INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1931
justification for the manufacture of solid carbon dioxide under such conditions from such a source” is absolutely meaningless in so far as giving a true picture of the possibilities of lime-kiln exit gases, unless the character of the lime-kiln plants is considered. With the type of kiln and methods of operation characteristic of the commercial lime industry generally, the statement has a more or less sound basis in fact; on the other hand, with kiln design and operation characteristic of the ammonia soda industry, the statement will not stand analysis for a moment. The ammonia soda manufacturer operates his lime kilns primarily for carbon dioxide, which, as previously stated, is an essential raw material entering directly into his product; lime, however, is not an essential raw material, nor does it enter in any way into the finished product, soda ash. Ammonia soda kilns are designed t o be operated under a rigorous chemical and physical control, unknown to the commercial lime industry. The control moreover begins in the quarry with the selection of stone of uniform chemical and physical characteristics, clean and free from dirt and broken to uniform size. The kilns are charged and drawn continuously, and, aside from constant attention to the character of stone supplied, particular care is given to ratio of fuel to stone, temperature, and draft; the result is an exit gas of uniform quality running somewhat in excess of 40 per cent carbon dioxide by volume when coke is used as a fuel. Moreover, the gas when cooled and scrubbed is odorless and free from all impurities that would contaminate the finished product. It is not a misstatement that solid carbon dioxide “has a great future,” and that it seems a “logical refrigerant for many purposes,” particularly in the field of economical distribution of frozen foods, fruits, etc. The statement that the solid carbon dioxide industry today is not a tonnage industry is, of course, true; moreover, it will never reach that stage on the basis of present selling prices. Radical reduction in manufacturing costs must be made, and what is equally vital, the industry must be standardized technically, utilizing the cheapest possible source of carbon dioxide, with standardized equipment. Manufacturing costs must be reduced to a point where the solid product can be sold a t a satisfactory profit on a basis comparable to water ice a t $4.00 to $5.00 per ton, or say $50.00 to $60.00 per ton delivered for solid carbon dioxide. An intensive study during the past four years of the problems involved in the utilization of the exit gas from lime kilns, designed and operated under the complete chemical and physical control characteristic of the ammonia soda industry, and producing high-grade chemical lime as a by-product, indicates that, a t a rate of 25 to 50 tons per day, solid carbon dioxide can be manufactured a t less than $10.00 per ton, including depreciation and superintendence. W. D. MOUNT P E O P L E S h-ATIOXAI.
LYNCHBURG, VA. May 15, 1931
BAXK BLDG.
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Editor of Industrial and Engineering Chemistry: I regret that I am not in a position to expand on the points in my paper on by-product carbon dioxide economics by citing cases, which is after all the only proper approach to reducing mere economic generalizations to sound conclusions as a basis for business judgment. One must recognize that the logic of many of Mr. Mount’s beliefs, divorced from supporting statistics, seems quite as good as my own contrary statements, likewise divorced from supporting statistics. I know of no necessity dictating commercial utilization of 40 per cent carbon dioxide gases, however clean and well controlled, as long as the quantity of relatively pure by-product gases, 95 per cent and over, far exceeds what the market will absorb. The contention that lower selling prices are essential to wide market expansion has of course an appeal to the manufacturer
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who knows he can manufacture carbon dioxide cheaply if he has a large ready market, but must resort to conjecture or prognosis in order to include the assumption of such a market in his plans for expansion. I freely confess error in the wording of my statement that manufacture of carbon dioxide from lime-kiln gases has been a dead issue for twenty years. I should have said “the manufacture of carbon dioxide for sale, etc.” I was not unaware of the use of lime-kiln gases in the ammonia soda process, and, in passing, would point out that substantial amounts of carbon dioxide for use in process a t concentrations less than 100 per cent are derived from lime kilns and other processes also in the manufacture of beet sugar, in recarbonation of city water supplies, and in the manufacture of synthetic methanol and urea. It can be contended, of course, in any industry that a sudden drop in selling prices will make possible a large increase in market and a substantial reduction in manufacturing costs, or these three circumstances may be linked together in any other order desired, any one being considered the cause and the others effects. I t has been my brief experience that sales-minded people are inclined to consider market development the cause, low prices and manufacturing costs merely effects of improved markets. The customer is prone to consider the reduction in selling price the cause, whereas the process engineer looks upon the manufacturing method as the prima causa, the other two merely links in a chain of circumstance. I am sure neither Mr. Mount nor myself can resolve this “trilemma” in your columns. As for the figures cited in the last two paragraphs, I should be interested to know the basis of the inference that Dry-Ice a t $50.00 to $60.00 is comparable to water ice a t $4.00 to 955.00, since it is still my impression that these figures cannot be called “comparable” without setting up an arbitrary basis of comparison. Incidentally, sale of solid carbon dioxide a t $60.00 is already afait accompli, and cause for no remark or surprise. Such a price, however, is profitable only when the margin is not wiped out by uneconomic practices. I t is unfortunately true that some solid is being sold on which trucking costs alone exceed this figure. Under these conditions, obviously, manufacturing cost could be reduced to absolute zero by the process engineer without profit to his principals. In regard to manufacture a t less than 510.00 per ton, we can only state that the DryIce Corporation long since reduced costs of manufacturing solid from carbon dioxide gas to considerably less than this figure. Problems of marketing, winter season standby costs, distribution losses, and assured continuity of supply loom so much larger in importance that the figure taken alone has no significance. Dividends are not paid on cost estimates but on results. C. L. JOXES DRYICECORP. O F AMERICA 52 VANDERBILT AVE. NEWYORK,N. Y. June 3, 1931
Prevention of Silica Scale with Sodium Aluminate Correction We desire to correct an error in our article entitled “Prevention of Silica Scale with Sodium Aluminate,” IND. ENG.CHEM., 23, 637-46 (1931). On page 639, column 1, line 5 , we wrote: “The amount of aluminate used was calculated to give the same sodium equivalent as in the sodium hydroxide series . . .” The t e x t should have read: “The amount of sodium aluminate used contained only 25 per cent of the sodium present in the sodium hydroxide ’ series. For an equivalent quantity of sodium from sodium alu-
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ISDUSTRIAL A1VD E S G I S E E R I S G CHEMISTRY
minate, it would have been necessary to use 174 p. p. m. instead of the 44 p. p. m. used in test Id.” On page 639, column 1, line 14, the statement: “With equivalent sodium contents, the added silica reduction is shown to be due to the presence of alumina” should have read: “Even with a reduced sodium and equivalent alumina content, the increased silica precipitation by sodium aluminate shows strikingly the influence of alumina in silica removal.’’
tivity
(:)
T-ol. 23,
KO.7
based upon weight of cake in the press rather than any
consideration of volume or thickness. Xumerous investigators have observed that filter resistance is determined to a large extent by particle size. Hatschek [ J . SOC.Chem. I d . , 29, 538 (1908)] expressed the belief that a microscopic examination of precipitates should be of considerable aid in predicting 1 filter behavior. -- as defined above has a very definite physiCY
cal significance and can be shown from theoretical consideraC. H. CHRISTMAS R26vy~ . A knowledge of the J. A . HOLMES tions to be a factor of the form H. THOMPSOK mean square radius of the particles in a suspension, their density, the pore volume with which they form a cake, a shape factor f , and a universal constant K should make i t possible to calculate from a theoretical basis the filter behavior of any suspenComments upon Recent Developments sion of solid matter in a liquid. Obviously, it would be a difficult task to measure these separate factors compared to the determinain Theory of Filtration tion of their product by means of a simple filtration test. However, the fundamental correctness of the relation and of this conEditor of Industrial and Engineering Chemistry: ception of filter resistance can be demonstrated very nicely by In the light of some recent papers by van Gilse, van Ginneken, studying the flow of liquids through layers of material such as quartz grains or crushed rock. and Waterman [ J . SOC.Chem. I n d . , 49, 444T, 483T (1930); 50, (4) The fact that there exists no sharp dividing line between 41T, 95T (1931) ] upon the subject of filtration, it seems advisable the sludges commonly regarded as non-compressible and those to call attention to an extensive program of research in the same which are highly compressible. Thus Equations 1 and 2 govern field which has been carried out in these laboratories during the the entire range of filter behavior in a manner of extreme simplicity as compared t o former equations (LValker, Lewis, and Mcpast two years and will be published as soon as released by the Adams, Zoc. cit., p. 366). faculty of the Graduate School of the University of Minnesota. (5) Recognition of the fact that non-proportionality between The authors cited worked a t constant pressures with a small rate of flow and pressure is due to an actual compressibility of suction filter having an area of 43 sq. cm. upon suspensions of dif- the filter cake which is a simple h e a r function of the pressure rather than a power function for constant pressure filtrations. ferent kinds of charcoal, over a pressure range of 0.2 to 0.7 kg. per -A theoretical basis for this has been derived which is confirmed sq. cm. (2.845 to 9.96 pounds per square inch). They concluded by our own researches and by the work of van Gilse, van Ginneken, and m’aterman. that the resistance of their filter cakes could be expressed as a linear function of the pressure-e. g., r = ao(1 pP)-a form Filtration tests have been made upon such materials as Sil-0which possesses decided advantages over the accepted Lewis Cel, calcium carbonate, calcium sulfate, ferric hydroxide, and equation (Walker, Lewis, and McAdams, “Principles of Chemical Engineering,” p. 366, McGraw-Hill, 1927) where r is considered lead chromate, using cotton cloth, monel metal cloth, and Filtros plate filter septums in a small-size filter press (9-inch diameter to vary as In the course of the work performed in these laboratories, modi- frame) with available pressures up to 60 pounds per square inch. The three different methods of performing filter tests-that is, fied equations governing filtration have been developed and a constant pressure, constant rate, and constant resistance-have number of methods worked out for analyzing the data of filter been investigated. tests, It has been found that the following equations, based upon For slightly Compressible material, the constant rate method Poiseuille’s law, are of general applicability : is superior for a number of reasons: ~
+
Equation 1 is the fundamental equation governing filtrations a t constant pressure, and Equation 2 is the form governing filtrations a t constant rate. The nomenclature is largely that of Walker, Lewis, and McAdams (Zoc. cit., p. 363) plus the addition of several new symbols. Success in this work has been due in large measure to the following nex conceptions and methods of treatment:
.
(1) The development of methods whereby the resistance offered to flow of filtrate by the supporting filter base can be separated from that of the filter cake proper in the treatment of data. It is because van Gilse, van Ginneken, and m‘aterman, in the work supporting their thesis, used a filter cloth of unusually low resistance (practically zero) compared to that of the filter cakes, that they were able to arrive a t their conclusions. When they made a similar filtration in a commercial-size press, they were forced t o conclude that it did not obey the same laws that governed filtrations in the suction filter. It can be shown easilv that in the latter test the resistance of the filter cloth was of more nearly normal proportions, and that when it is properly accounted for, the observed data do not depart more than 0.2 per cent from the theoretical behavior. 1 - 7s , whereby filtrate (2) The introduction of the factor ~
PS
volumes become a definite measure of the weight of filter cake in the press a t any time. (3) The conception of a specific cake resistance ( a )or conduc-
(1) h constant rate of filtration can be maintained with greater accuracy than can a constant pressure. (2) The small but unavoidable errors always present in constant-pressure filtrations due to turbulent flow of filtrate in the leads are avoided because in this method the pressure drop through the leads remains constant and can be measured. (3) The relation of resistance to pressure is given in a single test, whereas by constant pressure methods a number of tests must be made a t intervals over the desired pressure range. (4) The compressibility relation thus found is an average value which can be rechecked again experimentally with much greater accuracy than can any single value obtained by constant pressure test. The constant resistance method-i. e., a test conducted by running pure water through a cake previously laid down by a filtration-while theoretically the easiest method of carrying out a filter test, and capable of the greatest accuracy, is actually not a valid measure of any filter behavior. It is in large measure tests of this nature which have confused the minds of experimenters from 1912 to 1931 and which account for the differences in opinion from time to time as to whether pressure occurred in the filtration equation to powers of l / 2 , 1, 2, or decimal fractions. This phenomenon, which always seems to mask filter test constants when water is later passed through the cake, obeys a law just as definitely as does the filtration. Lacking its applications, many data in the literature obtained by running water through a filter cake or even a clean cloth, and utilized to determine filter behavior, have led to erroneous conclusions. It can be said with assurance that no other branch of chemical
