrogen-depleted solid char. The liquid contains significant amounts of nitrogen and sulfur compounds, as well as high-molec- ular-weight aromatic compounds such as as- phaltenes. It is difficult to upgrade this liquid to a fuel suitable for transportation uses. Fur- thermore, while the liquid might be used in boilers, it would pose severe problems of nitro- gen and sulfur oxide emissions from power plants. Research to date has uncovered few uses for the char, and it must be disposed of.
FRONTIERS ^~N CHE.~AL ENCINEERIiN'6 There are challenges and oppor- tunities in developing a process for in-situ pyrolysis of coal in which the char is the principal fuel. In direct liquefaction, coal is heated in the presence of hydro- gen and a catalyst such as cobalt- molybdenum or nickel-molyb- denum on alumina to give a greater yield of high-quality hy- drocarbons than that produced by pyrolysis. This hydrogenation process has been demonstrated in several 50- to 250-ton-per-day plants. Chemical engineering research on direct liquefaction has fo- cused on improving hydrogena- tion efficiency, for example, by treating a coal slurry in a hydro- gen-donor solvent in a high-pres- sure reactor. Basic knowledge of coal structure and reactivity as well as scientific understanding of hydrogen-transfer reactions has been crucial in improving the process. Equally important has been the realization that the de- sired reaction products can undergo secondary reactions that diminish yields and quality of final products. Accordingly, two- stage liquefaction with a catalyst in one or both stages is being tested on a small scale. Although catalyst performance is improv- ing, there is still a need for cata- lysts that will perform even bet- ter in such severely fouling conditions. New Raw Materials for Petroleum Refineries As the domestic mix of fossil fuel resources changes over the coming years, new challenges will emerge for the design and renovation of our nation's in- stalled base of refineries. While the practical aspects of this task must be left to the petroleum and gas industries, there is a need for funda- mental research to provide new design concepts and for trained engineering personnel to main- tain international competitiveness in these in
4~R ~ CESSING OF ENER ~ ~ A.~D HA TU~ ~ RES ~ ~ ~ CES dustries. The challenges arise from the prop- erties of the new raw materials that are already finding their way into the process mix. In comparison with the crudes for which refineries have been designed, both heavy crudes and shale oil contain hydrocarbons of higher molecular weight and higher car- bon-to-hydrogen ratio; more un- wanted sulfur, nitrogen, and cat- alyst-poisoning metals like vanadium and nickel; and a bi- tumen-like residue that is diffi- cult to refine. Such resources must be upgraded by chemically adding hydrogen or chemically removing carbon, by redistrib- uting hydrogen among hydrocar- bon fractions, and by removing compounds of heteroatoms and metals. Research has already led to improved thermal upgrading methods such as fluid-bed coking with coke gasification to remove carbon (Figure 6.9) and to cata- lytic hydrotreating schemes to add hydrogen and remove com- pounds of nitrogen, sulfur, vana- dium, and nickel (Figure 6.101. The co-processing of coal with heavy crude oil or its heavier fractions is being developed to lower capital requirements for coal liquefaction and to integrate processing of the products of coal conversion into existing pe- troleum refineries. This devel- opment appears to represent the main route by which coal-based liquid fuels will supplement and perhaps someday displace petro- leum-based fuels. At the low-molecular-weight end of the spectrum, a process newly commercialized by Mobil for converting methanol into gas- oline has significantly expanded opportunities in C-1 chemistry- the upgrading of one-carbon mol- ecules to multicarbon products. The process involves the use of ZSM-5, a shape-selective zeolite catalyst. (See "Zeolite and Shape-Selective Catalysts" in Chapter 9.) Since methanol can be made from coal, nat
Ural gas, or even biomass, the methanol-to- gasoline (MTG) process establishes a new link between these resources and liquid fuels. The MTG process is now operating commercially in New Zealand (Figure 6.11), where a synfuels FRO.~'ERS I.N' CHEMICAt EN.. FIGURE 6.8 A process known as integrated gasification combined cycle (IGCC) is shown. It begins with the heating of a slurry of coal and water in an oxygen atmosphere. This produces a fuel gas composed mainly of carbon monoxide and hydrogen. After the gas has been cooled, cleansed of solid particles, and rid of sulfur it can be burned to drive gas turbines and then produce steam for a steam turbine. An IGCC plant emits fewer pollutants into the air than conventional coal-fired plants do. Reprinted with permission from Scientific American, 257 (3), September 1987, p. 106. Copyright 1987 by Scientific American, Inc. plant converts natural gas into high-octane gas- oline at the rate of 14,500 barrels a day.4 Further developments in the process promise to extend the product slate to include other fuels as well as lubricants and chemicals. Through the coal
PROCESSING (3iF iENiElR~GY Aphid .~/LTfJIRAL RiESL3tJRCES 9] Tertiary Cyclones Coke Gas _ ~To Cleanup To F ctionator Robber (Start >~_ / \' |D~' Fines - - I, 1 ~ Steam - FIGURE 6.9 Flexicoking is a commercial process for refining petroleum that has been applied to heavy oil and tar sand fractions. The process employs circulating fluidized beds and operates at moderate temperatures and pressures. The reactor produces liquid fuels and excess coke. The latter is allowed to react with a gas-air mixture in the gasifies fluidized bed to provide a low-value heating gas that can be desulfurized and used as a plant fuel. Courtesy, Exxon Research and Engineering Company. to-syngas route, this C-1 technology makes it possible for the United States to convert its largest natural resource, coal, into many of the liquid products that are now derived from pe- troleum. There are a number of related challenges in research on new refining feedstocks. One is to find selective, economically viable catalytic methods to convert methane directly to liquids, thereby avoiding intermediate products such as methanol. A related challenge is to upgrade hydrocarbons in the range C-2 to C-4. These hydrocarbons are becoming more available as by-products from intensive refining processes used to make lead-free gasoline. Supplies of ~ ~ Slurry Venturi Scrubber ~Air 4~ Blower these light hydrocarbons are likely to become increasingly abundant as their presence in gas- oline is reduced by more stringent regulations to prevent hydrocarbon emissions to the at- mosphere. Municipal Solid Waste as an Energy Source The use of municipal solid waste as a source of energy fills a special need by partially elim- inating urban waste in an environmentally ac- ceptable manner while at the same time pro- ducing usable energy. Combustion of municipal solid waste, or of
~2 FROArTIERs IN COAL ENGi1~ING FIGURE 6.10 Hydrotreating plant. Courtesy, Amoco Oil Company. fuels derived from the waste, is a fledgling technology, especially in this country. There are currently 63 large plants for this type of combustion, and another 100 are starting up, under construction, or planned. Two different technologies are used. In one, which is em- ployed by about 40 percent of these plants, the raw refuse is fed directly to the boiler; in the other, the refuse is first processed to reduce piece size and to remove noncombustibles. About one-third of the plants produce low- pressure steam for heating, another one-third produce high-pressure steam for electric power generation, and the balance also exhaust low- pressure steam for industrial or municipal heat- ing.5 Process problems include slag formation, ash removal, and process control because of the heterogeneous solid waste feed. These problems have been managed to some degree by "over designing" the plant, with the result that com- bustion of municipal solid waste is not econom- ically competitive in areas where low-cost electricity or landfills for waste disposal are available. The future cost of electricity is diffi- cult to predict. However, the steady decrease in the availability of landfills portends increasing use of this process to dispose of municipal wastes, particularly in large cities. Nuclear Energy The phrase "nuclear power" covers a number of technologies for producing electric power other than by burning a fossil fuel. Nuclear fission in pressurized water-moderated reac- tors light water reactors represents the cur- rent technology for nuclear power. Down the line are fast breeder reactors. On the distant horizon is nuclear fusion.
PROCESSING OF ENERGY AND NATURAL RESOURCES FIGURE 6.11 The world's first methane-to-gasoline plant, located in Motunui, New Zealand. In operation for about a year, this plant has already met its strategic objective of reducing New Zealand's dependence on foreign oil. Courtesy, Mobil Research and Development Company. Nuclear Fission Nuclear fission accounted for 13 percent of the electricity generated in the United States in 1985. Plants under construction in 1985 will probably raise the proportion to 20 percent by 1993. However, overexpansion of electrical gen- crating capacity in this country, actual and imagined hazards of nuclear power plants, and negative perceptions of nuclear power by the public have combined to halt commitments to build new plants. New construction is not ex- pected to resume before the l990s. Development efforts in the nuclear industry are focusing on the fuel cycle (Figure 6.121. The front end of the cycle includes mining, milling, and conversion of ore to uranium hexafluoride; enrichment of the uranium-235 isotope;-con- version of the enriched product to uranium oxides; and fabrication into reactor fuel ele- ments. Because there is at present a moratorium on reprocessing spent fuel, the back end of the cycle consists only of management and disposal of spent fuel. 93 Fast breeder reactors continue to be devel- oped, although the level of support has fallen since the 1983 cancellation of the Clinch River Breeder Reactor project. Since that time, in- novative fast reactor concepts like the Integral Fast Reactor (IFR) have made considerable headway. The IFR is a self-contained, sodium- cooled, metal-fueled reactor system. Its key features include a closely coupled fuel cycle for recovery, purification, and recycling of the ura- nium-plutonium core fuel alloy and extraction of a plutonium concentrate from the uranium blanket, where new plutonium is generated, for reenrichment of the core fuel. Any fast breeder fuel cycle must include fuel reprocessing be- cause of the inescapably high concentration of fissionable materials in the used fuel. A novel aspect of the IFR reprocessing concept is the use of electrorefining rather than solvent ex- traction for recovery of fuel materials. Electro- refining technology will be carried out in a molten metal-halide salt electrolyte at about 500°C. To carry out electrorefining on a practical scale will require research and development in
so ~1 ~1~, 11 ,1r! ~ FUEL FABRICATION .~ E. it' h PROCESSING URANIUM MINING 1 1 :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-: ·:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-: ~_ I ~l_ ~_ r Van ~0 ~ · . o · · ~ · . b .. v ~ ~ . ;- ~ ;', '. MILLING . , . ~ such fields as pyrometallurgy, thermodynamics of metal-salt systems, ceramics, distillation of metals, waste processing, and remote process control technology. Nuclear Fusion Although generation of power from nuclear fusion has not yet been demonstrated, it is potentially a huge source of energy. Fusion power, the source of the sun's energy, results from the release of energy through the combi FR0~RS I,\ CHE1~AL E.\GINEERING l it, REACTOR I In SPENT FUEL STORAGE ~ ~ ~ ~ ,,_~_.~ SPENT FUEL SHIPMENT DEEP GEOLOGIC ISOLATION ·-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-: ·::::::::::::::::::: .................... .: . - · ~ ~ J ' :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-: in;- i'' 0;; _ ~ . 0.`'··O~ ·~> _ ~r=~ r _ ~1 O-0 ~. ;·0;~ ~';-02 ~° ACE FIGURE 6.12 Chemical process steps lie at the heart of the nuclear fuel cycle used in the United States. Uranium from mining and milling operations is converted to UFO, a volatile gas, which can then be separated so as to isolate the radioactive isotope of uranium. Further chemical steps are used to convert UFO into UO' for fuel assemblies, to isolate the long-lived isotopes generated in nuclear power plants, and to encapsulate these isotopes for eventual storage in nuclear waste respositories. Courtesy, Argonne National Laboratory. nation of light elements into heavier ones. The reaction currently under study is fusion of deuterium and tritium to produce helium and neutrons. Deuterium, an isotope of hydrogen present in low concentration in water, can be obtained by separating it from seawater. Tri- tium, another hydrogen isotope, is generated by reacting lithium with neutrons. In a fusion reactor, it would be generated in the lithium blanket of the reactor by neutrons derived from the reactor. Major research programs in the United States
~SSi.~76 OF ENERGY AND '\~^L RESOURCES and the Soviet Union have concentrated on proof of principle and on containment of the extremely high-temperature nuclear reaction. Although the research is expensive, U.S. federal funding had been relatively ensured until recent budgetary problems. Chemical engineering re- search is needed on the preparation of solid breeder materials, blanket tritium recovery, blanket coolant technology, high-temperature heat transfer, low-activation materials, and tri- tium containment. Electrochemical Energy Conversion and Storage The last 25 years have witnessed a rapid rise in the numbers and capabilities of batteries, fuel cells, and electrolysis cells. Developments in batteries and fuel cells open the way to new and improved schemes for energy storage and power generation (see "Fuel Cells for Trans- portation" in Chapter 91. Devices invented for military and aerospace applications have moved quickly into civilian use for example, in con- sumer electronic products, stationary energy storage, and electric vehicles. Significant ad- vances in electric vehicle technology could significantly reduce demand for transportation fuels derived from petroleum. The areas of important electrochemical en- gineering research can be grouped by the desired results. Initial costs can be lowered by devising better or less costly corrosion-resistant mate- rials for inactive components of cells, superior catalysts for electrodes in fuel cells and elec- trolysis cells, and innovations that reduce the complexity of electrochemical transformation systems. The useful lifetimes of systems can be lengthened by reducing corrosion, improving electrodes, and understanding how porous elec- trodes perform and change in structure during cyclic operation. Finally, battery and cell per- formance can be improved by developing new materials that permit higher electrode reaction rates and lower internal electrochemical losses and by new designs based on modern electro- chemical engineering. Solar Power The challenge of solar energy research is to discover or develop efficient processes for trans forming radiant solar energy into usable elec- trical energy or chemical fuels. Solar energy has inherent advantages over other energy sources. It is plentiful, ubiquitous, free, and continuously replenished, and it can be con- verted into electricity or fuels by processes that are environmentally benign. Yet, because it is a dilute and highly variable source of energy, conversion costs are high, and the construction of large (1,000-MW) commercial plants for elec- tric power or fuels production is not likely in this century. Solar energy falls on the earth at about 1 kW/m2 at noon on a sunny day. A solar cell area of about 40 km2 (15.5 mill would be required to construct a 1,000-MW power station with solar cells that are 12 percent efficient, assuming an average of 5 hours of sunlight per day throughout the year. The cost of siting and constructing a plant of this size, based on current technology, would be great. On the other hand, smaller photovoltaic arrays are being used as a peaking resource by some electric utility grids. For example, Southern California Edison main- tains an array of about 0.1 km2 to produce about 7.5 MW of power. It operates unattended and has low maintenance expenses. Solar power research should be continued to make smaller scale applications of solar energy more cost-effective. Specific research areas for chemical engineers in photovoltaics include de- velopment of low-cost methods for producing the cell materials and for fabrication of the other components that are proportional to cell area. Solar-induced temperature gradients afford op- portunity for ocean thermal energy conversion. Research is needed on convective and phase- change heat transfer as well as on biofouling control to minimize resistance to heat transfer on the seawater side. Development of low-cost materials and methods of construction for heat exchangers will be key to the success of this technology. Research on the molecular basis of photoex- citation and electron transfer, including inter- actions of electron donor and acceptor mole- cules, could lead to new photochemicals. Development of model photosensitive com- pounds and methods of incorporating them into membranes containing donor, acceptor, or in- termediate excitation transfer molecules, and .
~6 eventual development of photo- chemical reactor systems, could involve the use of sunlight to re- place less-specific energy sources. Geothermal Energy In some situations, it makes good economic sense to tap the earth's heat as an energy source. One approach to utilizing this resource is illustrated in the side- bar on this page. Plant Biomass as a Fuel Source During the 1970s, considerable attention was given to fuels from renewable resources. Use of wood-burning stoves increased markedly in some parts of the country. A subsidized industry to prepare ethanol as a gasoline supplement was started. But de- spite this, it is unlikely that plant biomass will ever satisfy more than a small percentage of U.S. energy demands. The reasons are primarily technological and lo- gistical. Much of the plant biomass pro- duced on U.S. land and water areas is used for food or forest products or exists in designated- use areas such as parks and na- tional forests. If 10 percent of the total biomass were available for conversion to energy, it would represent about 5.7 x 10~8 J/yr (5.4 x 10'5 BTU/yr). With the generous assumption of 50 percent conversion efficiency, this biomass would produce about 2.8 x 10'8 J/yr (2.7 x 10'5 BTU/yr) of energy as fuel, or only about 3 percent of current U.S. energy demand. Furthermore, the logistics of collecting today's available biomass would be forbidding. If a large area for producing biomass for energy were to be set up, it would almost certainly have to be on marginal land and would FRONHERS IA' ~L ENGI.~NG require greater than average use of fertilizer, irrigation, and mechanical work, all of which consume fuel energy themselves. The cost of net energy contributed by biomass will always be substantially higher than that calculable for the gross (i.e., apparent) fuel product. Thus, economic considerations would appear to rule out production of large amounts of energy from this source for the foreseeable future, and en
PROCESSING OF ENERGY AND ^~ATU^L RESOURCES gineering research for such schemes does not merit high priority. The development of bioreactor systems for the production of large-volume chemicals (see Chapter 3) could be the basis for reconsidering the production of biomass in limited quantities for fuel uses. This would require efficient mi- crobial organisms to catalyze fermentation, digestion, and other bioconversion processes, as well as efficient separation methods to re- cover fuel products from process streams. TECHNOLOGIES FOR EXPLOITING MINERAL AND METAL RESOURCES The technologies involved in the minerals processing industry can be broken down into those where the desired metal component~is in high concentration, such as scrap iron, iron ore, phosphate ore, and bauxite, and those where the concentration of the valuable constituent is low, such as gold and silver ore, lean copper ore, and certain types of scrap and wastes. High-Concentration Raw Materials The economics of extraction processes re- quire the primary raw materials industries to locate near the richest ore deposits. Many such deposits are now outside the United States. The combination of high U.S. labor costs, foreign government subsidies, and very dilute domestic ore deposits (0.5 percent or less) has driven a substantial portion of the copper mining and refining industry to foreign countries. The de- velopment or acquisition of technology abroad for the smelting of nonferrous metal sulfides, electrolytic extraction of zinc, and production of steel has drawn portions of those industries overseas. Although U. S. industry has been slow to adopt the new technologies, there are indi- cations that the pace of U.S. research and development in these areas may quicken. An example is the steel industry/federal government initiative in steel making. Projects now under way or planned include electromagnetic contin- uous casting, direct reduction of ore, and de- velopment of processes to remove copper and tin from scrap steel. 97 High concentrations of valuable elements ex- ist in the earth's crust but cannot be economi- cally recovered because they are buried too deep. The challenge to the chemical engineer is to develop methods for extracting these valuable materials in place without having to move and process enormous amounts of rock. The general concepts for in-situ recovery by solution mining or leaching are practiced for the recovery of uranium, soda ash, and potash. Despite these successes, most of the opportunities remain untapped because of technological barriers. Each mineral deposit has its own characteristics, and the processing environment deep beneath the earth's surface has been constructed by nature; our ability to modify it is limited. Many research needs in this area parallel those of in-situ processing of oil shale and recovery of heavy crude oils. Additional re- search needs cover a broad spectrum of mining, metallurgical, environmental, and chemical en- gineering: solids handling and comminution, separations and concentration processes for ore beneficiation, electrolytic processing, solvent extraction, and treatment and disposal of waste products. It will take the best-trained chemical engineers using the most sophisticated tools of chemistry, physics, and computer technology to unlock economically the vast reserves of metals and minerals that cannot be recovered at present. Low-Concentration Raw Materials The ascendant method for economically pro- cessing deposits low in the desired component is solvent extraction. This is most commonly done in processing plants above ground, and all the spent ore must be restored to its original location or otherwise disposed of in a way that meets environmental constraints. The restora- tion cost can be borne by high-value products like gold, silver, and uranium. In the production of moderate-value products such as copper from lean ores, magnesium from dolomite, and alu- minum from raw materials other than bauxite, the only economically viable way to process deposits may be to extract more than one product. This requires designing and building more complex chemical plants, with all the
attendant challenges to chemical engineering research. Waste Streams as Sources of Minerals and Metals Substantial quantities of aluminum, copper, and steel are reused as scrap. The challenge is to purify the scrap metal sufficiently to process it for reuse. There is opportunity for new pro- cesses that can remove unwanted elements- either alloyed or piece contaminants more ef- fectively and at lower cost than current pro- cesses. Many of the waste streams from U. S. process industries are water containing small quantities of metal ions that the law requires be removed before the wastewater is disposed of. There is an economic incentive to recoup at least some of the cost of wastewater treatment by recover- ing and selling the metal content instead of merely disposing of the metals as sludge. Be- cause the waste streams are dilute in desired materials, research is needed to devise efficient extraction and separation processes. Likewise, fly ash from power plant combus- tors often contains small amounts of metals or their oxides, which require costly disposal in the ever-shrinking number of approved hazard- ous waste landfills. Thus, there are economic incentives to recover the metal values as well as to reduce the costs of ultimate disposal. Here, too, the metal content is low, and research is needed to develop economical separation processes. In principle, advances in this area could be translated into recovery of metal values from mine tailings. INTELLECTUAL FRONTIERS The basic technologies used in the energy and natural resource processing industries have many elements in common, and the chemical engi- neering profession has a long history of finding and adapting basic technologies to the needs of diverse industries. No profession is better suited by tradition and training to attack the many difficult technical problems of these industries. And these problems must be attacked and solved if our country is to maintain its high standard FRO1~S IN CH^£~AL E\&~.~G of living and its position in the worldwide economy. The demands for energy and materials con- tinue to increase, and the accessibility of natural resources to meet them continues to fall as the most easily recovered fossil fuel and mineral deposits are depleted. The gap between rising demand and falling availability must be bridged by technology that improves the efficiency of extraction, conversion, and use of energy and materials. The development of such technology takes long lead times, and there is a paramount long-term need to maintain momentum in re- search on the frontiers of chemical and process engineering. The problems enumerated here offer challenges equal to any that chemical engineers have faced in the past. In-Situ Processing Available resources of fuels and materials in the accessible parts of the earth's crust are becoming increasingly scarce. The alternative to moving greater and greater amounts of crust, whether it is mixed with the valued substance or simply overlies it, is in-situ processing. A1- though this technology is well established in petroleum recovery, the long-term incentive to increase its efficiency is great. The incentives for other in-situ technologies vary but are bound to intensify in the future. The development of in-situ processes involves long lead times in research and development. Field tests are large- scale, prolonged projects that may last many months. The potential environmental problems are considerable. By the time the need for an in-situ process becomes acute, it is too late to commence research. The prize goes to those who are prepared. Problems with in-situ processing share certain elements. Fluid phases move through a vast, complex network of passages in a porous me- dium. The process is inherently nonsteady state. The physical transformations or chemical re- actions proceed in zones or fronts that migrate through the porous structure. The fluids interact physically with the solid walls that define the passages. The passages are irregular, and their dimensions and structure change with distance. This structural inhomogeneity imposes uncer
PROCESSING OF ENERGY AND NAP URAL RESOURCES tainties that make processing in situ riskier than processing in designed and constructed plants. Further, the potential adverse environmental impacts of in-situ processing have proved to be important barriers to the widespread commer- cialization of in-situ processes for oil shale and coal. Sustained research in the following areas is needed to reduce both environmental and process risks: · porous structures, both at the microscopic scale and larger; · methods for creating or enhancing perme- ability in nonporous formations of oil shale, coal, and ore bodies; · combustion processes under reservoir con . . citrons; · mechanisms of oil displacement; · the distribution and flow of viscous fluids in porous media and the motion of complex fronts; · surface and colloidal phenomena involved in fluid-rock interactions, such as wetting and spreading, and adhesion and release; · phase equilibria, phase thermodynamics, and chemical reactions between injected fluids and solids in the reservoir; · phase behavior, colloidal aspects, adsorp- tion, and rheology of surfactant formulations; · rheology and degradation of hydrophilic polymers and their interactions with rock; · the chemistry involved in winning a desired component from a given type of deposit; · separation of fines from produced mate- rials; · treatment and disposal of tailings; · mathematical models of such phenomena; and · process synthesis, design, management, and optimization with severely limited information. Geochemistry, geophysics, geology, environ- mental science, and chemical engineering must be more closely linked if advances are to be made in these areas. . Processing Solids Solids handling is ubiquitous in the processing of energy and natural resources. To liberate the desired components, crystalline solids (e.g., 99 rocks) must be broken into grains; these may have to be comminuted to yet finer particles. Current crushing and grinding processes are highly energy inefficient; typically 5 percent or less of the total energy expended is used to accomplish solids fracture. These processes also produce a broad distribution of particle sizes, including fines that are difficult to process fur- ther. Solids comminution could be greatly im- proved by a process that fractured crystalline solids selectively along grain boundaries. Fundamental understanding of crushing, grinding, and milling is deplorably limited. For example, there is no rational basis for the design of a ball mill, a commonly used industrial device. Mineral processing and chemical engineering researchers need a deeper understanding of solid-state science and fracture mechanics, just as they have mastered and are contributing to colloid and interface science. There are great challenges in devising chemical comminution aids as well as processes for handling solids that become plastic, sticky, or reactive at tem- peratures reached in comminution. Just as important as finding better ways to prepare granular solids and powders is finding ways to move them, to contact them with fluids, to allow them to react in chemical processes, and to separate the residues. A major study in 19816 showed that cost overruns on large proj- ects involving solids processing depended di- rectly on the throughput rate of solids. Much of the current equipment design for mineral processing dates from earlier times when ores were richer and costs of processing not as high. The handling of coal, oil shale, and ores would be improved by research on the mechanics of pneumatic and slurry transport of particulate solids, particularly on the mechanisms of failure through plugging, attrition, and erosion. Im- proved processes for coal liquefaction and gas- ification could come from research on particu- late transport in fluidized beds, including high- pressure gas-fluidized beds of large particles, ebulated beds, and liquid slurry reactors. We must also understand chemical reaction pro- cesses in systems of moving particles, especially at high temperatures and pressures. There are the related critical issues of particles being consumed or created by chemical reaction,
particle agglomeration and sintering, and trans- port and separation of hot sticky particles. For example, Plate 5 shows the stages of retorting of an oil shale particle as the temperature is increased by an external hot gas. The kerogen is reacted, cracked, volatilized, and coked in an idealized series of concentric volumes. Lib- erated products must flow through the coked zone to exit the particle. If the temperature falls too rapidly, particles can become wet and sticky. These problems are related to the more general problem of chemical reactions involving liquids or gases inside porous solid particles. Equipment design and scale-up present par- ticularly great challenges whenever solids are to be processed on a large scale. Consequently, advances in the basic understanding of solids processing will be for naught if they are not translated into practical, reliable designs. This will require close cooperation among the fields of mechanical, mineral, and chemical engineer- ing and between disciplines in the earth and physical sciences. Separation Processes Separations play a vital role in the processing of energy and natural resources.7 Improved separations can lead to improved efficiency of existing processes or to economical means for exploiting alternative resources. For example, the petroleum refining industry is based on separations of natural and synthetic hydrocar- bons. Improved separations could lead to better concentrations of aromatic hydrocarbons in gas- oline to enhance the octane rating and paraffinic hydrocarbons in jet fuel to improve burning characteristics. The winning of critical metals such as copper, uranium, and vanadium from low-grade domestic ores requires chemical ex- traction followed by recovery from the dilute extractant solution. More selective extractants are needed, as are better separations to remove fly ash, sulfur oxides, and nitrogen oxides from power plant and other gaseous emissions to protect air and water quality. Every separation process divides one or more feeds into at least two products of different composition. Separation processes that operate on heterogeneous feeds usually involve screen .~RS Alar CHE!~L F^NGlYEERING ing or settling. Those that involve physically homogeneous mixtures must use more subtle means to create products of different compo- sition. These latter processes are pervasive in industry; they consume large amounts of energy and require sophisticated research and design. Separation processes are based on some dif- ference in the properties of the substances to be separated and may operate kinetically, as in settling and centrifugation, or by establishing an equilibrium, as in absorption and extraction. Typical separation processes are shown in Table 6.1. Better separations follow from higher se- lectivity or higher rates of transport or trans- formation. The economics of separation hinges on the required purity of the separated substance or on the extent to which an unwanted impurity must be removed (Figure 6.131. Most methods of separating molecules in solution use direct contact of immiscible fluids or a solid and a fluid. These methods are helped by dispersion of one phase in the other, fluid phase, but they are hindered by the necessity for separating the dispersed phase. Fixed-bed adsorption processes overcome the hindrance by immobilizing the solid adsorbent, but at the cost of cyclic batch operation. Membrane pro- cesses trade direct contact for permanent sep- aration of the two phases and offer possibilities for high selectivity. There is already intensive research on mem- brane separations for energy and natural re- source processing. Applications have so far centered on organic polymeric membranes for mild service conditions, but research could lead to both organic and inorganic membranes that can operate under harsher conditions. Zeolites and other shape-selective porous solids like pillared clays appear to offer a fertile field of research for separation applications. Chemically selective separation agents that distinguish be- tween absorptive, chelating, or other molecular properties are also attracting study. Research should continue on traditional sep- aration methods. For example, there is a con- tinning need for more selective extraction agents for liquid-liquid and ion-exchange extractions. High-temperature processes that use liquid met- als or molten salts as extraction agents should have potential in nuclear fuel reprocessing and
PROCESSI;~G OF FNERGY ACID NA TURAL RE5~ES TABLE 6.1 Methods for Separating Mixtures Property Difference Examples of Processes Particle size Magnetism Density Solubility Surface affinity Solid/liquid phase Molecular character Molecular size Dielectric constant Solidification temperature Rate of phase change Ionic character Screening, mechanical jigging Magnetic separation Centrifugation, settling, jigging Extraction Adsorption Filtration Dialysis, membrane gas separation Molecular sieve separation Electrophoresis Zone refining Crystallization, distillation Ion exchange metals recovery; basic thermodynamic data on such high-temperature systems are lacking. Many of the ores of base metals are sulfide deposits. They must be milled to exceedingly fine size in order to free the wanted grains from the rest of the mineral. The desired grains are semiconducting colloidal particles, and the mechanisms of leaching and flotation-the pre 1o6 104 ~2 _~ - 2 1 lo-2 ferred methods of concentrating them depend on both their electrochemical and colloidal prop- erties. The separation processes leave a large quantity of unwanted fines that must be rejected as slimes. Better understanding of these pro- cesses should permit separation of complex sulfides and discovery of paths to recovering individual metals from dilute, impure solutions. / / · Radium Vitamin B-12 Penicillin ~ ~ / uranium from Ore Id Gold Copper ./ Magnesium from Seawater / .. ~ Bromine from Seawater Mined Sulfur Sulfur from Stack Gas / ~ Oxygen L Factor of 2 ~ differential in price 100 percent 1 percent 1 percent 1 thousandth of 1 millionth of 1 percent D I LUTI O N (expressed as percent concentration FIGURE 6.13 The importance of separation processes in determining the eventual cost of materials and products is illustrated in this figure. Product prices correlate with the degree of dilution of the raw material in the matrix from which it must be isolated. A factor of two in product price is shown in the figure. Courtesy, Norman N. Li, Allied-Signal Corporation. 1 billionth of 1 percent
A hypothetical separation of a homogeneous mixture, carried out in a thermodynamically reversible manner, would require the theoretical minimum expenditure of energy. In practice, however, separations of such mixtures need 50 to 100 times this minimum. Thus, there is significant opportunity for improvement of sep- arations by creating ways to reduce energy consumption without a commensurate increase in capital and operating costs. Researchers in separation science and tech- nology draw on and contribute to a variety of related fields, including · phase-equilibrium thermodynamics; · mass transfer and transport phenomena; · interracial phenomena, including surface and colloid chemistry; · mechanisms of chemical reactions, espe- cially complexation reactions; · analytical chemistry; and · computer-assisted process and control en glneerlng. Future progress in separation science and tech- nology will require continued cooperative re- search between scientists and engineers in these fields. Materials Research on materials can lead to more eco- nomical processing under extreme conditions and to reduced capital and operating costs. There are strong incentives to find construction materials for process units that are derived from domestic resources, that are less contaminating of process and environment, and that have the following properties: · greater strength and more resistance to abrasion and corrosion; · longer life and less subject to degradation by cycling conditions; · serviceability under more severe conditions of temperature, pressure, or neutron flux; and · greater resistance to hydrogen embrittle- ment. There are comparable incentives to develop new process-related materials that are more selective as catalysts, extractants, or separation FRON' - lERS IV CHExiUlCAL EM membranes and more effective in controlling flow in porous media. In addition, the devel- opment of materials that are less energy inten- sive in terms of production and use is a goal equivalent to other means of energy conserva- tion. The relatively mature technology of upgrading heavy oils by reaction with hydrogen is illus- trative. Reactors are required to withstand hy- drogen embrittlement at high pressures and temperatures. Present practice is to use foot- thick reactors lined with alloy steels. The largest of these can no longer be constructed in the United States because the cessation of nuclear power construction has led to the closing of facilities capable of such fabrication. Cheaper reactor materials would improve the economics of the process; better materials could lead to operability under more severe conditions that would provide higher conversions. Materials problems abound in the energy storage field. For example, cheap materials with large effective thermal capacity are needed to store thermal energy in solar heating systems. Some systems use chemically reactive materials and store energy as enthalpy of reaction. There is an opportunity to develop photosensitive catalysts to improve the coupling between the solar energy input and the energy converter. High-temperature thermal energy storage sys- tems confront corrosion problems aggravated by thermal cycling and temperature-sensitive solubilities that conspire to shorten system life, signifying the need for better materials. Like- wise, battery storage of electrical energy is limited by the cost of materials and by corrosion and microstructural changes aggravated by the inherently cyclic operation. Materials science is an intrinsically interdis- ciplinary field. Materials scientists include phys- icists, chemists, metallurgists, mechanical en- gineers, and chemical engineers. It is the latter who have the best opportunity to establish specifications for needed materials and to join in research on ways to meet those specifications. Advanced Methods for Design and Scale-up Many of the shortcomings of energy and natural resource processing arise from lack of
PROCESSING OF ENERGY Aide .~A TURAL RESOURCES sufficiently powerful design and scale-up pro- cedures for the practicing chemical engineer (see Chapter 81. A goal of research is to design large units from first principles and small-scale experiments. This has been done in the past; scale-up factors of 50,000 are common in petro- leum refining technology. However, in much of energy and natural resource production, there is such complexity and lack of basic data, especially for large-scale solids processing, that empiricism will continue to prevail until pilot plants and demonstration projects are success- fully modeled. Scale-up factors in solids pro- cessing typically range from two to five. For example, research on moving hot solids will require individual pieces of equipment, then whole systems, from which reliable data for scale-up can be obtained. Costs of such research would be out of reach for all but the largest industrial and government laboratories. The application of research results to improved com- mercial oil shale retorting would require large pilot plants costing tens of millions of dollars, followed by demonstration plants or single com- mercial modules costing hundreds of millions. Experimentation on an equivalent scale can be imagined for a new steel-making technology, for in-situ leaching of uranium, or for solution mining of hydrothermal mineral deposits such as soda ash. Such research will require inter- disciplinary teams and sustained activity over periods of years. The problem of ever-increasing construction costs dates from the mega-project concept of World War II and the race toward an overnight synthetic fuels industry. Nevertheless, large construction projects will be needed to bring coal gasification, coal liquefaction, and oil shale processing to fruition. Construction costs are not small in the minerals processing industries, and the more dilute the ore, the larger must be the economically viable plant. The incentives are great to develop lower cost designs and construction methods. Noteworthy ideas in- clude modular construction from preassembled units and the organization of construction into multiple small projects. Substitute materials that can be produced with lower energy or raw material cost need to be developed. Chemical engineers must be cognizant of the construction cost implications when they select construction materials and prepare flowsheets. Other Important Research Many additional intellectual challenges for chemical engineers are relevant to energy and natural resource processing. These include res- ervoir modeling (Chapter 8), combustion (Chap- ter 7), catalysis (Chapter 9), and electrochemical engineering (Chapter 94. A recent report entitled Future Directions in Advanced Exploratory Re- search Related to Oil, Gas, Shale, and Tar Sand Resources discusses some of the chemical engineering research challenges described in this chapter in a broad, multidisciplinary context that includes the earth sciences. [Finally, it should not be forgotten that chem- istry plays an important role as a fundamental science for these industries. Major contributions to be expected from chemistry in energy and natural resources are discussed in the 1985 report Opportunities in Chemistry.9 IMPLICATIONS OF RESEARCH FRONTIERS Each of the generic research areas discussed in this chapter has a strong multidisciplinary character. While the underlying fundamentals of some are amenable to investigation by indi- vidual chemical engineers, in many cases col- laboration will be required between chemical engineers and other scientists and engineers skilled in geology, geophysics, hydrology, me- chanical engineering, physics, mineralogy, ma- terials science, metallurgy, surface and colloid science, and all branches of chemistry. It will be necessary to generate creative interactions that overcome traditional academic compart- mentalization of outlook, experience, and ed- ucation. Academic departments of chemical engineering should take the lead in establishing interdisciplinary teams to carry out fundamental research in these high-priority research areas. They should seek ways to involve government and industrial scientists in interdisciplinary ac- tivities. There must be freer flow of information between industry, university, and government; professional disciplines; and academic depart- ments.
The educational background of chemical en- gineers makes them particularly well suited to solve problems in the areas discussed herein. Chemical engineers are used to working with concepts from all the related fields, and their training has evolved to cover most of the skills needed to solve technical problems. Interdis- ciplinary research in the relevant areas can only strengthen the chemical engineering cadre in the energy and natural resource processing industry. The funding needs for the research described in this chapter will be large and long term; they can be met only by some combination of gov- ernment and industry. Government support is appropriate because efficient processing of en- ergy and natural resources is key to continued national growth and prosperity. Appropriate initiatives for the Department of Energy, the U. S. Bureau of Mines, and the National Science Foundation all in cooperation with industry- are laid out in Chapter 10. Industry support is mandatory because commercialization is a goal and because companies will be the eventual profit-driven proprietors of the technology de- veloped. There is advantage and precedent for companies to band together in consortia or through institutes to provide continued funding particularly of basic research. In addition, such long-term commitment will allow academic re- searchers the freedom to set up ongoing pro- grams to feed basic data and concepts into the centers and consortia without fear of sudden shifts in funding priorities. FRONTIERS IN CHEMICAL ENGINIEJE]R]!1~G NOTES 1. U.S. Department of Commerce, Bureau of the Census. Statistical Abstract of the United States: 1987, 107th ed. Washington, D.C.: U.S. Govern- ment Printing Of lice, 1986, Table 1310. 2. U.S. Department of Commerce, Bureau of the Census. Statistical Abstract of the United States: 1987, 107th ed. Washington, D.C.: U.S. Govern- ment Printing Office, 1986, Tables 1233, 1215. 3. National Petroleum Council. Enhanced Oil Re- covery. Washington, D.C.: National Petroleum Council, 1984. 4. J. Haggin. "Methane-to-Gasoline Plant Adds to New Zealand Liquid Fuel Resources.'' Chem. Eng. News, 65 (25), 22 June 1987, 22. 5. E. Berenyi. "Overview of the Waste-to-Energy Industry." Chem. Eng. Prog., 82 (11), November 1986, 13. 6. E. W. Merrow et al. Understanding Cost Growth and Performance Shortfalls in Pioneer Processing Plants (Report R-2569-DOE). Santa Monica, Calif.: Rand Corporation, September 1981. This section draws, in part, on a recent NRC report; Separation and Purification: Critical Needs and Opportunities. Washington, D.C.: National Academy Press, 1987. 8. National Research Council, Board on Chemical Sciences and Technology. Future Directions in Advanced Exploratory Research Related to Oil, Gas, Shale, and Tar Sand Resources. Washington, D. C.: National Academy Press, 1987. 9. National Research Council, Board on Chemical Sciences and Technology. Opportunities in Chem- ist~y. Washington, D.C.: National Academy Press, 1985.