Applied Catalysis B: Environmental 40 (2003) 9399

A new method to clean industrial water fromacetic acid via esterification

C.L. Bianchi a,, V. Ragaini a, C. Pirola a, G. Carvoli b,1a Department of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, I-20133 Milano, Italy

b La Chemial SpA, Cavagli (BI), ItalyReceived 7 October 2001; received in revised form 1 June 2002; accepted 1 June 2002

Abstract

The valorisation of very low concentration of acetic acid (6%, w/w) was investigated by reacting with n-butanol and2-ethyl-1-hexanol taking advantage of the different solubilities of acetic acid and acetic ester in water. The esterification of verydiluted solution of acetic acid with alcohol is a reversible reaction and the conversion is greatly restricted by equilibrium limi-tation. Therefore, the peculiarity of the present method is to shift the reaction equilibrium towards the ester and not towards thereagents, thermodynamically favourite, because of the large amount of water. Different acid catalysts were tested in both homo-geneous (sulphuric acid) and heterogeneous phase (polymeric resins and sulphated zirconia) to optimise the process efficiency. 2003 Elsevier Science B.V. All rights reserved.

Keywords: Acetic acid recovery; Water purification; Esterification reaction

1. Introduction

Aqueous solutions of acetic acid are produced asby-products of many important processes, such asin the production of cellulose esters, terephthalicacid and dimethyl terephthalate. Moreover, reactionsinvolving acetic anhydride either as a reagent (e.g.acetylations) or as a solvent (e.g. nitrations) can pro-duce a large amount of acetic acid containing waste.Among the industrially relevant examples, the pro-cess for the manufacture of cellulose acetate fromacetylation of cellulose by acetic acid, acetic anhy-dride and sulphuric acid, is typically associated with

Corresponding author. Tel.: +39-2-50314253;fax: +39-2-50314300.E-mail address: claudia.bianchi@unimi.it (C.L. Bianchi).

1 Present address: Oxon SpA, Via Sempione 195, 20016 Pero,Italy.

35% (w/w) aqueous solution of acetic acid as a wastestream. The process for the synthesis of glyoxal fromacetaldehyde and nitric acid has a relatively diluteacetic acid stream (1320%, w/w) as a by-product[1]. Acetic acid is also a product in the destructivedistillation of wood (18%, w/w).

Moreover, it is necessary to remember the impor-tance of such an acid in the food industries: vinegar,for example is a sour liquid consisting mainly of aceticacid and water, produced by the action of bacteria ondilute solutions of ethyl alcohol derived from previ-ous yeast fermentation. In these cases, very dilute acidstreams are produced, due to many rinses of the de-vices during the production cycles.

Therefore, the recovery of acetic acid is a large prob-lem not always solved because of economical reasons.In fact, the conventional physical separation methodssuch as distillation and extraction suffer from sev-eral drawbacks. Distillation is generally uneconomic

0926-3373/03/$ see front matter 2003 Elsevier Science B.V. All rights reserved.PII: S0 9 2 6 -3373 (02 )00144 -3

94 C.L. Bianchi et al. / Applied Catalysis B: Environmental 40 (2003) 9399

because of the high costs involving in vaporising wa-ter, that is the most volatile compound and is present invery large proportion but possesses a large latent heatof vaporisation. Extraction is limited by phase separa-tion and distribution of the components involved in thereacting system. These are the reasons why acetic acidis often very much diluted in water and discharged insewer.

In our technology, water containing acetic acid issimply purified by means of an esterification. Aceticacid reacts with an alcohol in the presence of an acidcatalyst to give the corresponding ester. The same ideawas exploited by Saha et al. [2] who purified a streamof contaminated water by an esterification reaction.However, in that case the reaction was performed usinga more concentrated solution of acetic acid (30%) bymeans of a reactive distillation column.

In the present paper, the recovery of very low con-centration of acetic acid (6%, w/w) was investigatedby reacting with n-butyl or 2-ethyl-1-hexyl alcohol ina simple glass reactor taking advantage of the differ-ent solubilities of acetic acid and acetic ester in water.

n-Butyl acetate is primarily used as a solvent forcoatings, in the photographic industry, as a reactionmedium for adhesives, as a solvent for leather dress-ings, as extraction solvent and as process solvent invarious applications and in cosmetic formulations.2-Ethyl-1-hexyl acetate has a good market value as araw material in cosmetic formulations for sunscreencreams and anti-ageing creams.

The peculiarity of this recovery method is the pres-ence of large amount of water at the beginning ofthe reaction, and thus, the ability to shift the reactionequilibrium towards the ester (to the right) and not to-wards the reagent (thermodynamically favourite). Dif-ferent acid catalysts were tested in both homogeneous(sulphuric acid) and heterogeneous phase (polymericresins and sulphated zirconia) to improve the processrate towards the equilibrium efficiency.

2. Experimental

2.1. Materials and catalysts

All the chemicals used in the present paper are Flukaproducts: acetic acid, n-butanol and 2-ethyl-1-hexanol,sulphuric acid (96%), Amberlist 200 and Amberlist 15

(both macroreticular-strongly acidic cation exchang-ers), Amberlist IR120 (a microreticular-strongly acidiccation exchanger), Nafion NR50 (a super-acidic per-fluorinated resin). Sulphated zirconia was prepared asdescribed in [3].

2.2. Experimental procedure and apparatus

Two kinds of experimental procedures were per-formed: (a) runs at complete evaporation condensa-tion; (b) runs with distillation and reaction productremoval. In the case (a), all the evaporated specieswere condensed and thus directly re-introduced intothe reactor, avoiding the removal of any chemical dur-ing the reaction. By working with the procedure (b),the ester was continuously removed from the reactionmixture to allow the shift of the reaction equilibriumtowards the right by distillation of the organic phase(alcohol and ester).

All the tests were performed in a glass reactor pro-vided with breakwater, a mechanical stirrer (300 rpm),a thermometer and a coil condenser for equipment (a)(see Fig. 1a) or a Markuson head (distillation columncoupled with an azeotropic head) for equipment (b)(Fig. 1b).

Standard conditions: 400 ml of bi-distilled waterwas contaminated with acetic acid (6%, w/w) and200 ml of the selected alcohol was added. The reac-tion was performed at a constant temperature of 99 Cadding the correct amount of catalyst when the stirredsolution reached the right temperature.

All the experimental data reported in the paper areexpressed in terms of process efficiency (E%) calcu-lated as following:

E% =[

molAcOHin molAcOHfinmolAcOHin

100]

aq

where molAcOH denotes the acetic acid molespresent, in the aqueous phase (aq), at the beginning(in) and at the end (fin) of reaction.

2.3. Analysis

To follow the reaction product and also to calculatethe efficiency of the process to produce ester, a GC(ThermoQuest) equipped with a capillary SE52 col-umn ( = 0.53 mm, L = 25 m, T=150 C) was used

C.L. Bianchi et al. / Applied Catalysis B: Environmental 40 (2003) 9399 95

Fig. 1. Scheme of the used equipments. For both configurations:(1) mechanical stirrer, (3) withdrawal from both phases, (4) ther-mometer. For procedure (a): (2) coil condenser; for procedure (b):(2) Markuson head (distillation column coupled with an azeotropichead).

to analyse acetic acid, n-butyl acetate, n-butyl alcohol.The amount of acetic acid was also cross-checked bytitration with NaOH 0.01 M using phenolphthalein asindicator.

In the reaction with a large amount of n-butylalcohol or 2-ethylhexyl alcohol, two phases were ob-served inside the reactor. The analyses of the upperorganic phase (n-butyl alcohol, n-butyl acetate and2-ethylhexyl acetate, 2-ethylhexyl alcohol, respec-tively) were carried out by gas chromatography. Theamount of water in this organic phase was performedby KarlFischer instrument.

The presence of acidity in the aqueous phase (lowerphase) was monitored by subsequent titration withNaOH 0.01 M using phenolphthalein as indicator.

3. Results and discussion

The esterification of acetic acid with an alcohol is athree-step chemical reaction and all the three steps areequilibrium limited reactions. The overall reaction is:

CH3COOH+ ROH CH3COOR + H2Owhere

R = CH2CH2CH2CH3 orR = CH2CH(C2H5)CH2CH2CH2CH3Therefore, the esterification of 6% aqueous solutionof acetic acid with alcohol is obviously a reversiblereaction and the conversion is greatly restricted byequilibrium limitation.

Just to confirm this limitation, a first run withn-butanol was performed taking advantage of the sol-ubility of this alcohol in water (Table 1). The reactorwas filled with 400 ml water, acetic acid (6%, w/w),n-butanol (6%, w/w) and Amberlist 200 as an het-erogeneous catalyst and temperature raised to 99 C(procedure (a)). A single phase was present insidethe reactor. The process efficiency after 6 h was only3.4%.

By increasing the amount of n-butanol present in thereactor, a double phase was immediately generated.Therefore, a second run was performed at the sametemperature as above with the same amount of waterand catalyst, but 200 ml of alcohol was loaded. Theprocess efficiency after 6 h was 67.9%.

96 C.L. Bianchi et al. / Applied Catalysis B: Environmental 40 (2003) 9399

Table 1Solubility data

Compound Formula T (C) Sa (mass%) Sb (mass%)n-Butanol C4H10O 25 6.4 [4]Butyl acetate C6H12O2 25 0.68 [4] 1.2 [5]2-Ethyl-1-hexanol C8H18O 25 0.01 [4]2-Ethylhexyl acetate C10H20O2 25 0.55 [5]

C.L. Bianchi et al. / Applied Catalysis B: Environmental 40 (2003) 9399 97

Fig. 2. (a) Process efficiency vs. reaction time for reaction performed with procedure (a) (2-ethyl-1-hexanol as upper phase, 6% AcOH ()or 1.2% AcOH (), H2SO4 as acid catalyst). (b) () Ester, () acetic acid (6% at the beginning of the run) and () 2-ethyl-1-hexanoltrend vs. reaction time.

towards the right with even greater efficacy (proce-dure (b)). All these kinds of runs were performed us-ing 2-ethyl-1-hexanol and sulphuric acid as an acidcatalyst to avoid all the problem due to the decreaseof the process efficiency using a solid catalyst.

The acetic acid removal is complete after 55 hof reaction as shown in Fig. 3. The main disadvan-tage of this method is due to the co-presence of twoazeotropic mixtures (alcohol/water, water/ester) witha boiling point lower than both the pure alcohol andester (Table 3). This fact causes the removal not onlyof the reaction product (the ester), but also of thealcohol and of the water. For this reason, the distil-lation column was coupled with an azeotropic head

to separate the organic phase from the aqueous one(Fig. 1b). The removal of the organic phase and notof the aqueous one was decided for two main reasons:first of all the water acts as an azeotropic mixtureagent to the ester and therefore its removal wouldprevent the distillation of this component; secondly,the presence of the only alcoholic phase into thereactor may cause a bad dispersion of the catalyst,especially when it is solid, due to the alcohol highviscosity.

Therefore, the quantity of alcohol into the reactionmixture tends to decrease and for the future it wouldbe necessary to plan the possibility of its continuousaddition through the whole reaction.

98 C.L. Bianchi et al. / Applied Catalysis B: Environmental 40 (2003) 9399

Fig. 3. Process efficiency vs. reaction time coupling the classical reaction to the distillation of the evaporated phase (procedure (b), H2SO4as acid catalyst, 2-ethylhexyl alcohol as upper phase).

Table 3Thermodynamic data

Compound bp ( C) A (wt.%) B (wt.%) C (wt.%)BuOH 117.7 100 2-Et-esOH 183.5 100 BuOAc 126.2 100 2-Et-esOAc 198.6 100 H2O/BuOH [6] 92.7 42.5 57.5 H2O/BuOAc [6] 90.2 28.7 71.3 H2O/BuOH/BuOAc [6] 90.7 29.0 8.0 63.0H2O/2-Et-esOH [6] 99.1 80.0 20.0 H2O/2-Et-esAc [6] 99.0 73.5 26.5 H2O/2-EtesOH/2-Et-esAc [6] 99.0 74.0 10.0 16.0

4. Conclusions

The valorisation of very low concentration of aceticacid (6%, w/w) was investigated by reacting withn-butanol or 2-ethyl-1-hexanol taking advantage ofthe different solubilities of water with the acetic acidand with the acetic ester.

The reaction is equilibrium limited. The peculiar-ity of the method is the presence of large amount ofwater at the beginning of the reaction, and thus, theability to shift the reaction equilibrium towards the es-ter and not towards the reagent (thermodynamicallyfavourite).

Two kind of experimental procedures were per-formed: (a) runs at complete evaporation condensa-tion; (b) runs with distillation and reaction productremoval. In this latter configuration, a complete re-moval of acetic acid was achieved.

References

[1] I. Kroschwitz (Ed.), Kirk-Othmer Encyclopedia of ChemicalTechnology, 4th Edition, Wiley, New York, 1991, pp. 124178.

[2] B. Saha, S.P. Chopade, S.M. Mahajani, Catal. Today 60 (2000)147.

C.L. Bianchi et al. / Applied Catalysis B: Environmental 40 (2003) 9399 99

[3] S. Ardizzone, C.L. Bianchi, M. Signoretto, Appl. Surf. Sci.136 (1998) 213.

[4] D.R. Lide, H.P.R. Frederiske (Eds.), Handbook of Chemistryand Physics, 78th Edition, CRC Press, New York, 1997,p. B-260.

[5] H. Stephen, T. Stephen (Eds.), Solubilities of Inorganic andOrganic Compounds, Pergamon Press, Oxford, 1963, p. 1799.

[6] L.H. Horsley, in: R.F. Gould (Ed.), Azeotropic Data III,Advances in Chemistry Series 116, ACS, Washington, 1973,pp. 1339, p. 469.

A new method to clean industrial water from acetic acid via esterificationIntroductionExperimentalMaterials and catalystsExperimental procedure and apparatusAnalysis

Results and discussionConclusionsReferences