INSTRUMENTATION FOR PILOT PLANTS p H Control Aids in Production of Sweet Potato Starch R. M. PERSELL, E. F. POLLARD, W. F. GUILBEAU, L. H. GREATHOUSE, P. R. DAWSON, AND E. A. GASTROCIC Southern Regional Research Laboratory, New Orleans, La.
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4TH. SHAKER SCREEN 0.50 G.P.M
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T h e application of instrumentation fundamentals in the development of a process concept into a practical working plan is presented. The pH control of the plant flow eliminated certain major difficulties encountered in the pilot plant development of an improved process for the production of sweet potato starch. Manual control by experienced operators made possible the continued operation of the process and gave a greater recovery of starch.
T
HE need for a knowledge of instru-
TO PRE
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PULP WASTE' WATER 2N$ GRADE I ST GRADk STARCH FOR RECOVERY OF PROTEIN STARCH FOR FOR FOOD AND CATTLE FEED AND PRODUCTION OF INDUSTRIAL USE INDUSTRIAL USE FEED YEAST Figure 1. Pilot Plant Process for Manufacture of Sweet Potato Starch
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mentation in the development of a process concept into a practical working plan has been recognized and a resume of the fundamentals has been presented (4). The application of these fundamentals often makes possible the technical development of an economically attractive laboratory process into a practical plant operation. The application presented here shows how modern electronic equipment for p H control has facilitated the development in the pilot plant of improved processes for the extraction and the refining of sweet potato starch. During the past 18 years processes have been developed for the production of high quality starch from sweet potatoes. For 11 years the product was manufactured on a commercial scale by a farm cooperative and for 2 years by a large private enterprise (1-3, 6-7). Sweet potato starch became a market commodity because its particular suitability for a number of specialized applications opened up an extensive demand. When sweet potatoes can be
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Figure 2.
Pilot Plant Instrument Panel
grown and harvested at a cost justifying their use for starch manufacture, these processes .sill become of permanent importance to southern agriculture and inlustry. A t,ypical flow diagram of the process for extraction and centrifugal refining of sweet potato starch is shown in Figure 1. The sweet potmatoesare washed and ground with water to liberate the starch granules from the cells in which they are enclosed. The starch is separated from the ground sweet potato slurry by screening, and the crude starch in suspension is freed of soluble and solid impurities by successive centrifuging and fine screening. p H control is important in all the processes vhich have been proposed or used for extraction and refining of sweet potato starch. The first process which yielded a high quality white starch (1) depended on the use of a weak solution of sulfur dioxide during the grinding, screening, and initial refining stages, followed by a weak sodium hydroxide solution in the later refining stages. RIost effective removal of discoloring pigments and other nonstarch impurities required careful adjustment of both acid and alkaline concentrat'ions. In the second process (5) alkaline sodium sulfit,e solut'ion of carefully adjusted reaction was used t,hroughout the process. It combined the reducing properties to sulfur dioxide to retard formation of dark discoloring pigments by oxidation and the alkalinity of sodium hydroxide bo retard precipitation of contaminating substances. When this process was carried to factory scale in continuous operation, difficulty was encountered with clogging of the screens by coalescing and hardening on the mesh of organic substances, dispersed in t'he process slurry because of the sodium alkalinity, and by progressive hydration of the pulp during t'he screening process until it was difficult or impossible to dewater in the press. The original process using sulfur dioxide and sodium hydroxide in succession was then tried but proved unsatisfactory in a plant designed for an alkaline medium. Finally, use of a clear saturated solution of lime water was found to give the best results ( 3 )and has remained the standard practice in factory and experimental operation. Such use of lime water maintains a sufficiently alkaline reaction to keep pigments in solution until the starch is separated from the process water; a t the same time the calcium ion flocculates pectin and other substances, keeping them with the pulp when the starch is washed out and facilitating drainage of the pulp in screening and pressing. The optimum pH varies somewhat with t8hecondition of the sweet potatoes being processed but falls within the range of 9.0 to 10.0. Below this range, difficulties develop in the pulp pressing operation and in screening due to fouling. Above t,his range, the viscosity and
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other properties of the starch are adversely affected, and the costs of lime are increased. In setting up a pilot plant to develop further improvements in processes for extraction and refining of slTeet potato starch with the use of recently developed and improved equipment and to produce starch for other phases of research, advantage was taken of the most modern electronic equipment to facilitate control of pH. An analysis was made to determine the optimum points for measuring the pH and for making required adjustments. The selection of these points was complicated by the many operations performed on the plant flow. To control the pH throughout the system nith a low velocity-distance lag would have required several measurement and control points. As an alternative, it was found possible to control manually the pH of the slurry in contact with the coarse pulp so that the pectinlike material and gums coagulated and thus could be handled efficiently by the screens and the press. About 20% of the lime water Ras added to the first grinder and the remainder to the collecting tank ahead of the third shaker screen. The control was based on measurements of the pH of the starch milk underflow from the first shaker screen (the last point a t which the starch milk comes in contact with the coarse pulp). The addition of the lime water at two points made it possible to make immediate corrections to the p H a t these important points which vxre separated by a considerable velocity-distance lag. Corrections 'I\ ere made to the system pH by varying the lime vater feed to the collecting tank ahead of the third shaker screen. Temporary corrections were made to the pH of the plant flow at the first grinder by varying the lime water feed a t that point and maintaining the correction until the correction to the system pH became effective. Conventional equipment was found satisfactory for this application of pH control. This consisted of an automatic pH indicator, a strip chart recorder, and a flow-type electrode assembly. The indicator and recorder a ere mounted on an instrument panel (Figure 2) with rotameters and valves to regulate the flow of lime water into the system. The pH indicator was of the automatic, fully-balanced, electronic potentiometer type with a 3 to 10 pH scale graduated in 0.1 division. Temperature compensation was provided for electrode temperatures from 32" to 212" F. by means of a iesistance thermometer installed with the electrodes. The strip chart recorder was connected to the indicator and was of the automatic, fully-balanced, slide wire potentiometer type Rith a 2 to 12 p H scale. The flowtype electrode assembly was directly installed in the flow line and >!as fitted \iith glass and calomel electrodes suitable for conthe range of 50" to 140' F. rol made possible the continued operation of the press and gave a greater recovery of starch due to more efficient functioning of the scieens. Manual control by experienced operators was satisfactory for pilot plant runs. Automatic control would be preferred for continued production. ACKh-OWLEDGMENT
The authors wish to thank F. H. Thurber and IT. 0. Gordon for their assistance in reviewing the technology of the starch process, and Jack E. Hamkins for preparing Figure I. LITERATURE CITED
(1) Balch, R. T., and Paine, H.
s., IKD.ESG
CHCM.,
23, 1205-13
(1931). (2) Bourne, B. A., and Thurber, F. H., Southern Power and Ind., 64, SO.5,44-7,98,100 (1946). (3) Paine, H. S.,Thurber, F. H., Balch, R. T., and Richee, W. R., IND.ENG.CHEM., 30,1331-48 (1938) (4) Pollard, E. F., Persell, R. M., Molaison, H. J., and Gastrock, E. A, Ibzd., 42, 748 (1950). ( 5 ) Thuiber, F. H., Ibad., 25, 919-20 (1933). (6)Thurber, F. H., Gastrock, E. A., and Guilbeau, W. E U . "3. Dept. A ~ T Yearbook, ., in press. (7) Thurber and Paine, ISD. ENG.CHEM. 26, 567-9 (1934). RECEIVED January 9, 1950.
