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BACKGROUND OF THE INVENTION
The term solvent recovery generally refers to a process in which a liquid
is separated, purified and reused as a solvent. Some typical
solvent-recovery processes are:
(1) The recovery of solvents vaporized in the drying of synthetic fibers
and films, plastics and rubber products, smokeless powder, impregnated
fabrics, adhesives, printing inks, paints, lacquers, enamels and other
organic coatings;
(2) The recovery of solvents used in the solvent extraction of natural fats
and oils or in other solid-liquid extractions;
(3) The recovery of solvents used in the solvent refining of mineral or
vegetable oils in other liquid-liquid extractions, in extractive and
azeotropic distillations or in gas-absorption processes;
(4) The recovery of solvents used for degreasing fabricated parts for dry
cleaning or for other washing operations; and
(5) The recovery of solvents used as mediums for chemical reactions or for
the precipitation and crystallization of solids.
If an organic liquid is separated and purified, but not returned for reuse
as a solvent, the operation cannot be strictly referred to as solvent
recovery. However, except for the use to which the liquid is put, such
operations may resemble solvent-recovery systems in every respect. Typical
of such closely related processes are (1) the recovery of natural gasoline
and light hydrocarbons from natural gas; (2) vapor recovery operations in
petroleum refining; (3) the recovery of alcohol from fermentation gases;
(4) the recovery of organic liquids from wood distillation and from coal
tar distillation operations; and (5) the recovery of products or of unused
reagents from chemical synthesis.
The growth of solvent-recovery systems has followed closely the growth of
processes which manufacture and use organic solvents. While condensation
of alcohol from vapor-laden air has been practiced since ancient times,
some 99% of today's solvent recovery systems were put into operation after
1930. An excellent discussion of solvent recovery is found in Kirk-Othmer,
Encyclopedia of Chemical Technology, Second Edition, Volume 18, pages
549-64 (1969), which is hereby incorporated by reference.
SUMMARY OF THE INVENTION
The present invention provides a method for recovering solvent vapor
evaporating from a storage tank holding an organic solvent. More
particularly, the present invention relates to a process for recovering an
organic solvent by absorption, and regenerating the absorbing liquid
without using a high vacuum system.
It is an object of the present invention to provide an economical process
for recovering solvent vapor by absorption.
It is a further object of the present invention to provide an absorption
process for recovering solvent vapor which regenerates the absorbing
liquid without using high vacuum.
It is a further object of the present invention to provide an absorption
process for recovering a solvent vapor which includes absorbing the
solvent vapor in a first absorbing liquid, desorbing the solvent vapor
without using high vacuum, and absorbing the solvent vapor again in a
second absorbing liquid, which may be under greater than atmospheric
pressure.
Yet another object of the present invention is to provide a process for
separating and recovering two mixed solvent vapors which includes
preferentially absorbing a first solvent vapor in a first absorbing
liquid, desorbing and recovering the first solvent vapor without using
high vacuum, and absorbing the second solvent vapor in a second absorbing
liquid, and desorbing and recovering the second solvent vapor without
using high vacuum.
Other objects of this invention will be apparent to those or ordinary skill
in the art from a consideration of the entire specification and claims.
Surprisingly, it has been found that the objects of this invention can be
achieved without using a high vacuum system by desorbing a portion of
solvent vapor from a liquid in a regenerator under low vacuum, and
recycling the liquid through the regenerator, liberating a portion of the
solvent vapor at each cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior process in which a gas containing a solvent vapor
enters an absorption column 1 from inlet pipe 5 and is brought into
contact with an absorbing or scrubbing liquid, supplied from pipe 8. The
vapor to be recovered is absorbed in the absorbing liquid and leaves the
absorption column by means of outlet pipe 7. The absorbing liquid,
enriched with the vapor to be recovered, enters flush tank 2 by means of
outlet pipe 7. In flush tank 2 the vapor which was absorbed by the
absorbing liquid is evaporated under high vacuum and passed through pipe 9
under the influence of high vacuum system 3. Passing through the high
vacuum system the vapor to be recovered enters gas recovering apparatus 11
from pipe 10. Gas recovering apparatus 11 recovers the solvent vapor by
any of the conventional methods, such as cooling, condensation, absorption
or a combination thereof.
FIG. 2 shows the process of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a process for absorbing a solvent
vapor in an absorbing liquid, subjecting the absorbing liquid to a reduced
pressure to strip the solvent vapor therefrom, recovering the solvent
vapor and reusing the absorbing liquid, all without using a high vacuum
system. A prior process has been briefly described with respect to FIG. 1.
Turning now to FIG. 2, a mixed gas consisting of an organic solvent vapor
and air in a storage tank is stored in reservoir 32. This mixed gas is
forwarded to a water-sealing compressor 51, via pipe 50 and valve 26, and
is compressed in a water-sealing compressor 51, the compression takes
place without elevation of temperature (the temperature of the compressor
being about 45.degree. C.). Using a water-sealing compressor, it is
possible to effect a safe cool compression and it is not necessary to use
a saturator. A satisfactory compressor is disclosed in our copending
United States application, Ser. No. 693,975, filed June 8, 1976, the
specification and disclosure of which is hereby incorporated by reference.
The compressed mixed gas passes through a line 52 in a mixed gas-liquid
phase together with sealing water into a separator 33. The mixed gas is
separated from the sealing water in separator 33, and the mixed gas if
forwarded to a first absorption column 53 by means of pipe 36. The first
absorption column 53 comprises an upper absorption portion and a lower
portion for separating organic solvent from water. The mixed gas from type
36 is brought into contact with a countercurrent liquid flow in the upper
portion of absorption column 53. This liquid countercurrent flow is a flow
of cool organic solvent which is a first absorbent descending from pipe 22
provided at the upper portion of the first absorption column 53. The cool
organic solvent countercurrent flow absorbs most of the solvent vapor in
the solvent vapor-air mixture entering the first absorption column. The
cool organic solvent liquid countercurrent, enriched with absorbed solvent
vapor, descends downwardly into the lower separator portion of the first
absorption column 53.
The lower separator portion of the first absorption column 53 is maintained
at a temperature slightly above the freezing point of water. The enriched
cool organic solvent liquid is further cooled in the lower separator
region to assist in the removal of any residual moisture present in the
enriched organic solvent flow. The lower separator region of the first
absorption column separates moisture from the enriched organic solvent
flow by making use of the different specific gravity of water and organic
solvent. The water is discharged outside of the reaction system by means
of pipes 42. The dried enriched organic solvent flow is forwarded to a
recovered solvent storage tank 41 by means of pipe 39. The liquid solvent
stored in recovered solvent storage tank 41 is circulated through a pipe
20, a pump 55, a pipe 21, a cooler 56 and a pipe 22. From pipe 22, the
cool solvent is again circulated as a countercurrent flow through the
upper absorbing portion of the first absorption column 53.
Optionally, it is possible to recycle the dried enriched solvent from the
first absorption column 53 via pipe 39 directly into pipe 20, without
passing through recovered liquid solvent storage tank 41.
The volume of solvent in liquid solvent storage tanks 41 naturally
increases during operation of the recovery system. In addition, the
absorbing ability of the solvent gradually declines as it is recycled.
Therefore, recovered solvent is periodically removed from the recovered
liquid solvent storage tank 41 by means of discharge pipe 19, pump 7 and
discharge 18. Fresh solvent is periodically supplied to the system via
liquid solvent storage tank 41 from a pipe 40.
Sealing water separated from the mixed gas-liquid mixture in separator 33
passes through pipe 17, liquid surface control tanks 34, pipe 37, pipe 54,
pipe 16, sealing water cooler 35 and pipe 38, returning into water sealing
compressor 51.
Although most of the solvent vapor entering the upper portion of the first
absorption column 53 is absorbed by the solvent countercurrent flow from
pipe 22, nevertheless, a portion of the solvent vapor, mixed with air,
passes through the top of the first absorption column 53, by means of pipe
12. Pipe 12 forwards this vapor to a second absorption column 57. The
second absorption column 57 is in the form of a combination of an upper
absorbing portion and a lower storage portion. In the upper absorbing
portion, the vapor forwarded by means of pipe 12 and valve 27 is brought
into contact with a counter current flow of a cooled solvent liquid,
preferably a cooled, heavy hydrocarbon liquid which is supplied from pipe
25 at the upper portion of the second absorption column 57. By means of
this heavy hydrocarbon liquid countercurrent flow, the solvent vapor not
absorbed in the first absorption column 53 is substantially completely
absorbed. Any unabsorbed solvent vapor, and air, passes through the top of
the second absorption column 57 by means of pipe 13 and may be vented by
means of valve 30, or recycled by means of valve 29 and pipe 14 returning
to the water-sealing compressor 51.
The heavy hydrocarbon liquid countercurrent flow, having been enriched by
absorbing solvent vapor, passes into the lower storage portion of the
second absorption column. Therefrom it passes through pipe 23, pump 58,
pipe 24, valve 31, cooler 59 and is recycled into the second absorption
column by pipe 25. The operating temperature of the second absorption
column should remain slightly above the freezing point of water, to avoid
freezing moisture therein. The operating temperature of the second
absorption column may be controlled by cooler 59.
The heavy hydrocarbon liquid counter current flow in the second absorption
column continuously becomes more enriched as the liquid is recycled. The
absorbing capacity of the heavy hydrocarbon liquid gradually declines as
it becomes increasingly enriched. It is, therefore, necessary to
regenerate the heavy hydrocarbon liquid from time to time.
Pressure control device 43 measures the pressure inside the first
absorption column 53 and controls valve 28. The purpose of pressure
control device 43 is to maintain the pressure in the first absorption
column 53 above atmospheric pressure.
Pressure control device 44 controls the pressure inside the second
absoption column 57. Pressure control device 44 measures the pressure
inside the second absorption column and controls the degree of opening of
valve 27. Valve 27 controls the supply of vapor transferred from the first
absorbing column 53 to the second absorbing column 57.
In order to regenerate the heavy hydrocarbon liquid in the second absorbing
column 57, the following procedure is used. Valves 26 and 27 are closed.
Valves 28 and 29 are opened. Valve 30 is closed. At this time, because
valve 27 is closed, no vapor can travel from the first absorption column
53 to the second absorption column 57. Water-sealing compressor 51 now
functions as a low vacuum pump to draw gas from the top of the second
absorption column 57, placing the second absorption column under a low
vacuum. In accordance with the present invention, the low vacuum pressures
contemplated are from about 560 mm Hg to about 160 mm Hg. Valve 31 is
closed to bypass cooler 59 so that the heavy hydrocarbon liquid is not
cooled as it is recycled through pipes 23, 24 and 25. Thus, the heavy
hydrocarbon liquid recirculating through the second absorption column 57
gradually becomes warmer and it is placed under a low vacuum. Therefore,
absorbed solvent vapor becomes less soluble in the heavy hydrocarbon
liquid and is caused to evaporate therefrom. The solvent vapor leaving the
heavy hydrocarbon liquid is transferred through pipes 13 and 14 and into
the first absorption column 53. The pressure in the first absorption
column 53 is maintained at greater than atmospheric pressure by pressure
control device 43 and valve 28. The solvent vapor forwarded from the
second absorption column 57, is absorbed into the cooled organic solvent
in the first absorption column 53. Thus, in regenerating the heavy
hydrocarbon liquid of the second absorption column 57, the first
absorption column 53 recovers the solvent vapor liberated from the heavy
hydrocarbon liquid. The solvent vapor not absorbed in the first absorption
column may be returned to the reservoir 32 by pipes 15. To reduce the
vacuum in the second absorption column 57, a portion of the vapor may be
transferred through pipe 12 and valve 27 into the second absorption column
57. It is, of course, more economical to regenerate the heavy hydrocarbon
liquid used in the second absorption column 57 with the minimum effective
vacuum.
During normal operation of the solvent vapor recovery system valves 26 and
27 are open, valves 28 and 29 are closed and valves 30 and 31 are open.
During normal operation, the water-sealing compressor 51 operates as a
compressor, as described above.
As mentioned above, the heavy hydrocarbon liquid in the second absorption
column 57 may be generated under relatively low vacuum (i.e., only
slightly below atmospheric pressure), by recycling the heavy hydrocarbon
liquid until the desired removal of absorbed solvent vapors is achieved.
By using this recycling technique, it is possible to achieve the same
effect at relatively low vacuum as would otherwise require a high vacuum
(i.e., very low pressure) with all of the disadvantages which are inherent
in the high vacuum system. For example, a low vacuum system, as in the
present invention, may use a one-stage vacuum pump whereas a high vacuum
system would require a more complex two-stage pump. Further, less power is
required to achieve a low vacuum and it is, therefore, more economical.
Further, it is easier to construct a low vacuum system and to prevent
leakage therein. A particularly significant advantage of the low vacuum
system described is that the circulating pumps associated with a low
vacuum system are far more simple and economical than circulating pumps
associated with a high vacuum system. In this regard it is instructive to
compare FIG. 1, pump 4 with FIG. 2, pump 55. FIG. 1 illustrates a high
vacuum system. In such a high vacuum system absorbing liquid circulating
pump 4 must be installed at a position lower than that of the vacuum pump
3. Furthermore, flush tank 2 must be positioned higher than vacuum pump 3.
However, in a low vacuum system as illustrated in FIG. 2, the circulating
pump and flush tank may assume any convenient position, thereby making a
compact apparatus. All of these factors are significant in reducing the
cost of constructing the apparatus, and cost of operating same.
Although the above-detailed explanation was a general process of recovering
solvent vapor, it will be appreciated that the present invention is
applicable to the recovery of solvent vapors in a wide variety of organic
substances. The process of recovering solvent vapors of the present
invention is particularly suitable for the recovery of gasoline vapor, jet
fuel vapor, naphtha vapor, crude petroleum vapor, and other vapors of the
hydrocarbon series. A wide variety of aromatic substances such as benzene,
toluene and aliphatic solvents such as pentane, and hexane and
cycloaliphatic solvents such as cyclopentane and cyclohexane may also be
recovered by the process of the present invention. Further, the present
invention is applicable to the recovery of solvent vapors of ketone
solvents such as methylethyl ketone and methyl isobutyl ketone, as well as
alcohols and esters useful as solvents. In using the solvent recovery
system described, it is possible to separate two solvents. For example, a
high vapor pressure solvent may be mixed with another solvent of
substantially lower vapor pressure such as an aliphatic, aromatic or
cycloaliphatic hydrocarbon. In such a case when the two-stage absorption
process illustrated in FIG. 2 is performed, solvent vapor from the high
vapor pressure solvent will be recovered in the first absorption column
and the second solvent of substantially lower vapor pressure will be
recovered in the second absorption column. As those in the art are aware,
the higher the vapor pressure of a solvent, the more easily it is
generally absorbed.
The process will be further illustrated by the following example, which is
intended to be illustrative only and meant to include all techniques
equivalent thereto.
EXAMPLE
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1. Experimental data:
1-1 Mixed gas
Gasoline vapor 35 mol %
Air 65 mol %
1-2 Gasoline vapor recovering performance:
a. Concentration of gasoline vapor at the
entrance (y.sub.i): 35 mol %.sup.*1
b. Concentration of gasoline vapor at the
exit of the first absorption column:
10 mol %
c. Concentration of gasoline vapor in a
discharge gas (gas at the outlet of
the second absorption column) (y.sub.o):
5 mol %.sup.*2
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.sup.*1 : There was dispersion of 15 - 50 mol % in the actual operation.
.sup.*2 : There was dispersion of 2.5 - 5 mol % in the actual operation.
d. Recovering ratio:
##EQU1##
1-3 Operating conditions
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At the At the
time of time of
recovering
regenerating
operations
operations
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Pressure of the
1st absorption column
1.8 kg/cm.sup.2 G
10 kg/cm.sup.2 G
Temperature of the
absorbent (gasoline)
of the 1st absorption column
2.degree. C
2.degree. C
Pressure of the
2nd absorption column
1.75 kg/cm.sup.2 G
-500 mm Hg
Temperature of the
absorbent (kerosene)
of the 2nd absorption column
10.degree. C
10.degree. C
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It is understood that various other modifications will be apparent to and
can readily be made by those skilled in the art without departing from the
scope and spirit of this invention. Accordingly, it is not intended that
the scope of the claims appended hereto be limited to the description set
forth herein, but rather that the claims be construed as encompassing all
the features which reside in the present invention, including all features
which would be treated as equivalent thereof by those skilled in the art
to which the invention pertains.
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