Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
6g~
A SOLVENT STRIPPIN _PROCESS
The present invention relates to a process for
stripping solvents, and particularly to a special
steam recycling during steam stripping operations
in solvent recovery plants.
In mineral oil technology as well as in gene-
ral chemical technology solvent extraction processes
are used for separating blends of materials by means
of the differing solubility of their components in
solvents and, respectively, in solvent mixtures.
The main proportion of the solvent contained in the
extract and also the small solvent quantities present
in the refined product have to be recovered, and
this is usually done in a multi-stage evaporation
plant.
From the solvent recovery of, say, a mineral oil
extraction plant there result , for instance, in
addition to a solvent stream a raffinate stream and
20 also an extract stream from which solvent has been :
removed to a large extent. However, these streams
still contain small amounts of solvent. These residual
amounts are separated in stripping columns in which
_ 1 _
the solvent components are stripped by means of
steam. ~s the specification for the products
require a removal to only a few ppm of solvent,
the stripping steam must be free of solvent.
5 Steam, which contains the solvent components, is
removed from the top of the stripping columns and
condensed in a condenser, and in a downstream
separator the condensate is separated into a
solvent phase and an aqueous phase. Both the
10 solvent phase and the aqueous phase contain small
amounts of oil. In accordance with the physical :
properties of the respective solvent the aqueous
phase also still contains some residual solvent.
Therefore, the aqueous phase is fed into a column
15 in which these residual solvent quantities are also
stripped by means of steam , and the solvent free
water is withdrawn at the bottom of the column and
is discharged from the plant.
~ecause of the possible contamination of these
20 volumes of water with oil they flow to the waste
water treatment plant. In some countries due to
government regulations which are based on the creator
principle, high waste water charges arelevied on the
waste waters. Furthermore, the waste water treatment
25 causes additional costs. It is therefore desirable
in connection with the raffination of mineral oils
-to minimize the amounts of waste wa.er or, where possible, to
even elimina-te the dlscharge of waste water.
One way of eliminating -the production o:E consiclerable
volumes of waste wa-ter in the stripping secti.on oE a solven-t
rc~covc:ry pl.ant :is the replacc-~ment o:E -the str:ipplng steam by
rc!c~c.l..c.~d st:r:i.pp:i.ncJ cJases. I-~owever, when strlpplng qases are
~Is,ecl the amount of power requlred :Eor the compression oE the
gases constitutes a drawback.
It was -therefore the object of the presen-t lnventlon
to provide a process for the stripping of solvents by means of
steam, whlch ellminates as far as posslble the dlscharge of
waste water into the waste water system.
The present invention provides ln one aspect especial]y
for solvent extraction plants a process for stripping residual
solvents by means of s-team ln an extract and a raffinate strlpp-
ing column, transferring the condensates of the solvent-
containing overhead vapours into a separatox, wi-thdrawing the
aqueous phase from the separator, and stripplng the solvent
quantltles remainlng ln said aqueous phase by means of steam in
a water stripping column and introduclng the condensate of the
overhead vapours into the same separator, sald process being
characterlzed in that the water, from which solvent has been
removed, is accumulated at the bottom of the water stripplng
column, is withdrawn therefrom and transferred into a vessel
where it is re-evaporated by lndirect heat exchange with hot
process streams withdrawn from the solvent recovery section and
is returned in form of steam into -the respective stripping
columns, while extraneous steam is supplied as stripping steam
to the respective stripping columns only during start-up and
extraneous thermal energy is supplied only when the amounts of
heat originating from -the process streams and required for
re-evapora-tion of the water are insufficient.
--3--
According to another aspect of the present invention
there is provided a process for separating solvent from a
charge mix-ture con-taining a hydrocarbon oil and a solvent
wh:lch comprises
steam st:rlpping said charcJe m:ixture in a solvent
str:i.pp:i.nc,l operat.i.on thereby formi.ng strippe~d bot-toms containing
hyclrocarbon oi.:l. and solvent stripping operation overhead con-
tainlng steam ancl solvent;
withdrawing stripped bottoms containing hydrocarbon
oil from said solvent stripping operatlon;
condensing said solvent stripping operation overhead
containing s-team and solvent -thereby forming a solvent stripp-
ing operation overhead condensate;
collecting said solvent stripping opera-tion overhead
condensate in a collection vessel containing an upper aqueous
phase and a lower solvent phase;
withdrawing said lower solvent phase;
steam stripping said upper aqueous phase in a
water stripping operation thereby forming a water stripping
operation overhead containing water and solvent and wa-ter
stripping operation bottoms containing water;
condensing said water stripping operation overhead
thereby forming a water stripping operation overhead con-
densate;
passing said water stripping operation overhead
condensate to said collection vessel;
passing said water stripping operation bottoms to
a reboiler;
heating said water stripping operation bo-ttoms in
said reboiler by indirect heat exchange against a hot stream,
thereby forming steam; and
passing said steam as the steam stripping medium to
said solven-t s-tripping operation and said water stripping
operat.ion.
Suitable solvents -to be recovered accordinc3 to the
E):l^ecsent l.nvention are ~:hose being partly or completely soluble
.in wal:c!r, wh:Lch can be steam stripped, e.cJ. methyl ethyl ketone,
.Eu:r:Eural, methylene chloride and dichloroe-thane.
In the process according to the invention practically
no eontamina-ted waste water is produced although stripping
steam is used Eor the solvent recovery. Only a small quantity
of water (e.g. up to 5 vol.-%, and 10 vol.-% at the most) has
-to be drained in intervals, similar to the "blowciown"
-4a-
from steam boilers,in order to avoid concentration
of undesirable components.
The watcr required for the stripping steam
ls recycled, and at the same time heat that has so
5 far not been utilized and is contained in the process
streams obtained in the solvent recovery (e.g. hot
solvent) is utilized for the production of stripping
steam.
An embodiment of the process according to the
10 present invention will be described with reference
to a mineral oil extraction plant and its flow dia-
gram shown in the accompanying drawing.
The oil fraction - either raffinate or extract-
which still contains residual solvent is introduced
15 into the stripping column 2 through line 1. During
the start-up phase line 3 delivers externally supplied
s~ripping steam to the column 2. In the bottom of
column 2 the oil stripped of solvent accumulates and
is withdrawn through line 4 and pump 5 to the storage
20 tank- The vapour withdrawn from the top of the column
2 comprises the supplied stripping steam, the residual
solvent quantities stripped from the oil and possibly
traces of oil; it is transferred through line 6 to
the condenser 7. The liquid phase produced in the con-
25 denser 7 is supplied through line 8 to the separator 9
--5--
6S~
where it is separated into the aqueous phase anda solvent-containing phase which may be returned
to the extraction zone. The aqueous phase is delivered
from the separator 9 through line 10 (the pumps re-
5 quired for transport are not shown in the drawing)to the column 11, to whlch in the start-up phase
extraneous stripping steam is supplied through line
12 to remove the residual amounts of solvent con-
tained in the aquoues phase from the top of column 11
10 through line 13.
After condensation in the condenser 14 the
aqueous, residual solvent-containing condensate is
also introduced into the separator 9. The solvent-
free water, which may, however, still contain small
15 amounts of oil, accumulates at the bottom of the column
11. This water is not discharged in the usual manner
to the waste water system but flows through line 15 to
vessel 16. The interior of this vessel 16 is provided
with a heating coil 17 through which hot (ca. 140C)
20 product from the solvent recovery section is passed.
This hot product, e.g. solvent, which is with-
drawn from the solvent recovery process, has to be
cooled,anyway, prior to its re-use in the extraction
stage. Cooling is normally performed by air or water
coolers in which the heat is lost without being uti-
lized because the commonly used processes do not
provide for full utilization thereof.
In accordance with the present invention, the
5 heat of these hot solvent streams is used to evaporate
the volume of water accumulating in vessel 16 at an
absolute pressure of 1.5 bar and a temperature of
110C. In most cases the amount of heat liberated
during cooling of the solvent from 140C to 120C
10 will be sufficient for the generation of steam. If
desired, additional heating with steam may be per-
formed by means of coil 18. The steam from vessel 16
flows through line 19 and valve 20 into the stripping
column 2, which operates at an absolute pressure of
15 0.3 bar. The amount of steam supplied through line
19 increases during the start-up period, and the
amount of steam supplied through line 3 is decreased
accordingly. Further volumes of steam from the vessel
16 are supplied through line 21 to column 11. ~essel
20 16 and column 11 operate at the same absolute pressure
of 1.5 bar. In the case of column 11, too, the
amount of steam supplied through line 1~ is decreased
in accordance with the increasi-ng-amount of steam pro-
duced in vessel 16.
--7--
6~
At the end of the start-up phase both the
steam line 3 and the steam line 12 are closed,
for then a steam/water cycle has been achieved which
normally does not require any additional extraneous
5 steam.
No waste water to be purified in waste water
treatment plants will be produced.
For purposes of simplification the flow diagram
shows only one stripping column for stripping the
10 residual solvent contained in the oil, wherein the
solvent-containing steam is removed overhead and
after condensation is passed to the separator.
In commercial processes it is, however, common
practice to supply the condensates from the extract
15 and from the raffinate stripper to the same separator
and to treat the aqueous phase obtained from the con-
densate of both stripping columns in one water stripp-
ing column.
The volumne of steam produced in vessel 16 is of
20 such amount that the steam is sufficient not only for
operating the water stripping column but a;lso for ope-
rating the extract stripper as well as the raffinate
stripper.
--8--
For instance, in the solvent recovery plant
of a mineral oil extracti.on system operated with
furfural the following quantities are obtained
per day:
360 tons of extract
840 tons of raffinate.
At the specified daily capacity the extract
stripper according to the prior art consumes
18 tons of steam,
10 the raffinate stripper consumes 35 tons of steam,
the water stripper consumes 32 tons of steam .
In accordance with the invention it is thus
possible to save 85 tons of steam per day, and the
production of 85 m3 of waste water per day is elimi-
15nated.
