~
Research Ideas and Cost Estimates f o r
UTILIZING THE POTATO INDUSTRIALLY M E L B O U R N E L. J A C K S O N
production in the United States is more than P13tat0million tons per year. Annual per capita consumption of potatoes is about the same as that for paper products excluding paperboard ! At least 25% of potato production is processed into food, starch, and other products. Biggest obstacles to successful development of new industrial products from the potato are its own physical characteristics. More than three fourths is water, making transportation costs high. Storage life is short. Many physical properties, including water solubility, and thus processing characteristics, depend on the nature of the starch in potatoes (about 80% of the carbohydrate content), in granules within the cell structure and whether the granule is penetrated and the structure destroyed. Carbohydrate portion is about 80% starch; remaining fractions are mostly polysaccharides of arabinose and galactose, and minor amounts of other sugars. The ash is made np of oxides of potassium in bulk, a significant fraction of phosphorus, and minor amounts of sulfur, magnesium, sodium, calcium, and silicon. The starch consists of about 25% amylose and 75% amylopectin. The amylose is a straight-chained mole-
M . L. Jackson, Professor and Head of the Collegeof Engineering, Uniunsity of Idaho, does a conriderable amount of comlting work on indwtrial uses of the potato. AUTHOR
50
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
cule with only slight branching and a degree of polymerization of 1000 to 5000. In contrast, amylopectin is branched with a high degree of polymerization of about 100,000, These fractions are separated commercially in Holland. The amylose finds uses for edible films in food packaging and adds water resistance to paper. Amylopectin strengthens fibers in textile finishing and may be used as a thickening agent in foods. Industrial use of the potato must depend on the unique physical form or the peculiar chemical properties of its starch. Starch and related products can command only a rather low selling price, and profit margins depend upon the use of low cost potatoes. The State of Idaho is the country’s largest potato producer; in 1960 it processed more than half the entire crop. In the Department of Chemical Engineering at the University of Idaho, considerable emphasis has been put on research into both industrial products from potatoes and processing of potato wastes. Polmlo Slarch Floeculanlr
Flocculants have become increasingly important in industrial processing and the polyelectrolyte types command a price of $1.50 per pound. Such materials are often effective in relatively low concentrations. Other products, such as guar, sell for perhaps one fourth this amount, but still at a relatively high cost. Flocculants involve surface effects and must be used under snecific
conditions for maximum effectiveness. Potato starch, properly modified, can he an effective flocculant. La Mer at Columbia University found that one of the best flocculating agents for treating phosphate slimes to recover uranium was a modified potato starch product known as Flocgel, developed in Holland to clear up ei3uents from washing coal. Work at the University of Idaho in this area has been on flocculating ability and gelatinization characteristics of potato starches. The starch was readily gelatinized by sodium hydroxide without heating, and for flocculation purposes was treated with 5 weight per cent sodium hydroxide at the desired starch concentration. This concentration was usually kept low to avoid highly viscous solutions. For gelatinization by salts, it is satisfactory to use equal parts of salt, potato starch, and water, followed by heating the mixture with stirring at 60" C. for 5 to 15 minutes. A solution of the desired concentration of starch was then prepared by adding water. By this procedure, gelatinization could he accomplished with a lower salt consumption. A series of sodium salts was used to determine gelatinization characteristics of various anions-Le., minimum quantities to produce gelatinization. These follow a Hofmeister lyotropic series with sodium hydroxide being most effective, sodium chloride least. NaOH
> NaGH60s > NaCNS > Nal > Nab > NaCl
Sixty times more chloride than hydroxide was required to produce the same effect. Results with a common anion (chloride) showed the monovalent cations to he much less effective than the divalent: aluminum chloride was very effective. A mixture of equal parta of zinc chloride. and calcium chloride is hetter than calcium chloride of equal weight, hut not enough to justify the extra cost involved. The conditions under which potato starch would he an effective flocculant have also been investigated, using phosphate tailing supplied by the Tennessee Valley Authority. This tailing, similar in size and chemical composition to that described in the literature w s r e suspended by hydrating in water with agitation. T h e , fine material in the suspension would not settle appreciably, even after a period of hours, and was considered a difficult material to settle. , , Using this phosphate tailing and various flocculants, suspensions were .prepared by rapidly mixing (two minutes) with identical agitation equipment as shown in Figure 1. Potato starch, gelatinized with calcium or zinc chlorides or sodium hydroxide was the best flocculant and produced both rapid settling and a clear supern liquid in a matter of minutes. On the other hand, flocculants gave rapid settlmg of the larger partic1 left a turbid solution after considerable time. The settling rates of potato starch flocculants'were at 50 p.p.m. or less. The starch, gelatinited by calc~u VOL 54
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6
DRUM-DRIED TRAY-DRIED POTATO STARCH POTATO STARCH IUNMODlFlEDI lUNMODlFlEDl
F i p r e 7 . M o d f i d potato starch is a mpniorJ?malmzi for n phorphotc tniling which wtthout pocnrlont would not settle for hours. In the beakers, sellling time is 5 min.; Jiocnrlant concentration, 2W p.p.m,; 7.5% suspension of phosphate tailing. Potato starch w m
ALUMINUM SULFATE
FERRIC CHLORIDE
gelatinized with egualparfs of calciumond zinc chlorides. Much loww concentrations of potato stnrch can be effscfiue. In the graph bdow, other nwdged starches, prepared with the reagents indicated, also gave rapid setfhng of the mpenrion
io0 P.P.M. !OO P.P.M.
TIME, MINUTES chloride, was not as effective at low concentrations, but after five minutes results for the three flocculants at concentrations from 10 to 400 p,p,m, did not differ greatly. Only in the early stages did the rates of settling increase with flocculaut concentrations. Without a flocculant the materials would not settle significantly in hours. The suspensions had better clarity if a small amount of calcium hydroxide solution was added. Unmodified potato starch showed low effectivenessas a flocculant except when used in rather high concentration. In use, this material was gelatinized by heating at about 70' C. until it went into solution. Oxidized Starch
Potato starch is widely used as a filler and in coatings for paper manufacture. Certain physical characteristics give it an advantage over competing forms of starch, such as tapioca starch. When used for coatings, starch is put into solution but at the desired concentration of 50% potato starch has a high viscosity. Therefore to reduce this viscosity, the starch is oxidized with hypochlorite. This increases film strength, gives better penetration, improves clarity, and reduces cooking time. The decrease of viscosity is caused by a change in molecular strncture-it is believed that glycol groupings are oxidized and the ring structure of the anhydroglucose unit is broken. Evidence is cited for this in that amylo52
INDUSTRIAL AND ENGINEERING CHEMISTRY
..
pectin does not form gels, but the straight-chained amylose does readily. Amylose has a linear structure and the molecules can be closely packed. It is suggested that the amylose, when oxidized, becomes more branched. It can no longer undergo close packing with strong bonding, and therefore does not form gels as readily. The starch remains insoluble in cold water although a small fraction is solubilized. Sodium hypochlorite may be produced during electrolysis of sodium chloride by permitting the chlorine produced at the anode to mix and react with the sodium hydroxide produced at the cathode. Other reactions occur, as the concentration of hypochlorite increases, to deplete the hypochlorite and form chlorate and perchlorate. Without starch present, the concentration of hypochlorite increases with time to a maximum (Figure 2). Further oxidation of hypochlorite occurs, and at the limiting condition, is oxidized at the same rate as produced. To make the most effective use of the hypochlorite it should be utilized, as soon as formed, by , reducing substance to minimize side reactions. Neal employed a batch reactor made from a mal Vwce cell with the diaphragm omitted. The graphitt: anodes were satisfactory but the iron cathode had to be replaced with Type 18-8 stainless steel to prevent corrosion. Analyses were made for hypochlorite, chlorate, and total chlorine and the final starch viscosity was measured by a Brookfield viscometer.
Various voltages were used, hut reaction times were adjusted so that a fixed quantity of electricity was passed (120,000 coulombs per 1000 f i m s of starch). Starch, suspended in the salt solution, reacted with the hypochlorite as it was formed. Starch losses through solubilization increased as voltage increased; however, both equipment size and total cost per 100 pounds of oxidized product decreased. A minimum of about 4 volts was indicated, hut reaction rates and gas evolution caused excess foaming, and therefore 3.5 volts at 65 cents per 100 pounds of oxidized starch was considered satisfactory. I n conventional chemical processes for oxidization, chemical costs alone amount to $1.00 per 100 pounds of product. The best temperature for operation was considered to he the highest feasible without gelatinization. For this work, 40' C. was used, hut a somewhat higher temperature could have been used.
An Electrochemical Batch Process Can Produce Oxidized Starch Economically (Based MI a conrnvotiw scnolc-up fufm from laborotay dofa) Cost per 100 Lb. Pccduct, I visc., Totals cp. ment Materials Energy Voltage
2.8 3.1 3.4 4.1
20 27 65 88
0.50 0.29 0.19 0.12
0.39 0.43 0.46 0.49
0.01 0.01 0.01 0.01
0.90 0.73 0.66 0.62
Evans continued the oxidation studies by an electrochemical process using a douhle-pipe cell Constructed of l'/ainch stainless steel pipe for the cathode and I/sinch Karhate impervious graphite pipe for the anode. The pipes were electrically insulated at either end, and the reactor length was 5'/t feet. Salt solution (and later starch) was circulated continuously through the annular space of the cell. The mixture of salt and starch reacted in the cell hut starch oxidation continued in a holding tank. Although the over-all operation was hatchwise, production of hypochlorite was continuous and provided accurate control of hypochlorite concentration and starch viscosity. The inside of the Karbate tube was equipped for passage of cooling water hut this was not necessary for room temperature operation. About 3.5 volts applied to the cell was considered best (voltage drop across the cell was ahout 0.5 volts) because preliminary tests showed that higher current efficiencies were obtained at an average value of 3.2 volts as compared to 2.8 volts. Successive runs with the same salt concentration and operatingconditions were reproducible to about 15% with respect to the amount of hypochlorite produced. Hypochlorite production (with starch absent) increased with salt concentration for a fixed input of current (Figure 3, page 56) in accord with experience that current efficiency increases with current density. Even though low salt concentrations gave lower current efficiencies,salt economy dictates low concentrations. Also, the starch granules tend to swell somewhat in the presence of the salt. Energy costs are minor (Table I) and during oxidation, starch losses were about 5%. O n heating with water, a high-quality oxidized starch produces a clear light yellow solution which changes only slightly in viscosity after standing or after a change in temperature. Also, the dried starch should have a low moisture content to prevent deterioration, and a pH consistent with the viscosity desired. Viscosity of the solution produced is affected appreciably by pH, hut this can he adjusted by additives. The salt solutions should he discarded after a number of uses because reaction products, accumulated from oxidation and side reactions, cause viscosity and pH to drop continually with re-use. TABLE 1.
Typical Oxidation Results in the DoublsPIpe Elechochemical Cell
(Potato stmck; salt corn.. 4%; sampling, 7 h.affm ulccfrolys&) Number of uses PH Visc.," Cp.
5.5 5.0 4.7 4.4
1 2 3 4
44 22* 30 26
At 7 5 O C. b AfIer 1 br., 21 ep.; after cooling to 32O C., 35 cp.
0
Iuu
2uu
TIME, MINUTES Fiwe 2. Hypochlorite production in a double-p$e elccfrochmical cell showed (I limiting value. No starch present; p H , 9.2-8.3
Preconstruction cost estimates were made for a plant to produce oxidized starch at the rate of 5 tons per 24hour day based upon the laboratory data. This r e quired 100 cells similar in design to the laboratory unit with operation at 3.7 volts input (in series and parallel VOL 54
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combination to obtain the desired voltage drop per unit). Other conditions considered optimum were a 4% salt concentration, a temperature between 20" to 30" C. (to minimize chlorate formatiox), a flow rate of 2.9 gallons per minute through each cell (pressure drop, 9 p.s.i.), and 10% starch concentration. Costs of equipment and operation were for use in connection with an existing plant and the drying step was therefore excluded. Thus, the additional cost to produce an oxidized starch compared to unmodified starch could be determined. Capital investment Fixed annual costs Materials, utilities, labor Raw starch, 105 lb. Cost of product, 100 Ib. Selling price of product, 100 Ib.
$35,000 4,600 16,700 4.80 0.90 6.90
to those of cornstarch binders, both in the laboratory tests and in actual castings, was prepared by grinding with the ratio of water to potatoes equal to 3.2, filtering, resuspending the pulp and starch so as to make about a 50% water slurry, and drying on rolls heated by steam at 60 p.s.i.g. (6-inch diameter drums, 2.1 r.p.m., 240' contact) for a residence time of 19 seconds. Castings made from cores were of better quality if the Bentonite and potato binder were added to the sand, mixed dry, and then mixed again after water was added. Excellent and superior casting results were obtained with 0.5% potato binder, 3% Bentonite, and 3% water. With lower and higher percentages of binder (0.25 and 0.75%) the results were not quite as satisfactory. When potato binder was substituted for an equal weight of commercial cornstarch binder, higher baked core strengths were observed. Hhyl Alcohol und Related Products
Advantages of the electrolytic process would be close processing control, and elimination of the hazard of using and storing chlorine. However, hydrogen must be vented and diluted to prevent explosion. Corrosion of the stainless and carbon pipe was not a factor in the laboratory runs but should be checked thoroughly for a plant. Core Binder
Starch products are used in significant amounts (200 million pounds per year) in making sand cores (molds of all types) for metal castings. The starch binder provides green strength of the core during handling prior to baking, but also it influences general quality of the casting. Binders from cornstarch are used. Core binders prepared from potatoes were tested with standard equipment and procedures as specified by the American Foundryman's Association. Equipment included an intensive mixer, a sand rammer, a permeability meter, and a test machine for green and baked compressive strengths and for baked tensile strengths. Actual castings were made by the research department of American Steel Foundries, East Chicago, Inc. Binders were prepared in the laboratory from potatoes by washing, grinding, washing, filtering (in some cases), drying on a drum drier, and grinding. Variations in preparation and drying conditions influenced the quality of the potato product obtained. Also, hinder prepared from starch with protein removed was superior in green compressive strength (0.8 p.s.i.) to that prepared from whole potatoes (0.5 p.s.i.) or starch with some protein present. This is contrary to the practice in preparing binders from cornstarch where the product contains gluten or protein in amounts up to 11.5%. The presence of protein is reported to influence the effect of heat upon starches, and further, the protein in cornstarch is more closely associated with the starch molecules than in potato starch. Potato starch binder without protein showed a greater increase in pregelatinization with increasing temperature of drying than that containing protein. The best binder, and one showing properties superior 54
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Production of ethyl alcohol from agricultural products containing starch and sugars is frequently offered as a solution to the grain and potato surplus problem. Synthetic alcohol produced from petroleum products is so cheap that this process determines the market price for industrial alcohol. Alcohol from grain or potatoes must sell at the market price established by synthetic alcohol and the cost of the starting materials for potato alcohol severely limits the profit margin, if any. Preconstruction cost estimates were made for a plant to produce alcohol with' accompanying by-products.
However, if cull potatoes cost 40 cents per 100 pounds instead of the assumed value of 25 cents used in the table, operating cost would exceed income. Use of alcohol for beverage purposes might increase the return and the plant estimate includes equipment to remove the higher alcohols and fusel oils. The assumed price of 50 cents per gallon for the alcohol is below the March 1961 tax-free quotations for 95oj, alcohol delivered in tank cars at 52 cents east of the Mississippi River. Production of alcohol from potatoes appears marginal unless advantage can be taken of special price situations as to equipment, raw materials, or markets. Experimental production of alcohol from potatoes has been reported. Products from Potato Plant Wastes
The economic value of certain industrial potato products would be improved if an economic return could be realized from waste materials. Further, pollution of streams by wastes from processing plants may become such a problem that plants will be required to reduce the pollution load. Wastes from starch and food processing plants consist of soluble and insoluble portions of the potato, such as protein, cellulose starch, sugars and
Estimated Cost of a Plant to Make Alcohol and By-products
(Capacity, 7 0 0 tons potatoeslday; ofleration, 250 dayslyr.; potatoes at 25 cents/?00 Ib.) Item
using cull
Dollars
Including working capital
750,000
GROSS INCOME
Alcohol, 95%) 545,000 gal. Carbon dioxide, 3,260,000 lb. Cattle feed, 2,490,000 lb. Total
272 ,400 97 ,800 49,800 420,000
ANNUAL OPERATING COSTS
Materials and utilities Labor, overhead and distribution costs Maintenance (10% of equipment cost) Fixed charges (taxes, depreciation, insurance) TOTAL
165,000 52,000
30,000 93, 000 340,000
PROFIT
Net income (gross income less operating costs) Corporate federal and state income taxes at 52%, 8%) 60%
a
b
80,000
48,000
7.5"
CAPITALIZED PAYOUT TIME, YR. CAPITALIZED EARNING RATE,
Dialysis of Caustic Peeling Solutions
Potatoes, and other fruits and vegetables, are peeled by exposure to caustic solution or steam, or a combination of the two. Caustic solution of 14 to 25% is commonly employed. Dissolved and suspended matter builds up in the peeling solution which has a viscosity corresponding to a thick sirup. Dialysis can be employed to recover the sodium hydroxide and such a process using the highly viscous peeling solution, was investigated. The experimental apparatus consisted of a Webcell laboratory continuous dialyzer which contained 13 cells separated by 12 diaphragms. Because of relative unavailability, and the presence of large amounts of suspended matter (which the plants intend to reduce by use of different equipment), peeling solutions were simulated by mixtures of water, caustic, and whole potato flour. This permitted a variation in caustic concentration and viscosity. Because dialysis is essentially a diffusional operation, the concept of the transfer unit was used as:
where : C C, AC,
H,
L
Nt Z
CAPITAL INVESTMENT
a
associated inorganic materials. Those plants using caustic to effect peeling will have effluent streams containing sodium hydroxide.
%
4.30
+
(Capital investment)/(net profit depreciation). (Net Profit X 100) = (capital investment).
concentration of the solution, g.-moles/liter concentration of the water stream, g.-moles/liter log mean concentration difference, based on the solution over-all height of a transfer unit, based on the solution, ft. flow rate per unit membrane area, liters/hr.-ft.* over-all number of transfer units, dimensionless height of cell, ft.
Subscripts 1 inlet condition 2 outlet condition s solution w water stream
The equilibrium line for the peeling solution-water systems was determined. The experimental points could be approximated by a straight line with a slope nearly equal to unity. The height of the cell was taken as its diameter (an approximation) of 0.58 feet and the transfer area for the cell was 3.2 square feet. The height of a transfer unit is a straight line function of flow ratio of the two phases as expected from the two film theory for mass transfer. I t also increases with water flow rate as predicted by diffusional theory. The height of a transfer unit increased with viscosity, though not to the extent which might be expected. At a viscosity of 210 cp., H , was 13Y0 greater than at 1.4 cp.; at 610 cp. it was 50% greater. The laboratory data appeared sufficiently consistent and in accord with theory to permit the scale-up of the dialysis recovery process. Assumptions as to the height of the circular cell and the variation in flow rates of the two streams limited the usefulness of the data. A typical operation was considered which used 1470 caustic and discarded 1000 gallons of solution per week. Operation of the dialysis cells was computed for 24 hours per day, six days per week for nine months per VOL. 5 4
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FEBRUARY 1 9 6 2
55
ton per day yeast plant based on the larger scale runs indicated that the process would not compete with seed meals for feeds, but would compete with fish meal. Protein Recovery
Noncarbohydrate materials found in processing wastes include protein and ascorbic acid. Ascorbic acid commands a significant selling price and the protein could be used as a food supplement for human consumption. It was found that 60% of the protein could be coagulated either by heat or acid. A temperature of 80' C., or above was effective and temperatures below 60" C. gave no coagulation. Above a pH of 5.8 no coagulation was observed and the most effective p H was 3.2. Acid coagulation appears preferable because it preserves the ascorbic acid, gives a more desirable product on drying, and inhibits foaming. The protein remaining after coagulation was recovered by ion exchange. Of several Rohm & Haas resins investigated, IRA-400 in the hydroxyl form removed over 90% of the remaining protein. Tests on the effluent showed that only the basic amino acids bad passed through and that the protein had precipitated out on the bed. It was readily removed, and recovered, by backwashing and filtering. Amino acids retained by the bed were removed by eluting with 4% acid. The ascorbic acid was recoverable by the acid form of IRA-400 or IR-4B. Protein water which had undergone coagulation and passage through IRA-400 (basic) and IR-4B (acid) contained less than 2% of the original solids and would present no pollution problems.
zu 2 x 16
0
0
I x Iff COULOMBS
PASSED
year. The annual value of the caustic recovered was equal to about 40% of the capital investment required for equipment. Optimum operation (membrane cost DS. unrecovered caustic) would be obtained with 14 membranes with payout time of about two years. Electrodialysis could be even more promising. Microbiological Utilization
Aerobic fermentations were conducted in a laboratory fermentor using starch plant effluent. For convenience the BOD/COD (biochemical oxygen demand/chemical oxygen demand), was established for the protein water at 0.64. The rate of carbon dioxide formation was used to follow the progress of the fermentation and the level of cell activity. A single bacterial type from a mixed bacterial inoculum, obtained from a sewage treatment plant effluent, was found to adapt to growth on protein water. Cell activity increased with repeated inoculations until a maximum was reached. A single type of bacteria was observed upon microscopic examination and was thought to be of the genus Surcino. The pollution characteristics were reduced 60 to 70% from an original value ranging from 6000 to 7000 p.p.m. for the COD. A time element of eight hours was required to obtain maximum reduction, although a reaction time of four to five hours would be nearly as effective. The concentration of recoverable solids in the residual liquid was 0.75%, or about half of the original solids in solution. The bacterial cells could be recovered by centrifugation, or by filtration and used for a protein feed additive. The propagation of torula yeast using starch plant protein water was investigated on a laboratory scale. The addition of supplemental nutrients such as nitrogen, phosphorus, or potassium was not needed. It was desirable to control the pH of the fermenting medium at a value of about five to obtain maximum solids recovery. At higher pH values the yeast cells were contaminated considkrably by bacteria. If left uncontrolled the pH would rise and become constant at a value of about 8.7. Several runs made in a larger, 40-gallon fermentor, gave results comparable to those obtained in the laboratory 30-liter unit. Yeast recovery at optimum conditions was 40% based on the original solids content. Equipment and operation costs, computed for a 4.556
INDUSTRIAL AND ENGINEERING CHEMISTRY
SUGGESTED READING
M.S.Ch.E. t h x s oooilnblc at t h Uniwrriy of Idaho, MOSCOW, Idaho: AUTHOR
SUBJECT
Evaluation @ Utilirotion of Wastes from Potato Plants; 7957 Ethyl Almhol from Cull Potatom'; 7947
Manufacture.of a Cord Bindn from Potato W&; 7050 A Double-Pipc Elechochmical Cell for t h Frodwiion of 0.tidized Potato Starch; 79W Foddn Ytost from Potato Starch Plant Wastes; 7953 Bmtniol Utilizution of Potof0 Starch Waster; 7958 Potato StarGh asan Indurtrial FlocnJnnt; 7957 Remum and Utilization of Rot& from Potato Wa&; 7950 Th Elcctrochmicol Oxidation of Potof0 Starch;
Ambrose, T. W. Seraford, H., Christenson, L.M. Chase, W. L.
Evans, D. R. Dufner, R. V.
Furgason, R. R. Hou, K. C . Humphrey, . . A. E. Neal, L. G.
1.I__ 0GO
Dinlysirin t h Recoucry of Wash Cawtic in t h Lyc Pding of Potatoes, 7949
Tipton, F. W.
Othr Publicahom
.(1) Le Toumcau, D., Agr. Food Chm. 4, 543 (1956). (2) Thompson, D., Vilbrandt, F. C., Im. lbo. CHEM.46, 1172 (1954).
..
Agr. Erpt Station E d . 241.
Major portion of work supported by spedal researcb funds provided by the state of Idaho. Circle N r 2 IIReadan' Sulica Carl 4
