Ind. Eng. Chem. Res. 1905,34,2464-2467
2464
Recovery of Very Dilute Acetic Acid Using Ion Exchange Frikkie L.D.Cloete* and Arno P.Maraid Department of Chemical Engineering, University of Stelknbosch, Stelknbosch, 7599 South Africa
Acetic acid can be recovered from 1% solutions using weakly basic ion exchange resins. The acid is adsorbed by the free-base form of the resin, which can then be eluted using a slurry of lime to give a solution of calcium acetate. This solution could either be evaporated to crystallize calcium acetate or reacted with sulfuric acid to form acetic acid and gypsum. Laboratory tests of the proposed process gave product solutions of 15-2096 acetic acid using pure 1%acetic acid as feed. Some measurements using a typical industrial effluent gave similar recoveries and showed that there was no initial fouling of the resins.
Introduction Acetic and related acids occur in many industrial wastewaters, often mixed with several other classes of organic compounds (Helsel, 1977;Kohn, 1979; Ricker et al., 1980). The recovery of very dilute acetic acid and its higher value homologues could be justised by adding the savings in effluent treatment to the value of the acid recovered. The process proposed in this article incorporates wellknown principles of ion exchange, as are comprehensively described by Hemerich (1962). A very dilute waste stream from a petrochemical complex containing 0.9% total organic acids was considered as a typical example. A weakly basic ion exchange resin in the freebase form could be used to extract acetic and related acids. The distribution coefficient is extremely high, since the extraction is based on the neutralization of a base by an acid. The ion exchanger is totally insoluble: there is no release of toxic solvents to cause problems further downstream. This achieves the objectives of previous workers, summarized by Althouse and Tavlarides (1992). The recovery of the acids from the resin is simply achieved by raising the pH with a suspension of lime in a fluidized or agitated bed. The lime suspension dissolves as reaction proceeds, forming a solution of the highly soluble calcium acetate in the range 15-25%. A similar technique was used by Hendry (1982)in a partial demineralizing process for effluents. The logical development of the work of Althouse and Tavlarides (1992)is to use a solid adsorbent for the recovery of acetic acid, which is then eluted with lime slurry. This is the basis of an article by Reisinger and King (1995),which has been drawn to our attention (King, 1994). There are considerable similarities between some of their results and the work reported here. A typical effluent could contain a mixture of organic acids as well as non-acids. Any non-acids adsorbed could be removed from the resin by washing with water. The acids could then be eluted from the resin by raising the pH. The subsequent separation of acids could be carried out using distillations similar to those used in the manufacture of acetic acid by the oxidation of hydrocarbons. Effluent streams containing significant amounts of suspended solids would be more easily handled using
* Address
correspondence
to this
author.
E-mail:
fcloet&maties.sun.ac.za. +
Present address: Process Development, SASTECH Ltd.,
P.O.Box 1,Sasolburg, 9570 South Africa. 0888-5886/96/2634-2464$09.00/0
ion exchange in a fluidized or agitated bed contactor. Such techniques have been used in the recovery of uranium from ore slurries even up to 40% suspended solids (Menitt, 1971;Cloete, 1981). This project consited of preliminary studies of the behavior of very dilute pure acetic acid when taken up by various weak base ion exchange resins and eluted with a slurry of lime. The promising results of this work were also briefly verified using samples of a typical effluent from a petrochemical complex.
Experimental Procedures and Results Standardization of Resins. The ion exchange resins considered most likely to function were weak base (Duolite A-368 and Duolite A-378)and intermediate base (Duolite A-379, from the Rohm and Haas range. The capacities of the resins were determined by loading to saturation by passing excess 1M hydrochloric acid through a bed of resin in a filter tube of 20 mm diameter, washing with distilled water, and eluting with 1 M sodium hydroxide. The flow rates used were about 4 bed volumes per hour and ambient temperature was 25 "C. Analytical quality reagents were used. The quantity of chloride eluted was measured by titration of the total eluate with silver nitrate, after adjusting the pH to between 7 and 10 with nitric acid. Potassium chromate was used as indicator. The capacity of each resin waa calculated on the volume of resin measured in the free-base form under water in a measuring cylinder. The results are given in Table 1 and show that the measured capacities were generally below the nominal values quoted by the manufacturer. More information appears in a thesis by Marais (1994). Preliminary Tests of Resins for Suitability in the Proposed Process. Samples of 125 mL of resins in the he-base form were measured under water and added to measured volumes of 1L of 0.15 M acetic acid (about 1%weight), made up from analytical quality glacial acid. The mixtures were tumbled in sealed glass jars on rollers for 2 h to allow the adsorption reaction to take place. Ekperimenta had shown that the reaction was complete after 1 h within the accuracy of the measurements. The resin was then filtered off and washed using a Buchner filter. The combined filtrate and washings were titrated with standardized sodium hydroxide using phenolphthalein indicator. A mass balance gave the amount of acetic acid adsorbed by the resin from the feed solution. The washed resin was transferred to another glass jar followed by a lime slurry, using a 10%stoichiometric 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995 2465 Table 1. Ion Exchange Resins Useda type of resin pore type functional groups matrix bead density (free base) nominal capacity measured capacity
Duolite A-375
Duolite A-368
Duolite A-378
macroporous tertiary amine polyacrylic 1.07 g/L 1.6 equivL 1.40 equivL
macroporous 90%secondary amines polystyrene-DVB 1.03 g L 1.7 equivb 1.58 equivL
macroporous 85%secondary amines polystyrene-DVB 1.05 g L 1.3 equivL 1.39 equivL
Manufactured by Rohm and Haas, supplied by ACWNCP Ltd., Germiston, South Africa.
Table 2. Recovery of Acetate as Calcium Acetate Crystals
type of resin
loading of acetate on resin (equivh)
av overall acetate recovery (%)
std dev of recovery values (%)
Duolite A-375 Duolite A-368 Duolite A-378
1.12 0.93 0.90
93 75 78
1 1 1
a Averages from eight cycles of adsorption by and elution from resin sample.
excess of powdered chemically pure calcium hydroxide, above the amount of acetic acid taken up by the resin. The jar was sealed and tumbled on rollers overnight to achieve complete elution. The resin was filtered off from the reaction mixture and washed. The liquid product consisted of a solution of calcium acetate, which was evaporated to dryness in an oven at 120 "C and weighed as Ca(CH$00)2. This weight was used to calculate the overall recovery of acetate achieved by the process and the loading of acetate recovered from the resin. The samples of resin were each recycled through eight cycles of absorption and elution to give an average percentage recovery, as reported in Table 2. The relative standard deviation in overall recoveries was 1%, which indicates good repeatability of the experimental procedures. Full details are given by Marais (1994). The best recovery was obtained from Duolite-375, based on tertiary amine functional groups, which is more strongly basic than the other two resins. The higher basicity accounts for the superior recovery of acetic acid on this resin. Details of Table 2, from Marais (19941, show that 98.5%of the acetic acid was taken up from solution by Duolite A-375 and 93% of the original acid present in solution was recovered as calcium acetate crystals. Comparative values for Duolite A-368 were 82.5%and 78%and for Duolite A-378 were 81%and 75%. Values for the elution of acetic acid from Amberlite IRA-35 at 60 "C with dolomitic lime slurry were obtained by Reisinger and King (1995) which correspond to those obtained here for Duolite A-375. These recovery values were merely the results of an arbitrary comparative test of the three resins with a stoichiometric excess of resin in each case (15%excess resin for A-375 and A-378 and 30% excess for A-368). They were not intended as a realistic process simulation experiment. Preliminary Tests on an Actual Effluent Using Duolite A-375 Resin. The effect of a typical effluent on the total ion exchange capacity of Duolite A-375 was tested using a sample which contained a mixture of acetic, propionic, butyric, and valeric acids and their isomers and other non-acids. The total initial strength of the acids was 0.21 N, and acetic acid comprized about 70% of the acids.
A sample of 100 mL of Duolite A-375 resin was put through eight cycles of adsorption of acids from effluent and elution with lime, followed by continuous tumbling in a large excess of effluent for 4 weeks, giving about 1000 h of total exposure to effluent. The various acids were recovered in the same proportions as those occurring in the effluent. After elution finally with a strong base (NaOH), the capacity of the resin was measured at 1.36 equivL, which was still 97% of the value measured initially for that resin, 1.4 equivL, as given in Table 1. Conversion of a Solution of Calcium Acetate to Acetic Acid. In some applications, where there is no market for calcium acetate as a deicing salt, the recovered acetate will have to be processed further at the cost of at least a stoichiometric amount of sulfuric acid. The conversion of the solution of calcium acetate eluted from the resin with lime to a solution of acetic acid was simply achieved by mixing with 98% sulfuric acid. The overall reaction of sulfuric acid with calcium acetate gives a precipitate of gypsum and a solution of acetic acid. The precipitate of white gypsum crystals formed was readily filtered on a Buchner filter and was washed with distilled water to recover acetic acid. The moisture content of the final filter cake was about 38%, and this accounted for a loss of about 5% of the acetic acid recovered. The specific resistance of the cake formed was low ( r = 11.4 m-2). The concentrations of acetic acid solutions finally produced, incorporating the cake washings, were in the range 15-25%. The calcium content of this solution was measured at 300 ppm, while sulfate was estimated at about 200 ppm from the solubility product. Equilibrium Data for Acetic Acid on Duolite A-375 Resin. Quantitative data on equilibria between dilute solutions of acetic acid and Duolite A-375 ion exchange resin were obtained for use in design studies. A solution of 0.15 M acetic acid was made up from analytical quality glacial acetic acid: various measured volumes of this feed solution were tumbled for 24 h in sealed glass jars with samples of 125 mL of resin in the free-base form. Ambient temperature was 25 "C. After this process, the contents were filtered and the clear filtrate was titrated with standardized NaOH solution as described above. The loading of acetic acid on the resin was calculated from the fall in concentration of the acetic acid. This loading was then taken as being in equilibrium with the final concentration of acetic acid in solution. A range of values was obtained by varying the quantities of acid initially used in the reaction. The data in Figure 1 show a sharp step function, indicative of strong adsorption of acetic acid, until a
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