Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR RECO~ERY QF OLEFINIC NIT~ILES
B~CKGROUND OF THE IN~ENTION
The present invention is an improvement over the
invention described in commonly assigned U.S. Patent
4,166,008.
U.S. 4,166,008 describes an improvement in the
known system for recovery and purification of acrylonitrile
and methacrylonitrile produced by the ammoxidation of propy-
lene or isobutylene. In accordance with this improvement, a
vaporous sidestream is removed from the lower fourth of the
column whose bottoms product is used as the quench liquid
(extractive distillation column or stripper) for quenching
the ammoxidation reactor effluent. By withdrawing this
sidestream, it was found that the concentration of heavy
organics in the quench bottom streams could be significantly
increased while at the same time the total mass flow of this
stream significantly decreased. Since the quench tower
bottoms are normally disposed of by incineration, this
drastic decrease in the amount of quench tower bottoms
represented a significant energy savings.
Unfortunately, a penalty associated with this
approach is that additional heat must be supplied to the
column from which the side stream is taken to account for
the associated heat loss. For example, as taught in column
4 of U.S. 4,166,008, a 12% increase in heat input was neces-
sary in the specific embodiment described.
Normally, heat is supplied to the extractive dis-
tillation column and stripper column by reboilers using low
1 1~.4~1~ (5468)
pressure steam as the heat source. With higher and higher
energy costs, it is always desirable to reduce the usage of
low pressure steam as much as possible so that excess low
pressure steam can be used for other beneficial purposes.
Accordingly, it is an object of the present inven-
tion to provide a new system for accomplishing the same
results achieved in U.S. 4,166,008, i.e. significant reduc-
tion in the amount of quench tower bottoms produced by a
technique which requires significantly less energy than the
technique described in that patent.
S UMMARY OF THE I NVENT I ON
Thi~ and other objects are accomplished by the
present invention in accordance with which a multi-effect
evaporator is employed to process the raw distillation tower
bottoms or stripper bottoms recycled for use as the quench
liquid in the quench column. By means of a multi-effect
evaporator, 50% or more of the liquid in the recycled stream
can be removed therefrom leaving a concentrated recycle
stream to serve as ~he quench liquid in the same way as
shown in U.S. 4,166,008. Because multi-effect evaporators
are so energy efficient, however, the overall energy costs
of this technique are much lower than the technique
described in that patent.
Thus, the present invention provides a improvement
in the known technique for the recovery of acrylonitrile or
methacrylonitrile from an ammoxidation reactor effluent con-
taining acrylonitrile or methacrylonitrile, acetonitrile and
~ (5468)
heavy organic impurities in which the reactor effluent is
cooled by contact with an aqueous quench liquid to produce a
gaseous quench effluent containing acrylonitrile or meth-
acrylonitrile, and acetonitrile and a liquid quench bottoms,
the quench liquid being obtained by contactin~ the gaseous
quench effluent with water comprising removing water from
said quench liquid by multiple effect evaporation prior to
contacting said quench liquid with said reactor effluent.
DETAILED DESC~IPTION
The figure is a schematic representation of the
present invention as applied to the recovery and purifica-
tion of acrylonitrile.
Referring to the figure, the reactor effluent gas
in conduit 100 containing acrylonitrile, HCN, acetonitrile,
water vapor and impurities is first passed to a quench
column 102. The gas is contacted with quench liquid 130 in
the quench column. A bottoms stream containing water and
impurities is removed through conduit 106 and sent to waste
treatment.
The cooled reactor effluent gases leave the quench
system through line 108 and pass as feed to the absorber
110. Wash water enters the absorber at tne top through line
112. Non-condensible gases are removed from the absorber
through line 114. An aqueous solution containing water,
acrylonitrile, acetonitrile and impurities are removed as a
bottoms stream through line 116 and passed to the extractive
distillation column 118.
~ (5468)
Solvent water is introduced to the top of column
lL8 throu~h line 120 to perform extractive distillstion.
Acrylonitrile and HCN are removed as an overhead vapor
through line 122 and sent to further purification (not
shown). A bottoms stream containing acetonitrile and water
is removed through line 124 and passed to stripper 126.
Heat is added to the stripper to remove acetonitrile as an
overhead vapor through line 128. The bottoms stream con-
taining water, heavy organics and other impurities are
removed through line 132 and sent back to the quench system.
A liquid stream may be removed from the lower half of the
stripper throu~h line 120 and used as solvent water ~o the
extractive distillation column.
( In accordance with the invention, stripper column
bottoms in line 132 are subjected to multi-stage evaporation
in a multi-stage evaporator unit generally indicated at 134
prior to being returned via line 130 to quench tower 102 as
the quench liquid.
Multiple effective evaporator 134 i~s composed of
four shell and tube heat exchangers 136, 138, 140 and 142
arranged in series. In each heat exchanger, liquid in the
tube side of the exchanger is partially evsporated producing
a vaporous effluent and a liquid effluent. The liquid
effluent is fed to the tube side of the next heat exchanger
in the series while the vaporous effluent is fed to the
shell side of the same heat exchanger causing additional
partial evaporation of the liquid. This technique is con-
tinued for as many stages as is necessary to remove the
desired amount of water from the stripper bottoms, which are
then sent to the quench column for use as the quench liquid.
1164~:~4 (5468)
In each stage, condensate produced when the heat-supplying
vapor is condensed through heat exchange is recovered and
either recycled for reuse or subjected to chemical or bio-
logical purification. The details of this procedure are
described below.
Stripper column bottoms in line 132 are passed into
the tube side of first heat exchanger 136 while low pressure
steam is passed through the shell side of ~his heat ex-
changer. Heat exchange therein causes the lower pressure
steam to condense and the stripper bottoms ~o partially
evaporate. Condensate is removed from first heat exchanger
136 for reuse via line 146.
Heating of the stripper column bottoms in first
heat exchanger 136 causes partial separation thereof into
vapor and liquid phases. The liquid phase is withdrawn via
line 148 and transferred to the tube side of second heat
exchanger 138, a portion of the withdrawn liquid being
recycled via line 150 to the bottom of the tube side of
first heat exchanger 136. Vapor produced in first heat
exchanger 136 is withdrawn and transferred via line 152 to
the shell side of second heat exchsnger 138. Heat exchange
in heat exchanger 138 causes condensation of the vapor on
the shell side and partial evaporation of the liquid in the
tube side, thereby producing liquid in vapor phases in
second heat exchanger 138. Condensate produced on the shell
side of second heat exchanger 138 is discharged to waste via
line 154. This condensate has a relatively low concentra-
tion of heavy organics such as polymer and the like and
hence can be rendered environmentally acceptable by conven-
tional biological or chemical treatments.
llfi401tl (5468)
The liquid phase remaining in the tube side of
second heat exchanger 138 is transferred via line 156 to the
tube side of third heat exchanger 140, a portion of the
liiquid being recycled via line 158 to the tube side of
second heat exchanger 138. Vapor produced in the tube side
of second heat exchanger 138 is transferred via line 160 to
the shell side of third heat exchanger 140. Again, heat
exchange in third heat exchanger 140 causes condensation of
the vapor on the shell side to form a condensate which is
withdrawn via line 162 and disposed of in the same way as
the condensate from second heat exchanger 138.
The liquid and vapor phases produced in the tube
side of third heat exchanger 140 are transferred via lines
164 and 166 to the tube and shell sides of fourth heat ex-
changer 142 with a portion of the liquid phase again being
recycled via line 168. The vapor produced in the tube side
of fourth heat exchanger 142 is withdrawn via line 170, con-
densed in condenser 172 and recovered in utility water
vessel 174. The condensate from the shell side of heat ex-
changer 142 is also transferred to utility water vessel 174
via line 176. Both these condensates are of such ~igh
purity that it can be used as conventional clean water such
as, for example, in the flushing of various process equip-
ment. Liquid recovered from the tube side of fourth heat
exchanger 142 is withdrawn via line 178 and after a portion
thereof is recycled via line 180 to the tube side of the
fourth heat exchanger is transferred via line 130 to quench
column 102 as the quench liquid.
By the above means, a significant amount, for
example, one-half or more of the water in stripper bottoms
6.
~ 4 (5468)
132 can be removed thereby providing a concentrated stripper
bottoms much more concentrated in impurities but having a
much lower mass flow rate in line 130 for use as quench
liquid. This in turn means that the quench tower bottoms
recovered in line 106 will likewise have a much higher con-
centration of impurities but a much lower mass flow rate.
Because of the high energy efficiency of the multi-effect
evaporator system, however, the energy requirements to
operate this system are much less than the system described
in U~S. 4,166,008.
COMPARATIVE EXAMPLE A AND EXAMPLE 1
An acrylontrile recovery process is performed sub-
stantially as shown in the figure. In the Comparative
Example A, all of the liquid bottoms stream from the
stripper is recycled to the quench column as quench liquid.
Example 1 is identical to Comparative Example A except that
the stripper column bottoms is processed in accordance with
the system shown in the figure by a four-stage multi-effect
evaporator operated so as to remove one-half of the water in
stripper bottoms.
The tables below show the weight percent polymer
and the COD contained in the various process streams of both
examples.
TABLE I
Wei~ht Percent Polymers
Strip. Quench 1st 2nd 3rd 4th
Column Tower Stage Stage Stage Stage
Ex. Btms. Btms. Btms. Btms. Btms. Btms.
Comp A 1.5 10 -- -- -- --
1 1.5 25 1.62 1.81 2.18 3.00
(5468)
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TABLE II
Wei~ht Percent COD
2nd 3rd 4th
Stage Stage Stage Condenser
l.x. Conden. Conden. Conden. Condensate
Comp A
l 0.20 O.ll ~.05 0.04
As can be seen in these tables, the condensates
produced in the second and third heat e~changers contain
extremely small amounts of heavy organics. Thus, they can
be directly processed by conventional biological or chemical
treatment to produce environmentally acceptable water.
Furthermore, the condensate produced in the fourth heat
exchanger as well as the condensate produced by the con-
denser are pure enough to be used for various process pur-
poses such as wash water without further treatment. The
condensate produced by the first heat exchanger, since it
does not contact any other process stream, is of course
highly pure. In addition, because of the removal of water
from the stripper bottoms the amount of quench liquid
returned to the quench tower is approximately 50% of the
stripper bottoms. This means that the amount of quench
tower bottoms produced i8 approximately 40% of the quench
tower bottoms produced in the comparative example. Finally,
because a four-stage multi-effect evaporator is so energy
efficient, the amount of low pressure steam needed to carry
out the present invention is only one-third to one-fourth
the amount needed to operate this system. Thus, it can be
seen that the present invention accomplishes the same advan-
tageous result as U.S. 4,166,008 using significantly less
energy.
8.
~ ~ 64~ 1 4 (5468)
Although only a single embodiment of the invention
has been described above, many modifications can be made
without departing from the spirit and scope of the inven-
tion. For example, any number of stages can be employed in
the multiple effect evaporator. Thus, three stages are
beneficial although four are preferred from the standpoint
of energy savings. Furthermore, although low pressure steam
is shown in the above description as supplying the heat
necessary for all the evaporations, any heat source can be
employed. In a typical acrylonitrile purifications and
recovery plant, however, low pressure steam, that is satu-
rated steam having a pressure of up to 100 psig, normally
about 20 to 60 psig, is readily available and is preferably
used. Also, the amount of water in the stripper column
removed by the multiple effect evaporator can be varied
depending primarily upon economics. Finally, it should also
be appreciated that the multiple effect evaporator of this
invention need not be restricted to use on stripper column
bottoms as shown in the above description, but can be
employed to concentrate any other process stream which is
recycled for use as the quench liquid. For example, a
multi-effect evaporator can be used to process the extra~-
tive distillation tower bottoms recycled in line 156 of
Figure 2 of U.S. 4,166,008. All such modifications are
intended to be included within the scope of the present
invention, which is to be limited only by the following
claims:
