March 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table XII. Sugar Balance through Process on Basis of 1 Ton of Cane Containing 12.94% Sucrose Sugar in Sugar out
258.8 lb. in 1 ton of cane 2.6 Ib. in pulp (100% on cane, assumed) iO:130/o on pulp). 1 0% on sugar introduced)
1.0 lb. in pulp water (lOOyo on cane, assumed) (0.05% on pulp water)
Total sugar lost in process
(0.4% on sugar introduced) 1.1 lb. in lime aake (133 lb./ton cane) (0 877 total sugar in cake, wet basis) (0:4$ on sugar introduced) 49.3 lb. in molasses (44.3 purity) (19.0% on sugar introduced) 204.8 lb. in refined suzar (unaccountable losses disregarded) ’ (79.2% on sugar introduced) 54 lb. (20.8% on sugar introduced)
635
treated sirup which has a higher purity and better color, from which a refined sugar of marketable quality can easily be refined.
ACKNOWLEDGMENT Thanks are due t o Charles Price, argonomist, United States Department of Agriculture, Riverside, Calif., for providing cane, seed, and agronomic information which helped make this work possible, and also t o the Meloland Experiment Station, University of California, for growin the cane. The authors are inde%ted t o the Silver Engineering Works, Denver, Colo. , for the experimental machine which successfully produced the cane chips. The authors also wish t o acknowledge the valuable assistance of R. M. Daniels, J. E. Maudru, James H. Turner, H. B. Henderson, R. C. Barr, Jess Carson, Curt Hansen, C. V. Arnold, and Harold Gill. LITERATURE CITED
lost in the lime cake was free sugar, much of which could be removed by efficient washing. The greatest loss was in the molasses, and a t this point the greatest additional recovery is obtainable. Perhaps future work will reduce this. The assumption that unaccountable losses could be disregarded was thought t o be valid, because this loss (usually thought to be due mostly t o inversion within the process) is ordinarily small in factories where operations are efficiently controlled.
SUMMARY Although admittedly these investigations were not intended to answer all the questions that might arise concerning the production of crystalline sucrose from the sorgo cane, the basic steps in the process were developed and a method of manufacture was devised which not only yields a satisfactory product, but also is capable of being adapted with few major changes t o the existing production facilities available in a beet sugar factory. Two of the processing steps are outstanding in their deviation from common cane sugar practice, even though they were suggested as early as 1883by Wiley (19): (1)diffusion of sucrose from the cane; and (2) clarification of the raw sorgo juice by carbonation. These two deviations offer a great many advantages. By the use of the diffusion method of sucrose extraction a much greater percentage of the sucrose present in the sorgo is recovered, and most of the troublesome starch is eliminated from the very beginning. Clarification of the raw juice by carbonation not only removes the greater portion of remaining starch, thereby possibly eliminating the necessity of enzyme treatment, but it also gives a
(1) Ambler, J. A,, Roberts, E. J., and Weissborn, F. W , Jr., U. S. Dept. Agr., Bur. Agr. Ind. Chem., Publ. A.1.C.-132, (1945). (2) Assoc. Offic. Agr. Chemists, “Official and Tentative Methods of Analysis,” 6th ed., No. 26.25, p. 389, (1945). (3) Brown, C. A., and Zerban, F. W., “Sugar Analysis,” 3rd ed., p. 591, New York, John Wiley & Sons, 1941. (4) Godohaux, L., 2nd, Sugar J., 11, No. 11, 3-4, 29-30 (1949). (5) Lauritzen, J. I., Baloh, R. T., and Fort, C. A., U. S. Dept. Agr., Bur. Agr. Ind. Chem., Tech. Bull. 939 (July 1948). (6) Murke, F., “Manufacture of Beet Sugar,” New York, John Wiley & Sons, 1921. (7) Orleans, L. P., and Cotton, R. H., Proc. Am. SOC.Hart. Sei., 54, 319-24 (1949). ( 8 ) Pigman, W. W. and Wolfrom, M. L., “Advances in Carbohydrate Chemistry,” Val. I, pp. 261-2, New York, Academio Press, 1945. (9) Sartoris, G. B., personal communication, Dec. 15, 1948. (10) Sherwood, S. F., IND.ENG.CHEM.,15, 727-8 (1923). (11) Spencer, G. L., and Meade, G. P., “Cane Sugar Handbook,” 8th ed., p. 49, New York, John Wiley & Sons, 1945. (12) Ibid., p. 91. (13) Ibid., p. 115. (14) Ibid., p. 117. (15) Ventre, E. K., Sugar J . , 3, No. 7, 23-30 (1940). (16) Ventre, E. K., and Byall, S., E. S. Patent 2,359,537 (Aug. 10, 1943). (17) Ventre, E. K., and Paine, H. S.,Ibid., 2,280,085 (March 24, 1941).
(18) W a l t o n , C. F., Jr., and Ventre, E. K., Intern. Sugar J . , 39, 430-1 (1937). (19) Wiley, H. W., U. S. Dept. Agr., Div. Chem., Bull. 2 (1883). (20) Willaman. J. J., and Davison. F. R.. IND.ENG.CHEM.. . 16.. 609-10 (1924).
RECEIVED April 6, 1950.
Fundamentals of Lime Burning I
Efficient production of quality lime involves far more than merely building a fire under a piece of limestone. Retention of uniformly sized material at properly controlled temperatures for a definite period of time and then removal from high carbon dioxide atmospheres is necessary if lime of high calcium oxide availability is to be produced economically. General calcining conditions necessary to achieve these objectives are discussed.
A
LTHOUGH the carbonates of calcium and magnesium are stable at normal temperatures, they may be decomposed
into carbon dioxide and the respective oxides by exposure t o temperatures ranging from 600” t o 1200” C. The exact temperatures will depend on the composition of the carbonate, the time of retention a t calcining temperature, and the composition of the atmosphere surrounding the heated material. Excessive temperatures and retention periods must be avoided t o prevent
W. A. CUNNINCHAM The Universitj o f Texas, Austin, Tex.
overburning, which is accompanied by densification and molecular rearrangement; exposure t o gases containing carbon dioxide a t temperatures below t h a t required for calcination may result in recarbonation of the lime. Most of the lime produced is calcined in either vertical shaft or rotary kilns, the basic characteristics of which are illustrated in Figure l ( 1 ) . Multiple-hearth, or shelf, kilns of the HerreshoffNichols type and the fluidized solids calciner developed within recent years have limited usage ( 7 ) . The fundamental problems of calcination are the same in all types of equipment. The reaction involved in the calcination of limestone is simply the decomposition of calcium carbonate into calcium oxide (quick lime) and carbon dioxide. The reaction is reversible, although, as shown in Figure 2, the equilibrium decomposition pressure of
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Both Conley ( 4 ) and Furnas ( 5 ) present data which show that calcination proceeds inwardly &..from the outside along a rather sharply defincd zone which is almost narrow enough t.o be called a PREHEATING AND CALCINING line. In general, this zone, or line, of calcination advances a t a constant linear rate dependent only on the driving force of temperature difference and is independent of the size or shape of the particles, the degree of calcination, or the amount of previous heating. T It must be pointed out that the rate of t,em.i os 0 perature increase toward the cent'er of the lump Ais not equal to the rate of travel of the line of calcination. At the advancing calcium carbonateFigure 1. Typical Zones of Vertical and Rotary Lime Kilns calcium oxide phase boundary, the decomposition requires that 43,000 calories be absorbed per formula weight of calcium carbonate. Thus, since this additional the carbon dioxide is quite small at^ temperatures below 650" C. heat must be supplied after the actual temperature of calcination Above that temperature, the decomposition pressure increases is reached and since the transfer of a given quantity of heat rcrapidly and reaches 1 atmosphere a t about 900" C. ( 9 ) . I n most quires a finite time, the uncalcined interior of the lump may operating kilns the partial pressure of carbon dioxide in the gases be a t or even slightly above normal calcining temperature bein direct contact with the outside of the lumps is substantially fore the advancing line of decomposition reaches it. less than 760 mm. of mercury, hence the initial decomposition These relationships indicate that kiln temprratures should bc temperature is somewhat less than 900" C. If the partial presheld as high as possible, limited only by the refractories in the sure of the carbon dioxide in the atmosphere in contact with kiln lining. However, heating the lime to temperatures well the limestone is above 760 mm. of mercury, the equilibrium above those required for complete dissociation may result in the temperatures required for decomposition may be well above production of an overburned lime which is more dense, which gooo C. hydrates slowly or not a t all, and n hich has a considerably deThe total heat required may be divided into (1) sensible heat creased percentage of available calcium oxide. Thcse condito mise the rock t o decomposition temperature, and (2 j latent heat tions apparently are the result of atomic rearrangement and closer of dissociat'ion. Theoretical heat requirements per t,ou of lime packing, as well as of reaction of some of the lime TT-ith impurities produced are approximately 1,300,000 B.t.u. for the former, and such as aluminum oxide, ferric oxide, and clilicon dioxide. 2,600,000 B.t.u. for the latter if the rock is heated only to a calcining temperature of 900" C. However, as will be explained later, practical considerations in actual calcining operations require that the rock be heated t,o between 1200" and 1300" C., thus increasing sensible heat requirements by some 350,000 B.t.u. per ton of lime produced. Theoretical total heat requirements in actual operation will be approximately 4,250,000 B.t.u. per ton of lime produced, of which about 40y0 is sensible heat and 60% is latent heat of decomposition. I n order to obtain complete calcination of limestone each individual lump must be heated to such a temperature and for a sufficient period of time t o cause heat to travel to the center of the lump in quantities not only adequate to raise it to decomposition temperature but also t o supply the necessary latent heat of decomposition. This decomposition temperature a t the center of the lump probably is well above 900" C., since there the partial pressure of the carbon dioxide not only is equal t o or near the t o t d pressure but also must, be high enough to cause the gas to move outward t o the surface of the lump where it can then pass into the gas stream. Heat can travel from the outside t o the inside of the lump only under the influence of a temperature differential, and t'he rate of heat transfer is directly proportional t o the magnitude of this 600' 700' 'C 500' 800. 9000 1000' difference in temperature. Figure 3 illust,rates qualitatively the 'F $32' 1112. 1292' 1472' 1652' 1832' conditions which exist when the center of the lump has reached calcination temperature. Since the gaseous phase within the Tamoerotura lump is essentially pure carbon dioxide, the partial pressure of Figure 2 which must, be above that of the normal kiln gases in order t o cause it, t o pass outv-ard through the lump, the temperature a t h properly burned lime which has been heated to only slightly the center must be above 900" C. The temperature of the outer above its actual decomposition temperature and retained a t that portions of the lump must be higher than that of the center in temperature only long enough to obtain complete calcination is order to cause an inward flow of heat to decompose the innermost light in weight, quite porous, and hydrates readily. The spacings calcium carbonate, while the partial pressure of t,he carbon diof the calcium and oxygen atoms are essentially the same as they oxide decreases t o only slight,ly above that of the kiln gases surwere in the original limestone, and the individual lump is only rounding the lump. slightly different in shape and size from the original limestone. When the limestone is fed into the kiln, the individual lumps The lime slakes readily, since the interatomic spacing permits are preheated by the hot Etack gases to or near decomposition easy penetration by the water molecules (Figure 4C j. temperature before t,hey enter the calcination zone proper. STORAGE
AND
DRYING
~
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1951
0"
3 1
$
fn
f c2
-
$ +
CENTER
time results in a n atomic rearrangement and, in the limit, produces a "dead-burned" lime. The interatomic spaces in the crystal lattice of this dead-burned lime are so much smaller that the water molecules penetrate the lump only slightly, and virtually no slaking occurs. Such lime may have densities of as much as 25'% greater than that of properly calcined lime (9). This condition is aggravated by the presence of acidic impurities in the lime which, a t the high temperatures, react with the basic calcium oxide to produce glasslike or cementitious materials which rather effectively seal the pores through the formation of a film or layer of these materials on the outside of the lump.
Although data on the quantitative effect of ferric oxide and aluminum oxide are lacking, it is known t h a t each 1% of silicon dioxide causes a decrease of about 3% in available calcium oxide. It is probable that this is the result of the formation of tricalcium silicate (3CaO.SiOz) which is one of the major constituents of portland cement. Although overburning itself is serious enough, sometimes lumps of lime which have a n overburned outer shell also contain a core of uncalcined limestone. This may be explained by the fact that, although the heat transfer through the denser overburned portion is greater, the resistance t o passage of carbon dioxide from the inside is also markedly increased. As a result, the carbon dioxide pressure within the lump may be so high t h a t complete decomposition of the interior calcium carbonate is prevented, even though the actual temperature may be above that normally required for calcination.
perature of approximately 1200O C., the individual lumpsparticularly the larger ones-frequently contain an uncalcined portion, or core, in the center. Under properly controlled conditions, these cores can be eliminated, but they can result in a material decrease in available lime in the finished product. Since the lime leaves the calcining zone well above the decomposition temperature, the lumps can be cooled by a transfer of sensible heat into the core where it furnishes the latent heat required for decomposition of the calcium carbonate in the core. If, in SO doing, the exterior portion of the lump of lime is not cooled below about 900" C., the carbon dioxide passes into the gas stream and
0000 000000 00000000 000 000 000 000 00000000 000000 0000 A
- MOLECULAR
CROSS SECTION LIMESTONE ENTERS KILN
€CARBONATED
B - DISSOCIATION ZONE PARTLY BURNED LIME
0000 000000 00000000
ooooooow
00000000 00000000 000000 0000 C - H O T ZONE SOFT BURNED LIME Figure 4.
637
CaO
E
-
-
RECARBONATION OF LIME DUE TO PRESENCE OF CORE
D - H O T ZONE HARD BURNED LIME
Hypothetical Molecular Spacing in Limestone and Lime at Different Stages of Calcination (2)
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 43, No. 3
requires a calcination time of approvimately 12 houi s as coniparcd with 1 hour for a 1-inch stone. A projection of this direct relationship between stone size and calcination time i ~ o u l dindicate that, if mechanical difficulties could be overcome, finrly crushed limestone (about -50 mesh) should be subject t o very rapid "flash" calcination. The adaptation of fluidized solid, t w h niques is definitely in this direction ( 7 ) . RATE O F C A L C I N A T I O N O F L I M E S T O N E - K N I B B S 1427
1371
I316
1260
0 w
( 3 r
I216
2 w
t
TIME, HOURS
Figure 5.
Time Required to Calcine Limestone
1149
Different sizes under different temperatures of surroundings 1093
thence out, with the other stack gases. On the other hand, if the outside lump temperatures fall below calcining temperat,ures, perhaps by exposure t o incoming air going into t,he combustion zone, the carbon dioxide from the decomposing intclrior core may recarbonate the lime previously formed on the outside of the lump, t,hus resulting in no net gain of available lime (Figure 4E). Although modern practice recognizes that the phenomena just described mould require the use of carefully sized st,one, inany kilns are operated on rock of markedly nonuniform size. As a result, the liniestone particles of smaller size are calcined completely, long before the larger lumps are calcined. Iience, in many instances, operat,ors are faced with two alternatives, neither of which is altogether desirable. The kiln may be draTvn a t a rate that viill permit full calcination of the larger lumps and risk overburning of t,he smaller pieces, or the kiln may \)e drawn more frequently t o avoid over-burning of the latter a,nd risk t,he possibilit,y of having an uncalcined core in t,he middle of the larger pieces. Best operating procedure today requires that the st,oiie charged t,o the kiln be as nearly uniform in size as is economical. Azbe ( 3 ) recommends the use of rock screened as closely as -6 inch t o +4 inch and -4 inch to +2 inch, each size to be charged to a separate kiln, In this manner more uniform calcinat,ion with minimum amounts of overburned and core materials are obtained. The size of the limestone particles fed to the kiln also has a marked effect on the production capacitv, since t,he smaller the lump, the shorter the necessary burning time. This nieuns t'hat with smaller lumps R kiln of a given size operating a t a given temperahre in the firing zone should have a greater capacity, provided a sufficient draft can be maintained. The latter point is the probable ultimat,e limiting factor on the modern vert'ical shaft kiln, since the pressure drop occasioned by the pdssage of stack gases through a bed of small limestone particles becomes too large for either forced or induced draft. Haslam and Smith (6),Furnas (6),and Knibbs (8) have published data concerning calcination rates as a function of stone size; some of these are shown in Figures 5 and 6. An examination of these curves indicates that, at about 1100" C. a 6-inch stone
1038
082
C A L C I N A T I O N TIME I N H O U R S
Figure 6.
Rates Calcination of Limestone Lumps of Different Sizes (8)
In minmaiv, then, it should be kept clearly in mind that the calcination of limestone involves not only heating t o decomposition temperature but alqo the retention of the individual particles at that temperature for a sufficient period of time to ensure the transfer of the hrat needed to brrak up the calcium carbonate into lime and carbon dioxide. Thr time requix-d t o accomplish this is invexselj pioportional to the temperature at which thc heat is supplied. The size of the limestone lumps and the rate a t which they are fed t o the kiln as well as the manner in RThich the lime is cooled after letLvirig the calcining 7onc h a w ronsiderable influences on the quality of the lime produced. Finallv, the, purer the limestone itself, ihc better the resultaiit liinr LITERATURE CITED (1) American Society for Testing Materials, E'hiladelphia, Pa., "Standards on Refractory Materials,'' p. 206 (1948). (2) Azbe, V. *J., Rock Products, pp. 86-89 (September 1948). (3) &be, 1 '. J., "Theory and Practice of Lime Manufacture," p. 388: Aebe Corporation, S t . Louis, Mo. (1946). (4) C o n k y , J. E., Am. Inst. Mining Met. Engrs., Tech. Pub. KO. 1037 (1939). (5) Furnas, C . C., IXD.EIW. CHEM.,23, 534--38 (1931). (6) Haslam, R . T., and Smith, V. C . , Ibid., 20, 17O-P4 (1928). ( 7 ) Kite, R. P., and Roberts, E. d., Chem. Eng., 54, KO. 9, 112-115 (1947). (8) Knibbs, N. T. S., Cement and Lime Manzcf., 10, 4---11, 4 9 - 6 2 (1937). (9) Smyth, F. H., and h d a m s . L. II., J . Am. Chem. Soc., 45, 116784 (1943). RECEIVED April 6, 1950.
END OF SYMPOSIUM
