Professor S. V. LEBEDEV

ON THE DRYING OF SLOP

at Siberian distilleries in general

and in particular at the plant in Novosibirsk

Tomsk—1929

Professor S. V. Lebedev.

On the Drying of Distillery Slop at Siberian Distilleries in General and in Particular at the Plant in Novosibirsk.

Distillery slop, being a waste product of distilling, has the significance of a valuable feedstuff. In this connection, slop is everywhere used directly as feed for cattle. The extremely high water content in slop, up to 93–95%, greatly limits its sale, not allowing it to pass beyond the narrow limits of the settlements nearest to the plant. However, with the prevalence of small distilleries of the agricultural type, this usually did not cause any inconvenience for the complete disposal of slop.

Only in the comparatively rare cases of greatly enlarged plants does there arise the threat that some part of the slop will not be used by the surrounding population. But here too there is always a way of using such surplus slop, which is fed in cattle barns specially arranged at the plant, where cattle are fattened for slaughter. Owing to this, the question of transporting slop and the related necessity of drying it arose only in isolated cases. In prewar times Russia had absolutely no interest in drying slop. At Siberian distilleries there had never been any drying of slop. At present, however, Gostorg is interested in the question of drying slop. By agreement between the People’s Commissariat of Trade, the Supreme Council of the National Economy, the Spirit Convention, and the meat-fattening organizations, it is now planned to conduct, in the European part of the Union, an experiment in installing slop dryers at three distilleries: of the Smolensk, Maikop, and North Caucasus trusts.

Plants are being selected in regions convenient for marketing dried slop or else close to export thresholds. It is proposed to install three different types of dryers. The cost of one installation, according to the proposals, is 100–120,000 rubles. The most important point in the choice of a drying method is connected with the question of whether the slop will be dried as a whole or whether, before drying, it will first be separated from the liquid. It is obvious that a considerable proportion of the nutrients of the slop will pass into its solution, so that, while drying is made cheaper by discharging the liquid part of the slop into the drain, the dried slop itself will be greatly devalued.

However, when the slop is dried as a whole, the costs of arranging the drying process increase considerably.

The experimental installations planned are as follows: one according to the method of Prof. Shustov and Bobrov, who propose returning the expressed liquid from the slop to production instead of water for mashing-in (in the case of work on grain); one according to the method of the firm Imperial (in Germany); and one according to the method of the American firm Louisville.

The procedure of the latter two installations amounts to separating the solid substances from the slop, drying the separate insoluble residues of the slop in special dryers, concentrating the liquid part of the slop to the consistency of syrup, subsequently mixing the previously dried solid part of the slop with the condensed liquid slop, and redrying this mixture to the state of a mass

with a moisture content of about 10%, yielding a loose, air-stable dry mass of slop.

The Louisville firm recommends installing equipment for concentrating the liquid portion of the production only several months (2–3) after the installation of the apparatus for separating and drying the insoluble part of the slop. This seems necessary in order to clarify the operating conditions on slop of unknown quality, and also to determine what dry product will be obtained, which it is desirable to establish for the selection of the subsequent method of drying the liquid portion of the slop.

From the foregoing it is clear that one of the first and principal aspects of slop drying is the question of separating the slop into an insoluble and a liquid part. In this respect the proposals are very diverse, but the principal ones are as follows: separation on filter presses, or on squeezing cylindrical presses, or else on vacuum filters.

The inconvenience of using filter presses as devices requiring constant manual labor is obvious. The advantage of automation remains with the pressing method, for example that of Louisville, and with vacuum filtration, for example that proposed by the firm Imperial and others.

The use of vacuum filters has recently become widely disseminated in the most varied industries, for example in installations for treating wastewater, in soda, sugar, and other production processes. The steadiness of operation and the almost complete absence of any need for living labor definitely speak in favor of this method. However, the specific property of slop—this plant mass, boiled under pressure, of colloidal character, and in general very difficult to filter—raises the question of the extent to which filtration in general, and vacuum filtration in particular, can be applied to slop. There are no data in this respect, and therefore the question of the applicability of vacuum filters for filtering slop requires special experimental clarification; at the same time, the general question is raised as to how filtration of slop can be carried out at all with respect to the conditions: temperature, filtering material, applicability of rarefaction, required surface area, etc.

The method of pressing slop for the purpose of dewatering it, by analogy with other industries where products with a very small content of insoluble substances must be dealt with—for example, beet pulp, etc.—promises very little, while at the same time requiring a large expenditure of power and special equipment. In connection with this, among the principal methods of preliminary dewatering of slop before drying, vacuum filtration deserves the greatest attention.

The proposal concerning the necessity and possibility for Siberia of export-dried slop, given the great distance of Siberian distilleries from the border and the low cost of dried slop, can hardly be viable, and in essence is fundamentally incorrect.

The principal task of Siberian export, as is generally accepted, is not to export cheap, non-transportable agricultural raw material, and still less its wastes, but rather products of comparatively high value, which in the present case should be meat, butter, etc.—products of animal husbandry, into which slop should already be transformed here in Siberia. Judging by the approximate data of the North Caucasus Trust of the Alcohol Industry, a pood of dry slop costs 40–45 kopecks. Obviously, a product of such a price is too cheap to be transported 3.5–4,000 versts. Under such conditions it is necessary

drying of slop to the utmost degree. In connection with this, it would inevitably be necessary to abandon the use of the liquid part of the slop, and this would further cheapen the dry slop, which in turn would make its export more difficult. Under such conditions the question inevitably arises of using the slop locally in Siberia in the form of raw material for obtaining highly concentrated feeds, so necessary to Siberian agriculture and especially to its butter-making industry.

Over time, when in Siberia, with the quite natural reorganization of Siberian distilling, greatly enlarged plants arise, there will inevitably be at them a large surplus of slop beyond the local demand for it. Then drying the slop will become appropriate and necessary, so that it can be transferred to the nearest Siberian butter-making districts.

In the absence of fixed capital for special equipment to process the liquid part of the slop, the latter should be directed to biological purification, by means of which the liquid waste of the slop is easily mineralized and made safe when discharged beyond the limits of the plant.

The draff separated from the slop may be mixed with bran, brewery malt sprouts, hay, which, as admixtures to the slop, can facilitate its filtration, and similar supplementary materials that convert the slop into high-value feedstuffs suitable for export to places of consumption. This task would be facilitated by the use of briquetting.

This idea, one must suppose, theoretically quite correct and expedient in its viability for Siberian distilling in its further evolution, can at present, however, find only partial realization in individual cases.

Such a case apparently exists, as a short-term phenomenon, in Novosibirsk, at its distillery No. 7, where in spring, during the period of thaw and impassable roads, a certain quantity of slop remains at the plant, temporarily without a market, and therefore is inevitably discharged into the wastewater.

Given the short duration of this phenomenon and the insignificance of the total quantity of slop during this period, it is not possible to make large expenditures on the construction of complete equipment for drying slop. In the present case, the matter could only concern separating from the slop its draff, in order to make the purification of its liquid part by biological means accessible.

As for the slop draff, the latter is already, in itself, a material which, by virtue of its concentration relative to the slop, can without difficulty find a market locally in Novosibirsk. Otherwise it would be necessary to resort to drying this draff by one method or another.

Both in the general question of drying slop for export and in the particular case of rendering harmless the slop going into the drain, the question that comes to the fore is above all that of methods for the practically convenient and technically accessible separation from the slop of its liquid part, or, what is the same thing, the extraction of its draff from the slop. As was already clarified above, theoretically the most convenient method appears to be filtration with the use of vacuum. However, given the unknown attitude of the colloidal mass of distillery slop toward this process, we are deprived of the possibility of making our conclusion on this question.

Cases of successful application of the filtration method in general, and in particular of vacuum filters in other industries, give us very much

little in view of the entirely different properties of the materials being filtered. The brewer’s mash, closest to stillage in its origin, differs sharply from it in its physical properties, possessing properties entirely different, and at the same time more favorable for filtration, than stillage. All this compelled us to undertake a series of laboratory works to study the conditions of filtration of distillery stillage both under ordinary pressure and under vacuum.*) The conditions and results of these experiments are given below. The work was carried out with stillage from distillery plant No. 7 in Novosibirsk. The experiments were conducted with three samples of stillage: No. 1 of 15/X, No. 2 of 7/XII, and No. 3 of 16/I—1929. The characteristics of the mashes, according to extracts from the factory journals, are presented below:

Table I.

The characteristics of the stillage samples obtained in the laboratory are given in Table II.

Table II.

With an almost identical quantitative composition of the mashes, they should have yielded the same stillage, which in reality did not prove to be the case. In this connection, the data presented indicate that the stillage samples delivered to the laboratory were not average samples for the entire mash, but individual selections of stillage taken either at moments differing in the operating conditions of the distillation apparatus, or else these samples were taken at different degrees of settling of the stillage in its common receiver. The conditions under which the stillage was obtained in the laboratory were as follows. The stillage samples, brought from Novosibirsk to Tomsk by rail, were delivered to the STI laboratory on the day after they had been taken at the plant. In Tomsk, after being poured into bottles, the stillage samples were again sterilized three times with live steam for one hour over the course of three days.

*) This part of the experimental work was carried out, under the author’s supervision, by chemist V. P. Malchanova.

These samples were used for all the work on studying the filtration conditions of the slop. Since the composition of slop in general is extremely variable, in order to characterize the Novosibirsk slop a complete chemical analysis was made of its sample No. 2; the filtrate was analyzed with respect to its organic composition, and a complete analysis of its ash residue was also given. Both of these analyses are necessary for resolving the question of biological purification. In addition, an analysis was also made of the spent-grain fraction of the same slop sample No. 2.

Table III.

The composition of the ash residue of the filtrate of slop No. 2 is presented by the data of Table IV.

Table IV.

The composition of the dried distillery-slop spent grains from slop sample No. 2 is given below:

Table V.

The composition of the distillery-slop spent grains indicates that, for direct mineralization as a constituent part of wastewater, it would present very considerable difficulties. Establishing the feed value by the Kellner method of calculating the starch equivalent of distillery-slop spent grains, we find that the latter, with a correction for fiber and with pentosans excluded, is determined by the following value:

\[ 36.63 \times 0.94 + 8.71 \times 1.94 + 6.89 \times 1.00 + 14.05 \times 1.00 - 14.05 \times 0.29 = 61.35. \]

Here 0.94 is the starch equivalent of protein, 1.94 of fat, 1.00 of nitrogen-free extractives and fiber, and finally 0.29 is the correction coefficient for the fiber of the feed product.

The value found indicates a sufficient feed value of dry distillery-slop spent grains. For comparison it may be noted that for brewer’s dry spent grains it is equal to 84.

In the absence of drying, the value of the distillery-slop spent grains decreases in accordance with the increase in the moisture present in it, which may be expected to be approximately 80%. Under such conditions the starch equivalent of the moist spent grains will come out to be around 13, i.e., it proves to be very small.

On the question of determining the capacity of distillery slop for filtration, experiments carried out in the laboratory of the STI pursued the aim of establishing the conditions for the maximum effect of this process with the least expenditure of time, while simultaneously achieving the minimum moisture content of the distillery-slop spent grains remaining on the filter. Preliminary experiments established that hot slop, without dilution, filters 4–5 times faster than cold slop.

With respect to the filtering material, the following were tested: filter paper, flannel, knitted material, various fabrics, coarse and fine canvas, and finally the ordinary screen of the filtering vat of a brewery. The latter gave extremely slow filtration, which moreover very quickly ceased altogether; the cause of this should be sought in the sharply expressed colloidal character of the slop, owing to its content of small particles of boiled husks, coagulated proteins, pectic and similar substances, which quickly and densely clog the relatively small openings of the brewery filtration screen.

Of the filtering materials tested, canvas proved the most suitable: sparse—11 × 9 threads per 1 sq. cm, and dense—13 × 16 (13 warp, 16 weft). However, even in this case, without vacuum, filtration even of hot slop proceeded very slowly.

All the experiments were carried out on Nutsch perforated porcelain funnels with round holes, with a total filtering surface of 90 sq. cm and a free cross-section per 1 sq. cm equal to 0.044 sq. mm.

Under these conditions, the rate of filtration without vacuum at 83° C for thin filter paper was 27 minutes for 30 cubic centimeters per 100 cubic cm of slop; the same for canvas was 19 minutes.

A series of subsequent experiments showed that without vacuum, over time and regardless of the character of the filtering material, filtration usually very quickly ceases altogether. In connection with this, experiments were set up on the filtration of slop under vacuum. It was found that a comparatively small vacuum of 200–300 mm of mercury is more favorable than a high one. The reason for this lies in the fact that with a greater vacuum, from the very first moment of filtration, an overly compacted layer of slop grist is formed directly on the filtering surface, which subsequently greatly hinders the further filtration of the slop.

The relationship between the vacuum, the duration of filtration, the thickness of the layer of grist formed, the amount of filtrate obtained, the moisture content of the remaining grist, and also the productivity in liters per minute of one square meter of filtering surface with its free cross-section of 22 and 33% is presented in Table VI.

Table VI.

The data from the experiments presented indicate that, with a layer of draff on the filtering surface of 0.2–0.6 cm, in a comparatively short time of 2–5 min., the moisture content of the stillage residue on the filter reaches 85–69%, while the minute output of one square meter of filtering surface, depending on the conditions and the open area of the filter, is from 3.5 to 0.9 liters per minute at an open area of 22%, and from 5.2 to 1.8 liters per minute at an open area of 33%. Raising the degree of vacuum to 300–350 mm Hg, by increasing the layer of draff deposited on the filtering surface to 0.6–1.0 cm, noticeably lowers the productivity of the filter, while at the same time increasing the moisture content of the separated stillage draff. This indicates that, if it is necessary to discharge the stillage to the drain and there is a possibility of dumping the stillage draff without drying it, filtration should be carried out with a thin layer of stillage draff and with a low vacuum. If, however, the work is carried out with drying of the draff, then in that case it would be inadvisable to dewater the separated draff by slowing the filtration. This task would be better accomplished by special pressing of the separated draff before its subsequent drying.

In the case obtaining under the conditions of Distillery No. 7 in Novosibirsk, where it is necessary only during the short spring period to send to the drain the residues of unused stillage, it would naturally be appropriate to stop at filtering the stillage in a thin layer, since under Novosibirsk conditions the stillage draff, even without drying, could be disposed of on site without difficulty.

Let us now turn to the question of the capacity of the required drum filter operating under vacuum.

Taking the diameter of the filtering drum as two meters and immersing it in the stillage over 40% of its lateral surface, we find the productivity of one meter of length of such a filter under various conditions corresponding to the data of Table VI. This productivity is given below in Table VII.

Table VII.

If it is assumed that the productivity of structurally developed and technically perfected industrial filters is 50% greater in working efficiency than has been established under laboratory conditions, then, under these conditions, for one mash at Novosibirsk Distillery No. 7, amounting to about 260 poods of grain and yielding 1500–1700 buckets of slop, about 1.5 meters of length of the filtering drum would be required, and for 6 mashes about 9 meters.

To what extent this theoretical calculation of the required filter capacity corresponds to the actual capacity and dimensions of individual filters of various machine-building firms would have to be clarified by direct communication with them.

According to information received from the firm Imperial in Meissen, dated October 5, 1928, No. 7069, the cost of one vacuum filter of 3 sq. m., without installation, shipment, or assembly, is set at 10,900 gold marks. Thus, the cost of the filters alone for Novosibirsk Distillery No. 7, with 6 mashes, would require an expenditure of about 15,000 gold rubles, not including shipping.

An installation for drying spent-grain draff for one mash at the Novosibirsk plant costs about 15,000 gold rubles, without means of transport and steam-power installations, and likewise not counting shipping costs.

Comparing everything set forth above, one may arrive at the following conclusions with respect to Distillery No. 7 in N.-Sibirsk:

1) In the drying of slop, the Novosibirsk distillery, at its present scale of production, could be interested only for a short time—only during the period of the spring thaw.

2) Under the conditions indicated in the first point, the installation of an expensive slop dryer must be considered entirely inexpedient.

3) The situation of the Novosibirsk distillery in the spring period with respect to slop indicates the desirability, for the possibility of discharging it into the sewer, of installing here a device (for example, with vacuum filters) for separating from the slop its spent-grain draff, with the aim of subsequently rendering its filtrate harmless by the method of biological purification.

4) The slop draff obtained in filtering the slop, being, in comparison with the slop, of high value and in relatively small quantity, will readily find a market on the spot in Novosibirsk in the raw state, without any further processing or drying.

In order to reduce the amount of slop filtrate going to biological purification, it appears desirable to return it to production, using it, in the case of grain distilling, instead of water for mashing in the mash tuns. The extent to which this is expedient depends:

1) on the changes that the slop filtrate itself may undergo during mashing in the mash tuns;

2) on the influence of the mashed slop filtrate on the fermentation of the mash;

3) on the degree of influence of the acids of the slop filtrate on the saccharifying capacity of the mash tuns themselves.

In all these cases the principal point is the question of the relation of the slop filtrate to its mashing under pressure. A known answer to this may be given by Table VIII, which presents the results of experiments on mashing slop filtrate at various pressures and durations. The mashing was carried out for periods from 1/4 to 1 hour, with pressures from one to three atmospheres above normal. One series of experiments was conducted directly in an acid medium,

i.e., at the natural acidity of the slop; the other series of experiments was carried out after neutralizing the slop filtrate with soda. In the initial and boiled slop liquid, the color was determined in a Stammer colorimeter—the thickness of the liquid column in mm corresponding to the color of normal glass; the acidity was established in Delbrück degrees; the amounts of maltose, dextrins, and the sum of both, calculated per 100 cc of liquid, were determined by ordinary methods.

Table VIII.

The results of these experiments indicate:

1) the hydrolysis of dextrins,
2) the destruction of maltose—more precisely, of monosaccharides,
3) that this destruction proceeds more in a neutral medium, which increases its acidity during boiling,
4) that with an increase in pressure and duration of boiling the degree of destruction also increases,
5) that boiling the slop filtrate lowers its overall feed value.

The increase in color also indicates general destruction. It is evident that with repeated use of the slop filtrate—to which the constant return of slop to production will inevitably lead—the processes of destruction during boiling in the converters will increase rapidly. It follows from this that conducting the boiling of grain exclusively with slop liquid would be incorrect.

The return of slop or its filtrate to distilling production is, of course, also connected with the interesting, both production-wise and economically, question of the changes which the slop grain itself undergoes under boiling conditions.

Moreover, the effect of the slop on the starchy mash boiled in the converters is also significant, which may have subst—

...substantial production interest, but since the elucidation of this aspect would require appropriate special experimental treatment and a long period of time, this problem could not be included in the present work.

On the question of what should be added to dried slop meal in order to increase its value as a feed product and to make possible its transport over long distances, one may suggest first of all bran—the waste product of flour mills—and, on the other hand, dry brewers’ sprouts.

In order to give an idea of both, and also taking into account that bran may vary very greatly depending on the locality, the method of milling, etc., analyses of bran from Siberian mills are given below.

The same is also given for the composition of brewery-malt sprouts taken from the Tomsk brewery in Table IX.

Table IX.

The analysis of the ash of malt sprouts is as follows:

Total—93.81%; water and carbon residue—6.19%.

In the ash, by its weight:

Phosphoric acid—18.28%
Potassium oxide—24.43%

In general, however, the composition of brewery malt sprouts fluctuates within rather wide limits, as is evident from the appended Table X.

Table X.

The chief value of bran consists in its nitrogen-containing substances. The amount of the latter is very considerable and, in any case, no lower than in the grain from the grinding of which the given bran is obtained. Meanwhile, the cost of bran in most cases has usually amounted to about half the cost of the corresponding grain.

The valuable constituent parts of bran, besides proteins, are carbohydrates and fats; all this amounts to about half the weight of the bran.

Table X.

The composition presented for malt brewers’ grains and for bran from Siberian mills clearly indicates that, by combining dried distillery slop with brewers’ grains and bran, it is possible to obtain a feed of high quality. This feed will find a market everywhere, and above all in Siberian dairy farming. Nevertheless, given the present scale and conditions of Siberian distilling, the drying of slop is as yet inexpedient.