Econoimcs of Acetic Acid Concentration Produced From Partial Oxidation of Ethane PDF
Econoimcs of Acetic Acid Concentration Produced From Partial Oxidation of Ethane PDF
Econoimcs of Acetic Acid Concentration Produced From Partial Oxidation of Ethane PDF
A. H. Fakeeha, M. A. Soliman
Department of Chemical Engineering ,College of Engineering, King Saud University
ABSTRACT
In the process of ethane partial oxidation to acetic acid, hot gases coming out the
reactor contains acetic acid vapors that need to be recovered and concentrated.
This acid is first scrubbed and then concentrated in an azeotropic distillation
system. In this paper we will study the economics of acetic acid concentration
using ethyl acetate to extract acetic acid. The simulation package PROCESS of
Simulation Sciences is used to carry out this study. Different configurations and
design parameters are studied. The study reveals that acetic acid concentration
from the scrubber should be as high as possible to reduce the cost of acetic acid
concentration.
KEYWORDS: Acetic Acid, Azeotropic distillation , Simulation package, Ethane oxidation
INTRODUCTION
A number of investigators have been studying the process of partial oxidation of
ethane into ethylene and acetic acid. Thorsteinson et al. [1] studied the oxidative
dehydrogenation reaction of ethane over a catalyst composed of mixed oxides of
molybdenum, vanadium, and niobium on which ethylene could be produced at a
temperature as low as 215 C. As the pressure increases, acetic acid is produced
in addition to ethylene. Burch and Swarnakar [2], Desponds et al. [3] continued
the studies on the MoVNb catalyst whereas Marzouki et al., [4] studied in
addition to the MoVNb system, another catalyst containing oxides of vanadium
and phosphorous. Fakeeha, et.al [5] simulated the oxidative dehydrogenation
reaction of ethane in a fixed bed reactor using a MoVNb catalyst.
The recovery and concentration of acetic acid from gaseous or aqueous streams
are industrially important, e.g., in the manufacture of cellulose acetate and vinyl
acetate.Ordinary binary distillation of acetic acid water solution, though possible,
is very expensive because it requires a column with many trays (over one
hundred) and a high reflux ratio, leading to high energy costs.
There are other processes for the recovery of acetic acid from water including
liquid-liquid extraction, azeotropic distillation, and extractive distillation.
Eaglesfield et al. [6] have carried out an economic study for acetic acid
concentration. They concluded that extraction with ethyl acetate is most
economic for a feed below 16% (by weight) whereas ethyl acetate-benzene mix
is most economic for feeds between 16 and 25% w/w acetic acid. Extraction with
isopropyl acetate is most economic for a concentration between 25-35%.
Azeotropic distillation is preferred for more concentrated feeds. Between 50 and
70% w/w acetic acid, extractive distillation could be used.
Brown [7] reached a different conclusion. He determined extraction to be the
most favorable approach except for feeds above 80% acetic acid, in which case
azeotropic distillation became preferable.
There are many choices for the extractant and the entrainer with different
performance depending on acid concentration, extractant boiling point,
distribution factor, low solubility in water, inreactivity with acetic acid, toxicity,
stability and price. They include, among others, ethyl acetate, butyl acetate,
isoamyl acetate, benzene, diethyl ketone, methyl isobutyl ketone, diethyl ether,
di-isopropyl ether, and MTBE.
King [8] reviewed the subject of acetic acid concentration by extraction. He
indicated the recent trend of using chemically complexing extractants which,
though more expensive, have higher distribution coefficient. This reduces the
solvent extraction rate and thus extraction cost.
Hostrup et al. [9] presented an approach for the design of environmentally benign
processes. They applied the technique to the selection of a solvent for acetic acid
removal from water. The best solvents, according to their criteria are di-isobutyl
ketone, and heptyl acetate.
In this paper, we will study the recovery of the acetic acid from the gas mixture
coming out of the reaction. The gas is flowing at the rate of 6233 Kmol/h, at a
pressure of 20 atm and temperature of 270 C with the following molar
composition, 4.6% acetic acid, 19.4% water vapour, 1.4% carbon dioxide, 2.6%
carbon monoxide, 66% ethane and 6% ethylene. This corresponds to annual
production of acetic acid of 136 kt (300 million pounds).
The gases are first cooled, contacted with water in a scrubber to condense acetic
acid and water. The acetic solution is then delivered to the extraction system
shown in Fig. (1).
2.
3.
ii)
iii)
iv)
The following formulae are used for calculating capital investment and
production cost (Peters and Timmerhaus, 1981 [10]).
Fixed capital investment = 4.83 * total purchased cost * (Saudi Arabia
location factor = 1.2)
Annual production cost = utilities cost + 0.369 * fixed capital investment.
RESULTS
Some preliminary simulation results are carried out for the direct introduction of
the gas mixture at 270 C to the scrubber [11] , Table I (cases I,II) . The number
of trays in the scrubber is changed. The optimum number of trays is about
twenty trays. Lower number of trays required the use of more water. This
dilutes the acid and causes the acetic acid concentration step to be more
expensive. The concentration of the acid goes from about 45% to 20% by weight
in the scrubber. With this number of trays for the scrubber the number of trays in
the extraction is changed to obtain the optimum number of trays in the extractor.
This comes to be about twenty trays bringing the acid concentration to about
62% (ethyl acetate free basis).
Then we studied the effect of cooling the gas mixture before it is introduced in
the scrubber. This turns out to be more economic. The results of these
calculations are shown in Table I(cases III, IV).
.
Fixed Capital
Investment
(Millions)
Utilities
(Million/Year)
Production Cost
($/Kg)
47.40
4.3
0.175
II
39.00
2.9
0.138
III
44.25
3.12
0.147
IV
34.00
2.04
0.121
28.05
0.8
0.085
Case I:
Finally we consider the case of condensing the acetic acid in a heat exchanger to
44 C. A very small percentage of acetic acid leaves with the gases and this is
scrubbed in a four trays scrubber. As shown in Table I (Case V ), this is the most
economic case. The main reason is that acetic acid is now introduced to the
concentration section at about 48% by weight concentration. This shows the
importance of introducing the acetic acid solution at the highest percentage
possible to the acetic acid concentration section.
From this study and that of Brown [7], we established the following formulae for
the calculation of the acetic acid concentration section purchased cost and
utilities.
Acetic Acid Concentration Plant Purchased Cost (2007, Saudi Arabia)
= 60 * 106 / X0.7
Utilities:
Electricity =
Steam =
50
Kwt.hr
X 1000 kg Acetic Acid
60
Kg Steam
X Kg Acetic Acid
Cooling Water =
2400
Kg Water
X Kg Acetic Acid
CONCLUDING REMARKS
The concentration of acetic solution requires a considerable amount of energy
even when direct distillation is not used. Using modern concepts in process
engineering could be of a help to reduce energy requirements. QVF company
[www.qvf.com] developed a modification which they claim to reduce the energy
required by 40%. In this process a side stream is withdrawn near the top of the
distillation column of the extracted phase and is guided back to the extraction
tower so as to achieve maximum acetic acid extraction.
MTBE should also be seriously considered as an extractant since it is locally
produced and has high distribution factor.
REFERENCES
[1]
Thorsteinson , E.M., Wilson, T.P., Young, F.G., and Kasai, P.H., 1978,
"The Oxidative-Dehydrogenation of ethane", Journal of Catalysis, 52,
116-132.
[2]
[3]
Desponds, O., Keiskie, R.L., and Somorjai, G.A., 1993, "The oxidative
dehydrogenation of ethane over molybdenum-vanadium-nobium oxide
catalyst ", Catalysis Letters, 19, 17-32.
[4]
Marzouki, M., Taouk, B., Tessier, L., Bordes, E. and Courtine, P., 1992,
"Correlation between Catalytic and Structural Properties of Modified
Molybdenum and Vanadium Oxides in the Oxidation of Ethane in Acetic
Acid on Ethylene", Proceedings of the 10th International Congress on
Catalysis, Budapest. Hungary, July , pp. 112-115.
Eaglesfield, P., Kelly B.K., and Short, J.F., 1953, "Recovery of Acetic
Acid from Dilute Aqueous Solution by Liquid-Liquid Extraction Part I",
Ind. Chemist, 29, 147-151 .
[7]
[8]
[9]
[10]
[11]