Short Communication

Recovery of acetic acid and propionic acid from aqueous waste stream

M. N. Ingale and V. V. Mahajani

Chemical Engineering Division, Department of Chemical Technology, University of Bombay, Matunga, Bombay, India

Keywords: acetic acid; propionic acid; recovery; waste stream; tributylphosphate

Introduction

Recovery of low-molecular-weight acids, namely, acetic acid and propionic acid, is of considerable commercial interest even from an environmental engineering view- point. Tertiary amines (trioctylamine), trioctylphosphine oxide and tributyl phosphate (TBP) have recently been found to be superior to conventional solvents such as ketones, aldehydes, and alcohols.

While using a system for extraction, the process engi- neer must ensure that the extractant and diluent (if any) do not contribute to chemical oxygen demand of the aqueous stream. Amine-base extractant may pose prob- lem for the biological degradation process. By and large, the waste streams from chemical process plants are likely to have inorganic salts such as Na,SO, or NaCl, and therefore it is essential for process design to have a prior knowledge of how salt affects distribution coefficient. There exists scant information in the published literature on recovery of acetic acid and propionic acid at very low concentrations from the aqueous stream containing salts such as NaCl and Na$O,. Wardell and King have mea- sured effectiveness of phosphoryl compounds as extracting agents and found that the equilibrium distribu- tion, K,, increases in the order phosphate (TBP) C phos- phonate (dibutyl butyl phosphonate) < phosphine oxide (tributyl phosphine oxide, TBPO). Considering the ease of availability and the price, TBP may be preferred over other phosphoryl compounds. Further, TBP has very low solubility in water (0.039 wt %). It was therefore thought desirable to study recovery of acetic acid and propionic acid from aqueous stream with and without Na,SOflaCl using pure TBP as an extractant.

In the present investigation the effect of concentrations of acids on KD was studied. Also, the effect of mixed acids on KD was studied. Further, variation in KD in the presence of Na2S04 and NaCl was measured.

Address reprint requests to Dr. V. V. Mahajani at the Chemical Engineering Division, Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India. Received 9 September 1993; accepted 28 October 1993

0 1994 Butterworth-Heinemann

Experimental findings

Reagents

All the chemicals (acetic acid, propionic acid, NaCl and Na,SO,) used were of reagent grade. Tributylphosphate, having purity of 99% was used as an extractant.

To find out the distribution coefficients, 25 mL of an aqueous solution of acid (acetic acid or propionic acid, as the case may be) was contacted with 25 mL of solvent in a 250-mL-capacity stoppered flask using a magnetic stirrer in a thermostated bath maintained at temperature 30C. The mixture was stirred for 3 h. This time was sufficient to establish the equilibrium between two phases. The mixture was then transferred in a separating funnel and allowed to stand for 2 h. The two phases, aqueous (rafftnate) and organic, were separated and ana- lyzed for acid content. In general, only the aqueous phase was analyzed for its acid concentration. The organic phase acid concentration was obtained from material balance. However, the organic phase was checked periodically for its acid content. The distribution coefficients for mixed acid system, namely, aqueous solution containing known concentration of acetic acid and propionic acid in 1: 1 proportion was measured. To see the effect of salt present in aqueous solution of acid, on the distribution coefficient, separate extraction experiments were carried out. For this a known amount of salt (Na,SO, or NaCl, as the case may be) was added to the aqueous acid solution. The amount of Na,SO, and NaCl was added in such a way that the ionic strength of resulting solution was the same. The ionic strength of salt used was varied from 1.05-4.22 kg ion/m3. Concentration of acids in aqueous phase was determined by using gas chromatograph (GC) technique. Both internal and external standard techniques were employed for the analysis of acids. Acetic acid was used as internal standard for the analysis of propionic acid and vice versa. For the mixed acid system, only the external standard method was used for the analysis of acetic acid and propionic acid. Carbopack BDA 4% carbowax 2 M @O/100 mesh) column in FID mode of operation was used to analyze acids on GC.

Sep. Technol., 1994, vol. 4, April 123

Recovery of acetic and propionic acid: M. N. lngale and

90

70 - S

3 z 50

F

x 2 30 a t

10

-10 1350 1300 1250 1200

WAVENUMBER Cd 1

Figure 1 FT i.r. spectra of TBP (---), acetic acid: TBP (---) and propionic acid: TBP (--). TBP = tributyl phosphate.

Results and discussions

Tributylphosphate is Lewis base and as a result can form complex with organic acids (acetic and propionic acid). The FT i.r. spectra (Bruker IFS-88 model) as indicated in Figure 1 do substantiate complex formation.

The extraction of monocarboxylic acids by strong sol- vating extractants such as the organophosphorus com- pounds, TBP, being a heterogeneous process involves several ionic reactions. The various steps involved in the extraction equilibria have been well reviewed by Kertest and King.

However, for the ease of utility to process environmen- tal engineers we defined the overall distribution coefft- cient, K,, as

KD=E] 08 aq

(1)

where [HA!, is total (analytical) concentrations of the acid in all its possible existing forms (dissociated and undissociated acid) in aqueous phase, and [HA], is the total concentration of acid in all its possible existing forms (acid as such, dimer acid and acid-TBP complex) in the organic phase.

System: Acetic acid-water-TBP

For aqueous phase concentration less than 1,000 mg/L of acetic acid (total concentration and not equilibrium

K K Mahajani

concentration), KD increases from 1.77 to 3.74 when total acid concentration increases from 250 to 1,000 mg/L and then falls to average value of 1.72 (Figure 2). This value agrees well with KD = 1.73 reported by Page1 and McLaf- ferty,6 and Page1 and Schwab at 30C.

System: Propionic acid-water-TBP

Like acetic acid in case of propionic acid, KD increases steadily for acid concentration from 250 mg/L to 2,000 mg/L from 4.55 to 12.33 and then remains steady at 12.78, for total propionic acid concentration > 2,000 mg/ L (Figure 2). This value also agrees well with that reported by Fahim et al8 (KD = 12.40 at 25C).

System: Acids-water-salt-TBP

Industrial waste stream may contain salts such as Na2S04 or NaCl. Therefore, the effect of salts on KD was studied. At acetic acid concentrations of 500 mg/L, KD decreases as ionic strength is increased from 1.056 to 4.22 Kg ion/ m3. From Table 1, all along it is seen that NaCl has a lower effect on KD as compared with Na$O.,.

At 1,000 ppm acid or more, KD values are observed to be higher in the case of NaCl compared with those obtained with Na$O+ The effect of NaS04 and NaCl on KD in the case of propionic acid is exhibited in Table 2. It is seen that for propionic acid concentration 2,000 mg/ L or more, presence of NaCl resulted in considerable enhancement in KD values. Figure 3 shows effect of ionic strength of Na2S0, and NaCl on distribution coefficient of acetic acid and propionic acid at concentration of 3,000 mg/L. Mixed acids equilibria have been presented in Fig- ure 4. The presence of propionic acid resulted in higher KD value for acetic acid (almost by a factor of 2). However, KD value for propionic acid is increased marginally. Because of the intrinsic higher value of KD, more propi-

Figure 2 Effect of acid concentration on distribution coefficient. AcH = acetic acid; PrH = propionic acid.

124 Sep. Technol., 1994, vol. 4, April

Recovery of acetic and propionic acid: M. N. lngale and K K Mahajani

Conclusions

1. FI-IR spectra of the acetic acid-TBP and propionic acid-TBP phase do indicate acid-TBP complex formation.

2. Ionic strength and nature of the ion have substantial

Table 1 Effect of ionic strength of salt on distribution coefficient of acetic acid at various concentrations

Acid concentration

(mg/U Ionic strength

(kg ion/m3)

Distribution coefficient, KD

Na,SO, NaCl

effect on KD for-both acids.

500 1.056 1.84 1.31 2.112 1.48 1.08 3.168 1.30 0.79 4.224 0.92 0.66

2.04 2.87 2.79 3.06 2.92 3.38 3.18 3.58

1,000 1.056 2.112 3.168 4.224

1,500 1.056 1.72 2.17 2.112 2.09 2.48 3.168 2.65 2.82 4.224 3.10 3.45

2.13 2.52

1.89 2.22 2.46 2.98

2,000 1.056 2.112 3.168 4.224

2,500 1.056

2.86 3.22 a

J 2.03 2.31 2.57

2.112 3.168 4.224 2.88

3.42 3.48 3.91 4.47

3,000 1.056 2.47 2.76 2.112 2.79 3.43 3.168 2.94 4.15 4.224 3.26 4.91

I

3750 Acid concentration. v/l

Figure 4 Mixed acids equilibrium: distribution coefficient. AcH = acetic acid; PrH = propionic acid.

Table 2 Effect of ionic strength of salt on distribution coefficient of propionic acid at various concentrations

.e Y F Acid concentration, 3000 w/l ;; u . coefficient, K. concentration Ionic strength

(mg/L) (kg ion/m3) Na,S04 NaCl .sf, -AcN+Nm SO, :

t ++mAotl+Na 1 E ..m.- Pr N+Nn SO.

: AAAAAP~P+N.~~

500 1.056 2.112 3.168 4.224

1,000 1.056

5.84 5.41 5.49 8.61 4.15 11.82 6.35 9.41

2.112 3.168 4.224

852 8.80 6.29

7.13 9.00 9.63

10.11

1,500 1.056 10.11 16.85 2.112 12.76 21.38 3.168 19.27 26.77 4.224 19.00 28.41

2,000 1.056 2.112

Figure 3 Effect of ionic strength on distribution coefficient. 3.168 AcH = acetic acid; PrH = propionic acid. 4.224

12.60 14.38 13.70 19.83 14.03 24.31 17.18 23.69

2,500 1.056 15.77 17.24 2.112 17.38 23.27 3.168 21.72 23.75 4.224 18.68 28.09

3,000 1.056 13.27 14.62 2.112 18.44 18.73 3.168 18.23 27.30 4.224 19.98 45.15

a OS . ; P .

:

21

Ionic Strength K&ion/&

onic acid goes into TBP phase and forms a complex with TBP the way acetic acid does. Further propionic acid may also be forming a complex in the TBP phase via hydrogen bonding with acetic acid. As a result, we obtain substantial increase in KD for acetic acid in the presence of propionic acid.

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Recovery of acetic and propionic acid: M. N. lngale and K Cc Mahajani

3. In the case of mixture of acids, the presence of propionic acid (higher KD) resulted in considerable enhancement in KD for acetic acid.

4.

5.

References 6.

1. Wardell, J.M. and King, C.J. Solvent equilibria for extraction of 7. carboxylic acids from water. J. Chem. Eng. Data 1978,2,144-148

2. King, C.J. Amine based systems for carboxylic acids recovery. Chemrech. 1992, 5, 285-291 8.

3. Ricker, N.L., Pittman, E.F. and King, C.J. Solvent properties of organic bases for extraction of acetic acid from water. J. Sep.

Proc. Technol. 1980, 2, 23-30 Rydberg, J., Musikas, C. and Choppin, G.R. Principles and Practices ofSolvent Extraction. New York: Marcel Dekker, 1992, PP. 32 Kertest, A.S. and King, C.J. Extraction chemistry of fermentation product carboxylic acids. Biotechnol. Bioeng. 1486.28.269-282 Pagel. H.A. and McLaffertv. F.W. Use of tributvlohosohate for extracting organic acids fromaqueous solution. A&i Chkn. 1948, 3, 272 Pagel, H.A. and Schwab, K.D. Effect of temperature on tributylphosphate extracting agent for organic acids. Anal. Chem. 1950,9, 1207-1208 Fahim, M.A., Qader, A. and Hughes, M.A. Extraction equilibria of acetic and propionic acid from dilute aqueous solution by several solvents. .I. Sep. Sci. Technol. 1992, 13, 1809-1821

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