Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
10738'~1
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This invention relates to the purifica~ion of a raffinate
phase and/or extract obtained by extraction of a mixture of
organic compounds, such as hydrocarbons, with a sulfolane-
type solvent.
In a typical extraction process employin~ a sulfolane-
type solvent, aromatic compounds such as benzene, toluene and
xylenes are removed from reformate or hydrotreated p~rolysis
naphtha feedstocks or similar products from petroleum hydro-
carbon conversion processes, such as thermal cracking or
catalytic cracking. ~Jikewise straight-run gasolines or
kerosines may be subjected to extraction. The mixture com-
prising the aromatic compounds is contacted with the solvent
in an extraction zone. Fat solvent is withdrawn from the
extraction zone and introduced into an extractive stripper
where heat is applied to remove any raffinate material ab-
; sorbed by the solvent. The raffinate-free fat solvent then
passes to a solvent recovery column where steam and/or reboil
are employed to remove the extract product from the solvent.
The overhead from the solvent recovery column, consisting
mainly of the extract product, water vapour, and a small
quantity of solvent, is then condensed, and a portion of the
condensed extract product is then employed as reflux in the
solvent recovery column-so as to reduce the amount of solvent
in the overhead of the solvent recovery column. ~lowever, the
extract product in the overhead stream still contains significant
amounts of solvent that should be recovered. Because the
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sulfolane-type solvent employed is water-soluble, it is
the usual practice to recover the solvent from the extract
product stream by contact with an aqueous stream in a
subsequent contacting means. The recovery of the solvent
from the extract product stream with water is undertaken in
any suitable liquid-liquid contacting means, e.g., in a
rotating disc contactor. The solvent is then readily re-
covered from the aqueous solution by distillation. Likewise,
the raffinate product stream taken as overhead from the
initial extraction zone may be contacted with water so as to
remove entrained quantities of solvent. Nevertheless, solvent
in significant quantities may still be lost with the raffinate
and extract product streams. The solvent that is withdrawn
with the extract and raffinate product streams should be
recovered, not only because the solvent may interfere with
subsequent processing of the two streams or the ul~imate use
of the extract or raffinate, but also because continual loss
of solvent in the raffinate and extract streams is an economic
waste. Heretofore it has been proposed to remove sulfolane-
type solvents from the raffinate phase and/or extracts by
contacting with solid adsorbents. The solvent may then be
recovered from the adsorbent by washing the latter with water,
and the adsorbent may be used again for adsorption of solvent ~;
after it has been subjected to an appropriate drying treatment.
~25 U.K. Patent Specification 1,168,027 refers to such a drying
treatment comprising drying by contact with the mixture of
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organic compounds to be subjected to the extraction with sulfolane-type
solvent.
It has now been found that in the regeneration of adsorbent, the
intermediary step of washing with water can be disposed of by contacting the
adsorbent directly, that is without previous washing with water, with at
least a part of the mixture of organic compounds to be subjected to extrac-
tion, provided the adsorption of the sulfolane-type solvent is effected at a
temperature of from 10-50 C and the regeneration of the adsorbent by contact
with the mixture of organic compounds is effected at a temperature of from
60-100 C. In this way the solvent is recovered from the adsorbent and dir-
ectly recycled into the extraction operation and separate recovery of solvent
from an aqueous solution resulting from an intermediary regeneration treatment
of adsorbent with water is no longer required.
Accordingly, the invention provides a process for the purification
of an extract obtained by extraction of a mixture of organic compounds with
a sulfolane-type solvent which comprises contacting the extract with a solid
adsorbent selected from silica gel and alumina at a temperature of from 10-
50 C and regenerating the adsorbent by contacting the latter with at least a
part of the mixture of organic compounds to be subjected to extraction at a ~ :
temperature of from 60-100C.
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1073~
The term "sulfolane-type solvent" as used herein relates
to compounds having the following structural formula:
1 ~ IH IH - R4
R2 ~ CH- CH - R3
wherein R1, R2, ~3 and R4 are selected from the group comprising
hydrogen, an alkyl group of from one to ten carbon atoms, an
alkoxy radical of from one to elght carbon atoms, and an aryl-
I alkyl radical of from one to twelve carbon atorrs. Slnce sulfolane,
i also known as tetrahydrothiophene-1,1-dioxide or tetramethylene
sulphone, is the principal sulfolane-type solvent employed in
commercial extraction processes, references to particular ex-
~10 traction processes and illustrative embodiments within the
patent specification employ sulfolane as the sulfolane-type
solvent.
The primary use of sulfolane is in the extraction of
aromatic compounds, such as benzene, toluene and xylene from
reformate or hydrotreated pyrolysis naphtha feeds. Other
applications of sulfolane include acid gas removal; extractive
distillation of closely boiling products, such as n-propyl
alcohol and sec.-butyl alcohol; fractionation of fatty acids
and oils into saturated and unsaturated portions; recovery of
sulphur dioxide; fractionation and decolorization of non-
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cellulosic wood products, such as pyroligneous liquids, tars, and tall
oil; and in the separation of a wide variety of mixed coal and agri-
cultural chemical products.
Preferably, the adsorbent is regenerated by contact with at
least part of the extraction process feed stream at a temperature of
from about 70C to 80C, and preferably the adsorption is effected at
a temperature of from 20-30C. In general, the greater the difference
between the adsorption temperature and the regeneration temperature,
the better the adsorption/regeneration cycle proceeds.
The adsorbent may be contained in 2 or more vessels, so that
while one adsorbent bed is being employed to remove solvent, the other
adsorption bed is being regenerated by passing extraction process feed
through the adsorption bed. If both raffinate phase and extract are
to be purified 4 vessels (2 pairs) may be used.
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10738Zl
The purification treatment leads to a very efficient
removal of sulfolane from the product streams. Typical
results are the reduction of sulfolane contents in
raffinate streams ranging of from 2 to 100 ppm to levels
of less than 1 ppm. This is of particular importance when
the extraction process aims at the production of hydro-
carbon feedstocks for the preparation of hydrogen with a
nickel catalyst where sulphur may act as a catalyst poison.
The invention is illustrated by the following Examples.
EXAMPLE I
This Example shows the effects of some solid adsorbents ~-
for removing sulfolane from n-heptane to a very low content,
even if the sulfolane is present in the liquid phase to be
treated in small amounts.
A quantity of n-heptane containing 50 ppm of sulphur in
the form of sulfolane (an imitated raffinate phase) was divided
into four samples. Each of the four samples was shaken for
5 minutes at 20C with 10%w of an adsorbent, after which the
sulphur content of the treated sample was determined. The
results are listed below in Table I.
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TABLE I
Adsorbent Sulphur content
of treated sample, ~-
ppm ~ :
_ .... .. -:
Silica gel 0.3
Molecular sieve, 1-
pore width A 0.8
Commercially avail- ~:
: B able as "Terrana" 1.4
l ,;,:
EXAMPLE II
A quantity of an aliphatic hydrocarbon fraction having
a final boiling point of 65 C and containing 0.5%w of sulfolane
and 5%w of benzene (an imitated raffinate phase) was divided
into two samples. Each sample was shaken for 5 minutes at
20C with 15%w of an adsorbent, after which the sulphur
content of the treated sample was determined. The results
are listed below in Table II.
TABLE II
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Adsorbent Sulphur content
of treated sample,
ppm
Silica gel 0.5
Activated alumina 2.2
~ / rQ,den7Qrk
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g
I EXAMPLE III
Saturation experiments were conducted in order to
determine the maximum absorption capacity of silica gel. A toluene
stream comprising 1.4%w of sulfolane was passed at a rate of
6.2 cm3/min. and a temperature of 25C through a sample
of 3.83 g of dried silica gel contained in a glass tube.
The concentration of sulfolane in the stream having passed
the sample was determined by GLC analysis coupled with
thermal conductivity detection. It was shown that all
sulfolane contained in the first 75 cm3 of toluene stream
i to pass the sample was absorbed, thereafler traces of
' sulfolane began to appear and when 185 cm3 had passed the -
concentration of sulfolane in the eluent of the sample
was back to 1.4%w, thus indicating that the sample was
saturated. From the concentration measurements and the
amount of toluene stream having passed the sample, it is
found that the maximum absorption capacity was 28 g sulfolane
per 100 g silica gel.
In the same way the maximum absorption capacities at
;2 ~ 25C of activated alumina ~Alcoa, F-20) and of activated
charcoal were found to be 7 g, respectively 2.~ g sulfolane/
100 g adsorbent.
EXAMPLE IV
'
In order to regenerate the adsorbent a stream of 50 vol.%
of toluene and 50 vol.% of heptane (an imitated extraction
process feed) was passed through samples saturated to maximum
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capacity with sulfolane. The concentration of desorbed
sulfolane contained in the eluent was monitored up to the
moment when no more sulfolane was released. Table III
lists the amounts of sulfolane released by the adsorbents,
expressed in gram sulfolane/100 g adsorbent and in
percentage of the maximum absorption capacity at 25C.
TABLE III
. .. _ . ... ___
Regeneration
with at g/100 g %
~_
Silica gel, 76C 6.3 23
ditto , 62C 4.2 15 ;
Alumina , 76C 4.8 68
ditto , 62C _ _ 41
, .'
It is shown that the efficiency of regeneration at a
-temperature of 51 higher than the adsorption temperature
is better than when the temperature difference is only 37.
EXAMPLE V
-
~ Regenerated silica gel samples were used to determine
their maximum absorption capacities in accordance with the
procedure described in Example III. It appeared that the
maximum capacities were not effected by the regeneration.
Nor is there any decrease of the maximum capacity over a
continuous cycle employing 30 absorptions and 30 re~ener-
ation runs.
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