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Ind. Eng. Chern. Prod. Res. Dev. 1980, 79, 422-429
Preparation of Cellulosics for Enzymatic Conversion Gary L. Horton, Douglas B. Rivers, and George H. Emert' Gulf Science & Technology, Shawnee Mission. Kansas 6620 1
Exploitation of the chemical and energy resources found in cellulosics by current technological approaches requires that the native state of cellulose be substantially altered. Enzymatic hydrolysis of cellulose is severely restricted by its natural environment and character. Many approaches have been considered for pretreatment, including biological, chemical, mechanical, thermal, and combinations of these methods. Although any one method, under appropriate conditions, may result in substantial improvement in the rate and extent of cellulose hydrolysis, economic considerations c a n preclude their successful implementatin. The sensitivity on the economics of various pretreatment methods for the conversion of cellulose to ethanol in the Gulf process has been examined. Additionally, practical considerations indicate that only certain types of pretreatment would be preferred on an industrial scale. Studies have been performed on various means for pretreating cellulosics for subsequent bioconversion to ethanol. Effects on susceptibility by these methods will be discussed.
Tapping the resources found in lignocellulosics is an old concept with new meaning. Under the duress of national emergencies, i.e., World War 11, processes for converting cellulosic materials for use as an animal feedstuff and chemical and energy resources were developed and implemented in several countries. Special conditions during those times caused certain conversion processes to be economically acceptable. We are now entering a new era where the traditional supplies of petroleum for chemical and energy needs are becoming reduced and/or expensive. In contrast, this is not due to global conflict, but rather a consequence of increased use and demand as well as political developments. Our own domestic national use of petroleum far exceeds our rate of production. A renewed interest in cellulose and other renewable resources is emerging. The basic challenges which faced the original investigators in cellulose conversion have changed little. However, new approaches have surfaced and progress has been made. Two of the major deterrents to the implementation of a commercial conversion process for cellulose have been: (1)collection of the raw materials and (2) preparation for conversion. Significant progress, however, has been made in the area of enzyme technology for cellulose hydrolysis and fermentation to ethyl alcohol (1,2),a convenient form for further use as a chemical feedstock or as a liquid fuel. Vast quantities of cellulose exist. It is the most abundant carbon-containing chemical form on earth. Cellulose is constantly being synthesized at enormous rates by the natural process of photosynthesis. Its production is a natural phenomenon and part of the life process on earth. The raw materials, COS, HzO, and light, for its synthesis are ubiquitous and free. Although enormous quantities of cellulose exist, the economy of concentrating supplies and preparing it for industrial use are fundamentally important to commercial development. An indication of the sources and quantities of cellulose wastes potentially available in the U.S. is presented in Table I. Some of these materials are residues from other industrial processing. The list presented represents sources in which cellulose is found in a variety of physical and chemical microenvironments. This environment is of paramount importance in determining the suitability of a particular material for a commercial process. This is in large measure due to the technical difficulty of selectively assessing the cellulose for processing. These factors will be discussed later in the presentation. 0196-4321/80/1219-0422$01 .OO/O
Table I. Lignocellulosic Wastes in United States source Collected municipal solid waste pulp and paper mill waste selected agricultural waste Uncollected forest residues wood processing agricultural residues
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Not only is the nature of the cellulosic material an important factor, but the quantity necessary to operate a facility of economical size is a critical consideration. There may be special conditions where a highly desirable type of cellulosic material is available which would warrant consideration of a relatively small scale commercial venture. However, broad scale implementation of cellulose as a feedstock for production of chemicals and energy must look to sources where material is available on the order of 1000 tons/day or greater. In this regard, it is especially important to consider municipal solid waste, a source where complementary disposal problems exist. Municipal Solid Waste as a Cellulose Source The cellulose found in municipal solid waste (MSW) primarily derives from paper products which, in turn, are produced from wood sources. The route from wood to ethyl alcohol, the target product for our discussion, via the collection and processing technology for conversion is depicted in Figure 1. Considerable processing and gross degradation of native cellulose occurs during the many and varied methods of pulp and paper manufacture. (Waste residues from these processes are an excellent feedstock for conversion.) Finished products from these industries pass through consumer use and a large portion is discarded as refuse. Current practices of consumers do not involve a large amount of segregation of cellulosics from other solids; hence, a mixture of solid materials winds up as a disposal responsibility of municipalities. The traditional means of landfilling this material is increasingly becoming an undesirable solution for its disposal. The technology for recovering useful resources from MSW is developing. Table I1 presents the general composition of typical municipal solid waste. The extremely heterogenous nature of this material requires isolation of the cellulose fraction for further processing. Classification
0 1980 American
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
v1/.a[l#bL
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by fermentation methods. The conversion process involves hydrolysis of the P-1,4-glycosidic bond between glucose units in cellulose (11). Hydrolysis may be catalyzed by
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35 15 15 11 9 7 3 2 2 1
methods, currently operating, are capable of producing fractions of other solids and one possessing a high percentage of cellulose. These classification methods do not segregate materials such as plastics and other synthetics from cellulose. Nevertheless, the capability for providing a cellulosic feedstock with a relatively large percentage of cellulose in large quantities exists. The technical considerations in processing the cellulose-rich classified fraction from MSW involves further cleanup, if necessary, and treatment to maximize conkersion rates. These two aspects, together with classification of MSW, comprise the concept of preparation of cellulosics for enzymatic conversion as discussed in this presentation. In our view, these factors should receive the most attention by investigators, since they represent significant opportunities for allowing an early commercialization of cellulose conversion technology. In order to more fully explore the factors involved in preparation, a brief discussion of the conversion process is presented.
Conversion The abundant chemical form in MSW is cellulose, a natural polymer of glucose (I). Any process for utilizing
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I1 acids or enzymes (cellulases). The process of producing glucose from cellulose is referred to as saccharification. Maximizing the rate and extent of saccharification is the fundamental concern in improving cellulose conversion technology. Alternately, the thermal conversion of MSW by direct conversion or pyrolysis to fuel boilers or to produce gases and long chain hydrocarbons is possible. However, because of the fact that MSW contains plastics, proteins, and a relatively high ash level, several problems must be encountered. Direct combustion poses air pollution problems with the emissions of stack gases containing corrosive chlorides and sulfur oxides. In addition, direct combustion operates a t an average 65% efficiency as compared with an 85% average efficiency for fossil fuels ( 3 ) . The Btu of air classified MSW is 550@-6500 when the moisture content is in the range of 15-35% by weight ( 4 ) . Pyrolysis products for fuel use can be controlled by the rate of temperature change and residence time. The primary product, charcoal, has a Btu/lb of 11000-14 000. With rapid heating a long chain carbon char is produced with 10 000-11 000 Btu/lb. Complete gasification in pyrolysis at 1000 "C produces a gas with 100-400 Btu/ft3 ( 3 ) . The saccharification and fermentation of MSW is, however, preferred because either liquid fuels or several industrial chemicals may be produced. The unconverted residue is an enriched mixture of plastics at 10000-17 000 Btu/lb ( 4 ) and lignin a t loo00 Btu/lb. Therefore, not only is a primary product produced but the remaining residue is acceptable as a boiler fuel. Several factors can be important regarding the susceptibility of native cellulose to saccharification. These considerations include essentially the same aspects whether the method of catalysis is acid or enzyme. The nature of acid hydrolysis, however, requires careful consideration of process control due to the kinetics of the reaction. Acid hydrolysis of cellulose does not result in glucose as a sole end product. Depending upon reaction conditions such as temperature and acid concentration, glucose will be converted to degradation products at different rates. Although reaction conditions may be controlled to maximize glucose concentration, increasing rates of production also result in increasing rates of degradation. Enzyme hydrolysis, on the other hand, is a very selective process that is specific for the substrate, cellulose. The product, glucose, is stable under the relatively mild reaction conditions and builds up in concentration as the reaction proceeds. As illustrated in Figure 1, ethyl alcohol is a target product obtained from cellulose which may be employed for chemical and fuel development. A direct process for producing ethyl alcohol from cellulose involves enzyme catalyzed hydrolysis of the cellulose to glucose with si-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
multaneous fermentation of the glucose to the product ( I ) . This direct conversion process results in increased rates and yields and simplified processing. This technology has been substantially developed and is ready for the demonstration plant phase (2). Factors Affecting Conversion R a t e of Cellulose The chemical reaction, hydrolysis, for converting cellulose to glucose was introduced in the previous section. The rate of this reaction, as catalyzed by either acids or enzymes and independent of other factors, is not a rate-limiting feature for the conversion process, but rather, access to hydrolysis sites on cellulose, the @-1,4-glycosidicbond, by the catalyst and the hydrolytic agent, HzO, is the fundamental limitation preventing enhanced conversion rates and yields. If cellulose could be placed in a truly aqueous solution, its resistance to hydrolysis would be essentially limited to thermodynamic and stereochemical principals of catalysis. Native cellulose exists in a form complexed and associated with other materials, lignin and hemicellulose. Lignin is most often described as a random polymer consisting of phenylpropanoid units. Hemicellulose is a polymeric material consisting mostly of pentoses and some hexoses. The exact nature of the association of lignin and hemicellulose with cellulose is unclear. However, their close association with cellulose is a major factor in controlling access to native cellulose. Hemicellulose is much less resistant to chemical degradation by hydrolysis and can be removed with relative ease. The @-1,4-glycosidicbond between individual glucose units in cellulose forms a polymeric structure having unique conformational features which allows hydrogen bonding between hydroxyl groups of different glucose units. A highly ordered arrangement of macromolecular cellulose results, in which most glycosidic bonds are imbedded in a cellulose crystallite and are not freely available for contact with the catalyst. This crystalline feature of native cellulose is a predominant factor controlling rate and yields of conversion. Hydrolysis processes for converting cellulose to glucose involves a reaction system containing soluble and insoluble species in an aqueous environment. Since available reaction sites are located on the solid surface of the insoluble cellulosic material, it is reasonable to expect that to a certain extent, reactions rates would be limited by the amount of exposed surface. This is especially true of native cellulose as found in whole plant forms. It is obvious that gross reduction of native cellulose as such must occur before processing. Not only does this reduction take place in pulp manufacture, but a considerable amount of additional physical and chemical degradation occur which promote the susceptibility of cellulose for hydrolysis. The cellulosic materials in municipal solid waste have been processed considerably by the pulp and paper industry. All of the factors mentioned previously have been affected to various degrees by this extensive processing. Delignification is a major objective in the production of many pulp products. Therefore, MSW contains a portion of cellulosic materials which has much less than the original lignin content. A large portion of classified MSW is composed of newsprint. This cellulosic material is derived mostly from mechanical pulp in which lignin removal is not accomplished. Nevertheless, classified MSW represents a source of cellulose which has been considerably pre-processed for conversion purposes. Although this pre-processing is a fortunate occurrence, additional opportunities for enhancing the hydrolysis rate of cellulose in MSW exist.
Experimental Materials a n d Methods Microorganisms. Trichoderma reesei QM 9414, obtained from the American Type Culture Collection, Rockville, Md., was used to produce cellulases for both saccharification and simultaneous saccharification/fermentation (SSF) (see procedures below). Permanent stock cultures were freeze-dried, and working stock cultures were maintained on potato dextrose agar plates a t 4 "C. Seed cultures of T . reesei were grown from spores in Mandel's medium (5) containing 2 % glucose to increase biomass. This seed culture was used to initiate cellulase production in submerged culture according to the methods outlined in ref 1. Candida brassicae IF0 1664, obtained from the Institute for Fermentation, Osaka, Japan, was the yeast used in SSF. Permanent stock cultures were freeze-dried, and working stock cultures were maintained on Difco YM agar, Difco, Detroit, Mich., at 4 "C. Seed cultures of C. brassicae were initiated by inoculating a medium ( I ) containing 20 g/L of glucose, 5 g/L of yeast extract, 5 g/L of malt extract, and 5 g/L of bacto-peptone with a part of the stock culture. The seed was grown at 28 "C for 18 h prior to use. Saccharification and SSF. Saccharifications were run in 250-mL Erlenmeyer shake flasks at 200 rpm with a 100 mL working volume. Temperature was controlled a t 45 O C and pH was maintained at 5.0 with a 0.05 M citrate buffer. A 6% w/v cellulose concentration was the substrate, and the flasks were autoclaved prior to reaction initiation with a whole culture T. reesei cellulase. The SSFs were run in 250-mL Erlenmeyer shake flasks a t 100 rpm with a working volume of 100 mL. Temperature was controlled at 38 "C and pH was maintained a t 4.75 with a 0.05 M citrate buffer. The SSF medium (1) contained a 6% w/v cellulose concentration. The flasks were autoclaved prior to reaction initiation with a 5% v/v C. brassicae broth and a whole culture T . reesei cellulase. Assays. Glucose and cellobiose were determined with a Varian 8500 HPLC and refractive index detector, Varian Instruments, Sunnyvale, Calif., equipped with a Partisil PXS 10125 PAC column, Whatman, Inc., Clifton, N.J., and 80% acetonitrile, Burdick and Jackson Laboratories, Muskeegon, Mich. Glucose was also measured with a Model 23A YSI glucose analyzer, Yellow Springs Instrument Co., Yellow Springs, Ohio. Total reducing sugars were determined by a modification of the Miller dinitrosalicylic acid method (6). Ethanol was determined with a Model 3920 gas chromatograph, Perkin-Elmer Corp., Nonvalk, Conn., equipped with a 3-ft column packed with Porapak Q, Waters Associates, Farmington, Mass., and a flame ionization detector. The oven temperature was 150 "C. Protein was determined by the Lowry method (7) using a bovine serum albumin standard curve. Pretreatment Methods The subject of pretreatment of cellulose to enhance its conversion to glucose has received a considerable amount of attention (8-11). A detailed discussion of all of the technical and economic facets of all types of pretreatment approaches that have been pursued would involve time and space beyond the scope of a presentation such as this. Selected aspects of various approaches to pretreatment of lignocellulosics which we have considered and investigated will be discussed. Chemical, Through the use of various kinds of chemical agents, it is possible to have a significant impact on enhancing the susceptibility of cellulose for conversion by either acids or enzymes. However, some very important
Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No. 3, 1980
considerations must be mentioned with regard to implementing the use of such chemical agents on an industrial scale. These considerations concern the various processing problems and costs involved. It is especially important to discuss some of these agents since a great deal of discussion has concerned the expected technical and economic breakthroughs that their use could provide (12-14). The objectives in applying the use of chemical agents to enhance the conversion of cellulosics include: (1)the isolation and/or disruption of cellulose from the other components, lignin and hemicelluloses, and (2) decrystallization of native cellulose. By isolating and/or decrystallizing the cellulose, access to the P-1,4-glycosidic bond is improved, and consequently, the rate and extent of hydrolysis of cellulose can be significantly increased. Certain chemical agents can be used to meet such objectives; however, the trade-offs in achieving a net economical gain must be questioned. A variety of chemical agents have been studied and reported previously (13). The use of NaOH, NH,, ClOz, SOz,and amines have lbeen investigated to variously effect swelling and delignification. Improvements in the hydrolysis of lignocellulosics treated have been observed, but economic considerations remain a problem. The cost for chemicals alone often precludes further development work. In most instances, the necessity for recovery and waste treatment systems for the chemical agents, which can be expected, brings additional processing costs. It has long been known that concentrated sulfuric acid (i,e., 72%) can be used to solubilize cellulose from its native state (15,16). This phenomenon has been exploited in the development of assays for determining the concentration of cellulose in lignocellulosic materials. Solubilizing cellulose can aid both in the extraction of the material from its native micro-environment, and at the same time cause decrystallization of cellulose resulting in a greatly improved accessibility to the desired reaction sites on the cellulose polymer. If one hopes, to exploit this phenomenon, however, a series of problems (e.g., items 1-5 below) must be solved which will allow implementation on a large industrial scale. (1) Recovery of H2S04. It is imperative that the solvent acid be recovered for recycle. Loss of the large quantities involved would not be economically acceptable. (2) Equipment Maintenance. This consideration could be the overriding concern. The use of concentrated sulfuric acid or any other concentrated acid would probably result in formidable corrosion problems in maintaining equipment. Maintaining the integrity of seals on various pieces of equipment is just one example. (3) Disposal of Residual H2S04. The use of concentrated acids would inevitably result in the necessity of disposing some residual HzS04. Hence, a disposal processing system would be required. (4) Conversion of Cellulose in the Solubilized State. Since it is not technically feasible to hydrolyze the cellulose in a solubilized state in the presence of concentrated sulfuric acid, it would be necessary to produce a decrystallized cellulose for further conversion. The proposal to use a solvent such as methyl alcohol to induce such a separation could itself result in significant problems (12). For example, products, from the chemical interaction between the solvent H2SO4 and the precipitating agent could be expected. It is possible that toxic materials may be formed which would present subsequent disposal and processing problems. (5) Production of Fermentation Inhibitors. Most scenarios for the utilization of cellulose involve the pro-
425
duction of ethyl alcohol as an end product. It is proposed that this be achieved by the use of fermenting yeasts. It is known that yeasts are sensitive to various chemical compounds during the fermentation production of ethyl alcohol. An example of a compound which decreases ethanol production is 5-hydroxymethyl furfural which can be formed from glucose when in the presence of acids (17). Another technically effective approach to enhancing the susceptibility of cellulose for bioconversions is the utilization of various solvating agents. Examples of these are complexes between iron and tartrate, cadmium and ethylene diamine, copper and ethylene diamine, etc. The solubilizing effect on cellulose of these complexes has been known for some time (18). It is possible to solubilize cellulose such that the crystallites in native cellulose have been essentially eliminated. Amorphous cellulose may be recovered from the solution. The resulting state of the cellulose thus prepared is highly susceptible to hydrolysis by either acid catalysis or cellulase catalysis (13). Again, when considering the utilization of such chemical agents, a variety of processing problems are encountered which must be solved before industrial scale implementation can be realized. Some of these considerations are listed below. ( I ) Cost for Chemicals. Large amounts of expensive chemicals would be involved. (2) Recovery of Chemical Agents. In order to implement a solvent process on an industrial scale, it would be imperative that such chemical agents be recovered with a very high efficiency. This could involve additional sophisticated recovery systems which would add an additional expenge to processing. (3) Toxicities of Residues. This consideration could be an overriding problem. In the case of the use of cadoxen (I 3), a complex between cadmium and ethylene diamine, residues from the process containing only trace quantities of cadmium could be a formidable problem. The problems with cadmium are well known. Not to mention the inhibiting effects that cadmium has on fermenting yeasts (19),toxic residues from industrial manufacturing have, in recent years, become an especially sensitive issue that must be addressed. Broad scale implementation of cellulose resource recovery and utilization must involve the use of the cellulosic materials from municipal solid wastes. In order to effectively do so, it is essential that all aspects of any processing concept be compatible with the types of materials that may accompany the desired cellulosic stream from a recycling facility. These additional noncellulosic solids consist of a myriad of chemical types of materials. In that regard, one must consider chemical interactions between chemical pretreatment agents and accompanying materials that would be present in a classified MSW stream. These may include the production of toxic and undesirable chemicals with accompanying disposal problems as well as the production of materials that could be detrimental or inhibitory to the process. Especially adverse effects may be encountered in a fermenting yeast system. The above comments are only some of the reasons why an approach involving the use of chemical agents as a pretreatment for cellulosics has been rejected. Electron Irradiation. The use of electron irradiation as a pretreatment to enhance conversions of lignocellulosics is an intriging concept that has been studied previously (9). The data of Millet et al. suggest that with increasing treatment, the susceptibility of cellulose to hydrolysis is increased. Their evaluations were conducted using acid hydrolysis or digestion by rumen organisms. Interpretation of the effects of electron irradiation appears to have been
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980 E l e c t r o n I r r a d i a t e d MSW
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based on residual unconverted cellulose rather than on product formation. Previous work in the area of electron irradiation of cellulosics had not involved the use of classified MSW. The effects of this type of treatment on the production of ethyl alcohol from cellulose using both cellulases and fermenting yeasts also had not been determined. Although previous estimates indicated an uneconomical dosage of 100 Mrads appeared optimum for acid saccharification (9), the nature of the evaluations would not allow a prediction of results using cellulase enzyme hydrolysis and fermentation. Air classified MSW was subjected to various radiation dosages ranging from 10 to 100 Mrads from a 100-kW electron beam accelerator. The material was irradiated a t a moisture content of 11% . Ethyl alcohol production was measured following the procedures outlined in the Experimental Materials and Methods section. The results are presented in Figure 2. It is clear that increasing irradiation results in a decreasing ability to produce ethyl alcohol from the cellulose in air classified MSW. The same trend was observed in the production of reducing sugars in experiments where yeast were not present (data not included). These results would not have been predicted based on the results and interpretations from earlier work (9). The complicating effect of dealing with lignocellulosics in air-classified MSW, however, was not addressed before. It is unclear how noncellulosic materials that accompany the air classified fraction are affected by electron irradiation and what the consequences would be for the biological system used for these conversion studies. Mechanical. As a pretreatment for waste cellulosics, conventional mechanical grinding techniques can be used to separate and disintegrate fibers. From a processing standpoint, there are positive aspects in the use of these techniques. These positive aspects include temperature control, wet or dry processing, no chemical cleanup or recycle, and a relatively simple operating sequence. Although there are many positive aspects, there are also negative points such as maintenance of the grinding me-
dium or mechanism, dewatering equipment in the case of wet grinding, and very possibly a high power requirement. Although a great deal of mechanical grinding experience exists, mechanical pretreatment of cellulosic wastes to enhance enzymatic susceptibility is a relatively new application of grinding technologies. Therefore, it is likely that opportunities for improvements will emerge. For example, disk refining is commonly used in the pulp and paper industry to separate natural wood fibers while maintaining their integrity for pulp production (201, but in the case of enzymatic conversion of cellulose, fiber strength is not desired. In the cement industry, ball mills are used for comminution of a highly friable material (21). Cellulosics are low in friability and are most resistant to comminution using standard methods. Therefore, new approaches to grinding may be required to produce a substrate highly susceptible to enzymatic hydrolysis while remaining economically feasible. Increased susceptibility to enzymatic hydrolysis is the major objective of pretreatment, but economic feasibility is of ultimate importance. In the search for a feasible mechanical pretreatment, equipment such as the two roll mill and the vibratory ball mill have been tested on the bench scale (8,9,22). We have examined several methods of mechanical treatment using commercially available equipment. The contribution of power requirements to operating costs have been determined, which should provide a framework and perspective for further development work. Disk Refining. For this study, disk refining was performed at the Jones Division, Beloit Corporation, Pittsfield, Mass. A blend of 50 parts of pulp mill wastes, 25 parts of sawdust, 12.5 parts of newsprint, and 12.5 parts of corrugated cardboard by dry weight was the test substrate. This blend was run through a Shark pulper and a Belcor junker prior to refining at 730 rpm and 4% w/v solids in a 20-in. DD 4000 Duoflo refiner equipped with B-J no. 37 stainless steel disks. Three passes were made through the refiner with samples taken after each pass. Power requirements were monitored continuously. The reducing sugars produced in subsequent saccharifications indicate a nominal improvement with each pass through the refiner (Figure 3) when compared with the hydropulped control. Since the kilowatt hours/bone dry ton (KWH/BDT) increases dramatically with each refiner pass, the cost of pretreatment also rises proportionately. In addition, the need for dewatering equipment as well as the blockage and increased wear of the refiner plates
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, MSW'
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caused by extraneous pieces of metal remaining in air classified MSW are problems that must be overcome if disk refining is to be a feasible pretreatment. Thermomechanical Pulping. Thermomechanical pulping (TMP) is used in the pulp and paper industry much the same as disk refining. Due to the use of pressurized steam, however, air-classified MSW can be refined under conditions that do not require a subsequent dewatering step. Although use of steam results in additional cost, sterilization of the feedstock is a positive consequence of this type of treatment. T M P testing was performed at the Bauer Division, Combustion Engineering, Springfield Ohio. Air-classified MSW was subjected to 100 psig saturated steam digestion for times ranging from 4 to 30 min. Two additional 100 psig saturated steam digestions were conducted in the presence of either 1.0% w/w NaOH or 1.0% w/w H2S04. Immediately following digestion, the MSW was refined in a C-E 418 double disk refiner at 15 psig. The plates used were C-E 36104/7R0869M. Samples were taken following both steam digestion and disk refining for evaluation in SSF and saccharification. Upon sampling, the MSW treated in the presence of either 1% w/w NaOH or 1%w/w HzS04was immediately neutralized. The SSF data (Figure 4) indicate no improvement in ethanol yield from 100 psig steamed MSW over untreated controls after both 24 and 48 h. The time of steaming does not have a noticeable effect on ethanol production. MSW steamed 4 min in the presence of 1% w/w H2S04also showed no improvement over the control; however, the MSW steamed 4 min in the presence of 1% w/w NaOH did show improvement in ethanol production at both 24 and 48 h. All samples were effectively sterilized by the 100 psig saturated steam treatment regardless of residence time. At $7.66/BDT of feedstock, steaming a t this pressure to increase ethanol production is not economical. The 24-h SSF data (Figure 5) indicate no increase in ethanol production from the steamed MSW following disk refining; however, the 48-h data indicate a 2.5 g/L increase in ethanol production at all steaming times except 10 min. No reason is apparent for the depressed results for the 10-min treatment. Also, no reason is apparent for the NaOH and HzS04treated samples failure to show any improvement. Since the average disk refining cost of $21.72/BDT coupled with the steam costs of $7.66/BDT is high, while producing only minimal increases in ethanol
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Figure 5. Effect on ethanol yield from cellulose in municipal solid waste in simultaneous saccharification/fermentation by 100 psig saturated steam followed by disk refining at $29.38/BDT. SSF of A g i t a t i o n B e a d M i l l e d , S i m u l a t e d M S W ' 0 -1
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yields, pressurized steaming and refining seem impractical. Additional problems were again encountered with extraneous pieces of metal blocking the refiner plates. Previous tests at lower steam pressures and either 15 psig refining or atmospheric refining also produced no positive effects (data not reported). Agitation Bead Milling. The agitation bead mill is commonly used in the food and pharmaceutical industries for fine grinding and fine dispersions. The grinding medium, either ceramic or steel balls, is continuously agitated by an impeller. A simulated MSW consisting of 50 parts of newsprint and 50 parts of corrugated cardboard was the test substrate. Processing was conducted a t Draiswerke, Inc., Mannheim, West Germany, using a Model PM 75sRS agitation bead mill equipped with a 75-kW motor. A 6% slurry of the simulated MSW was ground with 6-mm and/or 8-mm stainless steel balls. Power requirements were monitored continuously. The SSF data (Figure 6) indicate an increase in ethanol yields at both 24 and 48 h as more power is used with
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
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Figure 7. Effect on ethanol yield from cellulose in municipal solid waste in simultaneous saccharification/fermentation. Hammer milled at an initial 25% moisture.
increased processing time. The increase in ethanol production may be attributed to the production of a finely ground substrate with increased surface area and perhaps increased hydration of the cellulosic particles. The increases in ethanol production are encouraging, but opportunities for increasing efficiency exist which would offset operating and capital expenditures. Agitation bead milling as a pretreatment for SSF appears borderline at present but represents a processing method with promise. Hammer Milling. Hammer milling is used in a wide variety of applications for particle size reduction. The test substrate, air classified MSW, was hammer milled a t the Williams Patent Crusher and Pulverizer Company, St. Louis, Mo., in a Williams “Rocket” hammer mill through 1/2, 1/8, and l/s-in. screens, respectively, a t an initial 75% solids consistency. The SSF data (Figure 7 ) indicate an increase in ethanol production from samples passing smaller screen openings which correlate with greater power requirements. Although ethanol production increases within the set of hammer-milled samples, ethanol production remained lower than or equal to the control MSW. The loss in reactivity may be related to drying of cellulose during milling. Even though the capital and operating expenses for hammer milling are very reasonable, the absence of improvement in ethanol production makes this pretreatment impractical. Vibratory Rod Milling. The vibratory rod mill is commonly used for the milling of dry, highly friable materials as well as for making dispersions. A test substrate was prepared from air classified MSW that was hammer milled through a 1/4-in,screen. Tests were conducted in a batch phase Palla mill, Humboldt-Wedag, U.S.A., Atlanta, Ga., using either 12.5 mm $ or 20.0 mm $ steel rods. Power requirements were monitored for each batch. The SSF data indicate that ethanol yields are related to the initial moisture level in the MSW (Figure 8). At the same time, the operating costs decrease slightly as the initial moisture increases. However, after 24 h, ethanol production was only 0.85 g/L greater than the hammermilled control in the best case. By 48 h, all test cases were at least equal to the control in ethanol production. Again, however, the best case was only 1.7 g/L greater than the control. In another series of SSFs, the initial moisture level was constant at 9.0% and the power applied was varied (Figure
14
5
90
16 5
% M o i s t u r e at M i l l i n g I n i t i a t i o n I O m n i l e m I1 ng I 2 5 m m 0 R o d s
K W H / BOT 2
l%/BDTi
Figure 8. Effect on ethanol yield from cellulose in municipal solid waste in simultaneous saccharification/fermentation by vibratory rod mill with 12.5-mm diameter steel rods for 10 min; cost/bone dry ton in parentheses.
2oi
S S F of V i b r a t o r y R o d M i l l e d M S W
42
84
$/BDT I26 168
210
252
29,4
600
700
5 !
0
100
200
300
400
500
KWH/BDT I 90% M o i s t u r e
Figure 9. Effect on ethanol yield from cellulose in municipal solid waste in simultaneous saccharification/fermentation by vibratory rod mill at an initial 9.0% moisture.
9). The 48-h conversion data show that the MSW milled with 12.5 mm 4 rods yields increasing ethanol levels as applied power increased through 642 KWH/BDT. At the same conversion time (48 h), the MSW milled with the 20.0 mm $ rods produced a maximum amount of ethanol between 100 and 200 KWH/BDT. In all cases, the 24-h data were equal to or less than the hammer-milled control while the 48-h data were all greater than the hammer-milled control. The improvements were at best, however, only 3.0 g/L greater than the hammer-milled control. The data taken together (Figures 8 and 9) indicate that milling a t an increased initial moisture content may result in greater cellulose susceptibility. Although operating costs for vibratory rod milling may be reasonable, considerably greater ethanol yields must be attained before it is a feasible pretreatment. Summary Cellulose resource recovery and utilization to produce ethyl alcohol have been discussed. Two major deterrents to the utilization of cellulose as a chemical and energy raw material have been collection and preparation. But in the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980 Ethanol Production Costs v s F'retreotrnent C o s t s
/'
5
I5
25
35
45
% IBDT, Pretreatment
Figure 10. E t h a n o l p r o d u c t i o n costs vs. pretreatment costs for various yields f r o m cellulose. Calculated o n t h e basis of 50% cellulose in dry substrate.
context of using classified lignocellulosics from municipal solid waste,collection and part of the preparation problems are overcome. In this instance, a much more susceptible cellulose that has been substantially altered from its native state by industrial processing is available. Disposal problems for MSW are a complement to the technology for preparation and enzymatic conversion of cellulose to ethyl alcohol. Preparation of cellulosics for enzymatic conversion may also involve additional treatment to enhance the susceptibility of cellulose to hydrolysis. Several pretreatments have been considered and discussed in this presentation. (1)Chemical pretreatments that have been previously investigated involve chemical costs and processing concepts that appear prohibitive for industrial scale implementation. (2) Ethyl alcohol and reducing sugar production from electron irradiated MSW does not conform to the trends suggested by cellulose digestibility by acids or rumen organisms reported previously. A net decrease in product was observed as compared to a control with no irradiation. (3) Steam treatment of MSW a t 100 psig up to 30 min does not enhance ethanol production. However, concurrent steam treatment with 1% NaOH does result in a modest improvement in cellulose susceptibility. (4) Disk refining, hammer milling, and vibratory rod milling do not indicate cost effective approaches to preparing classified MSW for ethyl alcohol production. There is an indication that moisture content of the substrate may be a factor in the effects observed. (5) Agitation bead milling in a low solids slurry has shown some promise.. Current data do not include results from actual MSW material. The presence of noncellulosic solids in MSW may result in processing problems with this type of equipment. As with any process, each step involved is a factor in the production costs. Ultimately, any approach to the preparation and pretreatment of cellulosics must be measured against its overall costs to the process. Regardless of how
429
technically effective a pretreatment may be, it must be workable and cost effective before it can be implemented industrially. A perspective on the relationship of pretreatment costs to the production costs/gallon of ethyl alcohol may be obtained by examining Figure 10. The data for Figure 10 were calculated on the basis of a dry cellulosic feedstock having 50% cellulose. The differences in production costs between different % conversions reflect only operating cost consequences for a pretreatment step. Fixed charges would increase with decreasing 70 conversions, resulting in greater diversions in the production costs for ethyl alcohol. Finally, although many microbial, chemical and mechanical pretreatments may be technically feasible, they are economically not feasible. Our data indicate that with proper process development some form of thermomechanical pretreatment will provide a technically feasible, as well as economically feasible, pretreatment for the conversion of waste cellulosics to liquid fuels and industrial chemicals.
Acknowledgment The authors wish to express their appreciation to Ms. Thresa A. Martin, Mr. Robert S. Evans, 11, and Mr. Raymond A. Craft, 111, for their excellent technical assistance during these and ongoing studies. This paper was presented at the 11th Central Regional Meeting of the American Chemical Society in Columbus, Ohio, May 7-9, 1979.
Literature Cited Blotkamp. P. J., Takagi, M., Pemberton, M. S., Emert, G. H., AIChE Symp. Ser. No. 787, 74,85 (1978). Emert, G. H., Katzen, R., Joint Meeting, American Chemical Society and Chemical Society of Japan, Honolulu, April 1979, Preprints, p 488. Brink, D. L., Charley, J. A., Faltico, G. W., Thomas, J. F., "Thermal Use and Properties of Carbohydrates and Lignins", F. Shafizadeh, K. V. Sarkanen and D. A. Tillman, Ed., 1976. Brickner, R. H., Allis-Chalmers, personal communication, 1978. Mandels, M. H., Sternberg, D., Andreotti, R. E., "Symposium on Enzymatic Hydroiysis of Cellulose", Aulauko, Finland, M. Bailey, T. M. Enari, and M. Linko, Ed., 1975. Rivers, D. B., Gulf 011 Corporation, unpublished, 1979. Lowry, 0. H., Rosenbrough, N. H., Farv, A. L., Randall, R. J., J . Biol. Chem., 193, 265 (1951). Millet, M. A., Baker, A. J., Biotechnol. Bioeng. Symp., No. 5, 193 (1975). Millet, M. A., Baker, A. J., Sutter, L. D.,Biotechnol. Bioeng. Symp., No.
6, 125 (1976). Halliwell, G., R o c . Bioconversion Symp., IIT Delhi. 81 (1977). Ghose, T. K., "Symposium on Enzymatic Hydrolysis of Cellulose", Aulauko, Finland, M. Bailey. T. M. Enarl, and M. Linko. Ed., p 73, 1975. Tsao, G. T., Indiana Biomass Conversion Conference, West Lafayette, Ind., March 1979. Ladisch, M. R., Ladisch, C. M., Tsao, G. T., Science, 201, 25 (1978). Tsao, G. T., Process Biochem., 13, 12 (1978). Ritter, G. J., Seborg, R. M., Mitchell, R. L., Ind. Eng. Cbem., Anal. Ed.,
4,202 (1932). Saeman, J. F.,Bubl, J. L., Harris, E.E.,I n d . Eng. Chem., 17,35 (1945). Ferrier, R. J., Collins, P. M., "Monosaccharide Chemistry", Penguin Library of Physical Sciences, William Clowes and Sons, Ltd., London, 1972,p 93. Jayme, G., "Cellulose and Cellulose Derivatives". "High Polymers", Vol. 5, Wiley-Interscience, New York, 1971. White, J., "Yeast Technology", Chapman and Hall, Ltd., London, 1954. Danforth, D. W., "Handbook of Pulp and Paper Technology". K. W. Britt, Ed., Van Nostrand-Reinhold, New York, 1970. Rowhnd, C. A., "Applying Large Grinding Mills", Technical hess Bureau, Milwaukee, 1970. Tassinari, T., Macy, C., Biotechnol. Bioeng., 19, 1321 (1977). R e c e i v e d for review J u n e 18, 1979 A c c e p t e d M a y 19, 1980
