December, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table IV-Volumes of Oils between 15.56O and 400° C. Calculated from A B C D Figure 3 and equations: Vt = Vlr.ss[l A(t-i5.66)'+'B(1-15.d6)1] for range 15.56" to 260° C and Vt = V:#o[l f C ( t - 2 6 0 ) D(I-260)*1 for range 260' to 400' d:
+
+
(1) (2) (3) (4) (5) SP,gr 0.800 0.825 0.850 0.875 0.900 Gravity, A. P. I. 45.4 40.0 34.9 30.2 , 25.7 A X 10' 0.960 0.880 0.820 0.800 0.740 B X 108 1.265 1.130 0.99 0,875 0.760 C X l O ~ .... 1.050 DXlOS .... .... 1.90 Temp., " C Milliliters per Gram 15.56 1.0000 1.0000 1.0000 1.0000 1.0000 50 1.0346 1.0316 1.0294 1.0286 1.0264 100 1.0901 1.0824 1.0764 1.0738 1.0679 200 1.2201 1.2007 1.1849 1.1774 1.1624 260 1.3103 1.2826 1.2596 1.2479 1.2263 300 .... .... .... 1.2831 400 .... 1.4721
.... ....
....
....
....
....
....
....
....
(6)
0.950 17.5 0.685 0.54 0.83 1~. .73 1.0000
1.0242 1.0616 1.1447 1.1997 1.2429 1.3796
The accuracy with which the gravity-temperature correction tables of the 18th edition of the Tagliabue Manual, based on the U. S. Bureau of Standards' measurements,apply to California petroleum oils was determined by the author by means of standardized hydrometers a number of years
1283
ago under very carefully controlled conditions on a wide range of products between 10" C. (50' F.) and 37.8" C. (looo F.). The conclusions from those tests check closely with the results of the present measurements. That is, for distillates below about 50" -1.P. I. and for observed temperatures below 26.7" C. (80" F.) the bureau's results give the true value a t 15.56" C. (60" F.) within 0.1" A. P. I. Expansion a t High Temperatures Attempts to express the expansion of the oils over the entire range of temperatures, 15.56" to 400" C. (60" to 752" I?.) by a single equation were not successful up to the time this paper was written. Acknowledgment The author is grateful to R. A. Halloran, manager of the Development Department, and to J. B. Terry, chief chemist of the Standard Oil Company of California, for helpful advice during the course of this investigation; and to V. Lantz for assistance in the laboratory.
Manufacture of Calcium Citrate and Citric Acid from Lime Juice' By F. H. S. Warneford and F. Hardy GOVERNMENT LABORATORY, LEEWARD ISLANDS. AND IMPERIAL DEPARTMENT OF AORICWLTUR~. BRITISH
WEST
INDIES
3--Attempts to induce reHE manufacture of calA detailed laboratory study has been made of t h e tention of the phlobatannin of cium citrate and citric conditions of precipitation requisite t o the production lime juice in the cake of calcium acid from lemon juice of high-grade calcium citrate from lime juice, with a sulfate, formed when calcium citrate is treated with sulfuric has been fully described by view to elaborating improved methods for manufactheir mnufacture turing citric acid and citric acid sirups. I t was found acid. from lime juice follows esthat by cold liming followed by heating, instead of Purification of c~~~~~~~~~ Sentially the same course. hot liming throughout, a satisfactory calcium citrate Wet Calcium Citrate The methods of calcium Cib could be obtained, containing minimal amounts of A thoroigh washing of unrate manufacture as Practiced coloring matter, which could be largely removed by dried, freshly prepared comin the British West Indies washing. The use of adsorbent carbon a t a n intermediate rather t h a n a t a later stage in the procedure mercial calcium citrate with have beende~cribed.~Citric acid liquors, obtained by gave better products. A special process, based on the hot water Yields a product treating crude Commercial use of soda ash, calcium chloride, and Norit, is described, from which straw-colored citcalcium citrate, whether preric sirups can be obtained. pared from lemon or from This process, however, relime juice, with sulfuric acid, are dark brown in color, quires very large quantities of hot water, and although the and yield dark-colored crystals. To obtain white crystals solubility of citrate in hot water is small (0.042 per cent) it the crude crystals are generally redissolved in water, the soh- entails a considerable loss. tion decolorized with adsorbent carbon, the liquor concenA still paler citric sirup can be obtained from citrate purified trated, filtered, and finally recrystallized a t rest. by suspension in hot water and treatment with sulfur dioxide, The writers have shown elsewhere4 that the dark color of followed by several washings with hot water. But in this lime-juice products is largely due to the atmospheric oxidation process also there is considerable loss, since the solubility of of a polyhydroxyphenolic component akin to phlobatannin, citrate in dilute acids is greater than in water. and that both the rate and the degree of oxidation are especially. high when the reaction of the medium is alkaline. Preparation of Pure Calcium Citrate It has been found possible, however, to prepare calcium The retention of phlobatannin and its oxidation products citrate of such high purity that citric acid liquors derived (phlobaphenes) by calcium citrate prepared from citrous therefrom are pale in color and yield white crystals directly juices may be due either to adsorption or to actual chemical when concentrated. The methods elaborated by the writers interaction with the added lime, or to both these causes. are herein described. Three lines of investigation have been The chemical interaction between lime-juice phlobatannin followed, namely: and lime (Ca(0H)Z) seems to occur only when lime is used 1-Attempts to purify ordinary wet commercial calcium cit- in excess, as it is usually avoided when liming is carefully controlled. Therefore the main cause of contamination of rate. 2-Attempts to prepare calcium citrate of initial high purity. commercial calcium citrate by lime-juice phlobatannin and I Received June 13, 1925. phlobaphenes is probably surface adsorption. 2 THIS JOURNAL, 18, 554 (1921). general, adsorption from solution chiefly depends upon * West Zndion Bull., 2, 308 (1902); 7, 331 (1907); 8, 167 (1908); 9, theI nfollowing factors: (1) concentration of solute (adsorbate) ; 193 (1909); 12, 465 (1912). (2) specific active surface of adsorbent; (3) temperature; 4 THIS JOWRNAL, 17, 48 (1925).
T
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1284
(4) nature of changes of solute, adsorbent, and dispersion medium. The effect of these factors on the limed lime-juice system will be examined in turn. (1) Concentration of solute. I n commercial practice it is not feasible to lower the concentration of the phlobatannin of lime juice by diluting the raw juice with water, because dilution leads to increasing difficulty in recovering citric acid in the liming vat. (2) T h e sped@ active sulface of precipitated solids depends largely on the conditions of precipitation. In order to gage the effect of different conditions on the physical characters of precipitated calcium citrate, specimens of calcium citrate were prepared from ( a ) solutions of pure citric acid, (b) lime juice, previously defecated with Filter-Cell and (c) raw, unfiltered lime juice, a t ordinary room temperature (29” C.), In each case hydrated lime and a t boiling temperature. (containing 30 per cent CaC03) was added in amount equivalent to the total quantity of acid present in the solution. The specimens were first examined under a microscope fitted with a micrometer eyepiece. It was observed that the size of the individual crystals was greater in the specimen prepared a t ordinary temperature, but that the degree of aggregation of the crystals, and consequently the filtrability of the mass, was greater in specimens prepared a t boiling temperature. An approximate measure of the relative specific surface of the different specimens was next obtained by treating equal weights of the specimens with equal volumes of standard potassium permanganate solution and determining by oxalic acid titration the amount of permanganate reduced in a fixed time. T a b l e I-Relative
Speci5c Surface of Calcium C i t r a t e Precipitated a t R o o m T e m p e r a t u r e a n d at 100° C.
PREPARATION From pure citric acid From defecated juice From raw juice
Per cent Permanganate Time of Decomposed contact Hot Cold Ratio 1 hour 72 43 1.67:l 30 min. 77 46 1.68:l Reduction of permanganate too rapid t o follow, implying large amount of organic matter covering the surface of the crystals. which were very small
The results (Table I ) indicate that in two cases cold precipitation yields specimens of smaller specific surface area than hot precipitation, so that it would appear advisable to lime the juice cold, if contamination with adsorbed phlobatannin is to be minimized. (3) The effect of temperature on the adsorption of phlobatannin by calcium citrate prepared from Filter-Cel-treated lime juice was next examined. Two methods were employed, as follows: ( a ) The acid in the juice was precipitated by the requisite amount of lime, the resulting calcium citrate was filtered off, and the filtrates were treated with excess of calcium carbonate to remove the last traces of acid without rendering the reaction alkaline. The colors of the filtrates were compared in a colorimeter. ( b ) The juice was treated as in (a), but the specimens of calcium citrate obtained were decomposed with sulfuric acid, and the final citric acid liquors were concentrated to the same degree, and their colors compared.
.
Vol. 17, No. 12
These results support the assumption, based on theoretical ground, that the “cold-hot” process, which combined the benefits of reduced active surface with decreased adsorption a t higher temperatures, should yield the cleanest and purest citrate. Offset against this, however, is the fact that precipitation at boivng temperature yields a more readily filtrable citrate. Note-It has been stated that the earlier attempt3 to manufacture calcium citrate in Sicily by neutralizing lemon juice a t ordinary temperature were not commercially successful, owing to the difficulty of filtering the product. No difficulty of this sort has been experienced in this laboratory, but filtration is certainly quicker with preparations obtained a t the higher temperature, and retention of liquid somewhat less. These facts are of practical importance in connection with the washing of citrate to free i t from impurities contained in the mother liquor.
The effect of temperature on the course of neutralizatioa of lime juice or citric acid solutions by lime is noteworthy. Calcium hydroxide and calcium citrate are scarcely soluble in water a t boiling temperature. Both calcium citrate and calcium hydroxide, however, are fairly soluble in cold water and, moreover, the former tends to form supersaturated solutions in the cold. When citric acid is neutralized by lime a t boiling temperature there is, therefore, a precipitation of one almost insoluble substance at the surface of another, but when neutralization is carried out a t low temperature the tendency for this result to occur is lessened. For these reasons only the finest grades of powdered lime should be used in the hot process, whereas somewhat coarser grades can be used if neutralization is carried out a t ordinary temperature. The results in Table I1 demonstrate the effect of neutralizing lime juice with lime of different degrees of fineness, a t boiling temperature and a t ordinary temperature. In each case the quantity of lime used was that required by theory to neutralize all the acid in the juice. of Hot a n d Cold Precipitations Using L i m e of Different Deerees of Fineness
T a b l e 11-Comparison
GRADS 20 to 30 mesh 80 to 90 mesh Chemically precipitated
Per cent Acid Remaining Unneutralized in Juice Hot Cold
17.50
i :+4
1.86 1 90
1: 17
Per cent of Lime (as CaCOa) in Resulting Calcium Citrate Hot Cold
10.50 6:15
0.875 0.375 0.625
The greater ease with which the process of neutralization is effected in the cold is offset, however, by the fact already stated that citrate precipitated a t the boiling point is more easily filtered. (4) Nature of changes of solute, adsorbent, and dispersion medium, during neutralization of lime juice by lime. At first the dispersion medium is strongly acidic in reaction, and the solute (phlobatannin) is unoxidized. At the point of neutrality, on the other hand, the medium is neutral in reaction and the solute is partly oxidized. If neutrality is passed, oxidation proceeds apace, and brown, tarry, more or less insoluble substances are formed (phlobaphenes). These changes favor adsorption of the coloring matter, as the following experiment demonstrates : Five batches of calcium citrate were prepared, the first (1) from diluted and filtered commercial concentrated lime juice, and the rest (2, 3, 4, 5) from defecated raw lime juice. Quantities of lime, representing (1) 100, ( 2 ) 95, (3) 98, (4) 100, and (5) 120 per cent of the theoretical amount required for exact neutralization were used in the different instances. In all cases neutralization was carried out a t room temperature, and the mixture then heated to boiling before filtration. The citrate specimens were washed with similar amounts of hot water. Each batch was then separately converted into citric acid, and the colors of the final concentrated citric sirups were compared in a colorimeter.
The temperature conditions maintained in these two methods of procedure were (1)ordinary room temperature (29” C.), (2) boiling temperature, and (3) neutralization a t room temperature, then the mixture heated to boiling point for a few minutes before separating the citrate. No appreciable difference in color was observed in the filtrates obtained by method (a), but the citric sirup obtained by the “cold-hot” process (3) in method ( b ) possessed the palest color, whereas that obtained by the “hot” process The results are shown in Table 111,in the order of increasing (2) had the deepest color. The relative color-depth values color depth. were “cold-hot” 1.00, “cold” 1.47, and “hot” 1.80.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1925
Table 111-Effect of Varying Q u a n t i t i e s of Coloring M a t t e r R e t a i n e d b y Juice number (3) (4) Lime added (% of theory) 98 100 Color.value of final citric sirup 1.00 1.07 1.83
%
L i m e on A m o u n t of Citrate (1) (concd. juice) ( 5 ) 100 120
7.30
11.50
It will be noted that the color value increased progressively with the quantity of lime used. I n the case of' ( 5 ) , where 20 per cent excess of lime was added, there was actual precipitation of a yellow calcium-tannin compound on the particles of citrate, and the extremely dark citric sirup that resulted owed its color to subsequent decomposition of this substance, rather than to matter merely adsorbed. The sirup was also extremely viscous, and contained considerable pectin. I n the case of (1) the effect of concentration in increasing the amount of oxidized dark-colored matter that was subsequently largely carried down with the precipitated citrate is clearly indicated. R e t e n t i o n of Lime-Juice P h l o b a t a n n i n b y C a l c i u m S u l f a t e
I n the usual process for manufacturing citric acid adsorbent carbon is added a t a late stage to liquors whose reaction is highly acidic (pH 1.8 or less). This is probably not the best condition for adsorption, however, since most adsorbent carbons appear to exert their maximum effect in media of reaction of pH 4.0 to 4.2. The writers found that lime phlobatannin adsorbed by Norit from a solution of reaction p H 4.0, is strongly retained, even when the mixture is subsequently introduced into a solution having a high acidity. The following process is based on these observations: Lime juice is defecated by treatment with Filter-Cel, as advocated by Wilson. The clear juice is kept gently boiling and well stirred while milk of lime is added in amount sufficient to raise the pH value of the juice to p H 3.8-3.9. Norit (1 per cent by weight) is then added and the stirring is continued for 15 minutes, when milk of lime is again slowly added until the total added alkali is equal to that required by theory to'precipitate all the citric acid present in the juice. A small amount of calcium carbonate is then added to complete the neutralization, the stirring continued, and the hot mixture centrifuged through cloth to separate the solids. These consist of calcium citrate and spent Norit. The cake is washed with hot water, removed from the centrifuge, dried a t low temperature, and finally treated with the amount of sulfuric acid required for complete double decomposition. The filtered liquor contains finely divided Norit, and requires treatment with a small quantity of Filter-Cel to render it transparent. The final liquor yields colorless citric acid crystals on concentration.
This procedure gives a product superior to that obtained by the ordinary process in which Norit is used on the final citric sirup obtained by dissolving crude crystals. Moreover, only one crystallization is required, and the loss of citric acid is relatively low (about 2 per cent only). The main objection to the method is that the spent Norit cannot be revivified for further use, since it cannot feasibly be separated from calcium sulfate, with which it is mixed in the centrifuge cake. Suggested Modifications in M a n u f a c t u r e of W h i t e Citric Acid Crystals and Pale Citric Acid S i r u p s
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washed, preferably with hot water, and worked up to citric sirup or citric acid in the usual manner. I n this way, and without the use of adsorbent carbon, products fairly free from coloring matter may be obtained. The recovery of citric acid as calcium citrate may be less than in the usual process, however, because there is always a small residual acidity after liming, and the solubility of calcium citrate in slightly acidic mother liquors is higher than in neutral or alkaline media. Thus, in a laboratory trial in which 2 liters of lime juice containing 159 grams of citric acid were used, 145 grams of citric acid were recovered in the citrate. This is a recovery of 91.3 per cent as compared with a recovery of 94 to 95 per cent in the ordinary commercial process. On the other hand, the final recovery of citric acid crystals may be greater in the simple process, since in the same experiment 84 per cent of the citric acid present in the citrate was recovered in a marketable form by direct crystallization, while only 69 per cent of the citric acid contained in ordinary commercial citrate was recovered in the crude crystals and these required decolorization and recrystallization before yielding a marketable product. The losses due to recrystallization and to reliming of the mother liquors are therefore reduced. Moreover, the ordinary commercial process, besides requiring adsorbent carbon, necessitates the use of greater quantities of lime, sulfuric acid, and fuel than the simpler process: labor costs would also be lessened in the latter. SIMPLE INTERMEDL4TE NORITPRocEss-This process has already been described under Retention of Phlobatannin by Calcium Sulfate. Defecated juice is partly hot-limed (pH 4.0), treated with Norit, and the liming completed. MODIFIEDINTERMEDIATE NORITPRocEss-The modified intermediate Norit process is based on the fact that when one third of the total acid of lime juice has been neutralized to form a soluble, strongly ionized citrate, the ionization of the remaining acid is so depressed as to allow a satisfactory removal of phlobatannin by treatment with Norit a t that stage. Defecated lime juice is neutralized a t ordinary temperature with sodium carbonate to the extent of one-third of the total acid content. The reaction a t this stage is about pH 3.8. Norit is introduced (1 per cent) and the mixture boiled for 15 minutes, and then allowed to cool, and filtered. The cake of Norit is washed with cold water. Turbidity in the filtrate and washings may be removed by treatment with a little Filter-Cel. The liquid is then treated with calcium chloride in excess of the amount equivalent to the sodium carbonate used. The liquid is boiled, and milk of lime, followed by a small excess of calcium carbonate, is added to complete the neutralization. After boiling again for a few minutes the precipitated calcium citrate is collected, thoroughly washed, and finally converted into citric acid in the usual manner. The liquor is almost colorless and yields perfectly colorless citric acid when concentrated and crystallized. A typical laboratory trial of the process showed the recovery and losses of citric acid given in Table IV. Table IV-Citric
Acid Recovery a n d Losses b y Modified I n t e r m e d i a t e Norit Process (1100 cc. of juice used) Recovery Loss CITRIC ACID Grams Per cent Per cent ~~
The facts established by the experimental and theoretical results described in foregoing actions suggested the following improvements on existing methods for preparing pale citric acid sirups (of use perhaps in the soda fountain trade) and for manufacturing colorless citric acid crystals. SIMPLE LIMING PRocEss-Defecated lime (or lemon) juice is first treated at ordinary temperature with a thin cream prepared from fine-grained lime containing sufficient alkali for theoretically exact neutralization of the total acid present. The temperature is then raised to the boiling point, and after a short time the calcium citrate is collected, either by sedimentation, centrifuging, or filtration. It is a t once thoroughly
Loss during conversion into citric sirup mainly due to retention in the cake of calcium sulfate.
The process permits a high recovery of citric acid with use of less lime and less sulfuric acid than the usual commercial process, but soda ash and calcium chloride are introduced as additional cost factors. An attempt was made to eliminate the use of these extra substances by neutralizing one-third
