STAXDARD DOUBLE UNIT TOR DRY RECLAIYINQ CULORINATED CLEANINn SOLVENTS
RECLAIMING
CHLORINATED DRY CLEANING SOLVENTS LAWREW3E E. STOUT AND ARTHUR E. TILLMAN
BY ADSORPTION
Washington University, St. Louis, Mo.
QT
HE substitution of adsorption for the customary distillation process used to reclaim dry cleaning solvents offers certain distinct advantages. The 6rst cost of the dry cleaning plant would be decreased by eliminating the expensive distillation and condensation units. The time required for distilling the used cleaning agent cuts down the period of useful operation of the fluid. If a continuous filtration and adsorption system could replace it, the capacity of the machine would be increased. The tests described in this paper were run in conjunction with one of the well-known dry cleaning units on the market today. The solvent was an azeotropic mixture (8) of 70 mole per cent carbon tetrachloride and 30 mole per cent ethylene dichloride. During the process knoxn as dry cleaning, the solvent. accumulates a mixture of dissolved oils, fats, waxes, grea,scs, etc., irom the articles being cleaned. This extraction may he termed the “primary” cleaning action. In addition, there is a “secondary” cleaning action consisting of the removal of solid matter such as dirt, dust, lint, etc., from the textile fibers. Continuous filtration of the solvent during the operation of the machine removes such solids effectively. When the machine is put into operation with fresh solvent, the latter is colorless. The color gradually passes through transitional shades from light yellow to dark reddish brown as
the poundage of clothes cleaned increases. Not long after this reddish brown stage makes its appearance, the solvent has become unfit for use and the garments cleaned have a decidedly unpleasant, rancid odor. This color is due, in the main, to the dissolved oils which can be classified into saponifiable and unsaponifiable oils. The latter can be considered mineral oils whereas the former includes both vegetable and animal oils, and free fatty acids. It is generally conceded that the presence of mineral oils produces a softer feel to the cleaned clothes. Saponifiable matter, however, must he removed if satisfactory cleaning is to result. Fatty acids give a rancid odor to the cleaned clothes, and the vegetable oils deposit within the fibers and can develop rancidity after a period of storage. Under continued use of the solvent the oil concentration builds up until solvent rejuvenation is neoessary. Increme in oil concentration is not, however, a linear function of the amount of clothes cleaned. Complete extraction of the cleaned clothes is impossible, and a certain amount of solvent plus oils is removed with each load. This drag-out loss must be replaced by fresh solvent, which slows down the rate of increase of oil concentration. As the concentration increases, there must be some point at which the oils removed from the garments by the solvent will be counterbalanced hy the oils 22
JANUARY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
Experimental Procedure
23
,
removed by the garment after cleaning. Long before this point is reached in actual cleaning practice, the solvent is in such a condition that satigfactory cleaning is impossible. The existence of this point of equilibrium is the necessary pivot upon which to build the rejuvenation of used solvents by adsorption. If such a point did not exist, the concentration of dissolved oils would increase to such an extent that, even though in a sweetened condition, their presence would impart a greasiness to the cleaned clothes. On the other hand, if the dissolved oils can be kept in such a condition that their removal by the clothes being cleaned results in no offensive odor or other ill effects to these clothes, the problem is solved. This condition postulates a point of equal exchange of oil from garment to solvent and from solvent to garment, with the former yielding a large percentage of saponifiable and odoriferous material in exchange for an equilibrium concentration of unsaponifiable and odorless mjneral oils. The problem thus resolves itself into a preferential removal of vegetable oils and fatty acids, leaving the mineral oils in solution. Stated another way, it involves the separation of compounds with polar groups from compounds with nonpolar groups. Such a separation offers promise of accomplishment by adsorption. However, any adsorbing agent used must also possess desirable filter characteristics since it must serve as a filter medium either on a pressure leaf filter or in a percolation tower. Some definite work has been done in the field of adsorption from nonaqueous solutions, but no applications to the dry cleaning industry have been made. Holmes and Thor (6) investigated the adsorption behavior from a variety of solvents of an oleostearin melting at 45' C. Carbon tetrachloride was one of the solvents used. A specially prepared silica gel (6),Norit carbon, hydrated ferric oxide, and hydrated aluminum oxide were used as adsorbing agents. Silica gel was by far the best of the four in removing the fat from carbon tetrachloride solution. Gurwitsch (2) studied the adsorption of benzoic and valeric acids from benzene solution with charcoal, and Gutlorn (3) found that fatty acids could be removed from vegetable oils by lime. He concluded that the action was not chemical. Croner (1) showed that animal charcoal was a good adsorbent for fats in benzene solution but that an excess of carbon would not remove the last traces. Patrick and Jones (7) used silica gel in their study of adsorption of benzoic and acetic acids from carbon tetrachloride solutions, and Holmes and McKelvey (4) also used silica gel in their study of adsorption of acids from toluene solutions. They showed that Traube's rule is reversed in this solvent, the lower members of the homologous acid series being most strongly adsorbed.
Preliminary work on the purification of the contaminated solvent by adsorbing agents showed that the best results were obtained with activated carbon, silica gel, and activated magnesia, all obtainable commercially. For the laboratory investigations on contact adsorption, the adsorbing agents were passed through a 200-mesh screen. Granular magnesia (14 to 16 mesh) was used in the experimental study of bed percolation. LABORATORY TESTS. A known solution was prepared by dissolving 25 cc. of a mixture of 40 per cent mineral oil, 40 per cent cottonseed oil, and 20 per cent oleic acid (by volume) in the solvent and making up to a liter. The mixture had an acid number of 39.4 and a saponification number of 120. An unknown mixture was obtained by distilling a sample of solvent which had cleaned 630 pounds (285.8 kg.) of garments. This oil residue had an acid number of 46.4 and a saponification number of 146. In general the procedure for both oil mixtures was as follows: For contact adsorption a measured quantity of the solution was intimately mixed with a weighed quantity of adsorbent at 27" C. The suspension was then filtered, and a measured quantity of the filtrate was distilled until 78" C. was attained. The residue in the distilling flask was then transferred to a weighed evaporating dish, and the remainder of the solvent was removed over a small flame in a current of air. The dish and contents were then placed in an oven at 110" C . for one hour, and the quantity of residue was determined. The residue was tested for saponification number and acid number by the usual methods. The amount of oils removed and the change in acid and saponification numbers were taken as a measure of the effectiveness of any given adsorbent. The time of contact was limited t o 5 minutes which represents the maximum time that any adsorbent could remain suspended in the solvent before depositing on the filter. The temperature of 27" C. corresponds to industrial practice. Laboratory ercolation tests using granular magnesia were carried out on goth known and unknown oil solutions. A measured volume of the solution at 27" C. was percolated for 5 minutes through a bed of known weight of oxide. The changes in total oil concentration, acid, and saponification numbers were determined. PLANTTESTS. The laboratory tests were followed by test runs on a full-size cleaning unit. Silica gel was eliminated in the large-scale tests mainly because of price considerations. Powdered magnesia would not form an adherent coat on the pressure leaf filter. This necessitated interpoeing a tower filled with 20 pounds (9.1 kg.) of the magnesia between the filter and the washer. Fluid passed continuously from the washer to the filter, from the filter to the tower, and from the tower back to the washer, during the operation of the machine. The procedure of a test run was as follows: The cleaning system was charged with 52 gallons (196.6 liters) of fresh solvent which was circulated until it was clarified. The basket was then charged with a known weight of clothes and the cleaning operation carried out in the usual manner. Accurate records were kept of the quantities of clothes cleaned, and the volume of cleaning fluid was kept constant at all times. Gallon samples of solvent were removed for examination at periods indicated on the
Dry cleaning solvents accumulate a variety of oils during use. These oils may be classified into polar and nonpolar substances. Acids and saponifiable materials are detrimental to the continued use of the solvent while minerals in moderate concentrations are usually present. Commercial practice calls for periodic removal of these oily residues by distillation of the azeotropic mixture of carbon tetrachloride and ethylene dichloride. The substitution of a combination of adsorption and filtration for this distillation process has been recommended from time to time, but many men in the industry feel that insufficient performance data
are available to justify such a procedure on high-class work. This paper evaluates the adsorptive properties of three materials when used in such a dry cleaning cycle. Silica gel is an excellent general adsorbent but does not seem selective for polar substances. Activated carbon and activated magnesia are both selective in removing, preferentially, the undesirable constituents. With these agents the solvent cycle of use can be extended enormously. A determination of the acid number of the solvent is an excellent index of the usability of the solvent for further cleaning.
24
INDUSTRIAL AND ENGINEERING CHEMISTRY S i l i c a Pel
VOL. 28, NO. 1
, TABLE I. ACTION OF ADSORBINGAGENTS ON OILS DISSOLVED ~dSOLVENT Adsorbent
Total Oils Oils Removed Acid No.
Grams per liter
Saponification No.
Adsorbent Uaed
128 12s 126 126 124 126 123 128 120 123 120 117 117 115 128 125 123 119 116 109 105
Carbon
Grams
A.
FIQURE 1. EFFECT OF A D S O R B E N TUPON AMOUNTOF OILS REMOVED-AND ACID NUMB I R O F O I L SF R O M UNKNOWNSOLUTION
0 8 20 30 40 60 80 0 8 20 30 40 60 80 8 20 30 40 60 80
23.32 21.84 21.44 21.17 21.00 20.60 20.20 23.32 22.60 22.07 21.30 20.83 20.26 19.76 23.32 22.36 21.24 20.01 19.60 18.16 17.24
0 4 15 25 40 0 4 15 25 40 0 4 15 25 40
7.23 6.63 4.99 4.97 4.06 7.23 7.13 6.88 6.52 5.68 7.23 6.66 5.82 5.39 4.90
0
data sheets. At regular intervals (also indicated in the data) the liquid flow was reversed (when activated carbon was used), and the Hter cake blown off and removed from the system. After each blow-down of the Hter, fresh adsorbent was added to the system. When magnesia was used, the solvent was circulated continuously through the percolation tower until the magnesia was spent as an adsorbing agent.
Discussion of Results The data listed in Table IA illustrate the adsorptive capacities of specific quantities of adsorbing agents for an oleic acid-cottonseed oil-mineral oil mixture dissolved in chlorinated solvent. These data enable us to follow the selective adsorption process involved. Silica gel is best when only the total oils removed are considered. Magnesia ranks first when we consider the saponifiable oils and especially the free acid contents of the oil residues. The data for carbon show that it is slightly better than silica gel in removing acids but not as effective in removing sapopifiable matter. The data in Table IB show the adsorptive capacity of the three materials for the oils obtained by cleaning 630 pounds (285.8 kg.) of garments. The solvent was reddish brown in color, and the oil residue was distinctly rancid. The data for the unknown mixture make it evident that the dissolved oils in this case are different in character from those of the known solution. The residues from the unknown solution were semi-solid a t room temperatures whereas liquid residues were obtained from the known mixture. Figure 1 shows graphically the effect of quantity of adsorbent upon the oils and especially the acid content of the oils removed. In this case, as with the known mixture, carbon seems most effective in adsorbing or removing the acids from the fouled solvent. Treatment with adsorbents had a marked effect upon the oil residues obtained from the fouled solvent. With carbon the color changed from the initial dark brown to a very light
Pound0 o f Osrments CleQned
FIGURE 2. INCREASE IN OIL CONCENTRATION DURINQ CLEANING TEST USING No SPECIAL ADSORBENT
Known Oil Mixture 0.000 35.7 0.370 38.0 0.470 36.0 0.537 34.6 0.580 31.5 0.680 30.4 0.780 25.0 0.000 35.7 33.4 0.180 28.8 0.312 25.2 0.506 22.4 0.622 16.2 0.765 10.1 0.890 0.000 35.7 0.240 36.2 0.520 34.9 0.827 36.0 0.930 34.0 1,290 31.1 1.520 28.8 B . Unknown Oil Mixture 0.000 46.2 0.600 49.0 2.234 20.9 2.266 12.5 3.174 5.2 0.000 46.4 0.120 46.3 33.9 0.342 0.710 23.3 1.548 12.3 0.000 46.4 47.7 0.570 1.410 43.7 43.7 1,840 2.330 35.4
146 143 103 93 79 146 147 135 136 117 146 137 129
Magne+a
Silica gel
Carbon
Magnesia
Silica ge
...
118
yellow, and the rancid odor practically disappeared. With magnesia the residues brightened several shades and the odor decreased somewhat but, considering the ability demonstrated by Figure 1B to remove acids, the effect was not as great as was expected. This indicates that saponifiable matter other than free acids contributes to the rancidity of the dissolved oils. Silica gel gave the poo'rest results of the three adsorbents. Even though an appreciable amount of oils was removed (Figure lA), the residues had not benefited in color or odor. Little selective adsorption seems to result from the use of this material. Table I1 contains data for plant tests using (1) no adsorbing agent, (2) magnesia, and (3) carbon. Figure 2 shows the rate of accumulation of total oils when no adsorbing agent is used. The data for this run indicate a gradual accumulation of total oils with a negligible change in their saponification and acid numbers. Table I1 also contains data for the plant test using activated magneSia as an adsorbent. These data are illustrated in Figure 3; A shows a gradual but less rapid accumulation of total oils similar to that encountered when n o adsorbing agent was employed. However, B shows that magnesia was very effective in holding d o m the acid numbers of the oils until 700 pounds (317.5 kg.) of clothes had been cleaned. At this time the adsorbent was spent, and the acid numbers of the extracted oils rose rapidly. In this test run on magnesia a serious difficulty arose. After the solvent had been circulated for a few minutes through the system which included the percolation tower, it developed a light yellow tinge even though no garments had been cleaned. At first no significance was attached to this color change. However, when garments were cleaned in this ffuid it soon became apparent that this color change accompanied some very undesirable chemical change. The garments cleaned in this ffuidpossessed a most disagreeable odor, and this odor was especially marked in the oil residues. The
JANUARY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
25
IO
TABLE11. ACCUMULATION OF DISSOLVED OILS DURING CLEANING PROCESS Total Oils Garment6 Cleaned Removed Pounds (Kg.) Grams/liter 2.168 3.314 4.824 300 (136.1) 5.044 400 (181.4) 500 (226.8) 5.900 7.230 630 (285.8) 0.400 0.635 2.430 3.208 4.740
% [;:$]
6.180
100 155 212 250 250 300 367 367
(45.4) (70.3) (9fi.2) (113.4) (113.4) (136.1) (166.5) (166.5)
500 550 550 600 650 700 700 750 800 850 900 950 950 1000 1100 1100 1160 1220
(226.8) (249.5) (249.5) (272.2) (294.8) (317.5) (317.5) (340.2) (362.9) (385.5) (408.2) (430.9) (430.9) 463.6) i498.9) 498 9) 1526:2) (553.4)
t!X fE:3
0.000 2.477 2.033 4.530 3.770 3.960 4.655 4.920 3.525 3.724 4.442 3.818 4.072 4.220 4.426 4.908 4.282 4.632 5.104 6.184 6.004 6.206 6.678 7.060 7.562 8.088 7.374 7,948 9.604 6.646 6.386 7.162
Acid
Saponifioation No.
52.6 50.5 49.8
159 172 163 149 147 146 71 a2 142 135 110 106
No.
...
44.2 46.4
...
1.63 1.64 1.64 2.00 2.48
...
...
16.8 9.7 17.3 4.9 7.8 11.1 12.7 6.4 9.8 13.6 2.6 3.3 5.1 5.4 9.6 6.8 11.0 11.2 9.8 11.5 12.8 21.1 30.4 21.5 29.7 31.2 35.2 31.6 4.5 4.4
169 136 142 129 118 172 171 157 140 137 111 105 106 105 113 101 99 108 96 92.5 105 107 131 115 129 120 129 130 98 72 91
6.6
Adsorbent Used None
0.
L
5
8
3
6
$
4
> 4
$
2
0
Magnesia tower
Ir
B
J 3
30 20
c)
Carbon, 2.5 Ib. (1.13 kg.) added Carbon, 1.25 lb. (0.57 kg.) added Carbon, 2.5 lb. (1.13 kg.) added Carbon, 2 lb. (0.91 kg.) added
4
IO IO0 200
300
400 500 600 700 800 900 P o u a e of Garments Cleaned
TOO0
IIW
I200
FIGURE 4. EFFECTOF CARBON TREATMENT ON OIL CONCENACID NUMBEROF RESIDUAL OILS DURING LARGFISCALE TESTRUN
TRATION AND
Carbon, 2.5 lb. (1.13 kg.) added
Carbon, 2.5 lb. (1.13 kg.) added Carbon, 1.25 lb. (0.57 kg.) added
Carbon, 2.5 lb. (1.13 kg.) added Carbon 5 lb. (2.3 kg.) added
that each addition of carbon has a pofound effect upon the oil residues and upon their acid numbers. In fact, it seems that the acid number is an excellent index of the usability of the solvent. Samples giving high acia numbers had dark, rancid oily residues, whereas samples with low acid numbers contained light-colored oils with little rancidity. The rhythmic nature of these curves indicate the desirability of a percolation system such as was used in the magnesia test. Work is in progress on such a procedure a t the present time. This involves the use of an activated carbon having different adsorptive characteristics as well as different physical properties.
Conclusions trouble was traced down to a carbon disulfide impurity in the carbon tetrachloride which was used to formulate the solvent. Solvent containing no carbon disulfide gave satisfactory performance in the system. The plant test using activated carbon was especially attractive. Over 1200 pounds (544.3 kg.) of clothes were cleaned without distillation of solvent, and a t the end of the test very satisfactory results were still being accomplished. Figure 4 shows these results graphically. It should be noted
1. Activated carbon, activated magnesia (both granular and powdered), and silica gel all act as adsorbents for oils dissolved in an azeotropic mixture of carbon tetrachloride and ethylene dichloride. 2. Activated carbon and magnesia remove free acids and other saponifiable matter preferentially from the unknown mixture, leaving mineral oils in solution. Silica gel removes both saponifiable and unsaponifiable matter. 3. The removal of free acids and other saponifiable matter from the contaminated solvent causes decolorization and a change in the oil residues from a brownish black rancid oil to a light yellow, practically colorless, semi-solid oil.
Acknowledgment The authors wish to thank Halsey Dunwoody for placing the facilities of his company, Bandbox, Inc., a t their disposal for the period of this investigation.
2 3 0
- 4 2
e
::I
Literature Cited
0
B9 ' z
3
I 0
Pounda o f G a r p e n t
9
Cleaned
FIGURE3. EFFECTOF PERCOLATION THROUGH MAGNESIAON OIL CONCENTRATION AND ACID NUMBEROF RESIDUAL OILSDURING LARGE-SCALE TEST RUN
(1) Croner, F., Seifensieder-Ztg., 52, 637-48 (1925). (2) Gurwitsch, L.,2.physik. Chem., 87,323-9 (1914). (3) Gutlorn, L., KoZZoid-Z., 33,365 (1923). (4) Holmes, H. N., and McKelvey, J. B., J. Phys. Chem., 32, 1522 (1928). ( 5 ) Holmes, H. N., Sullivan, R. W., and Metcalf, N. W., IND. ENG. CHEM.,18, 386 (1926). (6) Holmes, H. N., and Thor, C. J. B.,Colloid Sumposium Annual, 7, 213 (1930) (7) Patrick, W. A,, and Jones, D. C., J. Phys. Chem., 29, 1 (1925). (8) Young, H. D., and Nelson, 0. A., IND. ENO.CEEM.,Anal. Ed., 4, 67 (1932). R ~ C E I V EJuly D 15, 1935. Presented before t h e Division of Industrial and Engineering Chemistry a t the 90th Meeting of the American Chemical Society, San Francisco, Calif., August 19 to 23, 1935.
