Ion Exchange in Beet Sugar Manufacture Three full scale ion exchange plants are now operating in the beet sugar industry, and these operations have posed many new problems, both technical and economic. The results of ion exchange operations are generally in good agreement with pilot plant predictions. These include additional extraction of granulated sugar, higher quality of product, and fewer processing difficulties. An economic comparison between ion exchange and conventional operation is made, and the effect of relative sugar and molasses prices on this comparison is shown. With present sugar and molasses prices, increased freight rates, and regenerant costs, economic justification of ion exchange is difficult at some locations; however, with new developments such as ammonia recovery from spent anion waste, recovery of valuable fertilizer elements from regenerant wastes, and a low lime treatment of juice rather than the conventional carbonation, ion exchange may pay an adequate return on the investment.
J. E. MAUDRU Holly Sugar Corp., Colorado Springs, Colo.
I
ON exchange has been known for about one hundred years, and
*
is essential to avoid repeptizing the colloid, and acidproof presses, lines, and pumps are necessary because the p H of the juice is of the order of 1.8 to 2.0, It can be shown, however, that treatment of raw juice by ion exchange is not as economical as treatment of defecated thin juice, because nonsugar solids in raw juice removable by lime may be eliminated more cheaply by lime than by ion exchange (Table I).
commercial application of this process is the only radical change in juice treatment in a great many years. Ion exchange of sodium for alkaline earths was accomplished through the use of natural greensands a t first and synthetic materials later. The process was used almost exclusively to soften water for such applications as boiler feed. As it replaced hardness-producing materials, calcium and magnesium, with sodium, little decrease in total solids was realized. Attempts to adapt this process t o beet sugar failed economically, because the residual sodium left in the juice is even more melassigenic than the calcium and magnesium removed. In other words, no increased yield of sugar could be realized and the only possible benefit was a reduction in the amount of scaling of heating surfaces, which would be a minor economic justification. In 1941, after the discovery in 1935 by Adams and Holmes ( 1 ) of synthetic ion exchange resins, these resins were used in the treatment of beet sugar juices. Ion exchange, besides being employed commercially in water treatment, is also used on an industrial scale in recovery of pectin from grapefruit waste (a),refining of dextrose from starch hydrolysis (@, and removal of oxygen from water (7). The first thinking in applying ion exchange to beet sugar manufacture was along the lines of removing only ash from the juice, taking advantage of increased extraction due to this ash elimination and consequent rise in purity. Early laboratory work, however, proved tfhat the resins removed not only essentially all the ash but also weakly ionized nitrogen compounds, zwitter ions, many color bodies, and even some colloids. Present-day commercial operation enjoys the following removals: Total nonsugar removal Total nitrogen removal Ash removal
Table I.
Comparative Operation Costs of Same Beds on Raw and Thin Juice
Beets, tons per cu. foot resin t o 7.0 p H anion break through Regeneration cost per ton beetsa Carbonation cost per ton beetsb After treatment
RawJuice
Thin Juice
0.08 0.75 0 0.05
0 12 0.50 0.17
I
0.80 a Based on acid a t $20 per ton and ammonia a t $100. b Based on lime rock a t $5 per t o n and coke a t $20 per ton.
0 __
0.67
COMMERCIAL OPERATION The Hardin, Mont., ion exchange unit consists of four pairs of cells, each consisting of a cation and an anion exchanger. At any one time only one pair of cells is used in the juice treatment cycle, while the other three pairs are in various stages of regeneration or other preparation. The cells are rubber-covered steel tanks with spun or dished heads designed with a working pressure of about 60 pounds square inch. A flat bottom supports or contains the bottom tribution s stem, of which there are many t Of whatever sufficient nontype the &tributor may be, it is essential clogging orifice area be provided, so that maximum flow from the cell may be realized without excessive pressure drop, and yet sufficient pressure drop must be maintained across the orifices t o ensure good distribution. If this last condition is not maintained, excessive dilution during sweetening on and sweetening off will be experienced. The cell also includes an upper distributor for introducing juice, water, and regenerant to the resin, a level control t o re ulate the li uid level to a point slightly above the resin level, an$ a means o?providing air pressure above the li uid in the cell to move the liquid from cation to anion.cel1 an% from anion to receiving tank. Free board above the resin bed is made 80 to 100% of the resin bed depth.
84%
Ki
56% 94 %
Attempts have been made t o treat raw juice from the battery by ion exchange and thus eliminate the carbonation step (6). Raw juice passed through a cation-anion cell combination is separated from all the materials mentioned above, but retains a dark colored colloid that may be eliminated by any one of several defecation techniques. Filtration between the cation and anion bed while the juice is in the acid state and a t the approximate pH corresponding to the acid isoelectric point of the colloid satisfactorily removes this contaminant; however, precise control of pH 615
ET
616
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 3
i The valves are air-operated Saunders patent and rubber covered; the air is controlled by a solenoid located close to the valve, and the valves are controlled from a central control board, which also includes the necessary flow meters, p H instruments, and timers. Each valve may be operated individually, or each cycle part may be initiated by pressing a button which operates the valves required. Full automatic operation is not favored, because it would be inflexible. Figure l is a schematic piping diagram showing location of air-operated valves and auxiliary tanks. Only one pair is shown. Actually headers are employed to connect the other three pairs. The remaining tanks and accessory equipment are of standard design.
Table 11.
Juice Sweeten off Back wash
Flow Down Down UP
Typical Cycle Data
Pair Separated No No
Time, Min. 45
Yes
l7 15
7 30
Regeneration
Drain
Down Down
Yes Yes
Rinse
Down
Yes
(3
Gal./ Cu. Foot/ Min. 1 3 1 0
Cycle Ended Manually Manually Time
1.1
Column level Tank level
0.4 1.0
Time
C ;. A u.7
C 10
DESCRIPTION OF CYCLES
Juice. Thin juice is pumped into the cation cell of a freshly regenerated pair. Two void volumes of water must be displaced before juice appears as anion effluent, and this water, being deionized, is saved in a tank to be used for ammonia dilution and rinsing. The anion effluent is changed from the water tank to treated juice a t about 1.0 brix. Juice is passed through the pair
until breakthrough occurs. The breakthrough is determined very satisfactorily by use of recording p H meters on both cation and anion effluents. Amounts of regenerating chemicals are so regulated that both cation and anion breakthrough occurs a t substantially the same time. The juice cycle is completed manually when the anion pH reaches 7 0, at which time cation p H is about 3.0, having broken from 1.8 to 2.0 during the cycle. Sweeten Off. At the completion of the juice cycle the juice flow is transferred to the next pair and water is introduced into the original pair to displace the juice. Sweetening off is continued until the Brix reaches some low figure in the order of 1.5. The sweet water may first be added to treated juice, but later, owing t o leakage of nitrogen compounds, must be returned to the thin juice feed. Backwash. Raw water is introduced through the bottom distributor into both the anion and cation cells and removed from a connection a t the top of the cell. Backwashing, besides removing mechanically held impurities, also reclassifies the resins and thus reduces pressure drop through the beds during the service cycle. Time of backwash is arbitrarily set to ensure removal of impurities, and rate of flow is such that the resin bed is in full titer and yet not so high that resin is lost out of the top of the cell. Used backwash water is either screened or settled in a trap tank to recover resins possibly removed from the cell during the backwash. The water, not being badly contaminated, may be re-used for such functions as fluming beets. Drain Down. At the completion of backwash the cell is completely full of water; the top valves are closed, the bottom drain valve is opened, and air introduced a t the top of the cell forces water down and out the bottom until the water reaches operating level.
March 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cation Regeneration. Sulfuric acid (5%) having been diluted from 66" BB. acid with cation rinse water is pumped down-flow through the resin bed. Stage regeneration is very often employed, in which once-used acid is pumped through the bed to waste, followed by fresh acid which then becomes once-used acid. Some installations employ three-stage acid regeneration with slightly lower acid consumption. Because regeneration is in principle a mass reaction, higher regeneration level may be attained by conducting the final stage with acid containing a minimum concentration of impurities which have been previously regenerated off the resin. Maximum flow rate used during regeneration is critical, as too high a flow rate (too short a time) reduces regeneration efficiency. Acid used in regeneration is normally 200 to 300% of the stoichiometric amount. Anion Regeneration. Aqua ammonia (2?&),produced by diluting 20 to 25% aqua ammonia with deionized water, is pumped through the anion resin bed a t a low rate. Stage regeneration is not employed and the amount of regeneration used is about 110% of theoretical. Rinse. The cation and anion cells are rinsed separately to remove excess regenerant. The cation cell is rinsed with raw water, which after passing through the bed is used as the diluent for strong acid. The amount of cation rinse should be sufficient to remove all excess acid from the cation cell, because any remaining acid would only exhaust anion capacity on the following juice cycle. The anion rinse is similarly carried out using deionized water, which after passing through the anion bed is used as a dilutant for strong ammonia. Raw water is not employed as a rinse for anion resin because of the danger of fouling the resin with magnesium hydroxide formed by the magnesium content of the water and ammonia. I n some installations a series rinse of deionized water follows the single rinses. Deionized water made during sweetening on and off is circulated through cation and anion back to the water tank. Series rinse removes last traces of regenerants and decreases conductivity of effluent. The operation of a beet sugar factory by ion exchange presents many problems not normally encountered in conventional operation. In every case, to date, the commercial ion exchange plant has been an addition to an existing factory, and so any change in the size of water supply, boilerhouse capacity, or evaporators requires a large investment. Water Supply. Ion exchange with backwash and rinses requires a large amount of water. This may be raw untreated water, but suspended solids and more especially dissolved solids should be as low as possible. Excessive suspended solids tend t o increase pressure drop through the beds and dissolved solids in cation rinse water use up some cation capacity.
Table 111.
thus steam economy from this score is increased. Amounts of massecuite compared to the conventional process are somewhat less, so that fuel, in total, appears t o be not greatly increased. Ellison reports a slight decrease in fuel requirements a t the Layton Sugar Co. (6). Inversion. Because of the extremely low p H encountered as the juice passes through the cation resin, serious consideration has been given to inversion losses. Figure 2 shows temperature of juice from the exchangers and invert sugar, per cent on dry substance in thick juice, plotted against weeks of operation. Excessive inversion may be noted during the first weeks of operation, while after the juice temperature decreased to below 20" C. inversion was not serious. The lower curve shows juice temperature and invert sugar. This is the lower part of a typical inversion curve.
25
0' 0
nr E
20
0
c 16
5 IO Weeks of Operation
Sweeten off Backwash Cation rinse
17 21
I5
0
16
Ion Exchange Water Usage Gal./Cu. Foot/ Regeneration
617
20
25
temp.,OC.
Figure 2.
Invert Sugar in Thick Juice os. Juice Temperature
15 53
I n a plant using 45-minute cycle time (32 cycles per 24 hours) and 400 cubic feet of resin per bed, the water required would amount to 678,400 gallons per day, of which 268,000 gallons as backwash are available for other use, such as beet fluming or condenser water. Resin Losses. Described in early reports as high as 6% per 1000 cycles, resin losses appear to be on the order of 2% with no visible attrition losses. Fuel. Although the dilution from sweetening off and on increases the load on the evaporators and thus increases steam demand, the evaporator tubes remain much freer of scale and
Bacteria. Some difficulty has been experienced with bacterial contamination of the cooled juice prior to ion exchange. These microorganisms are dextrin producers, and this white gummy material plugs the narrow juice passages of the heat exchangers. It has been necessary t o establish a routine of steaming and cleaning the heat exchangers. Little difficulty has been experienced in contamination of the resins by bacteria, because pH's of both cation and anion resins are unfavorable for growth. The few times trouble has been encountered with bacterial infection of the beds, the sterilizing technique of Cruikshank and Braithwaite ( 4 )has proved satisfactory. Given in Table IV is a sugar balance comparing ion exchange and conventional non-Steffen operation for a typical mill cutting 1600 tons of beets per day, and in Table V an economic balance
INDUSTRIAL AND ENGINEERING CHEMISTRY
618
Table IV. Typical Sugar Balance Comparing Ion Exchange and Conventional Operation
Table V.
Typical Cost Balance of Ion Exchange Factory (24-hour figures)
(24-hour figures) Daily Slice Coseette sugar, tons Sugar entering battery, tons Beet end losses, tons yo on beets 0.13 P$P 0.10 U ater Lime flume 0.02 Unknown 0.15 Total
Conventional Yon-Steffen
Ion Exchange
1600 16.0
1600 16.0
256
Conventional Non-Steffen
Ion Exchange
256
---
0.40
6.4 ___
6.4 ___
S e t sugar in thin juice, tons Ion exohange loss including svieetening off and inversion, tons
249.6
249.6
Sugar to sugar end, tons Purity thick juice Purity of molasses Sugar end extractiona, % Sugar bagged, ton Sugar in molasses tons Molasses a t 50 pdarization, tons &traction, yo on sugar in beets 100 S ( J - i M ) a % extraction = J(S-M) S = purity of sugar .l = purity of juice .li = purit,y of molasses
Vol. 43, No. 3
... ___
6.0 ___
249.6 90
243.6 96.5
83.3 207.9 41.7 83.4 81.2
94.5 230.2 13.4 26.8 89.9
60
Total costs Credits Extra sugar, 446 bags a t $6.50 Decreased molasses, 56.6 tons a t $20 ton Total credits Credits minus costs Return per ton beets Cost per bag addl. sugar
$1101
iiiz __ 1132
2900
.. ___ 2900 667 0.41 5.00,
60
covering the same operation. In this balance only factors shorn.ing a difference between the two methods of operation are coneidered and only operating items are listed-no fixed costs are included, such as depreciation, maintenance, taxes, and insurance. These costs are merely typical and do not represent costs at an actual mill. because many factors such as return for sugar, location of mill TTith respect to sourcee of regenerant supply, and molasses price so greatly influence such a picture. The main advantage of ion exchange is increased extraction due t o elimination of impurities, and by the same token, decreased production of mo1asses. Because the price of sugar as sacked u hite sugar is greater than the price of molasses, this is the main economic justification for the process Less important advantages are less scaling of heat transfer surfaces in evaporators, heaters, and pans due to a very much decreased mineral content in the juices The main disadvantage of ion exchange is the large capital outlay for the unit. Because nearly all exposed surfaces must be acidproofed and the resins used are of rather high price, the capital investment is so high that often it is difficult for the savings realized by ion exchange to pay the interest on such an investment. The cost of the regenerating chemicals is likewise high. Beet sugar factories are always located a t the center of a source of the raw material-beets. Freight rates on the regenerant chemicals t o those locations are often very unfavorable and greatly increase the operating costs of ion exchange. Recent research and development have brought to light several features that may greatly improve the present economic situation. Because all the ammonia used in regeneration of the anion resin IS present in the anion waste, this waste may be combined with the acid cation waste t o produce a dilute solution of ammonium sulfate. This ammonium sulfate, with other impurities in the
beets which include potash and trace elements, may be concentrated to form a liquid fertilizer or epray dried for a granular fertilizer. This fertilizer by-product is valuable and is a logical source of nitrogen fertilizer. il sec,ond development (3)which may reduce the ammonia regenerant cost is a process whereby the ammonia from anion spent regenerant is distilled off and reclaimed by boiling the spent regenerant with lime. A third, and possibly the most econoinically important development of the three, is the ion exchange treatment of a juice defecated with only 0.2 or 0.3% of calcium oxide on beets. This process would save a major part of the lime rock and coke now employed. Most of the purity rise in lime treatment of raw juice is effected by only a small amount of lime, the remainder of the lime being necessary for easy filtration of the juice and the formation of a cake which may properly be sweetened off. With improved filtration equipment the cake from the low lime treatment may possibly be removed from the juice and sweetened off with a low sugar loss. With the above process improvements and with other improvements in resins, such as increased capacity and regeneration efficiency, ion ,exchange may well become a process that pays a legitimate return on the invest'ment. The writer is indebted to the management of Holly Sugar Corp. for permission to publish and to the perBonnel of the Hardin, Mont., plant for assistance in obtaining and assembling the data. LITERATURE CITED (1) Adams, B . A . , a n d Holmes, B. L., J . SOC.Chem. I n d . , 54, 1-6 (1935); U. S. P a t e n t 2,104,501, (2) Beohner, H. L., a n d Mindler, A. B., IND. EXG.CHEM.,41, 448-52 (1949). (3) Cotton, R. H., et al., Proc. Am. Sor. Sugar Beet TechnoZ., in press. (4) Cruikshank, G. A4., a n d Braitha-aite, D. G., ISD. ESG.CHEM.,41, 472-3 (1949). (. 5.) Ellison, H.. Division of Sugar Chemistrv a n d Technologv. -. 115th Meeting, AMERICAN CHE~XICAL SOCIETY,San Francisco, Calif. (6) M a u d r u , J. E., Sugar, 43, 5 (1948). (7) Mills, G. F., a n d Dickinson. E. N., IXD.ENG.C H E M . ,41, 2842-4 (1949). (8) Newkirk, T. H., a n d H a n d e l m a n , M., Ibid., 41, 452-7 (1949). RECEIVED April 6, 1950.
