Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 457-465
457
Fermentation as an Advantageous Route for the Production of an Acetate Salt for Roadway De-icing Chester W. Marynowskl,' Jerry L. Jones, Daniel TWO, and Robert L. Boughton SRI International, Menlo Park, California 94025
Alternative processes and alternative biomass feedstocks were screened to identify a preferred process and feedstock for commercial production of calcium magnesium acetate (CMA). The process identified as having the greatest economic potential was one that used the anaerobic thermophilic bacterium Clostridium thermoaceticum to ferment biomassderived sugars to acetic acid and react the acetic acid wlth dolomitic lime to make CMA. Preliminary process designs were prepared, and detailed economic analyses were carried out for three alternative CMA processes. Cost sensitivity analyses were included. For the base case, the estimated selling price (plant gate) of CMA was 18-19@/lb (40-42@/kg), or about 7-8 times the current cost of road salt. Recommendations are made for further work Judgednecessary to attain the performance of the process assumed for the base-case economic analysis.
Introduction Approximately 8 X 106 t (t, metric ton) of de-icing salts, primarily calcium chloride (CaC12)and sodium chloride (NaCl), are spread on roadways in the United States annually. Except for the use of NaCl as a raw material for caustic and chlorine production, roadway de-icing is its biggest use and represents about 20% of annual consumption. Roughly a third of the total CaClz consumption is for roadway de-icing. However, as shown in Tables I and 11,CaCI2represents only a few percent of the total use of de-icing salts. At a delivered cost of $25 to perhaps as high as $50/t, NaCl is the economic choice compared to CaC12,which costs well over $100/t. Calcium chloride is used only when low temperature conditions require it. This massive use of chloride salts for roadway de-icing is the cause of serious corrosion problems and some environmental problems in the United States. These problems include deterioration of Portland concrete bridge decks through chloride ion corrosion of reinforcing steel, corrosion of structural members in bridge structures and other highway appurtenances, corrosion of vehicle chassis, pollution of aquatic habitats and drinking water sources by sodium and chloride ions in runoff, and harm to roadside vegetation. The costs associated with the corrosion problems are enormous. For example, the use of de-icing salts has been a major factor in the decay of the nation's transportation infrastructure. Recent estimates show that more than a quarter million bridges in the United States, 45% of the total, are structurally deficient or functionally obsolete. In 1983, the Federal Highway Administration (FHWA) estimated that the cost of upgrading the 263 OOO deficient bridges in the United States would approach $50 billion. Bridge deck deterioration caused by corroding reinforcing bars was cited as the most common problem. In January 1983, President Reagan signed the Surface Transportation Assistance Act of 1982 into law, thus providing $7 billion to help upgrade, repair, or replace deficient bridges (Eng. News Record, 1984). Although the damage resulting from the use of highway de-icing salts is very high, it must be balanced against the safety and economic benefits derived from keeping pavements clear during winter. The U.S. Department of Transportation's FHWA initiated research work during the mid-1970s aimed to develop substitutes for chloride salts in de-icing. In FHWA-funded research conducted by Bjorksten Research Laboratories, calcium magnesium
Table I. Consumption of NaCl in the United States (Million Metric Tons)''* Ye= roadway de-icing total all uses 1960 2.2 (10.7)e 20.6 1965 4.1 (17.7) 23.1 1970 7.1 (17.1) 41.5 1975 7.1 (19.0) 37.4 1976 8.1 (20.1) 40.3 1977 7.9 (20.0) 39.4 1978 7.7 (19.7) 39.0
nAbout 23 million metric tons or 59% of the total is consumed by the chemical industry for chlorine and caustic, sodium carbonate, and miscellaneous uses. Source: SRI International "Chemical Economics Handbook". cPercent of total NaCl consumption. Table 11. Consumption of CaClz in the United States (Million Metric Tons)' Ye= roadway de-icing total all uses 1961 0.118 (25)b 0.471 1964 0.242 (36) 0.595 1969 0.227 (33) 0.687 1972 0.163 (25) 0.651 1975 0.162 (27) 0.600 1977 0.192 (30) 0.640 1980 0.192 (35) 0.549
Source: SRI International "Chemical Economics Handbook". *Percent of total CaCI, consumption. Table 111. Selected Candidate Chemicals Screened as Replacements for NaCl in Roadway De-icing" inorganics NaHC03 K2C03 (NH&HPO4 Na2C03 KHZPO4 NH4HC03 NaHZPO4 KzHPO4 (NH41zC03 NaHP04 K4P207 NH3 KHCO3 NHIHZPO, salts of organic acids organics calcium formate methanol calcium acetate ethanol magnesium acetate 2-propanol calcium propionate acetone magnesium lactate urea glycine formamide dimethyl sulfoxide ethyl carbamate
Source: Dunn and Schenk, 1980.
acetate (CMA)and methanol were identified as potentially acceptable, noncorrosive alternative de-icing chemicals
OI96-432lf85f1224-0457$01.5QfQ 0 1985 American Chemical Society
458
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985
Table IV. Twenty-four Participating States in the Pooled-Fund Study on CMA California Maryland Oregon Massachusetts Pennsylvania Connecticut Rhode Island Idaho Michigan Illinois Minnesota Texas Indiana Missouri Vermont Nebraska Virginia Iowa Washington Kansas New York Wyoming Maine Ohio Table V. Consumption of Acetic Acid in the United States" year metric tons 1970 1975 1976 1977 1978 1979 1980
880 000 995 000 1126000 1177000 1277 000 1464 000 1332 000
Source: SRI International "Chemical Economics Handbook".
(Dunn and Schenk, 1980). Other candidates considered during that research program are listed in Table 111. Currently 24 states (listed in Table IV) and the FHWA are involved in the CMA R&D effort. The program consists of a number of parallel projects in the following three areas: determination of the environmental acceptability of CMA, development of manufacturing technology for CMA production during this decade from non-oil and non-gas feedstocks, and evaluation of technical and economic benefits and problems with the use of CMA. Fossil Fuels Feedstocks/Process Alternatives for CMA At present, no commercial souces of CMA exist. The individual acetates of calcium or magnesium are available as specialty products, but not in adequate quantities or at a low enough cost to be considered as potential raw materials for CMA for highway de-icing use. The availability of dolomitic limes or limestones is not viewed as a problem. Almost all parts of the United States, including the snow belt states, have adequate existing or potential sources. The acetate sources, however, represent a much different situation. Because the purchase price of CMA is likely to be close to 20c/lb ($440/t) compared with a price of 16-2$/lb ($22 to $44/t) for NaCl for road use, it is unlikely that CMA will ever completely replace NaCl use in the United States. The first uses for CMA are most likely to be on bridge decks (and on roadways at some distance on either side of the bridges, so as to minimize NaCl drag-on by vehicles to the bridges from the roadways). In a recent analysis of acetate feedstock alternatives for the FHWA, we arbitrarily selected a figure of 10% market penetration for CMA into the highway de-icing chemical market at a substitution ratio of 1.5 weight unit CMA/ weight unit NaCl (Marynowski et al., 1983). This represents a potential demand for acetic acid of -1 million metric tons/year, or more than 50% of the acetic acid production capacity in the United States in 1980 and 75% of the 1980 consumption (see Table V). The FHWA specified that the size range of CMA production facilities considered should be from 100 tons/day (90.7 t/day) to 1OOO tons/day (907 t/day). The acetic acid feedstock requirement for the largest CMA plant would be about 240 000 t/year, which is equivalent to the total output from a large modern acetic acid plant. Therefore, each of the larger CMA plants would probably require a dedicated acetic acid plant.
Another FHWA-specified goal was to develop a process that would allow the manufacture of C M A from feedstocks not derived from petroleum or natural gas. Given this goal, a logical first candidate for a feedstock would be coal, with both methanol and CO being produced from the coal and used in conjunction with the Monsanto methanol carbonylation route for conversion to acetic acid. At the time of our analysis in 1982, all US. methanol plants used methane as the feedstock, except for one new plant based on heavy petroleum liquids. Numerous methanol-fromcoal projects have been proposed for fuel production over the last decade, but none have been built and the likelihood of any significant number of such plants being constructed during this decade appears remote. Tennessee Eastman has recently constructed a plant to produce methanol by using Texaco coal gasification technology at Kingsport, TN. Plant production capacity is >200 000 ton/year (>180 000/t/year) of methanol from 1600 ton/day (1450 t/day) of coal. This size plant could supply the methanol for the CMA plant cited above (i.e., requiring 240000 t/year figure of acetic acid production). Acetic acid, however, is not the intended end use for the methanol produced at Kingsport. A representative of one major US. producer of acetic acid, who wishes to remain anonymous,has indicated that coal-based acetic acid plants probably will not be built in the United States until the late 1990s at the earliest. Recent published cost estimates by C-E Lummus indicate that although the estimated transfer price of acetic acid produced from coal could be as low as 20c/lb (446/kg) compared to 25@-26&/lb(55~/57@/kg) for acetic acid from a methane feedstock, the uncertainties in capital costs for the coal-based plant could easily lead to a transfer price of 246/lb. On the basis of the above consideration of existing market conditions and the uncertain prospects for lower cost acetic acid production from coal during the next 10year period, we decided to specify a 25@/lb (55$/kg) base-case price for acetic acid produced by chemical synthesis. The feedstock is an unspecified fossil fuel, and this base-case acetic acid source will be useful for economic comparisons with other alternatives using biomass feedstocks. Biomass Feedstocks/Process Alternatives for CMA SRI supplemented and updated its already extensive data base on biomass conversion to fuels and chemicals by a computerized search of Chemical Abstracts, Government Publications, and the Engineering Index for information regarding techniques for producing crude acetic acid from biomass. The search focused on selected combinations of the following keywords: wet-air oxidation, acid hydrolysis, anaerobic, aerobic, ferment(ation), digest(ion), acetic acid, fatty acids, volatile acids, biomass, calcium acetate, and magnesium acetate. This search yielded more than 1000 titles and abstracts, of which more than 150 of the most relevant references were ordered and reviewed. Biomass to Methanol to Acetic Acid. The results of numerous previous SRI studies of methanol production from biomass (wood or field crop residues such as wheat straw or corn stover) via thermal gasification processes indicate that methanol by this route is not likely to be competitive with methanol produced from fossil fuel feedstocks (Jones et al., 1980, 1981a; Kohan, 1980). The literature search provided no information that, in our opinion, would alter these previous findings. Biomass to Acetic Acid by Wet-Air Oxidation. Depending on the conditions of temperature, pressure, and
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985 459
Table VI. Comparison of Four Strains of C.thermoaceticum in pH Controlled Batch Fermentations range of values reported acetic acid/ yield of glucose consumed, net acetic acid glucose, acetic acid, strain PH g/L produced, g/L molar ratio wt% 1.85-2.55" 63-86 Ljungdahl (ATCC-39073) 5.8-7.2" 6.0-15.7n 4.0-15.00 6.1 13.0-18.0 9.0-15.0 2.0-2.5 68-85 woodb S-3 (MIT)b 6.0-7.0 12.0-20.5 10.0-20.0 2.1-3.3 72-100+ 6.0-7.0 9.8-30.3 8.5-20.6 1.92-2.61 65-88 1745 (Union Carbide)b
mass doubling time, h 7-5.7" 5 18.5 5
Values were obtained by SRI in 16-L batch fermentations with pH control. Adapted from a previous summary by Schwarz and Keller (1982).
reaction time used, noncatalytic wet-air oxidation can convert biomass to a variety of products, including acetic acid. On the basis of the published data available, it appeared that the operation of a wet-air oxidation unit at conditions that would allow high conversion of solid biomass feedstocks would probably result in low yields of acetic acid because of further oxidation to formic acid, COz, and HzO. Overall, we did not view wet-air oxidation technology for conversion of biomass to acetic acid to be technically or economically competitive with other options under consideration.
Anaerobic Digestion of Biomass To Produce Acetic Acid. Anaerobic digestion of biomass using mixed cultures of bacteria to produce a methane-rich fuel gas has been studied for many years. Some of the intermediate products in the process consist of a number of organic acids including acetic acid. Yield of the acetic acid is too low and the recovery from the mixture too expensive, in our opinion, to warrant consideration of this route.
Biomass Sources of Sugars for Fermentation. Monosaccharides and disaccharides produced from (1) sugar crops, (2) starch crops, or (3) lignocellulosic materials (wood or field crop residues) are potential feedstocks for the production of acetic acid from biomass by several fermentation routes. SRI and others have conducted studies on the use of existing and proposed new sugar crops in the United States for a fermentation feedstock to produce fuels or chemicals such as ethanol. On the basis of the results of these studies, we do not view sugar crops as being cost competitive with starch (in corn grain) during at least the next decade. Acid or enzymatic hydrolyses of lignocellulosic materials to sugars are possible process options for a fermentation facility feedstock. However, the hydrolysis of lignocellulosic feedstocks does not appear to us to offer sufficient economic benefits to justify the attendant risks (Jones et al., 1981a-c; Kinderman et al., 1980, Jones and Semrau, 1982). We believe that there are generally no economic alternatives in the United States to corn as the preferred feedstock for large fermentation facilities for the production of commodity chemicals such as ethanol (or acetic acid) during the 1980s. Fermentation Routes to Acetic Acid. After an extensive review of the literature, we concluded that only two fermentation routes offer significant near-term potential for the production of acetic acid in high yield from sugars (see Schemes I and 11). The actual yields of acetic acid shown in Scheme I1 are often above 80 w t 3' % (see Table VI) from both C6 and C5 sugars. Furthermore, the byproducts produced, other than additional bacterial cell mass, are negligible. Route 2 is potentially preferable economicallyto route 1because of its much higher yield of acetic acid from C6 sugars and its ability to produce acetic acid from some C5 sugars. C5 would represent a significant portion of the sugars produced by hydrolysis of lignocellulosicfeedstocks; these C5s.ugars are not fermented by yeasts currently used commercially in route 1 and thus would not be a source
Scheme I. Route 1-Commercially Available Technology for Vinegar Production (Vaughn, 1954) step 1 S. ceraisae
C6H1,06 (yeast) 2C,H,OH 92 kg 180 kg (glucose) (ethanol)
+
2C0, 88 kg
step 2 2C,H,OH t 2 0 , 92 kg (ethanol)
A . aceti _ _ _ f
(bacteria)
2CH,COOH
+
120kg (acetic acid)
2H,O 36 kg
Scheme 11. Route 2-Research in Progress o n Converting C,- and C, Sugars to Acetic Acid (Webb, 1964) C. thennoaceticum C6H1206
180 kg C,Hl,O, 1 5 0 kg
' 3CH,COOH 180 kg
C. thennoaceticum
' 5CH,COOH 1 5 0 kg
of acetic acid by route 1. Route 2 is thus the one we selected for experimental evaluation and detailed economic analyses.
Process Research Influence of pH on Product Quality and on the Choice of Raw Materials. Either dolomite (CaC03. MgC03) or dolomitic lime ("dolime", CaO-MgO) dissolves readily in an excess of concentrated acetic acid. However, when the resulting acidic solution is evaporated to dryness, the residual solid CMA contains traces of free acetic acid that would render the CMA unfit for use on concrete (Dunn and Schenck, 1980). Stoichiometrically pure CMA, a mixed salt of a weak acid and two strong (though only slightly soluble) bases, is slightly alkaline, being hydrolyzed to give a pH of -9 in relatively concentrated aqueous solutions. It is thus important that when a process for making CMA for highway use involves evaporation and drying of an aqueous CMA solution, it should produce a solution with a pH of 9 before evaporation takes place. An alkaline pH is also desirable because it helps keep evaporation losses of acetic acid negligible. Producing a stoichiometric solution of CMA from dolime or dolomite is complicated by the different reactivities and solubilities of the various corresponding calcium and magnesium compounds, particularly in weak acetic acid solutions such as those that might be produced by fermentation. Dolomite is essentially insoluble in neutral or alkaline solutions, and its rate of dissolution in well-agitated acid solutions is a negative exponential function of the pH (i.e., each decrease of one pH unit results in about a tenfold increase in the dissolution rate for a given particle size of dolomite). We have found that even finely pul-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985
verized (6 would produce a solution of calcium acetate only, not one of CMA. In contexts other than fermentation, either dolomite or dolime could be used as the principal cationic raw material to make CMA from commercial or waste acetic acid. However, the final pH adjustment to pH 9 would have to be made either with dolime or with ordinary high-calcium lime. If dolime is used, the insoluble MgO residue would have to be recycled to the low-pH dissolution stage where it would dissolve to form the acetate. If ordinary lime is used for final pH adjustment, the final CMA solution would have a slightly higher mole ratio of Ca:Mg than the nominally 1:l ratio. Such a CMA product would probably still be acceptable for highway de-icing, although it might exhibit a slightly higher eutectic temperature than that of 1:l CMA; therefore, it might be a less effective de-icer under the very coldest weather conditions. Although dolomite is less expensive than dolime, the difference cost is judged to be virtually insignificant compared to the cost of acetic acid. In addition, we have observed operating disadvantages with dolomite that may more than offset its lower cost; such disadvantages include its lower reactivity and its evolution of CO,, which can cause potentially serious foaming under some conditions and can also increase the loss of acetic acid vapor. We judge that dolime is the raw material of choice for the processes we have considered to date. CMA Crystallization Experiments. In our initial crystallization experiments we examined the behavior of each of the individual acetates of Ca and Mg, which were prepared from reagent-grade oxides and acetic acid. The two systems exhibited pronounced differences when isolated from each other. Calcium acetate crystallized (as the monohydrate) in the form of tangled dendritic clusters of microscopic needles that adhered tenaceously to each other and to adjacent heat transfer surfaces, forming a hard “scale”. Like many other calcium salts, calcium acetate has a pronounced negative temperature coefficient of solubility. Magnesium acetate, on the other hand, has a positive temperature coefficient of solubility and crystallizes as the tetrahydrate. Our preliminary attempts to crystallize it showed that it does not readily self-nucleate spontaneously, even when the solution is highly supersaturated and very viscous. Such a viscous solution, when cooled, produces an amorphous “glass” that gradually becomes sticky, apparently from absorbing moisture from the air. On the basis of this behavior, we judged initially that calcium acetate should be amenable either to spray drying
or to drum dryinglflaking, whereas magnesium acetate might be more troublesome unless crystal nucleation could be induced by seeding. However, those judgments were made in regard to the individual salts and might not apply if a mixture of the two acetates could be made to crystallize as a double salt or if calcium acetate crystals induced nucleation of magnesium acetate. Subsequent experiments indicated that equimolar mixtures of the acetates of calcium and magnesium crystallized more readily than magnesium acetate alone. At least some of the crystals resulting from very rapid crystallization (such as that caused by dripping the solution onto a hot surface) formed as platelets rather than as the dendritic needles characteristic of calcium acetate alone. X-ray dispersive analysis showed such platelets to have a high mole ratio of Mg:Ca-about 3:l. We could not be certain, however, whether those apparent platelets were truly crystals or a glasslike amorphous solid solution of perhaps variable concentration limits. We also performed X-ray dispersive analyses on various samples of crude CMA, dolomite, and reagent-grade Ca(OAc),.H,O and Mg(OAc),.4H20. Four samples of crude CMA obtained from the FHWA gave results that were initially surprising, in that they showed no trace of magnesium. (That is, their X-ray spectra and their needlelike crystal shapes were essentially identical with those for reagent calcium acetate.) We interpret this result to indicate that, at least under the slow crystallization conditions used for making those four batches, calcium acetate and magnesium acetate crystallize separately, probably at different times and in different regions of the crystallizing vessel. The samples we received were apparently taken from a limited region of each batch’containing only calcium acetate. This result is significant in that it serves as a warning that any batch of CMA produced by slow crystallization may have to be pulverized and then thoroughly blended before its analysis can be expected to indicate a reproducibly uniform product. On the other hand, fast crystallization (e.g., by spray drying or drum drying a saturated solution) should produce small aggregates of microcrystalline material that are reproducibly uniform in the overall composition of each aggregate and hence would be directly usable without paior blending. Spray-Drying Experiments. Our preliminary spraydrying tests on CMA solutions were quite encouraging. Those tests were made with a laboratory spray dryer manufactured by Niro Atomizer Ltd. (Copenhagen). It employs a rotary air-driven atomizer operable a t speeds between about 10 000 and 40 000 rpm, with the highest speeds giving the finest atomization. Our preliminary tests were made with a feed solution containing 0.5 mol each of calcium acetate and magnesium acetate per kilogram of solution (corresponding to 19.5 wt % of crystallizable hydrated solids, Le., about one-half saturated in calcium acetate and far less saturated in magnesium acetate). We had no difficulty obtaining a completely dry, free-flowing, spherical product when the effluent air temperature was maintained at 120 “C or higher. At high atomizer speed ( - 38 000 rpm), the median particle size of the collected product was about 20 pm, with a range of about 5-30 pm. At 13000 rpm, the product was less dusty, with a median particle size of about 50 pm and a range of about 30-70 pm. We judge that even larger spray-dried CMA particles could be obtained if desired but that a much larger spray dryer would be required to provide adequate retention time.
-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985 461 Lime Receiving Storage and Grinding
Lime
722 IbId Sulfuric Acid 1,296 Ibld Water 8,071 x lo3 lbld Storage and Whole Corn
Storage and Grinding
GlycoAmylase 1,376 lbld
11
I
Water
ylase
and :charification
Fermentation
Thickener
32.6
Water
Water
21.426 x lo3 lbld
4,336 x lo3 lbld
4
4
Evaporation
Drying and Crushing
CMA Storage 71%Active
Component Stsrch C6 Sugars Pentosan Fats and Oils Crude Fiber Ash Lignin Water
2,064
1
1103 lb/d C6 Sugars 2,141 Soluble Solids 665 Insoluble Solids1 292 Water 8,372 Total 11,462
I
Total
14,389 x
C6 Sugars
I
kdd
Soluble Solids Insoluble Solids
Water
Soluble Solids Insoluble Solids
2,312 x lo3 Ib/d
Insoluble Solids Water CMA Total
526
ru? 1
140 2.000 2,807
i ;so;ble 1. 1
Solids Separation
I
Animal Feed Storage
Solidf
Total
Figure 1. Block flow diagram for production of CMA from corn grain.
Spray drying is not the only method of obtaining CMA particles of uniform composition. A more economical alternative on a production scale would be simultaneous drying and flaking on a rotary steam-heated drum. The resulting flakes could be designed for any desired degree of coarseness and could be made to have a uniform Ca:Mg ratio. The coarser (- 50 ym) spray-dried CMA was subjected to both thermogravimetric analysis (TGA) and chemical analysis to detect calcium, magnesium, and acetate. Both methods indicated that spray drying at 120 "C produced a CMA with essentially no water of hydration. The element ratios agreed with those of the feed composition. Presumably, if desired, a hydrated product could be obtained at a lower drying temperature. However, there would be an economic trade-off between the cost of removing the water of hydration and the cost of shipping that water in the form of hydrated CMA. In our economic analyses of alternative processes we have assumed an essentially anhydrous CMA product. Fermentation. Our initial attempts during early 1983 to obtain fermentation kinetic data for process design repeatedly gave anomalously low acetic acid yields compared to literature data judged to be reliable. Because of time constraints required to satisfy FHWA deadlines, we did not perform the extensive and time-consuming tests required to positively confirm the identity and purity of our test culture until our experimental results made it clear that such confirmation was vital to the interpretation of our data. Our subsequent identification tests provided conclusive evidence that our experimental culture predominantly consisted of some organism other than C. thermoaceticum. On the basis of the various tests we performed, we concluded that the dominant organism present was probably C. thermohydrosulfuricum. During late 1983 we conducted numerous batch and continuous fermentations using the wild strain (ATCC-
37073) of the bacterium C. thermoaceticum and found that 270 w t % ' acetate yields could be readily achieved. These results are to appear in a future publication.
Process Designs and Economic Analysis Three cases were considered in the economic evaluation for a 1000 ton/day (907 t/day) CMA production facility: (1)conversion of whole shelled corn to CMA, (2) conversion of purchased glucose syrup (from corn) to CMA, and (3) production of CMA from purchased acetic acid produced from a fossil fuel feedstock. The economic analysis was based on our best judgment of the attainable performance of a fermentation process with a reasonable amount of further development, not on experimental fermentation results. The block flow sheet with approximate mass flows for the fermentation facility processing corn grain is shown in Figure 1. The many assumptions used in preparing the design are summarized in Table VII. The flow sheet for the plant producing CMA from purchased acetic acid from a fossil fuel feedstock is shown in Figure 2, and the design assumptions are summarized in Table VIII. The production facilities are assumed to be located in the upper Midwest region, such as northern Indiana or Illinois. These areas are close to (1) large developed dolomite deposits and calcining operations, (2) large developed coal deposits, (3) large supplies of corn, and (4)large potential users of de-icing chemicals in Wisconsin, Iowa, Illinois, Michigan, Indiana, and Ohio. Estimates of the plant facilities investment (PFI) (the total cost of the plant, erected and ready for startup) were developed by (1) consulting with equipment vendors that specialize in the construction of specific equipment or plant sections, (2) analyzing published cost correlations, and (3) using SRI cost files. The total investment includes the PFI, cost of land, working capital, and startup costs.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985
462
M~Ac~/HAc
Filter
Glacial Acetic Acid
w p-l
1-
R-2 MgO Slaker and Dissolver
k
’-4
T-2 Acetic Acid p-2 Storage
Slurry
Saturated CMA (PH -9)
-1
I
P-3
H20 c Condensate
Slurry
Flaked CMA
ST
t Surplus H 2 0 to Disposal
Washed Gangue to Landfill
Slaker and Dissolver
I
1
1
I
+
5-2 _ _
Rotary Drum Dryer and Flaker
Screened CMA (-1/2”1
CMA Storage (Covered Pads)
C-1 Crusher S-3 Screen
Figure 2. Flow sheet for CMA from purchased acetic acid and dolomitic lime. Table VII. Summary o f Major Process Design Assumptions for the Production of CMA from Corn Production of the Dried Feed By-Product
Starch Conversion
0
--
99% (of theoretical) convernion of rtarch to sugars 91% (of theoretical) conversion of starch to glucose 50 w t X of the corn protein solubilized i n cooking .
0 0
0
Fenentation C. thenoaceticum ferments only monosaccharides 75 ut% yield of acetic acid from glucose 5% of glucose unfermented 10 w t X yield of cell mans froa the glucose 10 vtZ loan of glucose to undefined organic by-products 250,000-gal fermentorr vith a working volume of approximately 225,000 gal Fermentor dilut on rate q u a l t o 0.6 v 0.12 h-’) maX “ M X or D 0.072 hUse of solubilized corn protein an the nole nitrogen source for fermentation Cell MOO protein content of 60 -7. on a dry basis Agitator pomr of 0.1 hpl1000 gal Substantially complete reaction of dolime in 13 h i n the fermentor at a pR of 6.0 Fermentor operating temperature of 60-C
-
i.
Solids Separation and Cewatering
0
0 0 0 0
1 1 1 1
gal hp lb it2 1 ft
0
Concentration of the CMA Solution 0 0
-
7 lb of H20 evaporated per 1 lb of steam, uaing a foureffect evaporator with a thecmocomprensor on the first effect No serious fouling of heat exchange nurfaces from inorganic or orRanic deposits, including insoluble corn nolidn or bacterial celln
Drying and Flaking of the CMA Product 0
0 0
Thickener hydraulic overflow rate f 75 galldlft’, vith s solids loading of 6 Lbldlft’ Thickener nide wall depth of 15 ft 95% solids removal i n the thickeners Thickener underflow solido concentration of 4 vtX Filtration rate of 4 lb/ft2/h for vacuum filters 1R w t X solids cake f r a vacuum filtcrn Eonentially complete recovery of CMA by cake n m h i n g on the vacuum filter drum by using about twice the water volume of the residual water i n the 18 wtX cake
---
0
Dryer exit gan flow rate of 1000 lb/h/ft2 Hot gaa uptake of 0.23 lb of 4 0 per 1 lb of dry gas No significant contamination o$ the feed product from direct contact drying with hot boiler flue gas. (The bailer flue gas paascn through cyclones and an electrontatic precipitator before entering the dryers) No significant nticking or clumping of nolids i n the dryer No overheating of solids i n the dryer
0
-
10 lbs of AZO evaporated per it2 of effective drum surface area No problems i n removal of the CUA from the drum an a result of the presence of soluble and innoluble corn aolids No clumping or agglomeration problem as a result of the presence of soluble and insoluble corn nolids in the CWA produc t No serious odor problem resulting from the degradation of putrescible corn solids present with the CUA A dried C I U product subatantially free of water of hydration, containing 24 wt% soluble and insoluble corn nolidn
-
0.003R m3 0.75 kW 0.454 kg 0.0932 0.305
AU monetary figures for investments and operating costs are given in constant, second-quarter 1982 U.S. dollars.
Product transportation facilities are not included in the PFI, and the product prices given are on a “plant-gate”
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985 463 PRODUCTION OF CMA FROM WHOLE CORN GRAIN
a
I
I
I
n kj
n In
f 0.30 -
I
5
t
0.25
3
sa W
0.20
3
z
W
5a
0.15
0
I
I
-
I
I
I
-
-
1
U
oI
I
I
- -- --
*u 2.60
2.80
3.00
3.20
3.40
0.10 0.12 0.14-0.16 0.18 0.20 0.22
A. FEEDSTOCK COSTS-
1-00
1.25
1.50
B. PERCENTAGE OF BASE CASE PFI
L_Ld
0.7 0.8 0.9
C. STREAM FACTORpercent time on stream
0.22
0.24
- 0.26
0.28
0.30
A. FEEDSTOCK COSTSdollars per pound of acetic acid
0.75 1.00 1.25 1.50 B. PERCENTAGE OF BASE CASE PFI
B. PERCENTAGE OF BASE CASE PFI
dollars per bushel of corn
0.75
0.20
A. FEEDSTOCK COSTSdollars per pound of glucose
0.7
0.8
0.9
0.75
0.7
0.8
1.00
1.25
1.50
0.9
C. STREAM FACTOR-
C. STREAM FACTOR-
percent time on stream
percent time on stream
1 Ib = 0.454 kg 1 bushel = 0.035 m3 1 ton = 0.907 metric ton
Figure 3. Summary of sensitivity analyses for three CMA processes. Table VIII. Summary of Major Process Design Assumptions for the Production of CMA from Purchased Synthetic Acetic Acid" complete reaction of CaO in dolime (pebble size, 51 in.) at pH -9 in the second-stage reactor/thickener, at an average retention time of 2 4 h no carryover of unreacted MgO solids from the second-stage reactor/ thickener complete reaction of the MgO with acetic acid in the first-stage reactor/thickener, at pH -24 and an average retention time of 20.5 h no carryover of unreacted gangue solids from the first-stage reactor/ thickener 25% insoluble solids content in the underflow streams from both reactor/ thickeners hydraulic overflow rate of 600 gal day-' ft-2 for both reactor thickeners thickener side wall depth of -15 ft filtration rate of 16 lb of solids ft-2 h-' for vacuum filter 50 w t % solids cake from vacuum filters essentially complete recovery of acetate (as salt or free acid) from vacuum filter cake by washing the cake with condensate returned from the drying area 1 in. = 2.54 cm; 1 gal = 0.0038 m3; 1 ft2 = 0.093 m2; 1 ft = 0.305 m; 1 lb = 0.454 kg.
basis. The annual revenue requirements were estimated for 100% equity-financed facilities by using a 15% discounted cash flow rate of return method, a 20-year project life, a 15-year depreciation period using a sum-of-the-year's digits depreciation methdd, a 50% total income tax rate, a 10% investment tax credit, and standard labor, fuel, and utility rates The estimated base-case plant-gate CMA prices (100% anhydrous C M A basis) are shown in Table IX. The
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relative prices (%), compared to the purchased acid case, are as follows: corn to CMA purchased glucose to CMA purchased acetic acid to CMA
relative price, % 69 95 100
As Table IX indicates, the cost of production of CMA is heavily feedstock dependent, with raw materials representing roughly from one-half to three-quarters of the total revenue requirements for the three cases. Capitalrelated charges (including income taxes) account for from 13% to 27% of the total revenue requirements. The sensitivity of the estimated CMA price to possible variations in feedstock cost is shown in Figure 3. Note that, even at the maximum projected annual corn price, C M A from corn could still be cheaper (at 21&/lbor 46t/kg) than CMA from purchased acetic acid. The price of purchased glucose (in a syrup) from corn is obviously directly related to corn prices (i.e., the glucose price would not be expected to decline with rising corn prices). Glucose would have to sell for
