Process for separating a volatile organic compound from a gas

Abstract

A process is disclosed for recovering a volatile organic compound at a recovery of 90 to 96% by volume from a gas generated from gasoline, kerosine, benzene, and alcohol discharged from storage tanks, tank trucks, and tank lorries. In the process comprising absorption stages followed by desorption stages for the volatile organic compound, the pressure at the desorption stages is controlled within the area below, for instance, curves C' and B in FIG. 1 of the drawings according to the concentration of the volatile organic compound in the gas to be fed to a first absorption stage.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technology by which a volatile organic compound can be recovered effectively from a mixed gas containing the organic compound in the state of gas to avoid the compound from being diffused in atmospheric air.

2. Discussion of the Background

Volatile hydrocarbons are diffused from storage tanks in atmospheric air when atmospheric temperature rises. Also, the volatile hydrocarbons are diffused in atmospheric air when they are flowed into storage tanks or when they are filled in tank lorries from storage tanks. Diffused hydrocarbons are said to form substances causing "photochemical smog". Accordingly, regulations have been carried out in Japan and other countries to control the concentration of the hydrocarbons in an exhaust or flue gas, and more severe regulations have lately been issued in several countries, the United States being a leading country.

For instance, a standard hydrocarbon concentration in the exhaust or flue gases at outlets is established under a prefectural regulation in Nagoya, Japan, which is the severest regulation in the past and by which the hydrocarbon concentration at the outlets is prescribed to be lower than 5% by volume (corresponding to about 80% by volume in terms of the recovery (as referred hereinafter) of hydrocarbon from a gas having a hydrocarbon concentration of 21% by volume at inlets). Under EPA in the United States, however, such harsh standards have been issuing that the discharge of VOC (volatile organic compound) shall be less than 35 mg/l gasoline for bulk gasoline terminals newly constructed or expanded after Dec. 17, 1980 and that recovery at vapor recovery systems shall be higher than 95% by weight for facilities storing petroleum products having a vapor pressure of 78 to 570 mmHg and newly constructed or expanded after May 19, 1978.

Several methods can be taken into consideration for recovering a hydrocarbon from a vapor. For instance, there have been proposed methods in which the hydrocarbon is separated from a vapor containing the hydrocarbon by adsorption with porous adsorbents such as activated carbons or separated by low temperature processing.

In the adsorption method, however, there is a danger of firing since a large amount of the heat of adsorption will be generated. In the low temperature processing method, the hydrocarbon vapor must be cooled down to a temperature lower than -35.degree. C. in order to increase the recovery percentage since the vapor has a very low liquefying temperature and thus this method is economically disadvantageous.

On the other hand, a method is known for separating a hydrocarbon from a hydrocarbon vapor safely and effectively wherein the vapor is washed with a liquid to absorb at an ambient temperature under an atmospheric pressure, the hydrocarbon absorbed in the liquid is separated to recover, and then the liquid is recycled to the washing.

Suitable liquids used for the absorption are ones which are insoluble in water, have a strong absorption power to hydrocarbon gas, and having such a low vapor pressure such that the liquids will not be lost when hydrocarbon is separated for recovery. Examples of the liquids comprise, as a major component, at least one compound selected from the group consisting of phthalic acid esters, silicic acid esters, phosphoric acid esters, fatty acid esters, alkylbenzene, alkylnaphthalene, and .alpha.-olefins. The liquids may additionally contain less than 75% by weight of a refined mineral oil having a viscosity of 5 to 20 cst at a temperature of 37.8.degree. C., a boiling point of 250.degree. to 450.degree. C., and an average molecular weight of 200 to 350.

According to the absorption method, the hydrocarbon concentration at the outlets can be controlled lower than 5% by volume by using a system composed of an absorption column, desorption column, recovery column, and vacuum pump. However, the absorption method has a defect that a severer standard regulated under, for example, the EPA in the United States can not be cleared.

An improved method for recovering a volatile organic compound from a gas by an absorption method has been proposed in U.S. Pat. No. 4,102,983 wherein a liquid mixture of an ester of phosphoric acid, silicic acid, or fatty acid with a mineral oil is used, an absorption liquid containing a volatile organic compound dissolved therein is subjected to flashing under a reduced pressure in a desorption column, and the liquid subjected to the flashing is recycled.

Another method has been proposed in Japanese patent publication 22503/1983 in which the flashing is carried out in two or more stages using two or more vessels kept under different reduced pressures.

However, in the method wherein the absorption liquid is recycled, the volatile organic compound will remain in the desorption column in an amount corresponding to the compound's vapor pressure under a reduced pressure at the stage of regeneration of the absorption liquid in the desorption column, and the remaining compound will obstruct the increase of the recovery percentage of volatile organic compound in the absorption column.

SUMMARY OF THE INVENTION

Thus, the object of the present invention is to provide a novel method for separating or recovering a volatile organic compound from a gas or vapor of gasoline, kerosene, benzene, or alcohol discharged from storage tanks, tank trucks, or tank lorries at a recovery of 90 to 96% by volume.

The term "gas" as used hereinafter is intended to have the meaning of gas, vapor and mist.

The present invention relates to a process for separating a volatile organic compound from a gas containing the organic compound by

introducing the gas into a first absorption column at a lower part in the first absorption column,

supplying a liquid for first absorption into the first absorption column at an upper part in the first absorption column to contact countercurrently with the gas to absorb a major portion of the organic compound in the gas,

introducing the gas leaving the first absorption column and containing a remaining organic compound into a second absorption column at a lower part in the second absorption column,

supplying another liquid for second absorption into the second absorption column at an upper part in the second absorption column to contact countercurrently with the gas from the first absorption column,

supplying the first absorption liquid leaving a lower part in the first absorption column into at a first desorption column at an upper part in the first desorption column,

recycling the liquid leaving at a lower part in the first desorption column back to the first absorption column as the liquid for the first absorption,

supplying the second absorption liquid leaving at a lower part in the second absorption column into a second desorption column at an upper part in the second desorption column,

recycling the liquid leaving at a lower part in the second desorption column back to the second absorption column as the liquid for the second absorption, and

recovering a remaining volatile organic compound,

the improvement which comprises controlling the operating pressure at the first and the second desorption columns within the area below the curves C' and B in FIG. 1 of the drawings relative to the concentration of the volatile organic compound in the gas to be fed into the first absorption column to reduce the amount of the volatile organic compound in a discharging gas to less than 10 when the amount of the volatile organic compound in the feeding gas was assumed to be 100 (Embodiment 1).

Also, the present invention relates to a process for separating a volatile organic compound from a gas containing the organic compound by

introducing the gas into a first absorption column at a lower part in the first absorption column,

supplying a liquid for first absorption into the first absorption column at an upper part in the first absorption column to contact countercurrently with the gas to absorb a major portion of the organic compound in the gas,

introducing the gas leaving the first absorption column and containing a remaining organic compound into a second absorption column at a lower part in the second absorption column,

supplying another liquid for second absorption into the second absorption column at an upper part in the second absorption column to contact countercurrently with the gas from the first absorption column,

supplying the first absorption liquid leaving at a lower part in the first absorption column into a first desorption column at an upper part in the first desorption column,

recycling the liquid leaving at a lower part in the first desorption column back to the first absorption column as the liquid for the first absorption,

supplying the second absorption liquid leaving at a lower part in the second absorption column into a second desorption column at an upper part in the second desorption column,

recycling the liquid leaving at a lower part in the second desorption column back to the second absorption column as the liquid for the second absorption, and

recovering a remaining volatile organic compound,

the improvement which comprises controlling the operating pressure at the first and the second desorption columns within the area below the curves A' and B in FIG. 1 of the drawings relative to the concentration of the volatile organic compound in the gas to be fed into the first absorption column to reduce the amount of the volatile organic compound in a discharging gas to less than 10 when the amount of the volatile organic compound in the feeding gas was assumed to be 100, while introducing air into the second desorption column at a lower part in the second desorption column (Embodiment 2).

Further, the present invention relates to the process according to Embodiment 1 wherein the operating pressure at the first and the second desorption columns are controlled within the area below the curve C in FIG. 1 relative to the concentration of the volatile organic compound in the gas to be fed into the first absorption column to reduce the amount of the volatile organic compound in a discharging gas to less than 5 when the amount of the volatile organic compound in a feeding gas was assumed to be 100 (Embodiment 3).

Still further, the present invention relates to the process according to Embodiment 2 wherein the operating pressure at the first and the second desorption columns are controlled within the area below the curves A and B in FIG. 1 of the drawings relative to the concentration of the volatile organic compound in the gas to be fed into the first absorption column to reduce the amount of the volatile organic compound in a discharging gas to less than 5 when the amount of the volatile organic compound in a feeding gas was assumed to be 100 (Embodiment 4).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the pressure in a desorption column and a concentration of a volatile organic compound contained in a gas to be introduced into a first absorption column.

FIG. 2 is a first flow diagram of a system for conducting the process according to Embodiments 1 and 3 of the present invention.

FIG. 3 is a second flow diagram of a system for conducting the process according to Embodiments of 2 and 4 of the present invention.

FIG. 4 is a flow diagram of a modified system of the second desorption column in FIG. 3 for conducting the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Now, the present invention will be described in further detail with reference to drawings.

In FIG. 1, line A is a graphic representation of equation A

P=190f/(1-0.95f) (A) wherein P is an operating pressure (mmHg) in a first and second desorption column, and f is a concentration (molar fraction) of a volatile organic compound in a gas to be introduced into the first absorption column at a lower part in the first absorption column.

The equation A shows the relationship between the concentration of a volatile organic compound in the feed gas and an operating pressure at thefirst and second desorption column keeping out of explosion limit of the second desorption column outlet gas by regulating air injection rate to the second desorption column at a recovery of 95% by volume of a volatile organic compound.

In FIG. 1, line A' is a graphic representation of equation A'

P=380f/(1-090f) (A') wherein P and f have the same meanings as in equation A except that the recovery is set at 90% by volume.

The equation A' shows the relationship between the concentration of a volatile organic compound in the feed gas and an operating pressure at thefirst and second desorption column keeping out of explosion limit of the second desorption column outlet gas by regulating air injection rate to the second desorption column at a recovery of 90% by volume of a volatile organic compound.

In FIG. 1, line B is a graphic representation of equation B

P=-70f+83 (B) wherein P and f have the same meanings as in equation A.

In the combination of the first absorption column with first desorption column, when an amount of a volatile organic compound contained in a rich oil leaving the first absorption column at a lower part in the first absorption column and being fed into the first desorption column is large,entrainment of the liquid will occur. If the entrainment occurred, the liquid will not only be lost but also flowed into a vacuum pump, leading to a cause of a mechanical trouble.

The amount of entrainment of the liquid accompanied with the evaporation ofvolatile organic compound will increase with increase in the amount of volatile organic compound contained in the rich oil, and 35% by mole (10% by weight) is an upper limit of the amount of the volatile organic compound which is permitted to contain in the rich oil, derived empirically through actual operation and from the view point of safety. When the amount is more than 35% by mole, the entrainment will occur such an extent that operation is impossible.

The concentration of the volatile organic compound in the rich oil will decrease with decrease in the concentration of the volatile organic compound in the gas to be fed to the first absorption column, since the amount of volatile organic compound accumulated in the liquid will decrease. Thus, the pressure in the first desorption column necessary to secure the amount in the liquid at lower than 35% by mole may become higher with decrease in the concentration of the hydrocarbon in the gas tobe fed into the first absorption column. The second desorption column is operated at the same pressure as the first desorption column.

In FIG. 1, line C is a graphic representation of equation C

P=3.86+45f+75.1f.sup.2 (C) wherein P and f have the same meanings as in equation

The line C shows the relationship between operating pressure P necessary when the air is not bubbled and concentration f of a volatile organic compound in the gas to be fed into the first absorption column at a lower part in the first absorption column at a recovery of the volatile organic compound of 95% by volume.

In FIG. 1, line C' is a graphic representation of equation C'

P=8.31+51f+200f.sup.2 (C') wherein P and f have the same meanings as in equation A except that the recovery is 90% by volume.

FIG. 2 is a first flow diagram of a system for conducting a first and thirdembodiments of the present invention. In FIG. 2, a gas containing the volatile organic compound, for example a hydrocarbon gas, is introduced through a line 11 into a first absorption column 1 at a lower part in the first absorption column, and contacted countercurrently with a liquid for first absorption introduced through a line 13 into the first absorption column at an upper part in the first absorption column to separate a majorportion of the volatile organic compound from the gas.

The gas thus treated is introduced through a line 14 into a second absorption column at a lower part in the second absorption column 2. The liquid which absorbed the volatile organic compound in the first absorption column 1 is introduced through a line 12 into a first desorption column 3 at an upper part in the first desorption column 3.

Operating conditions for the first desorption column are determined so thatthe conditions of the present invention are satisfied. In the first desorption column 3, the volatile organic compound is separated from the absorption liquid. Absorption liquid from which the volatile organic compound was separated is recycled through a line 13 back to the first absorption column 1 as the liquid for first absorption.

The gas containing the volatile organic compound separated in the first andsecond desorption columns is fed through lines 17, 18 and 19 to a recovery column 5 for recovering the volatile organic compound. A liquid for recovering the volatile organic compound is supplied through a line 20 to an upper part in the recovery column 5. The volatile organic compound is recovered from a line 21, and a treated gas is recycled back to the first absorption column 1 via line 22.

On the other hand, a liquid (second absorption liquid) which absorbed a remaining volatile organic compound in the second absorption column is introduced through a line 15 into a second desorption column 4 at an upperpart in the second desorption column. The second absorption liquid from which the volatile organic compound was separated is recycled through a line 16 back to the second absorption column 2. The gaseous organic compound separated in the second desorption column is fed through lines 17and 19 to the recovery column 5 for recovering the volatile organic compound.

FIG. 3 is a second flow diagram of a system for conducting a second and fourth embodiments of the present invention. The diagram is the same as that of FIG. 2 except that an air introducing pipe 23 for air bubbling is connected to a lower part in the second desorption column 4.

FIG. 4 is a flow diagram of a modified system of the process of the presentinvention in which a third desorption column 6 is provided below the seconddesorption column 4 shown in FIG. 3, and air bubbling is conducted in a third desorption column 6. Line 24 in FIG. 4 feeds a volatile organic compound to recovery column 5 after being desorbed by air bubbling in column 6.

According to the present invention, the relationship between the pressure (regeneration pressure) in each of the desorption columns and the concentration of a so-called inert component other than the volatile organic compound is not affected even if a gas flow rate was varied.

As a matter of course, in the present invention, air introducing rate and total flow rate in a vacuum pump are varied in proportion to gas flow rate.

EXAMPLES

Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by such specific Examples.

The points in each of Examples 1 through 4 are summarized as follows:

Example 1: Corresponding to embodiment 2 (area below curves A'-B, hydrocarbon recovery was higher than 90% by volume); refer to Tables 4 to 5; air was bubbled.

Example 2: Corresponding to embodiment 4 (area below curves A-B, hydrocarbon recovery was higher than 95% by volume); refer to Tables 6 to 7; air was bubbled.

Example 3: Corresponding to embodiment 1 (area below curves C'-B, hydrocarbon recovery was higher than 90%); refer to Tables 8 to 9; air wasnot bubbled.

Example 4: Corresponding to embodiment 3 (area below curve C, hydrocarbon recovery was higher than 95% by volume); refer to Tables 10 to 11; air wasnot bubbled.

All of the Examples were to achieve the hydrocarbon recovery of higher than90% by volume at a hydrocarbon concentration in a feeding gas (gas to be fed to the first absorption gas) of 10 to 40% by volume. Accordingly, the molar ratio of liquid/gas was suitably 8 at the absorption stages, operating pressure at the desorption stages was determined depending on the hydrocarbon concentration in the feeding gas, and a considerable amount of air was bubbled in the second desorption column in Examples 1 and 2 to accelerate the regeneration of the liquid for absorption.

First, the conditions and results in Examples 1 and 3 are explained specifically by comparing them with reference to Tables 4 to 5 and 8 to 9.In Example 1 (Tables 4 to 5), air was bubbled in the second desorption column at a gas flow rate of 1000 Nm.sup.3 /hr at the inlet in the first absorption column, and in Example 3 (Tables 8 to 9), air was not bubbled but the inlet gas flow rate was the same as in Example 1. The recovery of the hydrocarbon was aimed at 90% by volume in both Examples.

The pressure in the first and second desorption columns in Example 1 (Tables 4 to 5) was higher than that in Example 3 (Tables 8 to 9) since air was bubbled in Example 1. As the result, the necessary gas flow rate of vacuum pump was 98 m.sup.3 /min at maximum in Example 1 (Tables 4 to 5)while the necessary gas flow rate was 110 m.sup.3 /min in Example 3 (Tables8 to 9) at maximum. This indicates that the necessary gas flow rate of a vacuum pump was higher by 12 m.sup.3 /min in Example 3 wherein air was notbubbled.

Calculation was performed for the number of necessary vacuum pumps having arated capacity of 50 m.sup.3 /min (at a suction pressure of 25 mmHg) from the data shown in Tables 4 to 5 and 8 to 9 to obtain the results that 1.6 to 2.0 vacuum pumps are necessary to be used in Example 1 (Tables 4 to 5) while 1.8 to 2.5 vacuum pumps are necessary in Example 3 (Tables 8 to 9) as shown in detail in Table 1.

TABLE 1______________________________________Number of necessary vacuum pumps (rated capacity 50 m.sup.3 min,gas recovery 90% by volume)______________________________________Hydrocarbon 10.6 12.7 16.0 19.9 24.6 30 36.5 40.2concentrationin feedinggas (%)Example 3 2.5 2.4 2.4 2.2 1.9 1.8 1.6 1.7Air was notbubbled.Example 1 2.0 1.9 1.8 1.7 1.6 1.6 -- --Air was notbubbled.______________________________________

Next, the conditions and results in Examples 2 and 4 are explained specifically by comparing them with reference to Tables 6 to 7 and 10 to 11. In Example 2 (Tables 6 to 7), air was bubbled in the second desorptioncolumn at a gas flow rate of 1000 Nm.sup.3 /hr at the inlet in the first absorption column, and in Example 4 (Tables 10 to 11), air was not bubbledbut the inlet gas flow rate was the same as in Example 2. The recovery of the hydrocarbon was aimed at 95% by volume in both Examples.

The pressure in the first and second desorption columns in Example 2 (Tables 6 to 7) was higher than that in Example 4 (Tables 10 to 11) since air was bubbled in Example 2. As the result, the necessary gas flow rate of vacuum pump was 160 m.sup.3 /min at maximum in Example 2 (Tables 6 to 7) while the necessary gas flow rate was 200 m.sup.3 /min in Example 4 (Tables 10 to 11) at maximum. This indicates that the necessary gas flow rate of a vacuum pump was higher by 40 m.sup.3 /min in Example 4 wherein air was not bubbled.

As in the cases in Examples 2 and 4, calculation was performed for the number of necessary vacuum pumps having a rated capacity of 50 m.sup.3 /min (at a suction pressure of 25 mmHg) from the data shown in Tables 6 to7 and 10 to 11 to obtain the results that 2.5 to 3.8 vacuum pumps are necessary to be used in Example 2 (Tables 6 to 7) while 3.1 to 7.1 vacuum pumps are necessary in Example 4 (Tables 10 to 11) as shown in detail in Table 2.

TABLE 2______________________________________Number of necessary vacuum pumps (rated capacity 50 m.sup.3 /min,gas recovery 95% by volume)______________________________________Hydrocarbon 10.6 12.7 16.0 19.9 24.6 30 36.5 40.2concentrationin feedinggas (%)Example 4 7.1 5.8 5.1 4.5 4.4 3.6 3.3 3.1Air was notbubbled.Example 2 3.8 3.5 3.4 3.3 2.8 2.7 2.5 2.5Air wasbubbled.______________________________________

Results in respect of the number of necessary vacuum pumps are summarized as follows:

______________________________________ Number of necessary vacuumRecovery Air pumps (rated capacity 50(% by volume) bubbling m.sup.3 /min)______________________________________90 no 1.8 to 2.590 yes 1.6 to 2.095 no 3.1 to 7.195 yes 2.5 to 3.8______________________________________

As will be understood from the above, number of vacuum pumps to be used must be considerably increased in order to increase the gas recovery percentage.

From the data obtained in Examples 1 through 4, the following conclusions can be drawn:

(i) When the recovery of hydrocarbon from a gas containing 10 to 40% by volume of the hydrocarbon is to be kept at a level of higher than 90% by volume in a hydrocarbon gas recovery unit, operating conditions in the desorption columns will be dominant, and it will lead to the reduction of the cost of plant to maintain the operating pressure in the desorption column as high as possible.

(ii) Under such conditions, it will become possible to maintain the operating pressure in the desorption columns at a high level when a considerable amount of air is bubbled in the second desorption column.

Actually, a rational or effective plant can be designed by obtaining best operating conditions for a minimum plant cost through a simulation at a required gas recovery to be aimed.

For convenience, an example of an expected performance of a vacuum pump is shown in Table 3.

TABLE 3______________________________________ Discharge pressure 0.1 kg/cm.sup.2 g______________________________________Suction 60 55 50 45 40 35 30pressure mmHgVacuum flow rate 59 58.5 58 57 56.5 54.5 53m.sup.3 /min______________________________________Suction 25 20 15 10 9 8pressure mmHgVacuum flow rate 50 45 41.5 30 27.5 20m.sup.3 /min______________________________________

EXAMPLE 1

Corresponding to embodiment 2 (area below curves A'-B, recovery higher than90% by volume)

TABLE 4______________________________________Hydrocarbon 10.6 12.7 16.0 19.9concentration infeeding gas % byvolumeFirst absorption 1000 1000 1000 1000column, inlet gasflow rate Nm.sup.3 /hrFirst absorption 100.2 98.3 94.3 90.1column, inlet liquidflow rate T/hrFirst absorption 765 765 765 765column, operatingpressure mm HgFirst absorption 35.2 35.3 35.3 35.4column, operatingtemperature .degree.C.First absorption 954 934 907 871column, outlet gasflow rate Nm.sup.3 /hrFirst absorption 2.90 3.23 4.10 4.87column, hydrocarbonconcentration inoutlet gas % byvolumeFirst absorption 2.5 2.9 3.8 4.8column, hydrocarbonconcentration inoutlet liquid % byweightSecond absorption 98.1 95.5 91.5 86.0column, inlet liquidflow rate T/hrSecond absorption 760 760 760 760column, operatingpressure mm HgSecond absorption 35.1 35.1 35.2 35.2column, operatingtemperature .degree.C.Second absorption 925 905 876 838column, outlet gasflow rate Nm.sup.3 /hrSecond absorption 1.15 1.38 1.77 2.30column, hydrocarbonconcentration in outletgas % by volumeFirst desorption 23 26 31 36column, operatingpressure mmHgFirst desorption 35.3 35.5 35.5 35.7column, operatingtemperature .degree.C.Second desorption 23 26 31 36column, operatingpressure mmHgSecond desorption 35.1 35.2 35.3 35.4column, operatingtemperature .degree.C.Second desorption 24.5 23.5 22.4 20.8column, air bubblerate Nm.sup.3 /hrSecond desorption 67.3 65.7 59.9 56.7column, inlet gasconcentration in outletgas % by volumeHydrocarbon 74.8 77.1 77.6 79.4recovery(first absorptioncolumn) % by volumeHydrocarbon 61.0 58.1 57.9 54.0recovery(second absorptioncolumn % by volume)Hydrocarbon 90.0 90.2 90.3 90.3recovery(total) % by volume.Vacuum pump flow 97.9 95.5 92.9 92.7rate m.sup.3 /min______________________________________

EXAMPLE 1 (Continued)

Corresponding to embodiment 2 (area below curves A'-B, recovery higher than90% by volume)

TABLE 5______________________________________Hydrocarbon concentration in 24.6 30.0feeding gas % by volumeFirst absorption column, inlet 1000 1000gas flow rate Nm.sup.3 /hrFirst absorption column, inlet 84.6 79.1liquid flow rate T/hrFirst absorption column, 765 765operating pressure mm HgFirst absorption column, 35.5 35.6operating temperature .degree.C.First absorption column, outlet 836 786gas flow rate Nm.sup.3 /hrFirst absorption column, 6.19 7.46hydrocarbon concentration inoutlet gas % by volumeFirst absorption column, 6.3 7.9hydrocarbon concentration inoutlet liquid % by weightSecond absorption column, inlet 80.2 73.0liquid flow rate T/hrSecond absorption column, 760 760operating pressure mm HgSecond absorption column, 35.3 35.3operating temperature .degree.C.Second absorption column, outlet 800 748gas flow rate Nm.sup.3 /hrSecond absorption column, 3.05 3.98hydrocarbon concentration inoutlet gas % by volumeFirst desorption column, 44 51operating pressure mmHgFirst desorption column, 36.0 36.2operating temperature .degree.C.Second desorption column, 44 51operating pressure mmHgSecond desorption column, 35.5 35.5operating temperature .degree.C.Second desorption column, air 24.2 21.8bubble rate Nm.sup.3 /hrSecond desorption column, inlet 54.8 51.1gas concentration in outlet gas %by volumeHydrocarbon recovery (first 79.8 81.2absorption column) % by volumeHydrocarbon recovery (second 52.3 48.6absorption column % by volume)Hydrocarbon recovery (total) % by 90.1 90.1volumeVacuum pump flow rate m.sup.3 /min 90.5 90.4______________________________________

EXAMPLE 2

Corresponding to embodiment 4 (area below curves A-B, recovery higher than 95% by volume)

TABLE 6______________________________________Hydrocarbon 10.6 12.7 16.0 19.9concentration infeeding gas % byvolumeFirst absorption 1000 1000 1000 1000column, inlet gasflow rate Nm.sup.3 /hrFirst absorption 100.2 100.0 94.4 93.9column, inlet liquidflow rate T/hrFirst absorption 765 765 765 765column, operatingpressure mm HgFirst absorption 35.1 35.2 35.3 35.4column, operatingtemperature .degree.C.First absorption 944 925 893 862column, outlet gasflow rate Nm.sup.3 /hrFirst absorption 1.84 2.21 2.60 3.18column, hydrocarbonconcentration inoutlet gas % byvolumeFirst absorption 1.6 1.9 2.8 3.1column, hydrocarbonconcentration inoutlet liquid % byweightSecond absorption 98.7 96.1 92.0 88.3column, inlet liquidflow rate T/hrSecond absorption 760 760 760 760column, operatingpressure mm HgSecond absorption 35.1 35.1 35.2 35.2column, operatingtemperature .degree.C.Second absorption 921 901 869 835column, outlet gasflow rate Nm.sup.3 /hrSecond absorption 0.54 0.71 0.88 1.14column, hydrocarbonconcentration inoutlet gas % byvolumeFirst desorption 15 17 20 23column, operatingpressure mmHgFirst desorption 35.3 35.4 35.5 35.7column, operatingtemperature .degree.C.Second desorption 15 17 20 23column, operatingpressure mmHgSecond desorption 35.1 35.1 35.2 35.3column, operatingtemperature .degree.C.Second desorption 25.9 24.7 22.0 27.7column, air bubblerate Nm.sup.3 /hrSecond desorption 74.1 71.4 68.5 67.6column, inlet gasconcentration inoutlet gas % byvolumeHydrocarbon 83.6 83.9 85.6 86.2recovery (firstabsorption column)% by volumeHydrocarbon 71.1 68.5 65.4 65.2recovery (secondabsorption column %by volume)Hydrocarbon 95.3 94.9 95.0 95.2recovery (total) %by volumeVacuum pump flow 156.9 152.3 155.0 157.1rate m.sup.3 /min______________________________________

EXAMPLE 2 (Continued)

Corresponding to embodiment 4 (area below curves A-B, recovery higher than 95% by volume)

TABLE 7______________________________________Hydrocarbon 24.6 30.0 36.5 40.2concentration infeeding gas % byvolumeFirst absorption 1000 1000 1000 1000column, inlet gasflow rate Nm.sup.3 /hrFirst absorption 88.3 84.2 78.3 74.6column, inlet liquidflow rate T/hrFirst absorption 765 765 765 765column, operatingpressure mm HgFirst absorption 35.4 35.8 35.7 35.9column, operatingtemperature .degree.C.First absorption 818 821 709 673column, outlet gasflow rate Nm.sup.3 /hrFirst absorption 4.14 4.95 6.21 7.09column, hydrocarbonconcentration inoutlet gas % byvolumeFirst absorption 4.6 4.9 6.3 7.1column, hydrocarbonconcentration inoutlet liquid % byweightSecond absorption 81.8 75.8 68.3 63.7column, inlet liquidflow rate T/hrSecond absorption 760 760 760 760column, operatingpressure mm HgSecond absorption 35.3 35.3 35.3 35.3column, operatingtemperature .degree.C.Second absorption 789 735 676 638column, outlet gasflow rate Nm.sup.3 /hrSecond absorption 1.53 2.06 2.72 3.10column, hydrocarbonconcentration inoutlet gas % byvolumeFirst desorption 30 33 40 44column, operatingpressure mmHgFirst desorption 35.8 36.1 36.4 36.6column, operatingtemperature .degree.C.Second desorption 30 33 40 44column, operatingpressure mmHgSecond desorption 35.3 35.5 35.5 35.5column, operatingtemperature .degree.C.Second desorption 23.1 23.1 24.8 23.3column, air bubblerate Nm.sup.3 /hrSecond desorption 60.0 58.0 55.9 52.1column, inlet gasconcentration inoutlet gas % byvolumeHydrocarbon 86.4 87.4 88.0 88.1recovery (firstabsorption column)% by volumeHydrocarbon 61.6 60.1 58.2 58.5recovery (secondabsorption column %by volume)Hydrocarbon 94.8 95.0 95.0 95.1recovery (total) %by volumeVacuum pump flow 151.0 147.3 143.6 141.3rate m.sup.3 /min______________________________________

EXAMPLE 3

Corresponding to embodiment 1 (area below curves C'-B, recovery higher than90% by volume)

TABLE 8______________________________________Hydrocarbon 10.6 12.7 16.0 19.9concentration infeeding gas % byvolumeFirst absorption 1000 1000 1000 1000column, inlet gasflow rate Nm.sup.3 /hrFirst absorption 98.1 96.2 93.0 88.7column, inlet liquidflow rate T/hrFirst absorption 765 765 765 765column, operatingpressure mm HgFirst absorption 35.1 35.1 35.2 35.3column, operatingtemperature .degree.C.First absorption 922 903 874 839column, outlet gasflow rate Nm.sup.3 /hrFirst absorption 2.04 2.35 2.82 3.58column, hydrocarbonconcentration inoutlet gas % byvolumeFirst absorption 2.1 2.5 3.1 4.1column, hydrocarbonconcentration inoutlet liquid % byweightSecond absorption 94.1 91.5 87.3 81.8column, inlet liquidflow rate T/hrSecond absorption 760 760 760 760column, operatingpressure mm HgSecond absorption 35.0 35.0 35.0 35.0column, operatingtemperature .degree.C.Second absorption 903 884 854 819column, outlet gasflow rate Nm.sup.3 /hrSecond absorption 1.19 1.41 1.74 2.29column, hydrocarbonconcentration inoutlet gas % byvolumeFirst desorption 16 18 21 26column, operatingpressure mmHgFirst desorption 35.2 35.2 35.4 35.6column, operatingtemperature .degree.C.Second desorption 16 18 21 26column, operatingpressure mmHgSecond desorption 35.0 35.0 35.0 35.0column, operatingtemperature .degree.C.Second desorption -- -- -- --column, air bubblerate Nm.sup.3 /hrSecond desorption 46.9 37.2 31.4 27.7column, inlet gasconcentration inoutlet gas % byvolumeHydrocarbon 82.4 83.5 84.8 85.1recovery (firstabsorption column)% by volumeHydrocarbon 42.2 40.6 39.0 36.9recovery (secondabsorption column %by volume)Hydrocarbon 89.8 90.2 90.7 90.6recovery (total) %by volumeVacuum pump flow 103 105 110 109rate m.sup.3 /min______________________________________

EXAMPLE 3 (Continued)

Corresponding to embodiment 1 (area below curves C'-B, recovery higher than90% by volume)

TABLE 9______________________________________Hydrocarbon 24.6 30.0 36.5 40.2concentration infeeding gas % byvolumeFirst absorption 1000 1000 1000 1000column, inlet gasflow rate Nm.sup.3 /hrFirst absorption 83.4 77.4 69.2 67.0column, inlet liquidflow rate T/hrFirst absorption 765 765 765 765column, operatingpressure mm HgFirst absorption 35.4 35.7 35.8 35.9column, operatingtemperature .degree.C.First absorption 799 753 698 659column, outlet gasflow rate Nm.sup.3 /hrFirst absorption 4.66 6.06 8.19 8.46column, hydrocarbonconcentration inoutlet gas % byvolumeFirst absorption 5.5 7.1 9.5 9.9column, hydrocarbonconcentration inoutlet liquid % byweightSecond absorption 74.9 67.3 57.8 54.0column, inlet liquidflow rate T/hrSecond absorption 760 760 760 760column, operatingpressure mm HgSecond absorption 35.0 35.2 35.3 35.3column, operatingtemperature .degree.C.Second absorption 777 728 670 638column, outlet gasflow rate Nm.sup.3 /hrSecond absorption 3.06 4.04 5.46 5.49column, hydrocarbonconcentration inoutlet gas % byvolumeFirst desorption 33 42 55 55column, operatingpressure mmHgFirst desorption 35.8 36.3 36.5 36.8column, operatingtemperature .degree.C.Second desorption 33 42 55 55column, operatingpressure mmHgSecond desorption 35.0 35.3 35.5 35.5column, operatingtemperature .degree.C.Second desorption -- -- -- --column, air bubblerate Nm.sup.3 /hrSecond desorption 24.8 24.8 25.5 23.6column, inlet gasconcentration inoutlet gas % byvolumeHydrocarbon 85.0 84.9 84.4 86.3recovery (firstabsorption column)% by volumeHydrocarbon 35.4 34.7 35.3 37.1recovery (secondabsorption column %by volume)Hydrocarbon 90.3 90.2 90.0 91.3recovery (total) %by volume)Vacuum pump flow 104 100 91 101rate m.sup.3 /min______________________________________

EXAMPLE 4

Corresponding to embodiment 3 (area below curve C, recovery higher than 95%by volume)

TABLE 10______________________________________Hydrocarbon 10.6 12.7 16.0 19.9concentration infeeding gas % byvolumeFirst absorption 1000 1000 1000 1000column, inlet gasflow rate Nm.sup.3 /hrFirst absorption 100.0 98.2 95.4 91.9column, inlet liquidflow rate T/hrFirst absorption 765 765 765 765column, operatingpressure mm HgFirst absorption 35.2 35.5 35.5 35.5column, operatingtemperature .degree.C.First absorption 914 895 864 828column, outlet gasflow rate Nm.sup.3 /hrFirst absorption 1.18 1.46 1.78 2.26column, hydrocarbonconcentration inoutlet gas % byvolumeFirst absorption 1.3 1.6 2.1 2.8column, hydrocarbonconcentration inoutlet liquid % byweightSecond absorption 96.1 92.8 89.1 84.1column, inlet liquidflow rate T/hrSecond absorption 760 760 760 760column, operatingpressure mm HgSecond absorption 35.0 35.3 35.3 35.3column, operatingtemperature .degree.C.Second absorption 897 878 847 810column, outlet gasflow rate Nm.sup.3 /hrSecond absorption 0.54 0.72 0.92 1.23column, hydrocarbonconcentration inoutlet gas % byvolumeFirst desorption 9 11 13 16column, operatingpressure mmHgFirst desorption 35.3 35.4 35.5 35.6column, operatingtemperature .degree.C.Second desorption 9 11 13 16column, operatingpressure mmHgSecond desorption 35.5 35.5 35.5 35.5column, operatingtemperature .degree.C.Second desorption -- -- -- --column, air bubblerate Nm.sup.3 /hrSecond desorption 66.8 55.1 60.6 56.7column, inlet gasconcentration inoutlet gas % byvolumeHydrocarbon 89.9 89.9 90.5 91.0recovery (firstabsorption column)% by volumeHydrocarbon 54.5 50.9 48.7 46.1recovery (secondabsorption column %by volume)Hydrocarbon 95.4 95.0 95.1 95.0recovery (total) %by volumeVacuum pump flow 196.1 185.5 190.5 187.8rate m.sup.3 /min______________________________________

EXAMPLE 4 (Continued)

Corresponding to embodiment 3 (area below curve C, recovery higher than 95%by volume)

TABLE 11______________________________________Hydrocarbon 24.6 30.0 36.5 40.2concentration infeeding gas % byvolumeFirst absorption 1000 1000 1000 1000column, inlet gasflow rate Nm.sup.3 /hrFirst absorption 87.9 83.0 77.0 73.6column, inlet liquidflow rate T/hrFirst absorption 765 765 765 765column, operatingpressure mm HgFirst absorption 35.5 35.5 35.5 35.5column, operatingtemperature .degree.C.First absorption 783 734 673 639column, outlet gasflow rate Nm.sup.3 /hrFirst absorption 2.72 3.64 4.77 5.56column, hydrocarbonconcentration inoutlet gas % byvolumeFirst absorption 3.5 4.6 6.0 6.9column, hydrocarbonconcentration inoutlet liquid % byweightSecond absorption 78.3 71.4 63.3 58.7column, inlet liquidflow rate T/hrSecond absorption 760 760 760 760column, operatingpressure mm HgSecond absorption 35.3 35.3 35.3 35.3column, operatingtemperature .degree.C.Second absorption 765 714 652 617column, outlet gasflow rate Nm.sup.3 /hrSecond absorption 1.56 2.10 2.79 3.25column, hydrocarbonconcentration inoutlet gas % byvolumeFirst desorption 19 24 30 34column, operatingpressure mmHgFirst desorption 35.8 36.1 36.4 36.6column, operatingtemperature .degree.C.Second desorption 19 24 30 34column, operatingpressure mmHgSecond desorption 35.5 35.5 35.5 35.5column, operatingtemperature .degree.C.Second desorption -- -- -- --column, air bubblerate Nm.sup.3 /hrSecond desorption 52.4 47.2 40.6 36.6column, inlet gasconcentration inoutlet gas % byvolumeHydrocarbon 91.2 91.2 91.3 91.3recovery (firstabsorption column)% by volumeHydrocarbon 44.6 43.2 42.7 42.9recovery (secondabsorption column %by volume)Hydrocarbon 95.1 95.0 95.0 95.0recovery (total) %by volumeVacuum pump flow 191.7 181.9 174.6 168.8rate m.sup.3 /min______________________________________

Claims

1. In a process for separating a volatile organic compound from a gas containing the volatile organic compound by

introducing the gas into a first absorption column at a lower part in the first absorption column,

supplying a first absorption liquid into the first absorption column at an upper part at the first absorption column to contact countercurrently with the gas to absorb a major portion of the volatile organic compound in the gas,

introducing the gas leaving the first absorption column and containing a remaining volatile organic compound into a second absorption column at a lower part at the second absorption column,

supplying a second absorption liquid into the second absorption column at an upper part of the second absorption column to contact countercurrently with the gas from the first absorption column,

supplying the first absorption liquid leaving at a lower part of the first absorption column into a first desorption column at an upper part of the first desorption column,

recycling the liquid leaving at a lower part of the first desorption column back to the first absorption column as the first absorption liquid,

supplying the second absorption liquid leaving at a lower part of the second absorption column into a second desorption column at an upper part of the second desorption column,

recycling the liquid leaving at a lower part of the second desorption column back to the second absorption column as the second absorption liquid, and

recovering a remaining volatile organic compound,

the improvement which comprises controlling the operating pressure at the first and second desorption columns within the area defined below curve:

P=8.31+51f+200F.sup.2

where P is the operating pressure in mmHg and f is the concentration in molar fraction the volatile organic compound in the gas to be fed into the first absorption column, when the concentration is at or less than 0.379 but greater than or equal to 0.100 and within the area defined below line:

P=-70f+83

when the concentration is greater than 0.379 but less than or equal to 0.400, to reduce the amount of volatile organic compound in a discharging gas to less than 0.100 when the amount of volatile organic compound in a feeding gas was assumed to be 1.

2. In a process for separating a volatile organic compound from a gas containing the volatile organic compound by

introducing the gas into a first absorption column at a lower part of the first absorption column,

supplying a first absorption liquid for first absorption into the first absorption column at an upper part of the first absorption column to contact countercurrently with the gas to absorb a major portion of the volatile organic compound in the gas,

introducing the gas leaving the first absorption column and containing a remaining volatile organic compound into a second absorption column at a lower part at the second absorption column,

supplying a second absorption liquid for second absorption in the second absorption column at an upper part of the second absorption column to contact countercurrently with the gas from the first absorption column,

supplying the first absorption liquid leaving at a lower part at the first absorption column into a first desorption column at an upper part of the first desorption column,

recycling the liquid leaving at a lower part of the first desorption column back to the first absorption column as the first absorption liquid,

supplying the second absorption liquid leaving at a lower part of the second absorption column into a second desorption column at an upper part of the second desorption column,

recycling the liquid leaving at a lower part of the second desorption column back to the second absorption column as the second absorption liquid, and

recovering a remaining volatile organic compound,

the improvement which comprises controlling the operating pressure at the first and the second desorption columns within the area defined below curve:

P=380f/(1-0.90f)

where P is the operating pressure in mmHg and f is the concentration in molar fraction of the volatile organic compound in the gas to be fed into the first absorption column, when the concentration is at or less than 0.161 but greater than 0.100, and within the area defined below line:

P=-70f+83

when the concentration is greater than 0.161 but less than 0.400, to reduce the amount of volatile organic compound in a discharging gas to less than 0.100 when the amount of volatile organic compound in a feeding gas was assumed to be 1, while introducing air into the second desorption column at a lower part of the second desorption column.

3. In a process for separating a volatile organic compound from a gas containing the volatile organic compound by

introducing the gas into a first absorption column at a lower part in the first absorption column,

supplying a first absorption liquid into the first absorption column at an upper part at the first absorption column to contact countercurrently with the gas to absorb a major portion of the volatile organic compound in the gas,

introducing the gas leaving the first absorption column and containing a remaining volatile organic compound into a second absorption column at a lower part at the second absorption column,

supplying a second absorption liquid into the second absorption column at an upper part of the second absorption column to contact countercurrently with the gas from the first absorption column,

supplying the first absorption liquid leaving at a lower part of the first absorption column into a first desorption column at an upper part of the first desorption column,

recycling the liquid leaving at a lower part of the first desorption column back to the first absorption column as the first absorption liquid,

supplying the second absorption liquid leaving at a lower part of the second absorption column into a second desorption column at an upper part of the second desorption column,

recycling the liquid leaving at a lower part of the second desorption column back to the second absorption column as the second absorption liquid, and

recovering a remaining volatile organic compound,

the improvement which comprises controlling the operating pressure at the first and second desorption columns within the area defined below curve:

P=3.86+45f+75.1f.sup.2

where P is the operating pressure in mmHg and f is the concentration in molar fraction the volatile organic compound in the gas to be fed into the first absorption column, to reduce the amount of volatile organic compound in a discharging gas to less than 0.005 when the amount of volatile organic compound in a feeding gas was assumed to be 1.

4. In a process for separating a volatile organic compound from a gas containing the volatile organic compound by

introducing the gas into a first absorption column at a lower part of the first absorption column,

supplying a first absorption liquid for first absorption into the first absorption column at an upper part of the first absorption column to contact countercurrently with the gas to absorb a major portion of the volatile organic compound in the gas,

introducing the gas leaving the first absorption column and containing a remaining volatile organic compound into a second absorption column at a lower part at the second absorption column,

supplying a second absorption liquid for second absorption in the second absorption column at an upper part of the second absorption column to contact countercurrently with the gas from the first absorption column,

supplying the first absorption liquid leaving at a lower part at the first absorption column into a first desorption column at an upper part of the first desorption column,

recycling the liquid leaving at a lower part of the first desorption column back to the first absorption column as the first absorption liquid,

supplying the second absorption liquid leaving at a lower part of the second absorption column into a second desorption column at an upper part of the second desorption column,

recycling the liquid leaving at a lower part of the second desorption column back to the second absorption column as the second absorption liquid, and

recovering a remaining volatile organic compound,

the improvement which comprises controlling the operating pressure at the first and the second desorption columns within the area defined below curve:

P=190f/(1-0.95f)

where P is the operating pressure in mmHg and f is the concentration in molar fraction of the volatile organic compound in the gas to be fed into the first absorption column, when the concentration is at or less than 0.258 but greater than 0.100, and within the area defined below line:

P=-70f+83

when the concentration is greater than 0.258 but less than 0.400, to reduce the amount of volatile organic compound in a discharging gas to less than 0.005 when the amount of volatile organic compound in a feeding as was assumed to be 1, while introducing air into the second desorption column at a lower part of the second desorption column.