December 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY CONCLUSIONS
The principal conclusions which were reached in this investigation may be summarized briefly. The surface tension of a young surface controls the effect on heat transfer. Qualitative results of other authors on the general effect of surface tension have been verified-namely, decreasing the surface tension of a boiling liquid increases the heat transfer when nucleate boiling persists; and the critical temperature difference occurs a t lower values of the temperature difference as the surface tension is lowered.
2769
LITERATURE CITED (1) Cryder, D. S., and Gilliland, E. R., IND. ENG.Cxmi., 24, 1382-7
(1932). (2) Insinger, T. H., Jr., and Bliss, H., Trans. Am. Znst. Chem. Engrs., 36,491-516 (1940). (3) Jakob, M., and Linke, W., Physik. Z., 36, 267-80 (1930). (4) McAdams, w. H,, “Heat Transmission,,, 2nd New McGraw-Hill Book Co., 1942. ( 5 ) Morgan, A. R., M.8. thesis, University of California, Berkeley, Calif., 1949. (6) Stroebe, and Baker, E , M,, E N a . CHEM., 2oo-6 (1939).
w.,
RECEIVED January 15, 1949
Nitrogen Removal in Ion Exchange Treatment of Beet U
Sugar Juices u v’
RAY T.JACOBS AND FRANK N. RAWLINGS The Amalgamated Sugar Company, Twin Falls, Idaho
Approximately two years ago, a study of modifications of the ion exchange process and the characteristics of ion exchange resins was started to determine if the overall nitrogen removal from beet sugar juices could be economically improved. The need for this study was motivated by several factors: The nitrogen compounds represent 20 to 25% of the nonsugar impurities found in sugar beet juice; the browning of ion exchange treated juices is materially effected by their residual nitrogen content; and the nitrogen compounds in beet sugar juices have a considerable by-product value. The nitrogen removals that can be expected by several modifications of the ion exchange process are given, the special regeneration treatment of the cation bed for increased nitrogen removal is described, and graphs showing nitrogen removal for both cation and anion cells throughout a juice cycle, as well as graphs showing how the nitrogen compounds are eluted from the exchangers on regeneration, are given.
0
VER the period of years that work has been conducted on the
24
9
ion exchange purification of sugar beet liquors, the normal evolution for the application of any new purification procedure in any industry has been followed. At fast, a measure of the over-all purity rise as a criterion of process efficiency was satisfactory. As the mechanics of the process have been delved into further, it has become apparent that all classes of impurities are not removed with equal efficiency. This has been found to have a significant effect on the results from the application of this process so t h a t a study of the behavior of various classes of impurities involved was begun and this is leading now to the study of the individual compounds among the impurities. T h e present paper covers an investigation of a class of impurities. A very small fraction of the nitrogen in sugar beet juice is borne as inorganic nitrate nitrogen, but the primary nitrogen carriers are organic amino compounds. I n the original juice of the sugar beet, the largest proportion of the organic nitrogen exists as betaine and glutamine. Under the influence of the heat and alkalinity of the beet-end processing, there is a progressive conversion of glutamine t o pyrrolidone carboxylic form with the attendant liberation of ammonia which is largely liberated from
the juice in the gassing and evaporation steps of the process. The degree of this conversion depends on many factors which are variable from factory to factory and year t o year so that no generalization can be made safely. hluch smaller amounts of other organic nitrogen-bearing constituents exist and their true character is just being learned. Present evidence points t o the existence of very little nitrogen-bearing material more complex than the simple amines arid amino acids; there appears t o be very little polypeptide or true protein material. The first investigations into the nitrogen removal by ion exchange treatment rf sugar beet juice disclosed that the percentage of over-all nitrogen removal was considerably less than t h e removal of the inorganic ash with the type of operation being conducted in those earlier years. Everyone took this t o be t h e nature of the beast and worried about the then more critical functional aspects such as the proper ion exchange cell design, materials of construction, etc. Approximately two years ago, however, a concerted effort was undertaken to see if modifications of the ion exchange process could be found Which would economically improve the over-all nitrogen removal. This desire was motivated by several factors: The nitrogen compounds represent 20 t o 25% of the nonsugar impurities found in beet sugar juice; the subsequent browning properties of the ion exchange treated juices are materially effected by their residual nitrogen content; and these nitrogen compounds of the sugar beet juice have a considerable potential by-product value. As nearly as possible, complete transfer of these nitrogen constituenta to the ion exchange resins ttnd finally t o the spent regenerants, or fractions thereof, will lead to a high recovery of these amino products. Various combinations of bed sequence, exhaustion end points, and special bed regenerant treatments, with specific reference to their effect on nitrogen removal, have been examined. This paper reports the data on representative individual experiments rather than averages of larger numbers of repetitions. This work t o a great extent was carried out in pilot plants where repetition experiments reflect too greatly the quality of the operator’s control rather than the inherent behavior of the operation. It was felt that these data on known accurately controlled cases would more
2770
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 12
I l o
-
0I
FO' QJ
a2
i -
2.4
0021
-
- 2.0
Figure 1
50 ml. of an indicator solution made with bromocresol grccn arid 4% boric acid, using 0.05 gram of bromocresol green to 17 liters o i indicator solution, The distillation sample is titrated with 1/14 A7 sulfuric acid so that 1 ml. of acid equals 0.001 gram of nitrogen. NITROGEN EXTRACTION CHARACTERISTICS O F CATION A N D Ah'IOK kOLUMNS
VOLUME JUICE TREATED IML.)
Figure 3
truly reflect the true variables. A comparison of the test data and data from the full scale commercial plant which allows very accurat,e cycle control shows very close agreement. The results obtained from ion exchange treatment are sensitive to numerous vmiables of both bed regeneration and operation. On all experiments reported herein the cation beds were regenerated with 4 pounds of 66 O BB sulfuric acid per cubic foot of bed diluted to approxiinatrly 1.5 N. Thc anion beds were regenerated d t h 0.9 pound of anhvdrous ammonia per cubic foot converted to approxinmtely 1 iV ammonium hydroxide. The cation regenerant and rinse were passed a t t,he rate of 1.3 gallons per cubic foot per minute. T h e anion regenerant and rinse were passed a t the rate of 0.6 gallon per cubic frmt per minute. Bed exhaustion on both the cation and anion beds was a t the ratc of 0.75 to 1.0 gallon per cubic foot per minute. The cation regeneration was two-stage regeneration and the anion regeneration was single stage. The columna were thoroughly seasoned columns of Chemical Process Company. C-3 cation and A-3 anion exchanger. The nitrogen andyses herein reported are Kjeldahl analyses conducted in the following manner: Anion effluent, samples and anion spent regenerant samples are preboilid i n t,he KjPldahl flask with 25 ml. of wat,er and 5 ml. of 0.1 N sodium hydroxide until approximately the original sample volume is ohtainod to expel ammonia. All other samples are digested witshout pretreatment. For digestion, use 25 ml. of concentorattadsulfuric acid, 10 grams of sodium sulfat,?,and 0.3 gram of coppcr sulfat,e. Roil for a t least 1hour or until solution is clear. For distillntiori, cool digestion sample and add 200 ml. of water and 100 ml. of strong sodium hydroxide (400 prams of sodium hydrowidr per liter of solution), and boil until 7 5 nil. have distilled over. The distillate is collected in an Erlenmeyer flask filled with
It has been established, as will be borne out by evidence piesented here, that the nitrogen constituents in the carbonated beet juice divide into two fractions, those which may be taken out OD the cation columns, and those which may be taken out on the anion columns; also it is possible to accomplish essentially complete nitrogen removal from the juice. In this work the authors were interested in the translation of results t o factory practice and so the columns were given regeneration treatment which had been proved to be most economical in commercial practice. I n Figure 1, the characteristic variables of developed acidit) and p H of the cation effluent and p H of the anion effluent are plott,ed. If it is desired t o establish a uniform control of the ion exchange columns in industrial practice, a reproducible and measurable variable must be established first, preferably with a sharp inflection at the point of control. If one cannot measure directly the variable in which he i interested (as in this case the nitrogen content of the juice cannot be directly measured) a variable must be chosen which can b s measured and which uniformly reflects the behavior of the variable under consideration. The three cycle variables a h i c h are illustrated in Figure 1 arc commercially measurable. These variables show two sharp inflection points: The titration break on the cation cell where the exhaustioil front reaches the bottom of the column, and the acidity of the cation effluent begins to dccrease in developed acidity. Before this break is reached, the nitrogen content of the cation effluent is already higher than in the feed as shown in Figure 2. The p H break of the anion effluent shown in Figure 2, which. plots the nitrogen content of the cation and anion column effluents and apparent purity of anion effluent for the same cycle as is plotted on Figure 1. Owing to chromatographic selection the nitrogen constituents are banded selectively on the exhaustion front of the cation and prior to the titration break are flushed off in quantities exceeding the concentration in the input feed to the cation column. It is this characteristic of the cation which makes the accomplishment of high nitrogen removals by ion exchange treatment, difficult. On Figure 1, the anion pH curve has not been displaced t o wincide with the cation effluent pH and acidity curves on a throughput volume. On Figure 2, the plot of the anion effluent has been displaced to correct for the time lag of the Now of cation effluent through the anion column. Therefore, a vertical comparison on the plot compares the quality of the same unit of cation effluent after it has passed through the anion column.
5.0
0.rs
Second carbonation juice before cation treatment, 0.354 Na on drv substance. after double cation treatment, 0.11% Nz on dry sugstanoe. Bed volume 3b cubic inohm. 0
s
m
until after the titration break on the cation cell when the removal by the anion bed would be expected to fall off owing to incomplete acidification of the anions. This type of plotting gave the first indication that the nitrogen constituents were divided into two classes, those removable only on the cation columns and those removable only on the anion columns, and also that while the cation columns begin a progressively increasing nitrogen leakage of the potentially cation-removable nitrogen the anion column of equal volume carries on complete removal of its potentially removable constituents right up t o the titration break on the cation column. The anion column has become acid prior t o this point, but the leakage is some other class of compound than the amino class. T o confirm further the fact that a fixed fraction of the nitrogen constituents of the carbonated sugar beet juice are removable by the two respective beds and that the anion bed has essentially no capacity t o accept the cation fraction, the following data were accumulated. For the first run, second carbonation juice was given a double cation treatment with adequate bed capacity to accomplish essentially complete removal of the cation removable nitrogen. For the second run, a portion of the same second carbonation juice was given a single cation bed treatment to the titration brcak. Only the second half of this cation effluent was saved and composited For use in the tests. This second sample gave a juice which was completely converted to acid anions but had a nitrogen content actually equal to the untreated second carbonation juice. Each of these composited feeds was then subjected separately t o anion column treatment. This gave a feed of constant nitrogen content to the anion columns which would not be possible if the anion were connected directly to the cation as is ordinarily the case.
Treatment to Exhaustion End Point of Titration Break on Cation Column. With this background of the basic mechanics of nitrogen removal by the two columns, efforts were then directed to the utilization of this knowledge to the development of a practical method of economically accomplishing a high degree of nitrogen removal. Before proceeding with these data, another factor which must always be borne in mind in choosing a practical economical ion exchange treatment cycle should be set forth. From the preceding data, it is evident that cutting off the exhaustion cycle a t a progressively earlier end point increases the percentage nitrogen removal accomplished. This is an undesirable method of accomplishing high nitrogen removal which would bring increases in plant size and, therefore, plant cost. By virtue of this reduced throughput per unit of bed volume the dilution of juice is increased due to the higher ratio of columns t o be “sweetened on” and “sweetened off.” Also, the regeneration efficiency would suffer when regeneration is practiced on partially exhausted beds. The most simple bed operation Is t o operate with single pair treatment-i.e., juice treatment through one cation column followed by one anion column and to a n exhaustion end point of the titration break on the cation column. When operating in thie manner so as t o maintain a continuity and uniformity of juice flow t o the evaporators, the following sequence of changing from one pair of beds t o the next was used. As soon as the titration break is reached, the feed t o the exhausted pair is transferred t o the next pair of cells in line and water a t an equivalent rate is fed into the exhausted pair to “sweeten them off,” The effluent from this pair continues t o
INDUSTRIAL AND ENGINEERING CHEMISTRY
2772
TABLE11. PER CENT NITROGENREMOVALFROM SEcom CARBOTATIOK JCICE B Y SINGLE P A I R TREATMEKT (Operating to exhaustion point of titration break on cation cell a t the pilot plant a t Burley, Idano) Less t h a n 3 Days after Ammonia Longer t h a n 7 Days after Ammonia of _ Cation Bed Treatment Treatment of Cation Bed __ _~~ . Total % Kz Total % Sz removal by S o . of days, removal b y No. of days, cation-anion since aininonia cation-anion since ammonia treat inent treatment treatment treatment 46.5 65.3 57.0 53.5 44.2 44.6 53.6 44.0 48.0 48.5 66.5 55.0 61.0 Av. 5 2 . 9
0 0 1 2 0 0
Vol. 41, No. 12
column effluent. The same procedure of returning the end of the sweetening off juice for retreatment via the recycle tank is prapticed. Table 111with the data on an experimental cycle sets forth the expected nitrogen removal from thig type of operation. Tables IV and V give the incremental quality of both tho cation and anion colunin effluent over 5-minute increments of time for this same cycle. Table V I sets forth typical %-hour composite analysis on the feed and effluent juice from this commercial operation showing an average nitrogen removal of 67.9391, over the 7-day pcriod This is in close agreement I+Nith Table 111.
1
2 3 0 0 0 0
TABLE 1s;. CATIONEFFLUENT CYCLE (Single pair treatment of sulfured thin juice with a n exhaustion end point v? p H 7 on anion bed; treatment in commercial ion exchange plant)
Av. 4 2 . 1
Cycle ‘Time, hlin.
Effluent Acidity
Table I1 tabulates the data from such single pair treatment. The percentage nitrogen removal was calculated from 24-hour composites of feed and treated juice. An earlier examination of data on nitrogen removal had shown that there was a gradual decrease of nitrogen removal over a period of days and it was established that if the exhausted cation column m s periodically flushed with an alkali the nitrogen removal capacity mas completely restored. I n the commercial operation a t Twin Falls, the present practice is to flush the cation columns every 7 2 hours with 0.7 pound of ammonia per cubic foot applied as 1 AT ammonium hydroxide. The data in Table I1 illustrate the benefits which may be expected from this alkali treatment of the cation beds. These data also illustrate the normal expectation of nitrogen removal with this type of operation of 50% or slightly above. Treatment to Exhaustion End Point of pH 7 on the Anion EfPuent. Referring to Figure 2 , it is evident that, from the time the anion effluent reaches a p H of 7 a t 50 minutes to the time the titration break is reached a t 70 minutes, as far as nitrogen removal is concerned, the effluent essentially equals the feed. Thus, no nitrogen removal is accomplished over 29% of the cycle. The dual desire to increase nitrogen removal and to avoid production of acid effluent led t o the present commercial practice of establishing the exhaustion end point at a p H of 7 on the anion
TABLE 111. SINGLEPAIRTREATMENT OF SULFURED THINJUICE WITH AN EXHAUSTION ENDPOINT OF p H 7 ON THE ANION EFFLUENT=
(Composite samples; pilot plant treatment)
%
K-2
Effluent on D r y Acidity Substanoe Sample Feed, sulfured thin juioe 0.31 Cation effluent 0.164 0.059 .v 0.105 Anion effluent a Results: total nitrogen removal, 66%; percentage of total nitrogen removal taking place in the oration bed, 71%; percentage of total nitrogen removal taking place in the anion bed, 29%.
...
0.05 0.068 0.074 0.073 0.074 0.057 0.058 0.056 0.051 0.024
15 20 25 30 35 40 45 50s 55
% rin on D r y Siibstance
...
...
io
“treated juice.” As soon as the fresh pair have sweetened on to the desired brix, the effluent from the fresh pair is switched t o treated juice and simultancously the srvertening off from t,he exheusted pair which has not yet’ begun to fall off in brix is transferred to the recycle tank for the rest of the duration of sweetening off. From the recycle tank this residual sweetening off liquor is returned to the feed juice at a uniform rate over the next cycle, thus this partially treated liquor at the end of the cycle is returned for retreatment.
Effluent PH 1.80 1.68 1.62 1.62 1.61 1.75 1.85
0.096 0.109 0.118 0,109 0,122 0.158 0.207 0,270 0,298 0.444
1,92
2.00 2.25
Thin juice feed t o cation cell is 0.31% Nz on dry substanoe. Point at which cell was transferred t o sweetening off.
TABLE V.
ANIONEFFLLENT CYCLE
(Sinale pair treatment of sulfured thin juice with a n exhaustion end point pH 7 on anion cell; treatment in commercial ion exchange plant)
x Nz
Cycle Time, Min. 10
on Dry Substanot
9.20 9.15 8.95 8.80 8.75 8.60 8.40 8.05 7.35 6.50
15 20 25 30
35 40 45 50a
55 0
ill
0.254 0,206 0.132 0.056 0,043 0.046 0.069 0.119 0,149 0.180
Point a t which 0011was tramferred t o sweetening off
TABLE VI. TYPIC IL OVER-ALL NITROGEN REMOVLLS O N DAILY COMPOSITES FROM COMMERCIAL TREATMENT O F SULFURED T H I ~ .JUICE WITH SINGLE P A I R TRE4TMENTS T O AN EXHATJSTIOS I h O POINT OF pH 7 ON ANION CELL
% Date 12-15-48 12-17-48 12-18-48 12-1 9-48 12-20-48 12-2 1-48 12-22-48
Av
.
5 2 on Dry Substance in Feed
0.41 0.31 0.28 0.34 0.34 0.33 0.34 0.336
% Nz on Dry Substance in Treated Juice
Removal
0.11 0.12 0.09 0.11 0.13 0.11 0.09 0,109
73.5 66.8 62.0 71.0 68.0 61.2 73.0 67.93
70 Nn
DOUBLE PAIR TREATiMENT
Treatment to an Exhaustion End Point of the Titration Break on the Primary Cation Column. The possibility of utilizing multiple pair treatment of sugar beet juice has been investigated periodically from the very inception of this application in 1947. There have beon operational and process disadvantages t o this type of operation which have t o date offset any advantages in commercial application. I n t h e search for a method of economically accomplishing higher nitrogen removal this type of treatment was re-examined with specific reference to nitrogen removal b e havior. The following conditions were observed:
December 1949
a
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
PRIMARY CELLS. Double pair treatment in the sequence cation, anion, cation, anion. The primary cation and anion pair were operated t o a n exhaustion end point of the titration break on the cation cell. I n this manner the anion effluent of the primary pair reached a low of pH 5 a t the end of the cycle. None of the sweetening off effluent from the primary pair was advanced to the secondary pair. It was all returned to feed in the recycle tank. The primary cells were not previously used as secondary cells. SECONDARY CELLS. Cation cell was given an ammonia treatment prior to each regeneration to prepare it to be a most effective scavenger for nitrogen leakage from the primary pair. Both secondary cells otherwise regenerated in the standard manner. I n this experiment the secondary pair of cells was operated for three cycles of primary cells.
PAIRTREATMENT OF SECOND TABLE VII. DOUBLE CARBONATION JUICE (Pilot plant treatment)
Sample Second carbonation juice Juice effluent from primary pair of cells Secondary anion effluent, 1st cycle Secondary anion effluent, 2nd cycle Becondary anion effluent, 3rd cycle Yecondarv cation effluent. 1st oyde Secondary cation effluent, 2nd cycle Secondary cation effluent, 3rd cycle
li-
x
% Nz
% Nz Removal Based on Nr in Feed
% Nz
Removal Based on Total Na Removal
on Sucrose 0.276
pH 9.2
...
...
0.150
7.1
46.7
54.3
0,044
8.2
84.1
3.9
0.124
5.1
55.0
0.201
4.0
27.2
... ...
0.053
3.5
80.5
41.8
0.160
3.6
45.7
...
0.229
3.7
17.0
...
Table VI1 sets forth the typical data of this type of operation. A normal nitrogen removal of 45.7% of the nitrogen in the feed was accomplished by the three cycles of primary cells. The cations of these primary cells in this experiment had not received alkali treatment for 18 days. The secondary cation for the duration of the f i s t primary cycle accomplishes a high degree of removal of the nitrogen leaking through the primary pair. During the second primary cycle, however, the secondary cation removed no nitrogen and during the third primary cycle the secondary cation sluffed nitrogen back into the primary pair effluent acting t o reduce rather than increase nitrogen removal. The secondary anion column throughout the duration of the three primary cycles accomplished a small but steady nitrogen removal. I n the right-hand column of Table VII, the percentages of removal accomplished are calculated on the basis of the total nitrogen removed by the double pair treatment rather than on the basis of the nitrogen in the feed. It is seen that the primary columns accomplished just ever half of the total nitrogen removal accomplished by the over-all double pair treatfnent during the first primary cycle.. The secondary cation column removed 41.8% of the over-all nitrogen removed and the secondary anion only 3.9% of the over-all nitrogen removed, again demonstrating that the primary anion column even when exhausting to the titration break on the cation column allows only a minor nitrogen leakage. From the data of Table VII, it is evident t h a t from a nitrogenremoval standpoint, double pair treatment accomplishes nothing if the usual rotation is resorted to where the secondary pair of 'columns is advanced to the primary position as each pair of primary cells becomes exhausted. From the standpoint of nitrogen removal, there is very little gained by the use of a secondary anion column. From the data presented herein, the only feature disclosed which has increased the percentage of nitrogen removal other than to increase the cation column capacity relative t o the
2773
quantity of juice treated is the alkali treatment of the cation bed. So far the authors have been unable to avoid this limitation that the only way to increase the nitrogen removal is to cut off the cation column progressively earlier in the exhaustion cycle, thus accomplishing lower nitrogen leakage figures. DOUBLE CATION-SINGLE ANION COLUMN TREATMENT
Referring now, however, t o the potential nitrogen by-product extraction from the spent ion exchange regenerants, it became apparent from the results of the double pair experiments listed above that it might be possible to use certain features of this operation to advantage in the segregation of the nitrogen constituents for by-product recovery and a t the same time accomplish a high nitrogen removal on a profitable basis. Utilizing equal bed volumes of cation and anion resin and exhausting to the titration break on the cation cell has already been shown to accomplish essentially complete removal of the anionic nitrogen but only about a 25% removal of the cationic nitrogen. A second cation column to pick up the nitrogen leakage from the primary cation column would have the cpportunity to pick up the bulk of the cationic nitrogen and segregate it from the other impurity constituents. This double cation treatment followed by a single anion column would complete the treatment. Conditions of operation are as follows: The juice to be treated is passed through two cation columns in series and then through one anion column. The primary cation column in this case had gone 7 days without ammonia treatment; the secondary cation column is given an ammonia treatment prior to each regeneration Both cation cells and the anion cell are regenerated in the standard manner. The primary cation cell is operated to the titration break and all juice previous t o this break is sent through the secondary cation cell and anion cell. A typical result of this operation is portrayed in Table VIII which shows the expected segregation of cationic nitrogen on the secondary cation column and also realizes a high over-all nitrogen removal of SS%. More recently there has been evidence that by manipulation of the primary cation column regeneration, a more complete segregation of the rationic nitrogen on the secondary cation column can be accomplished. This is now being developed and will be reported later.
TABLE VIII. DOUBLE CATION-SINGLE ANION TREATMENT OF SECOND CARBONATION JUICE (Pilot plant treatment)
Second carbonation juice Primary cation effluent Secondary cation effluent Anion effluent
0.344 0,258 0.105 0.043
9.2
... ...
7.4
...
25.0 69.5 88.0
...
28.6 50.8 20.6
COLUMN RELEASE OF NITROGEN CONSTITUENTS DURING REGENERATION
An examination of the typical data on the rate of flushing of the nitrogen from the cation columns in Figure 5 does not disclose a sharp segregation of the nitrogen constituents in any fraction of the spent regenerant but rather indicates a slow displacement of these nitrogen constituents over the whole of the regeneration cycle. To understand better the plotting of this graph, it should be recalled that the two-stage regeneration of the cation columns is being practiced. As the regeneration cycle is started, the first 300 cubic feet of solution displaced from this column are discarded because they represent the water in the voids of the column. The volume of fresh acid and once used acid for this regeneration were 665 cubic feet each so that the next 565 cubic feet of solution displaced from the column represent the spent acid collected tor by-
2774
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 41, No. 12
the nitrogen concentration in the feed. Khen an anion bed volume equal to the cation bed volume is utilized, the anion bed will give a very complete removal of the anion removable nitrogen lip to the titration break on the cat,ion bed.
1.6
