DESALINATION
ELSEVIER
148 (2002) 267-273
Desalination
Membrane aromatic recovery system (MARS): lab bench
to industrial pilot scale
Frederic0 Castelo Ferreira”, Sheijiao Han”, Andrew Boamb, Shengfu Zhangb,
Andrew G. Livingstona-b,*
“Department of Chemical Engineering, Imperial College of Science, Technology and Medicine,
London, SW7 28x United Kingdom
“Membrane Extraction Technology Ltd,. room 437 Sherfield build, Imperial College of Science,
Technology and Medicine, SW7 2BE London, United Kingdom
Tel. +44 (207) 5945582: Fax +44 (207) 5945429; email: [email protected],uk
Received
1 February
2002; accepted 29 March 2002
Abstract
This article describes a novel process for recovery of aromatic amines and phenolic compounds form wastewaters,
the membrane aromatic recovery aromatic system (MARS). Laboratory work on wastewaters containing aniline and
phenol will be presented, including data demonstrating removal and recovery of each chemical in a sufficiently pure
form to allow recycling into a chemical production process. This article also describes successful scale-up and
operation of the process through pilot trials at Solutia, UK. Process economics are discussed and data showing the
potential for application of the process to a wide range of organic chemicals are presented.
Keywords: Aniline;
Phenol; Recovery; Membrane
separation;
1. Introduction
Phenolic compounds are used in phenolic resins,
polycarbonates,
biocides and agrochemicals.
Aromatic amines are used in a wide range of consumer products, including polyurethane foam,
*Corresponding
dyes, rubber chemicals and pharmaceuticals. The
factories that manufacture and/or use these types
of chemicals often create aqueous waste streams
containing significant (0.1-10 wt%) amounts of
aromatic amines or phenolic compounds. These
wastes are typically disposed of off site or treated
using expensive
author.
Presented at the International
July 7-12, 2002.
Wastewater treatment
Congress on Membranes
and Membrane
activated
Processes
OOI I-9164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved
PII: so0 I I-9 164(02)00709-9
carbon.
(ICOM),
Toulouse, France,
FYC.Ferreira et al. /Desalination 148 (2002) 267-273
268
Various membrane processes, for example,
pervaporation and liquid membranes, have been
proposed for treatment,
but none has found
widespread industrial application. Further references about these processes can be found elsewhere [I ,2]. Previous works have used acid-base
reaction to maintain the driving force for aromatic
acids and bases membrane extraction. Li and coworkers [3] report an emulsion liquid membrane
for phenol extraction with a caustic receiving
phase. Klein et al. [4,5] report phenol and aniline
dialyse through solid membranes to a sodium
hydroxide or a sulphuric acid solutions, respectively. These studies showed the possibility
of using acid base dissociation to extract aromatic
molecules. However, the molar concentration of
acid or base was higher than the molar concentration of anilinium or phenolate respectively in
the stripping solution, and organic recovery was
neither possible nor attempt. The membrane
aromatic recovery system (MARS) is a newly
commercialised membrane process able to recover
aromatic amines [ 1] and phenolic compounds [2],
2. MARS description
MARS is characterised by low energy consumption and simple and stable operating configurations. The MARS process is shown schematically in Fig. 1.
Wastcwatcr
out
stripped of dissolved
Amincs (phenols)
HCI
(NaOH)
NaOH
(HCI)
R-NH,+
(R-O-)
Nonporous
Mcmbranc
R-NH,
recovered
amines
(ROH
rccovcrcd
phenols)
Saline aqueous phase
Wastcwatcr containing
dissolved amincs (phenols)
Fig. 1. Schematic diagram of MARS process showing
operating principles.
2.1. Extraction stage
Aromatic molecules are extracted across a
membrane into a stripping solution where they
are converted into ionised form and concentrated.
The stripping solution is acidic for aromatic amines,
and alkaline for phenolic compounds anilinium
and phenolate (aromatic ionic forms) are total
miscible in water. The membrane-separating layer
is a nonporous elastomer material that allows
permeation of the organic species but is impermeable to ionic species and water. In the stripping
solution, an acid-base reaction takes place and the
aromatic molecule is converted into an ionised
form (anilinium chloride or sodium phenolate).
The membrane plays the key role of separating
the two aqueous solutions,
wastewater
and
stripping solution, which would otherwise mix,
and allows the pH differential to be maintained.
A 500 pm thickness silicone rubber tube, composed of 30 wt% fumed silica and 70 wt% poly
(methylsiloxane) or PDMS, was used as membrane
in this study. The membrane module consists in
tube coils immersed in a membrane tank. MARS
can be operated continuously or in batch, as it is
illustrated in Figs. 2 and 3.
2.2. Recovery stage
The stripping solution is periodically collected,
and pH is adjusted in order to recover the nonionic
form of the aromatic molecule by acid-base
reaction (i.e., alkaline conditions for amines and
acid conditions for phenolics). Anilinium and
phenolate (the ionised forms of the aromatic) have
virtually infinite solubility in water. However the
solubility of nonionic forms of aromatic amines
and phenols is usually less than 5 wt%. Hence,
when the aromatic ion (highly concentrated in the
stripping solution) is neutralized to the nonionic
form, the resulting concentration greatly exceeds
the aqueous solubility limit and yields organicrich phases, which can be re-used in chemical
manufacture. NaCl is a by-product in both cases,
but this salty underlayer can be simply recycled
EC. Ferreira et al. /Desalination 148 (2002) 267-273
Wastewater
container
Stripping
Membrane
tank with
solution
stirring and heating
Acid for
Stripping
pH control
vessel, level rises
solution
stirring
as aniline
Wastewater
transfers
and acid added
overflow
Membrane
tank wtth
and heating
outside
membrane
Fig. 2. Laboratory and pilot plant continuous extraction
configuration, with wastewater inside membrane tubes.
Fig. 3. Pilot plant batch extraction configuration,
wastewater outside membrane tubes.
to the wastewater feed as shown in Fig. 1. Furthermore, the NaCl produced has a positive effect on
the phase separation, simultaneously decreasing
the water concentration in the organic phase and
the aromatic concentration in the aqueous phase.
3.1. Laboratory scale
3. Results
The overall mass transfer coefficient
calculated using the expression:
(Kay) is
(1)
Here L is the membrane tube length, F, the wastewater flow rate inside membrane tube, ri the
membrane tube internal ratio, C,, is the aromatic
concentration at membrane tube inlet, C,,r is the
aromatic concentration at membrane tube outlet
and C”, is the nonionised aromatic concentration
in the stripping solution. C”, is calculated based
on pH, aromatic dissociation constant (Kc,) and
the total concentration of aromatic in the stripping
solution (ionised plus nonionised form). It is
recognized that Eq. (1) ignores the more complex
effects of the nearly instantaneous and reversible
reaction occurring in the film layer at the stripping/
membrane interface. However, it suffices for the
purposes of this paper.
with
Experimental work at laboratory scale has used
aniline as an example of an aromatic amine and
phenol as an example of a phenolic compound.
In aniline experiments, a synthetic wastewater
containing 5 g.L-’ aniline was continuously fed
to the process, and 10.45 wt% HCl solution was
used to control the pH in the stripping solution at
pH 1. At steady state, total aniline concentration
(anilinium and nonionic aniline) in the stripping
solution was 218.5 g.L-I, 44 times higher than the
aniline concentration
in the wastewater.
An
overall mass transfer of 4.8x 10T7m.s-’ for aniline
across a 500 l.trn silicone rubber membrane tube
was obtained at 50°C. After recovery, an organic
rich product phase containing 96.5 wt% aniline
and 3.5 wt% water was obtained.
In phenol experiments, a synthetic wastewater
containing 10 g.L-I phenol was fed to the process,
and a 12.5 wt% NaOH solution was used to control the pH of the stripping solution at 11-13. At
steady state total phenol concentration (phenol
and phenolate) in the stripping solution was
25 1 g.L-‘. After recovery, an organic rich product
phase containing 86.5 wt% phenol and 13.5 wt%
water was obtained. An overall mass transfer of
1.4x 1O-’ m.s-’ for phenol across a 500 pm silicone
rubber membrane tube was obtained at 50°C.
Fig. 4 shows inlet, outlet and total phenol concentrations in the stripping solution over time. The
EC. Ferreira et al. /Desalination
160
80
40
0
0
10
30
Tinfi
--
inlet -*
outlet
40
(day)
--stripping
solution
Fig. 4. Evolution of phenol concentrations in the inlet, outlet
and stripping solution over time.
pH effect in the driving force is shown by outlet
concentrations. At pH 11, the outlet concentration
increases with increasing phenol concentration in
the stripping solution. From calculations based
on the pKn of phenol, at pH 11,9% of the phenol
in the stripping solution is present as nonionic phenol
and 9 1% as phenolate ion. Therefore, when the
concentration in the stripping solution increases,
the nonionic phenol concentration becomes high
enough to have a significant negative effect on
148 (2002) 267-273
the driving force. At pH 13 only 0.1% of the phenol
is in the nonionic form, the driving force is restored,
and outlet concentrations drop to lower values,
as shown in Fig. 4.
3.2. Overall mass transfer coejficients for a range
of aromatic acids and bases
In order to illustrate the applicability of MARS
to recover compounds other than aniline and
phenol, the overall mass transfer coefficients of a
range of compounds across a 500~pm silicone
rubber membrane tube were measured; the results
are shown in Table 1.
Dimethylamine,
4-nitrophenol,
and 2,4,6tris(dimethylaminomethyl)phenol
exhibit low
overall mass transfer coefficients,
and their
extraction from wastewaters will be difficult using
silicone rubber tubes as membranes. Mass transfer
rates for phenol, 4-nitroaniline and hydroquinone
have intermediate values, and the extraction of
these three compounds from wastewaters
by
MARS technology is possible using silicone rubber
tubes as membranes, but it will use relatively large
membrane areas. All other compounds tested have
higher overall mass transfer coefficients, and so
they can be relatively easily removed from wastewaters by MARS technology using silicone rubber
tubes as membranes.
Table 1
Overall mass transfer for a range of aromatic acids and bases compounds across 500 pm thickness silicone rubber tubea
Compounds
Kov xl O’, m.s-’
Compounds
Kov x10’, m.s-’
Aniline
4-chloroaniline
2,4-chloroaniline
4-nitroaniline
4-fluoroaniline
2,4_fluoroaniline
Triethylamine
Dimethylamine
Benzyldimethylamine
Dicyclohexylamine
8.20
11.60
6.36
4.34
10.27
9.33
20.00
0.72
18.00
16.50
Phenol
4-chlorophenol
2,4-dichlorophenol
4-nitrophenol
4-cresol
Hydroquinone
3.10
9.30
14.70
0.48
7.04
2.37
2,4,6-tris(di-methylaminomethyl)phenol
0.64
“The stripping solution pH was kept below 1 to aromatic bases and above 13 to aromatic acids
EC. Ferreira et al. /Desalination
3.3. Pilot plant
Production of 4-nitrodiphenyl amines at Solutia
UK results in a wastewater containing around
6 g.L-’ aniline. Membrane Extraction Technology
Ltd., carried out a successful pilot trial, in which
aniline was recovered from the wastewater, Initially
the pilot plant was configured for continuous operation, in which the wastewater flowed inside the
membrane tubes and aniline was accumulated
outside the tubes, in an acidic stripping solution
(pH = 1.5). However, a solid precipitate formed
in the wastewater and blocked the tubes, interrupting
operation.
The plant was re-configured for extraction of
aniline in batch, with the acidic stripping solution
flowing inside the membrane tube and wastewater
outside. In each batch, 1 m of wastewater was
treated, and results are shown in Fig. 5. An average
removal efficiency of 90 % was achieved. A total
of 150 kg of aniline was extracted from 29,500 L
of wastewater, and 128 L of organic phase was
recovered. This organic-rich phase has a composition of 95 wt% aniline, 2.4 wt% toluidine and
2.6 wt% water.
From November 2001 to January 2002, this
pilot plant was applied to recovery of phenol from
a second process stream at Solutia UK. Critically,
the recovered phenol was added back to the
production process (manufacture of phosphate
esters), and produced product of normal quality.
1 2
3
lAn~hne
4
5
6
inlet
0
7
6
9 10 1112 1314 151617
Anl~neoutlel+Anhs
removal
271
148 (2002) 267-273
This shows that the material recovered via MARS
can be re-used directly in-process, which has a
positive major impact on both environmental
borders and economic viability.
4. Economic and environmental
aspects
As a theoretical exercise, the concentrations
of different species involved at different points
of the process can be calculated based on a mass
balance for the MARS trial at Solutia, UK. The
amounts of extracted aniline and mass of organic
phase are comparable between the calculated
values and data (respectively
162.1 kg against
150 kg and 167.4 kg against 128 L).
The MARS process will produce 1 g of salt
(NaCl) per 1.6 g of aniline recovered. However (1)
this salt is considerably less toxic on a mass basis
than aniline, (2) many of the waste streams emanating from chemicals manufacture already contain
sufficient salt, such that the extra salt produced
by MARS is insignificant. In the example illustrated
in Fig. 6, this difference it is of two orders of magnitude. Since the original waste counts 30 wt% KCl.
Environmentally, the main feature of MARS
it is the ability to remove very toxic compounds
at a low energetic cost from aqueous wastewaters
in a purity that can be re-used in chemical
manufacture. This achievement is translated to an
16192021
BatchNumber
Fig. 5. Batch operation pilot plant data: aniline initial and
final concentration in each batch, aniline removal in each
batch.
Fig. 6. Theoretical mass balance for MARS process applied
to Solutia UK aniline recovery trial.
272
Table 2
Case study -
EC. Ferreira
et al. /Desalination
148 (2002)
267-273
process economics of MARS process using fluoroaniline as an example
Item
Comment
Annual cost (benefit)
MARS plant
Membrane replacement
Acid (33% HCl)
Base (50%)
Power (3 kW)
Steam
Labour
Capital charge factor = 0.3
2-year lifetime
17ty-‘(@e113t-’
12 t y-’ @ E226 t-’
8000 h y-’ @ 0.08
82 t y-l @ El6 t-’
10% of one staff
e50,ooo
e 8,000
E 1,921
6 2,712
e 1,920
E 1,312
E 12,000
Recovered material
15 t y-’ (90% recovery)
(~120,000)
Total
Benefit
(e 42,135)
economical advantage by cutting costs of reagents
and the wastewater detoxication process required
by environmental legislation.
The price of aniline at the time of writing is
about e 1.2 kg-‘, and hence it is not one of the
more valuable aromatics in the market. Nevertheless, due its environmental
impact, aniline
removal is necessary and MARS provides a considerable economic advantage over alternative
recovering processes, although details cannot be
provided here for commercial reasons. However,
an example of MARS to recover a more valuable
chemical is given in the case study contained in
Table 2. The commercial value of fluoroaniline
is ten times the aniline value (around e 8 kg-i). In
the example given the treatment of 10 m3.d-’
wastewater with a 5 g.L-’ fluoroaniline concentration is considered. The MARS process in this
case is able to deliver a net benefit through the
value of the recovered fluoroaniline.
5. Conclusions
MARS is a novel process coupling detoxification and recovery. It is capable of achieving high
recovery efficiencies and producing a relatively
pure stream of recovered organics. At the laboratory scale, MARS has proven to be a successful
process for removing and recovering aniline and
phenol, with water contents of 5 and 13.5 wt%
respectively
At pilot plant scale MARS was
proven able to recover aniline from an industrial
wastewater in a good purity. MARS was shown
to be easily scaled-up based on membrane area.
Using different configurations, MARS was adapted
to deal with a key problem for membrane technology applied to chemical processes, that is, membrane blockage by tarry solids or organics precipitation.
The MARS process can utilise very simple
nonporous rubber tubes as membranes. MARS
has low energy requirements because it exploits
the acid-base functionality of aromatic acids and
bases to produce a driving force based on the
chemical energy contained in NaOH or HCl. The
process can be carried out at conditions of pressure
and temperature that are near ambient throughout
all items of equipment. MARS does not rely on
volatility of organics (or any phase transition), so
one can recover organics that membrane technologies such as pervaporation cannot reach. Finally,
MARS has been shown to be promising in the
industrial application to recovery a larger range
of aromatics than phenol and aniline. Examples
include amines, phenolics and pyridines.
Acknowledgements
F.C. Ferreira acknowledges financial support
from Funda@o para a CiCncia e Tecnologia, grant
PRAXIS XXI/BD/21448/99.
This work was
funded by the UK Engineering
and Physical
EC. Ferreira et al. /Desalination
Sciences
Research Council (EPSRC), grant GR/
L935.53.
[3]
References
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[2]
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Res., 2002, in press.
S. Han, EC. Ferreira andA.G. Livingston, Membrane
aromatic recovery system (MARS) - a new
[4]
[5]
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