Separation Technologies for the Industries of the Future (1998)

The U.S. forest products industry comprises both the pulp and paper industry and the wood products industry. In 1993, pulp and paper industry products were valued at $247.4 billion, and wood products were valued at $103.6 billion (U.S. Department of Commerce, 1994). The forest products industry as a whole employs more than 1.4 million people and is one of the top 10 manufacturing employers in the United States (AFPA, 1994a). The annual energy consumption of the forest products industry is about 3 quadrillion Btus, and its hazardous waste production is about 219 thousand tons (U.S. Environmental Protection Agency, 1993).

The pulp and paper industry is highly capital intensive, with a new integrated mill costing about $1 billion (AFPA, 1994a). Research and development on new separation processes must, therefore, be targeted to overcoming the barriers of capital investment and required return on investment. This industry would benefit greatly from the optimization of existing processes. Natural products, such as the wood used for papermaking, are very complex and variable. Fundamental research on the physico-chemical properties of the materials and waste streams of the forest products industry could improve existing processes with significantly less capital investment than would be required for the development of new separation processes. The multicomponent nature of all streams and materials in pulping and papermaking must be kept in mind when concepts for separation are investigated.

Of the several pulping processes used in the United States today, the kraft pulping process is used for producing the largest volume of wood pulp (more than

80 percent) (AFPA, 1994b). The presence of contaminants at different stages of the pulping process results in a number of separation requirements. Inorganic materials not needed for the pulping process (i.e., nonprocess elements [NPEs]) include potassium, chloride, calcium, phosphorous, metals, and transition metals. An accumulation of NPEs in the pulping process can cause problems, such as scaling, corrosion, plugging of heat exchanger passages, increased use of bleaching chemicals at later stages, and problems with filtration. The forest products industry would benefit from technologies to separate NPEs from the pulping stream.

During the digesting process, wood fibers are liberated from other wood components, such as lignin (Kocurek, 1983). A filtration step, called ''brown-stock washing,'' is subsequently used to separate the wood fibers from inorganic chemicals and dissolved wood components. This is an important step for minimizing the carryover of pulping chemicals and NPEs into the subsequent bleaching operation. A better understanding of the fundamental science behind the brown-stock washing process would benefit the forest products industry. Areas of interest include fluid dynamics during washing and the mechanisms and kinetics of absorption and adsorption of organics and inorganics on wood fibers. The goal of this research would be to predict the path that a given organic or inorganic material would take under different process conditions, which could lead to optimized operation protocols for existing washers and presses; new, more efficient, low-cost washing processes; and a better understanding of how the behavior of NPEs is affected by changes in process conditions, such as temperature and concentration.

Black liquor is the effluent stream that contains the unwanted components of wood (mostly lignin) and the majority of spent inorganic chemicals. This effluent must either be disposed of or reused. Currently, the inorganic components of black liquor are recovered via thermal oxidation in a kraft recovery furnace. The organic components are burned and their fuel value is recovered in the production of electrical energy and steam. The recovery furnace is an example of a separative reactor where sulfates are reduced to sulfide and discharged as an inorganic smelt, together with carbonates. Organic carbon is simultaneously oxidized to CO₂ and discharged with the flue gas.

Alternatives to the kraft recovery furnace are under development. These alternatives, which involve adding incremental black liquor processing capacity to existing operations, may ultimately replace the kraft recovery furnace with more efficient technology. A major challenge for some of these alternative

technologies is the removal of sulfur and other compounds from hot gases that are to be used in the gas turbines (Larson and Raymond, 1997).

A better understanding of crystallization and precipitation from multicomponent aqueous inorganic solutions, or from mixtures of inorganics and organics in water, could increase the efficiency of NPE removal. Black liquor, for example, is an aqueous organic/inorganic mixture with multiple interactions between the organics and inorganics. One significant area for research is the highly complex scaling behavior of black liquor as it is concentrated in evaporators to obtain solids for firing in the recovery furnace (Frederick and Grace, 1979). As a result of recent changes in digester technology, scaling problems in multiple-effect evaporators have increased. Scaling problems could potentially be controlled if the physical chemistry of the system were understood better. The goal would be to achieve benign NPE purge streams with NPEs in a form that could be discharged directly to the environment, sold as by-products, or used as raw material for other industries (e.g., potassium for fertilizers).

The pulp and paper industry is facing new challenges as a result of the need to recycle, rather than discharge, water. Inorganics, replacement chemicals, and recycled effluents from the bleaching and papermaking processes increase in concentration in the liquor cycle unless selective purges are used (Ulmgren, 1997). Figure 9-1 illustrates the path of one NPE, chlorine, in the liquor cycle. If efforts are made to decrease water use by minimizing or eliminating the effluent streams that serve as purges, this concentration may be increased. For example, reducing the effluent from kraft pulping has led to significant problems in controlling sulfidity (the sulfide/hydroxide balance in the wood-digesting liquor). The solution to this problem will require robust methods for separating sulfur compounds from liquor streams. The same technology would be useful for obtaining liquors with different sulfidities for the kraft cooking process. Some alkali-insoluble elements (green liquor dregs) in the liquor cycle can be purged with existing purge streams if the settling process currently used is executed efficiently or if one of the newer available technologies, such as pressure filters, is implemented. However, the removal of alkali-soluble elements, such as potassium and chloride, will require new selective purge methods.

Some types of wood pulp undergo bleaching processes to remove additional lignin and brighten the pulp. These bleaching processes produce effluent that can be detrimental to the environment. In an effort to reduce effluent from bleaching processes, wastewater is recycled countercurrently through the process and reused for brown-stock washing. One problem with this, however, is that bleach effluent

Figure 9-1

Schematic illustration of bleached pulp production emphasizing chlorine, a nonprocess element (NPE).

is thus introduced into the kraft chemical recovery process where it can cause problems.

Currently, the high-pH portion of the bleach effluent, which carries some dissolved inorganics and the majority of organics extracted during the bleaching process, can be recycled successfully and the organics incinerated in the kraft recovery furnace. In newer systems, the bleach effluent with the highest content of inorganics is produced after the first bleaching stage, which uses chlorine dioxide (Vice and Carroll, 1998). This effluent is acidic and contains substantial amounts of dissolved organics, NPEs (e.g., chlorides, metals, and transition metals), and small amounts of suspended solids (e.g., fibers). The industry would benefit from methods for removing inorganic contaminants from bleach effluent before it is recycled to the brown-stock washing and kraft chemical recovery processes. The organic contaminants could then be thermally destroyed in the recovery furnace and problems arising from inorganics entering the recovery process could be avoided.

Methods for dewatering bleach effluent, other than evaporation, would also be beneficial. If the organics in the effluent could be concentrated, they could be incinerated in the kraft recovery furnace and the remaining effluent could be treated and discharged directly to the environment.

The forest products industry would benefit from separation technologies to decrease emissions of methanol and other VOCs from pulping and bleaching

processes (Pinkerton, 1998). Methods of separating VOCs listed as hazardous air pollutants from both wet air streams and aqueous streams are of great interest to the industry. If environmental regulations regarding gaseous and vapor emissions are significantly tightened in the future, hazardous air pollutants will have to be separated from the pulp before it reaches the paper machine to avoid releasing them in dilute form via gaseous or aqueous streams, from which they will be much more difficult to remove. Research is therefore needed on the basic behavior of VOCs in process equipment and on separation methods that can be combined with modifications to existing pulping and bleaching processes (Rezac, et al., 1996). If hazardous air pollutants could be removed earlier in the bleaching process, when they are highly concentrated, the high cost of installing systems to remove these pollutants from dilute effluent streams could be avoided.

In the papermaking process, a thin layer of cleaned, bleached, and diluted wood pulp is applied to a rapidly moving fine mesh belt. Water is removed from the wood pulp, first by mechanical means and then by evaporation. The mechanical means used for water removal include application of a vacuum by various means, and pressing. These processes extract most of the water and interlace the fibers to form a paper sheet (Sell, 1992). The resulting wastewater stream contains dissolved organics, papermaking additives, and inorganic salts. When the rate of water discharge is high, dissolved materials are continuously removed from the system and do not cause problems. Efforts to reduce water discharge can result in both organic and inorganic contaminants building up in processing equipment and causing problems, such as scaling, corrosion, and poor product quality. The industry would benefit from separation processes to remove dissolved organics and inorganics continuously from papermaking process water under low effluent conditions. Because mechanical water removal methods cannot remove water to the low level needed, the final stage of papermaking includes water removal by evaporation. Evaporation, however, is a slow process, and more efficient methods for removing water would be welcomed by the industry.

Currently, 40 percent of the paper used in the United States is recovered for recycling (AFPA, 1994a). In-house waste paper and postconsumer paper can both be recycled. Postconsumer paper, which must be collected and transported to recycling plants, has a higher level of contaminants than virgin wood pulp and must be more thoroughly cleaned before it can be reused for papermaking. After the initial, coarse cleaning, an aqueous stream is produced containing wood fibers,

small amounts of dissolved organics and inorganics, and suspended contaminants, such as adhesive residues, waxes, inorganic fillers, and the polymeric inks used in xerography and laser printing. If these suspended contaminants remain in the recycling stream, they can cause deposits, scaling, and faults in the final paper product. Centrifugal cleaning, screening, and flotation are all currently used to improve the quality of the recycled fiber furnish.

Removal of suspended contaminants from recycled fiber furnish is challenging. Fiber loss must be minimized to avoid problems with sludge disposal and recycling efficiency. Density-and size-based separation processes are of limited use because the density difference between contaminants and fibers is insignificant and the size distribution of the contaminant is too broad. Flotation is sometimes used, but the fluid dynamics and physical interactions of gas bubbles and suspended and dissolved materials often result in high fiber losses (up to 20 percent). Research into methods of optimizing the removal of suspended contaminants in recycled fiber streams, as well as efficient and cost-effective separation processes to separate contaminants from wood fibers, would be of great interest to the forest products industry.

The main output of the wood products sector of the industry is lumber. Lumber is typically dried in kilns using steam as a heat source. A significant quantity of VOCs is released from the wood during this drying process. The industry would benefit from technologies for the cost-effective capture of VOCs for disposal or reuse and from drying strategies that either minimize the release of VOCs or generate waste streams sufficiently concentrated so that they can be treated easily.

The technologies that would be most effective for separating NPEs from pulping streams are based on physical or physico-chemical methods. Technologies with the potential to remove inorganics and alkali-soluble elements, such as potassium and choride, from pulping streams include ion exchange, electrodialysis, and crystallization. Gas separation membranes and absorption or adsorption technologies are promising methods for removing sulfur and other compounds from black liquor gasification effluent gases intended for use in gas turbines.

Potential technologies for separating organics from bleach effluent streams include recent advances in membrane materials, which may be able to separate and

concentrate the organics so that they can be incinerated and the remaining effluent discharged to the environment. Alternatively, if inorganics were removed from the bleach effluent, the remaining effluent could be incinerated. Ion exchange, electrodialysis, and crystallization are separation technologies with the potential for removing inorganics from bleach process effluent streams.

Membrane vapor permeation, adsorption, and biological remediation are promising technologies for the removal of VOCs from wet air streams and aqueous effluent streams. Filtration, membrane filtration, electrodialysis, and precipitation could be developed for the continuous removal of dissolved organics and inorganics from paper machine process water.

Research into flotation mechanisms, surface chemistry, and custom chemistries for specific contaminants may improve the removal efficiency for suspended and dissolved contaminants in the recycled fiber stream. Overall, a focus on underlying principles, rather than on optimized solutions for special cases, would be most effective.

Impulse drying has the potential to be a more efficient at removing water from the papermaking process than evaporation. Biological beds represent a potential technology for the cost-effective capture and disposal or use of organics from lumber drying processes. In addition, the industry would benefit from drying strategies that minimize the release of organics or generate concentrated streams that can be treated easily.

The forest products industry has a variety of separation needs including the following:

• separation of inorganics in pulping streams
• separation of alkali-soluble elements, such as potassium and chloride, from the kraft liquor cycle

• improvements in the basic understanding of crystallization and precipitation from multicomponent aqueous inorganic solutions and mixtures of inorganics in water
• fundamental research on brown-stock washing, including fluid dynamics and the kinetics of absorption and adsorption of organics and inorganics on wood fibers
• separation of sulfide compounds from liquor streams (sulfidity control, split sulfidity, kraft cooking)
• removal of contaminants (e.g., sulfur compounds) from hot gases from black liquor gasification
• removal of inorganic contaminants from bleach effluent
• removal of VOCs from wet air streams and aqeous streams
• research into the basic behavior of VOCs in process equipment (equilibrium and nonequilibrium)
• separation processes to remove dissolved organics and inorganics continuously from closed-cycle paper machine process water
• separation concepts for VOCs that may combine separation processes with modifications of existing pulping and bleaching processes
• optimization of the removal efficiency for suspended and dissolved contaminants from the recycled fiber stream
• cost-effective capture and disposal or use of organics from lumber drying processes or drying strategies that minimize release or generate concentrated streams