National Academies Press: OpenBook

Separation Technologies for the Industries of the Future (1998)

Chapter: 2 Chemical Industry

« Previous: 1 Introduction
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

2
Chemical Industry

The U.S. chemical industry includes producers of industrial gases, large-volume commodity chemicals and polymers, chemical products for agricultural and medicinal uses, and performance-targeted chemical and polymer specialties. The industry produces more than 70,000 products, including raw and basic materials, intermediate materials, and finished products (OIT, 1997a). The industry shipped $375 billion worth of products in 1996, comprising 10 percent of the value-added for the U.S. manufacturing sector and 1.9 percent of the U.S. gross domestic product. The chemical industry is the nation' s largest exporter, with exports totaling $61 billion in 1996. The industry employs more than one million workers (OIT, 1997b).

The chemical industry obtains raw materials from the petroleum refining, natural gas, and mining industries, as well as from biological sources. In 1994, the chemical industry consumed 5.8 quadrillion Btus, or 7 percent, of the total energy consumed in the United States. Air emissions for that year included 4.3 million metric tons of sulfur dioxide (SO2), nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), and particulates.

Traditional Chemical Engineering Separation Processes

Chemical products are made by a combination of processes that include synthesis, separation, and purification. The traditional chemical engineering methods of separation and purification include distillation, crystallization, adsorption, membrane processes, absorption and stripping, and extraction. These technologies are briefly described below.

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Distillation

Distillation and its companion processes, azeotropic and extractive distillation, are by far the most widely used separation processes for mixtures that can be vaporized. Distillation is a process for isolating components from a mixture based on differences in boiling points. Vapors are generated from liquids or solids by heating and are then condensed into liquid products. In azeotropic distillation, a compound is added to form an azeotrope with at least one of the components of the mixture. That component can then be more readily separated from the mixture because of the increased difference between the volatilities of the components. Extractive distillation combines continuous fractional distillation with absorption. A relatively high-boiling solvent is used to selectively scrub one or more of the components from a mixture of components with similar vapor pressures. Distillation processes are widely used for the separation of organic chemicals and for the separation of gases, usually at cryogenic temperatures, as in the production of oxygen and nitrogen from air.

Crystallization

Crystallization is one of the oldest unit operations in the portfolio of separation techniques used for industrial and laboratory processes. Crystallization is used to achieve several functions: separation, purification, concentration, solidification, and the production of a crystal that can be used to determine molecular structure. Because the heat of crystallization is typically much lower than the heat of vaporization, considerable energy savings can be realized in applications where crystallization is an effective means of separation.

Solutes can be recovered from solutions by reducing the solubility through cooling, heating, evaporation, chemical reaction, or by adding a nonsolvent to the mixture. Alternatively, separation of a chemical species from a mixture of similar compounds may be achieved by melt crystallization. In such operations, the mixture is cooled and the species allowed to solidify differentially according to their melting points. Melt crystallization is an important means of separating para-xylene from ortho-and meta-xylene. An example of the purification of a chemical species is the manufacture of L-isoleucine, in which the material crystallized from a fermentation broth that has been filtered and subjected to ion exchange may contain undesirable impurities. The crystals are then redissolved and recrystallized to enhance purity. Concentration of a solution can be accomplished by crystallization of the solvent. For example, fruit juice is concentrated via the crystallization of ice.

Product requirements are the criteria for determining the success of the crystallization process. These requirements are based on how the product will be used and the processing steps between crystallization and the recovery of the final product. Key determinants of product quality are size distribution (including mean and

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

spread), morphology (including habit and polymorphic form), and purity. Crystal size distributions determine several important processing and product properties, including appearance, the separation of crystals from liquor, reactions, dissolution, and other processes and properties involving surface area, transportation, and storage.

Adsorption

Adsorption is a method of fractionating mixtures using microporous solids (adsorbents) that have strong affinities for one or more of the components in the mixture (adsorbates). The adsorbates held by the adsorbent solid are subsequently desorbed and the adsorbent freed for further adsorption. The process is necessarily cyclic, alternating between adsorption and desorption. Desorption involves weakening the bonds between the adsorbates and the adsorbent or reducing the driving force for adsorption. Desorption can be accomplished by increasing the temperature, reducing the pressure, adding another component that competitively adsorbs with the adsorbate, or a combination of these strategies.

Membrane Processes

Separation processes involving membranes require two bulk phases that are physically separated by a third phase, the membrane. In all membrane processes, the feed is separated into two phases: the permeate (the materials that go through the membrane) and the retentate (the portion of the feed retained by the membrane). The transport of materials between the permeate and retentate phases is controlled by the membrane and the operating conditions. One or more of the species in the feed mixture are allowed to pass through the membrane in preference to others, that is to say, the membrane is selective for these species. The permeate phase is enriched in these species as the retentate phase is depleted of them.

Most commercial membranes consist of thin, selective, active layers or skins (about 0.1 to 5μm) on porous support layers that provide mechanical strength. The active and support layers can be formed in a single operation from a given polymeric material. In the case of composite membranes, the active layer can be a coating on the support layer. The transport of any species across the membrane relies on one or more forces, such as those created by a gradient in chemical potential or electrical potential. Membrane processes typically do not involve a phase change and therefore do not involve a specific heat of vaporization (like distillation) or a specific heat of crystallization (like crystallization). Because there is no phase change, highly selective membranes can, in a number of circumstances, accomplish separations with considerably less energy than other methods.

In addition to gas separation, a number of membrane separation processes are used. Dialysis is the transfer of solute molecules across a membrane by diffusion

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

from a concentrated solution to a dilute solution. In the electrodialysis process (e.g., the concentration of brine) a typical electrodialysis stack consists of a series of anion-exchange and cation-exchange membranes arranged in an alternating pattern between an anode and a cathode to form individual cells. A reverse osmosis membrane separates the various low-molecular-weight molecules and ions from the solvent by forcing the solvent or major component to pass selectively through the membrane by applying pressure greater than the normal osmotic pressure. Separation occurs based on the size, solubility, and/or charge of the various species. Ultrafiltration is another pressure-driven membrane process capable of separating somewhat larger solution components on the basis of molecular size and shape. Under an applied pressure difference across the membrane, the smaller molecules pass through the membrane and are collected as permeate while the larger molecules are retained by the membrane. The microfiltration process is similar to the ultrafiltration process, except its effective separation range is from 1,000 Å to 100,000 Å in molecular size whereas the ultrafiltration range is from 10 Å to 1,000 Å.

Absorption and Stripping

Absorption refers to the transfer of one or more components of a gas phase to a liquid phase in which the gas phase is soluble. Stripping is exactly the reverse, the transfer of a component from a liquid phase in which it is dissolved to a gas phase. There are three types of absorption processes: separation based on physical solution; separation based on reversible chemical reaction; and separation based on irreversible chemical reaction. Absorption processes require the generation of extensive areas of liquid surfaces in contact with gas phases.

Extraction

Liquid-liquid extraction is a separation technique involving two immiscible liquid phases. During liquid-liquid extraction, one of the two phases, the solvent phase, extracts the solutes from the other liquid phase. Solvent recovery and raffinate cleanup follow the separation. There is usually more than one possible method of purifying the solvent and raffinate phases, so process design is important. Solvent selection is based on liquid-liquid interfacial tension.

Separation Needs

In the chemical industry, many separation technologies are already highly developed and more than one technology option is available for most processes. However, these separation technologies could be improved in terms of energy

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

efficiency, raw materials use, or cost effectiveness. In addition, changing technologies and changing customer demands continually create new needs for the chemical industry. Opportunities for improving separation technologies for the chemical industry are identified and discussed later in this chapter.

Production of Industrial Gases

The chemical industry would benefit from more efficient and cost-effective separation methods for producing oxygen from air. Although oxygen is relatively inexpensive compared to typical organic compounds, current production methods are too expensive for it to be used in a number of production and combustion processes. Examples include hydrocarbon oxidation processes to produce materials like ethylene oxide, acrylic acid, and other oxygenated products. Inexpensive oxygen would benefit the chemical industry by providing a much-sought-after feedstock, especially in the production of synthetic fuels via the reforming of natural gas. Less expensive oxygen could also be used in a number of applications in other industries where, currenfiy, it is not always cost-effective. For example, oxygen could improve chemical and biological oxidative processes used for industrial production and waste remediation by increasing chemical efficiency, making recovery easier, and reducing investment. Finally, the use of oxygen would decrease energy use in combustion processes by eliminating the need to heat the nitrogen in air.

Nitrogen and the noble gases argon, krypton, and xenon are also produced by the separation of air (the noble gas helium is most often removed from natural gas). Nitrogen is used to produce inert atmospheres for industrial processes and storage facilities. A more efficient method for separation of air would also improve nitrogen production. Argon, krypton, and xenon are typically distilled from the residues in the fractionation of air, but more efficient methods of recovering these gases would be beneficial to the industry. A number of promising technologies under development could produce industrial gases, such as oxygen, nitrogen, and these noble gases, more efficiently and at lower cost.

Reactive Metal Complex Sorbents

Reactive metal complex sorbents may provide a means for producing oxygen more efficiently and at lower cost (see Chapter 4). A great deal of work has been done on the use of coordination complexes of cobalt (II), which reversibly bind oxygen for the separation of air (Ramprasad et al., 1995). Although this technology has been well demonstrated in a laboratory environment over relatively short periods of time, the economical production of oxygen by such means at the industrial scale has not yet been realized.

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Polymer Membranes

Current commercial polymer membranes, mainly polysulfone, polyimide, ethylcellulose, and polycarbonate, can produce nitrogen-rich air with 95 to 99 percent, or more, nitrogen. However, the oxygen-rich air has only 45 to 50 percent oxygen. Membranes with higher selectivity and flux could decrease compression costs and membrane module size. Recently, an improved membrane based on polyaramide and polyimide has been developed with an O2/N2 selectivity of about seven (Koros and Walker, 1993).

Sorbents

Methods that utilize specific sorbents to recover noble gases efficiently would benefit the chemical industry. For example, argon might be recovered from O2/Ar mixtures by using a sorbent to remove the oxygen. In addition, it may be possible to develop physical sorbents selective for shape or other characteristics for the recovery of krypton or xenon from liquid air fractions. Existing sorbents can be either reversible or irreversible and may provide avenues for the removal of trace contaminants in process streams. Specific sorbents could be used to remove contaminants from noble gases and to purify nitrogen for use as a production gas.

Production of High Purity Gases

With changing technologies, the demands for high-purity and ultra-high purity gases have increased. For instance, the progressive miniaturization of semiconductor devices has resulted in more stringent requirements in contaminant-free manufacturing, which in turn has increased the demand for ultra-high purity gases (see Table 2-1). The chemical industry needs more economical separation technologies for producing gases in ultra high-purity form, particularly oxygen, nitrogen, hydrogen, and argon. Demands for specific gas mixtures for use as analytical standards have also increased. One of the major challenges in this case is to maintain the composition of the prepared mixtures.

Dense Perovskite-Type Oxide Membranes

Inorganic polymer membranes, such as dense perovskite-type oxide membranes, may be developed to produce high-purity oxygen. This would require stable, thin (less than 2,000 Å) membranes with high oxygen productivity that can be operated at 800°C with common metallurgical hardware. The bismuth-based system with high oxygen ion conductivity and the thin-film coatings of Bi4V2O11

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

TABLE 2-1 Projected Sizes of Semiconductor Devices and Gas Purity Requirements for Contaminant-Free Manufacturing

Year

1997

1999

2001

2003

2006

2009

2012

Device size (nm)

250

180

150

130

100

70

50

Bulk ambient gases Key impurity levels (ppt)

100–1,000

100–1,000

<100

<100

<100

<100

<100

Particles > critical size (per liter)

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

Specialty gases POU particles > critical size(per liter)

2

2

2

2

2

2

2

 

Source: Adapted from Semiconductor Industry Association, 1997.

on a porous ceramic substrate by the sol-gel technique are of particular interest (Pell et al., 1995).

Inorganic Membranes

Thin, nanostructured, dense metal and metal oxide membranes might be developed for the separation of gases, for example, high purity hydrogen, oxygen, and nitrogen. Indications are that membranes have been developed at Oak Ridge National Laboratories, but the work remains classified, and these membranes are, therefore, not available for commercial testing and exploration. Although highly H2-selective palladium metal membranes are well known, problems, such as short lifetimes and poisoning by S, CO, and other substances, must be overcome. Future work in this area should be directed toward the development of molecular templale-directed synthesis of a nanostructured zeolite or inorganic material (Tanev and Pinnavaia, 1996; Yang et al, 1996) and the development of an economically feasible method of synthesizing a carbon molecular sieve with a uniform small pore size (Ioannides and Gavales, 1993; Trocha and Koros, 1994) for hydrogen or O2/N2 separation.

Polymer-Inorganic Hybrid Membranes

It may eventually be possible to produce high purity gases using high-selectivity polymer-inorganic hybrid membranes, such as polymer-zeolite membranes. The inorganic segment of the membrane could be used either to increase selectivity or to control membrane swelling (Ho and Ying, 1997). Other materials, like ceramics

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

or silica, could be used. Future research should focus on tunable synthesis and the fabrication of low-cost membrane modules.

Removal of Acid Gases

The chemical industry would benefit from improved methods of separating out and removing unwanted acid gases, principally CO2, H2S, COS, and SO2, from process streams. In the manufacture of ammonia and hydrogen, for example, carbon dioxide must be separated from mixtures with hydrogen, carbon monoxide, nitrogen, and steam. In some process streams, acid gases are present in large quantities as by-products that must be removed. In others, only low contaminant levels must be removed. Although many processes for carbon dioxide recovery are already in use, interest in the recovery of carbon dioxide has increased recently and the field is still open to innovation.

Polyimides, polyaramides, and polypyrrolones are among the polymers that are emerging or already in use (Baker et al., 1990; Zolandz and Fleming, 1992; Costello et al., 1994) and that could be used for the separation of acid gases from natural gas. Membranes made with polyvinylbenzyltrimethylammonium fluoride and polydiallyldimethylammonium fluoride have, uniquely, a high CO2/H2 selectivity (about 40 to 100) but low CO2 fluxes due to thick active layers (Laciak, 1994; Quinn et al., 1995). Thinner membranes appear to lose this selectivity. In general, more novel methods will have to be developed to produce very thin membranes for a high flux of the permeating acid gases.

Distillation Technologies

In spite of the high energy required for distillation, this process is often chosen over other separation processes because of the relatively low initial capital investment required and because it can yield high purity products. Reducing energy costs for this separation process would yield great benefits. Other areas for research are the separation of substances with similar boiling points, separation processes where constant boiling azeotropes are formed (e.g., the separation of water from ethanol), and the concentration of mineral acids (e.g., HCI and HNO3) beyond their constant boiling mixtures. The separation of close boiling liquids will require new and improved methods of distillation.

Heat Cascading

One method of reducing energy consumption in distillation is to use the energy released in the condenser of one column as energy for the reboiler in another

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

column, so-called ''heat cascading.'' This process requires that the temperatures be adjustable (by manipulation of column pressures) so that the condenser temperature of one column is higher than the reboiler temperature of the other. One example of this approach is a seven-column commercial ethanol-refining distillation method in which steam is fed to just one reboiler, and the remaining column reboilers are fueled by condensers from the other columns (Humphrey and Keller, 1997). A second, more common, example is the double column used in air separation.

Vapor Compression

Distillation technologies based on vapor compression could be designed for more efficient use of energy. The high investment needed for the compressor will, however, limit the use of this technology to processes where energy costs are very high (e.g., cryogenic separation).

Coupled Chemical Synthesis and Distillation

Separation efficiencies could be improved by coupling distillation with chemical synthesis. One such combination is catalytic distillation where a solid catalyzed reaction and the distillation of its reactants and products occur simultaneously within a distillation column (Podrebarac et al., 1997). This method is used industrially in the synthesis of the fuel additive, MTBE, by the acid catalyzed reaction of methanol and isobutylene. The economic advantages of performing two unit operations in a single piece of equipment are obvious. In addition, the combination of separation and reaction, which allows the simultaneous selective removal of products, can lead to higher conversions in equilibrium-controlled reactions. This process has some limitations, however, such as the requirement that the reaction proceed in the liquid phase because the catalyst particles must remain wetted. Another way to facilitate the separation of a mixture in distillation would be to add a homogeneous selectively reactive component. This technique is referred to as reactive distillation (Terrill et al., 1985).

Membrane Separation and Distillation

Humphrey et al. (1991) have examined the possibilities of combining membrane separation and distillation. Although most membrane processes cannot produce high-purity products, it may be possible to take advantage of the energy efficiency associated with them to perform part of the separation. In one study, simulations were performed to identify systems with a high probability of using one-third less energy than current distillation operations. Table 2-2 shows the results of that study,

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

TABLE 2-2 Candidates for Energy Savings of 33 Percent through Hybrid Technologies Involving Membranes and Distillation

System

Potential Savings (1012 Btu)

Propane/propylene

13

Natural gas dehydration

12

Deasphalting of oil

10

Ethane/ethylene

6

Sour-water stripping

6

Inorganic acid dehydration

5

Acetic acid dehydration

3

Ammonia manufacture

2

MTBE manufacture

2

Urea manufacture

2

which did not identify the specific types of membranes or process conditions that could be used to achieve the energy savings. Clearly, research on this and similar topics has the potential to yield significant benefits.

Drying Technologies

Drying, the removal of a solvent (such as water) from an existing solid body, is one of the most widely used separation techniques in the production of large-volume and specialty chemicals. For example, drying is used in the devolatilization of polymers, where unreacted monomers and solvents are removed. Drying is also used in the production of pharmaceuticals and bioproducts in powder form. Currently, drying processes are highly energy intensive. In fact, they are second only to distillation in terms of energy usage. Large capital investments are required for the equipment. Increased drying rates could lower equipment costs and, in some cases, increase energy efficiency. More energy-efficient drying processes would greatly benefit the chemical industry.

Production of Chiral Compounds

The chemical industry needs new separation technologies for the production of single-isomer chiral compounds. Chiral compounds are molecules that are mirror images of each other, or enantiomers. Some single-isomer chiral compounds are

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

produced by selective chiral synthesis. However, the chemical synthesis of compounds that can form enantiomers usually leads to the production of racetalc, or 1:1, mixtures of the enantiomers. In these cases, specific enantiomers must be obtained by separation of the racemic mixtures.

Although enantiomers have identical chemical properties towards nonchiral substrates, their biological activities are usually very different from each other. Both single-isomer chiral compounds and racemic mixtures are increasingly in demand for pharmaceutical, agrochemical, and biotechnology applications. Drugs produced as single-isomer chiral compounds include antibiotics, anticancer agents, and analgesics (Chemical and Engineering News, 1997). An area for research is the separation of the single optical isomer from drugs that are currently marketed as racemic mixtures, for example, (S)-(+)-ibuprofen, the active single isomer of the familiar analgesic. Potential separation processes for chiral compounds include high-performance liquid chromatography, crystallization, and selective chiral permeation through membranes.

High-Performance Liquid Chromatography

High-performance liquid chromatography with chiral stationary phases is used on both an analytical and preparative scale to separate racemic mixtures of chiral compounds. A relatively recent innovation is the use of high-performance liquid chromatography columns consisting of resins that have been "imprinted" (much like fossilized leaves) with chiral molecules to make them selective toward the adsorption of those compounds. This technology could be further developed, as could other innovative, lower cost methods of optical isomer resolution, such as facilitated transport membranes that use chiral carrier molecules.

Crystallization

One technique for separating racemic mixtures of enantiomers is enantioselective crystallization of conglomerates or compounds that form mixtures of crystals of individual enantiomers. All other separation techniques for racemic mixtures rely on the interaction of the chiral component or its synthesis intermediate with another chiral substance. Thus, racemic mixtures may be resolved by reaction with a chiral reagent to form diastereoisomers, which are, in principle, separable by crystallization or other separation techniques.

Separation of Components from Dilute Streams

The chemical industry would benefit from better technologies for the separation of components from dilute product or waste streams. In some cases, the

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

objective is the recovery of valuable components. In other cases, the objective is the removal of contaminants from an otherwise useful orrecyclable stream or from a stream that will enter the environment. The industry needs highly specified methods that can operate on minor components.

The commodity chemical and specialty chemical industries produce dilute effluent streams containing waste products that must be separated either for recovery and recycling or for destruction. Problems involving the separation of low concentration components, such as metal salts, inorganic compounds, and particulate matter, from aqueous streams are common. For example, the chemical commodity industry produces effluent streams containing low levels of valuable metal ions, such as copper, silver, mercury, gold, palladium, and platinum. Ideally, the effluent stream would be detoxified and the metals recovered simultaneously.

The chemical industry also needs better separation technologies to remove VOCs and nonvolatile organic compounds from effluent streams. The current method for the removal of VOCs involves purging the aqueous stream with air followed by the adsorption of the organic vapor in a carbon bed. The removal of nonvolatile organic compounds is often accomplished by biotreatment or by adsorption of the organics in carbon beds, followed by the regeneration of the carbon where practical.

Finally, the use of living organisms to synthesize chemical products (e.g., fermentation) is likely to increase. Biological processes often produce aqueous solutions with less than 10 percent of the desired substance. The recovery and purification of this substance presents unique challenges made more difficult by the potential fragility of the substance. Increasing the concentration of the solution could destroy the organisms that generate the products. In order for the bioprocessing sector of the chemical industry to grow, it needs separation technologies that enable the recovery of valuable compounds that differ only slightly in molecular structure from the other components in a dilute solution.

Promising research for the separation, removal, or recovery of components from dilute streams includes: reactive metal complex sorbents; gas separation membranes; the use of selective reducing/oxidizing agents; electrically aided membrane separation, such as electrodialysis; continuous adsorption processes; air oxidation combined with absorption; and the use of selective adsorbents to remove the products from bioprocessing streams.

Reactive Metal Complex Sorbents

Reactive metal complex sorbents may offer a means of separating out components of dilute effluent streams, such as recovering metals from dilute solutions by extraction with a liquid that contains a metal-ion specific ligand. Once the metal is recovered as the metal-ligand complex, it can be processed further (see Chapter 4).

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Gas Separation Membranes

The removal of VOCs from air or industrial process gases is an emerging application for gas separation membranes. Current silicone rubber type membranes are sometimes only marginally permselective for VOCs. Thus, new membranes with higher selectivity, possibly achieved through the control of swelling, would lower costs and increase applications.

Reducing Agents

It may be possible to detoxify effluent streams containing low levels of valuable metal ions by reduction with a low-cost reducing agent, such as H2. The metals could be recovered simultaneously.

Electrically Aided Membrane Separation

Materials might be recovered from dilute streams by electrically aided membrane processes. If the pressure driving force and electric fields are additive, the transport of a species across a membrane could be selectively enhanced or retarded. A combined-effect membrane could be constructed with sufficient strength to withstand a significant pressure field and with ionic transport capability (see Chapter 4).

Continuous Adsorption Processes

The vast majority of adsorption processes involve fixed beds of granular adsorbents that pass periodically through adsorption and desorption cycles. With the advent of polymer-based sorbents, combinations of moving and fluidized beds, in which the sorbent moves between sorbing and desorbing zones, have been developed. Thus, much of the process complexity associated with cycling fixed beds has been eliminated, but the physical stability requirements for the sorbent are now greater. The development of large, ceramic, monolithic adsorbent wheels has resulted in a new "moving sorbent" concept for recovering and concentrating small amounts of organic contaminants in gas streams. The wheel rotates at a rate of a few revolutions per hour, subjecting each part of the wheel alternately to adsorbing and desorbing conditions. Units have now been commercialized that can process several million cubic feet of gas per hour. This technology also eliminates much of the complexity associated with fixed-bed, cyclic operation.

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Air Oxidation Combined with Absorption

A useful technology for the separation of constituents from dilute gas streams would be a combination of absorption and air oxidation of the collected organic contaminant. One technique is wet air oxidation, in which the whole liquid stream is heated in the presence of air or oxygen and the contained organic is incinerated. Problems to be overcome include the high energy cost and fouling of the reactors with inorganic residues. The former limitation could be mitigated by catalytic systems that enable wet air oxidation to take place at lower temperatures and pressures. Membrane pervaporation is an emerging technology for the isolation and recovery of VOCs from aqueous streams.

Adsorbent Specificity for Separation of Bioprocessing Streams

Affinity-based separation processes have grown more important in recent years because of their application in biological processes or on biologically produced species. The technique most often uses a chromatographic operation, which makes it possible to remove a specific species from solutions containing a number of other slightly different compounds (see Chapter 4).

Recycling Polymeric Materials

The growing number of consumer products manufactured from commodity organic polymer materials has led to problems in disposing of these materials. Recycling is one option for dealing with waste polymeric materials, such as polyethylene (PE), polypropylene, polyethylene teraphthalate (PET), and nylon. A substantial fraction of consumer polymer materials, such as PET bottles, are already being recycled. Some materials, such as polystyrene foam, are not economically recyclable in small quantities because of their low density. However, if these materials are used in sufficient quantities, the supply source is centralized, and local recycling facilities exist, they can be profitably collected and recycled. Recycling polymer materials (principally nylon) from used carpets is a promising new area that is currently being extensively investigated.

The physical similarities of commodity organic polymer materials poses problems in separating them for recycling and reuse. Mixtures of PE and PET can be separated by density, but problems arise if the polymers contain fillers. The identification and separation of polymer components will require more discerning separation techniques and instrumentation that can identify polymers to enable automatic sorting. The situation is complicated by the existence of a large number of polymer types and the fact that objects made out of polymers have a large variety

Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×

of sizes and shapes. Further complications arise from the fact that polymers differ from each other not only in chemical composition, but also in more subtle respects, such as molecular weight, branching, order of monomer units in chains, and stereochemistry. These characteristics cannot be easily detected and can seriously affect the quality of the final product.

Summary

Although the chemical industry already uses many highly developed separation technologies, they could be improved in terms of energy efficiency, raw materials use, and cost effectiveness. Areas for research include the following:

  • more efficient and cost-effective separation methods for producing oxygen from air
  • more efficient methods of recovering nitrogen and noble gases from cryogenic air separation fractions
  • more efficient separation technologies for producing gases in high-purity form, particularly nitrogen, hydrogen, and argon
  • improved methods of separating out and removing unwanted acid gases, principally CO2, H2S, COS, and SO2, from process streams
  • more energy efficient distillation technologies and practices
  • more efficient distillation of substances with similar boiling points or when constant boiling azeotropes are formed
  • more efficient use of energy in drying techniques for the production oflarge-volume and specialty chemicals
  • separation technologies for the production of single-isomer chiral compounds
  • technologies for the separation of components from dilute product or waste streams, including metal salts, inorganic compounds, and particulate matter from aqueous streams; valuable metal ions, such as copper, silver, mercury, gold, palladium, and platinum, from effluent streams; and VOCs and involatile organic compounds from effluent streams
  • separation technologies for recycling chemically similar commodity organic-polymer materials
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 13
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 14
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 15
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 16
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 17
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 18
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 19
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 20
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 21
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 22
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 23
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 24
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 25
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 26
Suggested Citation:"2 Chemical Industry." National Research Council. 1998. Separation Technologies for the Industries of the Future. Washington, DC: The National Academies Press. doi: 10.17226/6388.
×
Page 27
Next: 3 Petroleum Industry »
Separation Technologies for the Industries of the Future Get This Book
×
Buy Paperback | $50.00 Buy Ebook | $39.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Separation processes—or processes that use physical, chemical, or electrical forces to isolate or concentrate selected constituents of a mixture—are essential to the chemical, petroleum refining, and materials processing industries.

In this volume, an expert panel reviews the separation process needs of seven industries and identifies technologies that hold promise for meeting these needs, as well as key technologies that could enable separations. In addition, the book recommends criteria for the selection of separations research projects for the Department of Energy's Office of Industrial Technology.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!