Dry Gelatinized Starch Produced on Different Types of Drum Dryers Hartwig Fritze Escher Il-yss GmbH, Ravensburgt It'urtt, W e s t Germany
The production of dry pregelatinized starch from maize starch and maize flour using four different drum drying processes i s described, and the results have been analyzed. A comparison i s made among the following items: the twin-drum sump dryer; the single-drum dryer with top applicator rolls and starch slurry feed ahead of the first roll only; the single-drum dryer with top applicator rolls and starch slurry feed ahead of each roll; and the single-drum dryer as described under the preceding item, but preceded b y a starch cooker. In particular the dryer capacity, water absorptivity, viscosity, cold water solubility, bulk weight, and degree of gelatinization were investigated as functions of the feed concentration of the starch slurry and partly also of the drum speed. It could b e demonstrated that advantages are offered in particular b y pregelatinization before drying, and that as a result thereof the physical properties could b e varied.
E v e n in ancient times the hydrothermal alteration of starches and flours into starch pastes and glues aiid the utilization of these pastes as adhesives and water absorbents were familiar to all civilized peoples. While for thousands of years the starch pastes had to be prepared shortly before use by heating st,arch or flour suspensions, the large-scale industrial production of dry, pulverized or flaked gelled starches lias not been adopted until the current century. Tlie first names to appear in the patent literature describing a process for the production of special pregelatinized starch on drum dryers were lIahler and Supf (1921, 1932). These patents were followed later by patents in the name, for t h e most part, of Corn Products. -4t first two heated, counterrotating drums mere used-the so-called sump dryer, which originally, about the turn of the century (Kehl, 1932), had come over from -4merica to Europe as a milk dryer. To satisfy the growing demand for hydrothermally altered, gelled starch in many industries, such as tertiles, paper arid foodstuffs, the process of producirig dry pregelatinized starches capable of being recoiivtituted into pastes by mashing with water offered considerable advantages and exciting nen- allplications. Now new drum-drying techniques using different types of drum dryers have gained importance. These various types of drum dryers will now be compared to each other in terms of drying capacity and properties of the end product. Drum-Drying Processes
The idea hehind the production of dry pregelatinized starch on drum dryers is to exploit, the heat transferred from the surfaces of the steam-heated drums to the wet product to convert a t the yame time the starch particles into paste. 011 the drum the gelling of the starch takes place first in the wet produrt sump bet\?-eeri the applicator roll and the drying drum, then tlie 1)aste is eveiily spread over the drum surface in n thiii film hy applicator elements (properly adjusted to obtain the desired film thickness), and finally dried. Tlie four best-known types of drum dryers are shown schematically in Figure 1. Twin-Drum Sump Dryer (Figure 1, 1). This dryer basically consists of two counterrotating steam-heated 142
Ind. Eng. Chern. Process Der. Develop., Vol. 12, No. 2, 1 9 7 3
drums. oiie of which can usually be adju5ted a t right angles t'o its axi.*, so that the gap between the two drums may be varied. The starch 'uspelision is fed into the nedgeshaped space between the drum3 (also called the sump) by means of a distribution pipe. Under the influence of the heat transferred from the hot drum surfaces to the starch suspension fed into the sump, the starch is converted into paste and is applied onto the drums. For the drying process the most efficient wet-product application is obtained with a gap of 0.2 mm between tlie drums. For the comparat'ive study, n twin-drum sump dryer of 350 mm drum diameter and 600 mm useful drum length \vas used. Single-Drum Dryer Equipped with Applicator Rolls, a Wet-Product Feeding System and Doctors (Figure 1, 2 ) .
On this dryer t h e wet' product
i q iiitroduced ahead of the first uppermost applicator roll. Agaiii the starch is gelled by the effect of heat from the drum, and by the first aplilicator roll it' is applied onto t h e drying drum. T h e first applicator roll is adjusted for a gap of about 1-2 mm lietween the roll mid drum surfaces so that a film of gelled product call forin on the applicator roll. This film is scraped off continuously hy flat-iron doctors with their edges set against the applicat,or rolls. It drops into blie wedge-shaped space ahead of the next applicator roll. Here the same cycle is repeated, and a furt,her product film is superimposed on the filni 011 the drum. Finally the last applicator roll is set so close to the drum that only a narrow gap is left betweeii drum and roll, preventiiig the formation of a thick layer on the applicator roll, but smoothing simultaiieouslj- the film 011 the drying drum. This process ensures a gradual concentration of the gelled starch on its way to the last applicator roll. Further details on the illfluelice of this gradual concentration on the product aiid the process will be given elsewhere iii this paper. For the tests which yielded the results described here, a siiigle-drum dryer with a 650-mm drum tlianieter aiid a 600mm useful drum length was used, equipped tvith three applicator rolls of 140 mm diameter mounted a t the top.
Single-Drum Dryer w i t h Applicator Rolls and WetProduct Feed Ahead of Each Roll. In this process the
starch suspension is fed ahead of each applicator roll by
-
tarch siurry teed
Starch slurry feed
W 11
I' r TWIKDRUM SUMP DRYER to the drum the wet stock by
3 Rolling out of the wet stock
1 SINGLE-DRUM DRYERwith applicator rolls on starch slurry feed before each roll
pregelatinized
Figure 2. Sump dryer: phases of product application in sump
1 . Twin-drum sump dryer. 2. Single-drum dryer with top applicator rollst one starch slurry feed, second and third rolls fed b y doctor blades. 3. Single-drum dryer with top applicator rolls, starch slurry feed ahead of each roll. 4. Single-drum dryer with top applicator rolls ond.'pregelatinization; starch paste feed ahead of each roll
1 . Contact and adhesion to the drum. 2. Slide fracture of the wet product by T B = f ( r . w ) .3. Rolling out the wet product
Figure 1. Processes for producing starch with drum dryers
dry
a n oscillatiiig wet-product distribut'or. T h e suspension flow through each feed line can be adjusted either b y use of valves or by means of metering pumps. The applicat'or rolls are set so close to the drum that a gap of only 0.2-0.3 m m if left, again to preclude the formation of a thick product film on the applicator rolls. From a n operational point of view this method has t,he advantage that' the product flow ahead of the individual applicator rolls can be controlled and, as is usually desirable, kept small. This control of the product flow aims a t a better gelling of the starch and a more efficient application. Moreover, the extremely thin product film on the surfaces of the rolls makes the internal cooling system of the applicator rolls much more effective, preventing caking and iricrustation of the product on the applicator rolls. I n addition, this process is superior on hygienic grounds. The tests were carried out on a single-drum dryer with three top applicator rolls; the drum diameter n-as 650 mm, the useful drum length 600 mm. Single-Drum Dryer with Applicator Rolls and Precooked Wet-Product Feed Ahead of Each Roll (Figure 1, 4). Here t h e gelling of t h e starch i3 not accomplished 011 the d r u m d q - e r itself; instead, t,he druni dryer is preceded by :I cooker ill which heat, is transferred t o t'he htarch by direct steam injection (Fritze, 1969). An Escher Wyss cooker is employed (Xaurer, 1962). The gelling temperature of this cooker is maintained by a PI governor which adjusts tlie steam valve to the desired set point'. The starch suspeiisioii is passed through the cooker by a lIohno pump, and by n second l l o h n o pump is fed t o the oscillating wetproduct distributor preceding the applicator rolls. The second IIolino pump in this system is indispensable in overcoming the pipe heat losses \\-here the line from the cooker to the drum dryer is relatively long and starch slurries n i t h concentratioiis of u p t o 407, have to be handled. The product flow t o the individual applicator rolls is regulated by means of ball-type cock? in conjunction with the two synchronously operating l l o h n o pumps. Recently this process of applying the precooked and pregelatinized product to the drying drum has gained more and
more importance. The reasoning behind its adoption is that the starch applied onto the drying drum in a n aqueous suspension cannot be gelled completely, since the short' cycle time on the drying drum is not sufficient to ensure a perfect gelatinization and since the wet product is continuously cooled by the inflow of more wet product. Of course, precooking of the product can be done in a n indirectly heated cooker, which, however, calls for a high-power input. T h e necessary tests for this study \vere again carried out on a single-drum dryer with three top applicator rolls; drum diameter 650 mm, useful drum lengt'h 600 mm. Theory of Product Application on Drying Drums
T o a l l o r a better understanding of the results discussed, we give here a brief esplanation of the physical processes taking place during the product application by rolls or in the sump as the case may be. The product application can be divided into two distinct phases. T h e first phase is that of product take-up, which results from the interplay between the adhesion forces acting between drum surface and product (but also in the product itself) aiid the shear stresses created by the motion of the drum. Extremely complex phenomena are involved here n-hich do not lend themselves to mathematical espressioii because the product properties are changiiig all the time. They are best compared to the flow behavior of elastoviscous substances which possess the propert,ies of both elastic bodies and viscous fluids. The shear stresses are primarily functions of the speed of tlie drum aiid of the viscosit'y of the product. The second phase of the product application is the even spreading of the sheared-off wet product to obtain a product, layer of uniform thickness needed to ensure homogeneous drying. T h e compression forces prevailing in the gap are governed by the viscosity of the wet product, the gap width: and the speed and the radii of the drum and the applicator rolls. For every product there is a most appropriate gap width, which will create the compression force needed to achieve a n even spreading of the product. Figures 2 and 3 are schematic representations of the product application processes for sump dryers arid single-drum dryers with applicator rolls. O n the sump dryer the wet product is held in the wedge-shaped sump and does not move. The adInd. Eng. Chem. Process Des. Develop., Vol. 12,
No. 2, 1973
143
and sliding detachmsnt by shear forces
2 Rolling w t of the wet stock
la1 Deformation by shear stresstzf (r.w)
1 b)Sliding detachment
Figure 3. Single-drum dryer: phases b y means of applicator rolls
of product application
1. Adhesion and slide fracture b y shear forces. 2. Rolling out the wet product
__c
Drum speed Lrpm)
Figure 5. Bulk weight vs. drum speed
be boosted by increasing the speed, with the result that applied film thickness and film density drop as the speed rises (Figure 5). At high speeds the wet product is detached a t less distance from the drum surface than a t low speeds. These facts will be repeated when discussing further results. Methods of Analysis and Measured Values
I
15
IO
15
I
20
25
30
Drum speedlrpm)
Figure 4. Specific drying capacity vs. drum speed
liesiori forces cause the product to adhere to the drum, and when they exceed the inherent molecular forces of the product, the product is sheared off b y the prevailing shear forces (Figure 2). On the single-drum dryer (Figure 3), with many viscous wet products such as pregelatinized starch, a product roll is formed which spins between the drying drum and the applicator roll and obtains its sense of rotation from the latter. As the roll of wet product rolls off on the surface of the drum, revolving in the opposite direction, product adheres to the surface arid is sheared off at once owing to the shear forces acting when the inherent molecular forces of the paste are exceeded. It may be imagined that the sheared off paste scraps are then drawn into the gap between drum and roll and flattened as shown in Figure 3 a t position 2. On the basis of these considerations, the followirig three theoretical assumptions can be made: The product take-up by the drying drum, and hence the specific drying capacity (Figure 4),increases as the concentration of solids increases, as long as this has the effect of increasing the adhesion forces. Up to certain speed ranges, the specific drying capacity may 144 Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 2, 1973
Maize starch was employed first for the tests. For comparison, corresponding values were determined for maize flour, since these values are particulary useful for the manufacture of cereal products. To evaluate the various drying processes, the following test readings were taken and tests made on the dried pregelatinized starch comminuted to a medium-fine flour in a cross-beater mill with about a 2-mm mesh : T h e drying rate as a function of the speed and starch slurry concentration. The water-absorbing capacity of the dry product. These values were determined by stirring in each case 10 grams of absolutely dry pregelatinized starch in 150 ml of water arid letting the solution stand for half a n hour to obtain a satisfactory gelatinization. Thereafter the solution was passed through a standard filter followed by a rinsing with 50 ml of water. T h e amount of water filtered out was read off after 12 hr. The viscosity of the pregelatinized starch reconstituted with water to a solution. T h e test readings were taken on a starch solution prepared from 0.6 gram of absolutely dry pregelatinized starch arid 100 ml of water. The starch solution was stirred for 10 miri inside a closed round flask immersed in boiling water, after which it was cooled down to 20OC and measured in this state by use of the IIoeppler viscometer. Comparative readings were taker? by means of a Urookfield viscometer on 5 and 10% pastes. The bulk weight and dry product film thickness as functions of the drum speed and starch slurry coticentra t'1011. The degree of hydrothermal alteration of the pregelatinized starch. The cold water solubility. Tmo grams of powder pregelatinized starch were suspended in 100 in1 of water and stirred for about 1 hr a t 20°C. The suspension was centrifuged for 10 min. a t 2500 rpm, and 50 ml of the supernatant were evaporated in a drying ovcti. The dry residue was weighed.
E 26f
Coding water outflow 90.C
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4
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Wer wfflow 60'C
Starch slurry concentration 35%
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-Starch
slurry concentration(%I
15
20
25
30
Drum speed ( r p m )
~~~
Figure 6. Specific drying capacity vs. starch slurry concentra tion
~
Figure 7. Influence of applicator roll cooling
Comparison of Drying Proeesses
In Figure 6 the specific drying capacities of the four processes examined are shown as functions of t h e starch slurry concentrations applied on t h e drying drums. These drying rate figures were established by use of single-drum dryers operating at a speed of 18 rpm, the customary operating speed. On the sump dryer, a speed of 3 rpm was chosen because at higher speeds the dry-product film became too wet, while a t the same time a residual moisture of the dry product of about 5% was required for this comparison. T h e steam pressure and coiisequently the drum surface temperature were matched t o the speed, so t h a t the same residual moistures (about 5%) were obtained in the end product. When producing pregelatinized dry starch on sump dryers, t h e operation is generally limited to this speed range, regarded as the most appropriate, so t h a t a comparison with the single-drum dryers appears justified. It now appears t h a t t h e production capacity increases in each case with the content of solids in t h e starch suspension; this is esplained by the increase in the adhesion force described earlier in this paper. T h e production capacity of t h e sump dryer is markedly inferior t o that of t h e single-drum dryers because of the single very thick film applied. T h e film has a lorn heat conductivity and therefore greatly retards the drying speed. For the three single-drum drying processes under discussion, the characteristics of the production capacity curves are very much similar though t h e differences are recognizable. I n the medium concentration ranges t h e wet-product takeup curve shows higher values ahead of each applicator roll because with this drying method t h e highest viscosity values in the paste are obtained ahead of each applicator roll-Le., in the gelatiiiization process the first viscosity maximum is passed ahead of each roll, which is a decisive factor for the product application conditions. I n the Jxocess with a single wet-product feed, this condition prevails only ahead of t h e first applicator roll. I n this case the specific drying rate is lower, owing t o the further reduction of the viscosity due to the shear forces and temperature levels while the product passes through the succeeding rolls. As the conceiitration increases, the differences tend to disappear, however, since the influence of the increasing concentration due t o water evaporation makes it,self increasingly
-
Starch slurry concentratlon l%l
Figure 8. W a t e r absorptive capacity as function of the starch slurry concentration
felt while the product passes through t h e succeeding applicator rolls. T h e fact that variation of the viscosity strongly affects t h e dryer capacity is best illustrated by t h e drying curve for precooked wet product. T h e specific drying rates are much lower here, because application takes place at paste viscosities which are probably after the first maximum of the gelatinization curve. Figure 7 shows the influence of cooling the applicator rolls on the specific drying capacity. The diagram shows that low temperatures of the outflowing cooling water-i.e., a high cooling water throughput-reduce the application rate and hence the drying rate. An explanation for this phenomenon is the increased condensate formation on the roll surface. T h e condensate is absorbed by the outermost layer of the wetproduct roll where it reduces the viscosity. Naturally this process functions all t h e better the more completely the starch is hydrothermally altered and the narrower the gap is between applicator roll arid drying drum. Consequently thiq phenomenon is most marked with the drying process employing precooked starch. U p to a certain extent, conclusions may now be drawn on t h e degree of hydrothermal alteration or gelatinization of Ind. Eng. Chem. Process Des. Develop., Vol. 1 2 , No. 2, 1 9 7 3
145
-
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Figure 9. Viscosity of an O.6Y0 solution as a function of the starch slurry concentration
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Starch slurry concentration (%)
Starch slurry concentration ( % I
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. 15
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Starch slurry concentration 1%)
Figure 10. Cold water solubility vs. starch slurry concentration
the pregelatinized starch by examining the water absorption capacity and viscosity of the starch solution prepared from the dry pregelatinized starch because these factors are influenced by the increase in the volume of the starch granule during the gelatinization. Figure 8 shows the mater absorption capacity as a function of the starch slurry concentration lvhen applying the product on the drying drum operabing a t the favorable operating speeds mentioned. Here the superiority of the process with starch precooking stands out clearly: It gives a much higher water absorption capacit,y thanks to the better hydrothermal alteration of the starch. The drop in the water absorption capacity with a n increasing starch slurry concentration has been noted by other authors too (Takahashi, 1969) and might be attributed to a poorer gelatinization and also t,o the modified drying characteristics. Owing to the lower water content with high concentrations, the st'eep initial drying phase is completed faster, so that the flat final drying phase is longer (Kirschbaum, 1956).K i t h this process, starch particles may be overdried aiid thus encrust, possibly leading to a reduced mater absorption. might he 146 Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 2, 1973
~
~~~~
Figure 1 1 . Bulk weight vs. starch slurry concentration
expected, the drop is least with the precooked starch. The viscosity curves for the starch solutions prepared from pregelatinized starches produced by the diff ereiit processes are similar to those for water absorption and also to those for the cold water solubility. Figure 9 shows the viscosity of the starch solutions as a function of the concentration of the starch slurry applied on the drying drum. Despite different feeding arrangements, the single-drum dryers with applicator rolls show only slight differences, while drying 011 a sump dryer and drying preceded by precooking deviate widely from each other. Here again precooking has definite advantages, for a higher water absorption capacity and higher viscosity mean a better reconstitutional yield of the dry product. The slight drop in t h e viscosity noticeable with a n increasing starch slurry concentration and evident with all processes may be related to the drying characteristics which, at higher concentrations, may already induce dextrinization of the starch. Figure 10 shows the cold water solubility of the pregelatinized starches plotted against starch slurry concentration. This diagram also shows the already known cycle. Yet, it is obvious that, in the case of the sump dryer, a particular drop of the cold mater solubility as a function of the higher starch slurry concentrations can be observed, whereas a t lojver concentrations even higher values were measured than n i t h the triple feeding arrangement on a single drum dryer. It can be assumed that a t lower coiicentrations a relatively sufficient hydrothermal alteration can be obtained on a sump dryer because of the bigger sump volume. At higher concentrations that process will be retarded because of the high viscosity of the paste which does not permit a homogeneous mixing of the just-fed-in starch slurry with the already pasted product in t'he sump. Thus, pregelatinizing temperatures and the time of reaction mill get out of coiitrol as will be demonstrated later. The advantages of precooking the starch are offset, hoivever, by zi lower bulk weight, as showii in Figures 11 and 12. The lower bulk weight with precooking can be attributed to the greater volume aiid to the porosity of t'he paste applied on the drying drum, owing to the inclusion of steam bubbles. The higher bulk \veight wit'h the single-drum dryer having one wet-product feed can probably be explained by bhe thickening effect as the starch paste passes the various applicator rolls. -1s may lie seen from Figure 11, the bulk weight
-
Drum speed I r D r n )
~~~
Figure 12. Bulk weight vs drum speed; film thickness
at a given speed increases ivith the starch slurry concentration. The high bulk weight obtained with t h e sump dryer running a t only 3 rpm is probably attributable to this circumstance. Similarly high bulk weights may also lie obtained with a single-drum dryer having several applicator rolls, though this would entail operating speeds of the order of 3 rpm and the use of as many applicator rolls as possible. Figure 12 s h o ~ sthe influence of the druni speed on the bulk weight and 011 the film thickness of the dry product. The film thickness must not necessarily correspond to the bulk weight which, as already stated, is a function of the structure of the wet product :ipplied on the drying drum and is again most clearly visible with the precooked product. Figure 13 shows the influence of the precooking temperature on the properties of the pregelatinized starch. Whereas the viscosity of a11 0.6% pregelatinized starch solution hardly changes with the gelatinization temperature, a direct increase in the water a1)sorptive capacity can be noted. T h e bulk weight and the capacity have not' changed either. T h e radical change of the water absorptive capacity allows the conclusion t!iat the gelatiiiizatioii of the starch granules is still more intensive :it higher cooking temperatures and that the graniiles are already partly disintegrated. It may be concluded, that the slight drop of the water absorptive capacity and of the cold water solubility, which could be observed a t higher starch slurry conceiitrations, even when the product had been precooked, may be compensated by yet higher cooking temperatures. Further esperimeiits in t,his direction are in the preparation stage.
Figure 13. Water absorption, viscosity vs. gelling temperature
It was also intended to investigate within the scope of this study t'o what extent ungelled starch wa end product. It has become apparent, however, that no nongelatinized particles were detectable in any of the examined which were within the selected starch slu centration range of 15-40%. From thi:, it must be concluded that a pasting or gelling of the starch take5 place lvith each of the four processes investigated, but that the gelatiiiizatioii of the starch is stopped a t certain stages by the drying process, and that furthermore the properties of t h e starch are affected by secondary physical and chemical reactions such as destrinization, incrust,ation, or retrogradation. Because this indication of the degree of gelling 'iyas unsat clear discrepancies repeatedly appeared when masliiiig the starch pastes with water (with the same admisture of v a t e r the precooked paste product had a puciding-like structure, for example, xhereas the other pregelatiiiized starches yielded a viscous liquid), the viscosity of the starch pastes was determined again with the aid of the Brookfield viscometer. I n the nest to the last column of Table I are the viscosities measured in a pregelatinized starch produced from a 35y0 starch slurry a t a speed of 18 rpm ( 3 rpm ill the case of the sump dryer). The viscosities of the pregelatiiiized starches obtained hy the first three processes vary in line with the values indicated previously, and no viscosity change could be noted eveii wlieii t,lie viscosity rendiiigs ivere continued for a period of several miiiutes. On the other hand, the precooked starch showed a very high initial viscosity, which with the same shear stress conditions dropped rvitliin a few
Table I. Comparison of Processes at Optimum Speed and 35% Starch Slurry Concentrution (Maize Starch)
Drying process
Sump dryer Single-drum dryer with 1 wet-product feed with wet-product feed ahead of each roll with precooking
Specific drying capacity, kg/m2 hr
Water absorptivity, cm3/g
7.0
9.5
25 2
11.4
25.6 23.0
11.1 14.2
Solubility in cold water,
%
Viscosity, HEppler, CP
Viscosity, Brookfield, CP
Bulk wt, SI!.
1.43
80
390
12.2
1.50
95
280
10.5 13.4
1.55 1.62
9
102 Start after 55 min 520 258
Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 2, 1973
265 245
147
Table 11. Comparison of Processes at Optimum Speed and 35% Feed Concentration for Gelled Maize Flour
Drying process
Sump dryer Single-drum dryer with top applicator rolls and 1 wet-product feed with wet-product feed ahead of each roll with precooking
Specific drying capacity, kg/m2, hr
8.1
Water absorptivity, cmvg
Viscosity, Brookfield,
6.8
128
CP
Bulk
2:‘ 369
operation of single-drum dryers to such a n extent that their advantages in respect of quality and hygiene can be fully exploited. Because of these two aspects, preference should be given to the two processes with wet-product feeds ahead of each applicator roll when planning new installations in the future. Either process will alternatively be possible with one and the same installation. Conclusion
19.2
7.3
20.0 18.1
7.0 138 323 8 . 2 Initially after 232 30 min 715 250
142
252
seconds to a level substantially higher than the comparative values. Moreover, with other shear stress conditions, a strongly thixotropic behavior was observed in the paste just described. This behavior of the starch paste from the precooked and subsequently dried pregelatinized starch leads us to believe that the gelatinization is most advanced with this process, so that the disintegration of the gelatinized starch granules will more readily achieved under the action of mechanically induced shear forces. I n Table I all the parameters already discussed for the various processes are set out once again. With high drying capacities and a high degree of hydrothermal alteration, the process of producing pregelatinized starch with the preceding precooking process offers big advantages, while with its low drying rate and poor quality of the end product the sump dryer hardly meets the requirements of a modern production plant-apart from special products perhaps. T o be fair, however, i t must be said that the operation of a single-drum dryer with applicator rolls demands more attention from the personnel than the sump dryer. Nevertheless by using modern control systems it is possible to simplify the
I n this paper the processes are compared taking maize starch as the only example. Similar results can also be deduced for other products containing starch, as shown by Table I1 giving the figures for gelled maize flour. Other flours and tuberiferous starches have already been investigated as well-even oxidized starches-and in nearly every case it has been possible to control and vary the properties of the pregelatinized starches or gelled flours using one process or the other. Pregelatinization has not always proved to . be advantageous, namely in those cases where the starch had already sustained enzymatic attack. I n such cases a marked drop in viscosity was noted. This work represents only a beginning. I n the course of the tests numerous new avenues and possibilities offering worthwhile objectives for further investigations have been noted. Acknowledgment
Thanks are extended to m y assistants, Crsula Zoller and Wilhelm Soll, for their valuable assistance in carrying out the tests and analyses. Literature Cited Chemische . .. . Fabrik Mahler. SuDf. German Patent 403.076. 1921. ,
I
Chemische Fabrik Mahler,’Sup’f,’zbzd.,554,945, 1932. Fritze, H., Escher Wyss Mitteilungen, Vol. 42/43, Yo. 2, 1969. Kehl, E., zbzd., KO.3, 1932. Kirschbaum, E., Baumann, R., Chem. Ing. Tech., pp 520 ff, 11956). Maurer,’W., Sturke, 14, 197-208, (1962). Takahashi, R., Ojima, T., ibid., 21, 318-21, (1969). RECEIVED for review March 17, 1972 ACCEPTED December 4, 1972 Presented at the 22nd Starch Meeting, Detmold, West Germany, 1971.
Preliminary Economic Analysis of Donnan Softening Lawrence Dresner Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
D o n n a n softening (Eisenmann and Smith, 1970; Smith, 1971) is the name given t o a dialytic method (Wallace, 1964, 1966, 1967) of removing divalent cations from the feed water of desalination plants. The feed to the plant and the brine from it flow countercurrent to each other in adjacent channels, the common wall of which is a cation exchange membrane. Divalent cations migrate from feed to brine, while univalent cations migrate in the opposite direction. In a variation of the process, the brine is enriched in a salt of the univalent cation 148 Ind. Eng. Chem. Process Des. Develop., Vol. 12, NO. 2, 1973
from a n external source. Donnan softening of the feed water would be extremely valuable in suppressing scale formation in desalination plants. It is the purpose of this paper to estimate the cost of this pretreatment. Description of Process, Simplifying Assumptions, and Notation
Figure 1 s h o w a schematic diagram of the process. Two channels are shown, one marked “feed” and the other “brine.”
