Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-07-19
2002-05-14
Smith, Duane (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S055000, C096S007000, C096S009000
Reexamination Certificate
active
06387157
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a concentration regulating method and a concentration regulating apparatus for carrying out the concentration regulation of a mixed gas by gas separation, and particularly to a useful technique of decreasing the concentration of a hardly permeable gas as in such a case that the concentration of a mixed gas consisting of nitrogen gas and hydrogen gas is regulated to produce a feed material for ammonia synthesis.
DESCRIPTION OF THE RELATED ART
A technique of carrying out the purification of a mixed gas by utilization of a gas separation membrane has been hitherto known, and the mechanism and principle thereof are as follows. By use of a gas separation membrane made of a material different in permeability to the respective components of a mixed gas as a feed material is used, namely, the mixed gas fed as it is pressurized is separated to a permeate gas which is permeated through the same gas separation membrane and a retentate gas which is retained on the feed side. At that time, a difference (a difference in partial pressure) between one (a gas partial pressure) obtained by multiplying the whole pressure of the feed side (the high pressure side) by the mole fraction of each gas component and the other (a gas partial pressure) obtained by multiplying the whole pressure of the permeation side (the low pressure side) by the mole fraction of the same gas component becomes a motive power of permeation, and the permeation of each gas component is caused at a ratio obtained by multiplying this difference in partial pressure by a permeability and a membrane area. In the gas separation, accordingly, an easily permeable gas higher in permeability and a hardly permeable gas lower in permeability are separated as they are concentrated on the permeation side and on the feed side (the retentate gas side), respectively.
In a case where the concentration regulation of a mixed gas is carried out by utilization of such a gas separation technique as mentioned above, on the other hand, the concentration of a hardly permeable gas can be decreased, for instance, by leading out a part of a mixed gas as a feed material and separating it to a permeate gas and a retentate gas by a gas separation membrane, and then introducing the permeate gas, in which an easily permeable gas has been concentrated, into the mixed gas again.
In such a concentration regulating method, the following characteristics are particularly taken into consideration.. Namely, the concentration of an easily permeable gas in a retentate gas is lower and the higher the recovery of the easily permeable gas is, the higher the efficiency of gas separation is, and this becomes an important factor in a case where a compensation to the loss of the easily permeable gas is higher. In a case where the concentration of the easily permeable gas in a permeate gas is extremely lower, although it is not so important, on the other hand, the hardly permeable gas become apt to be entrained in the permeate gas and this is disadvantageous because a power for the gas separation therefore increases. When the pressure ratio (the feed side/the permeation side) of a gas separation membrane is low, a power expense for the pressure raising becomes smaller, but an equipment expense will increase because the recovery of the easily permeable gas becomes lower or the membrane area must be increased for maintaining the same recovery.
A case where a mixed gas consisting of nitrogen gas and hydrogen gas is subjected to the concentration regulation by the aforementioned concentration regulating method to produce a feed material for the ammonia synthesis, will be concretely described on the basis of FIG.
4
.
A mixed gas as a feed material is produced by another process, and it has, for example, a whole pressure of about 25 bar (absolute pressure, this unit will apply to the following), in which the mole ratio of nitrogen gas exceeds a mole ratio (⅓) suitable for ammonia synthesis, and hence the concentration of nitrogen gas must be decreased. Since it is required in an ammonia synthesis process that the feed pressure of a mixed gas is generally 60~200 bar, on the other hand, the pressure thereof is raised to the same pressure in turn by a first booster C
11
and a second booster C
12
. In this example, the pressure raising is performed to 50 bar in the first stage and to 100 bar in the second stage.
After a gas partially led out of the outlet side line of the second booster C
12
is separated to a permeate gas and a retentate gas in a gas separation section S
0
having a gas separation membrane M
0
, the permeate gas is introduced into the inlet side line of the first booster C
11
so as to be joined with the feed material mixed gas. At that time, the retentate gas, in which hardly permeable nitrogen gas has been concentrated, is discharged out of the system and removed from the mixed gas, and hence the concentration of the hardly permeable gas in the mixed gas can be decreased. Thus, a mixed gas having a desired mole ratio can be obtained.
However, if the area of a gas separation membrane is made larger and namely the number of membrane modules connected in parallel is increased in the aforementioned concentration regulating method, the recovery of hydrogen gas will become higher, but there is contrary a problem of increasing the equipment expense. If the pressure ratio is lowered in order to restrain the power expense of a booster in such a case as mentioned above, where the stage number of gas separation sections is one stage, the recovery of an easily permeable gas will lower, and hence it has been difficult to make compatible the maintenance of the recovery of an easily permeable gas and the restraint of the power expense. In the conventional method using one stage of the gas separation section, namely, there has been such a limitation as mentioned above.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a mixed gas concentration regulating method and a concentration regulating apparatus, in which the maintenance of the recovery of an easily permeable gas and the restraint of the power expense are compatible.
The aforementioned purpose can be achieved by the present invention, as follows. Namely, the mixed gas concentration regulating method according to the present invention comprises feeding a mixed gas consisting of two or more gases different in permeability to a gas separation membrane to a line having a booster so that the pressure thereof is raised, leading out a part of the mixed gas from said pressure raising line and separating it to a permeate gas and a retentate gas by a gas separation membrane, and then introducing said permeate gas into said pressure raising line, thereby regulating the concentration of the mixed gas, and is characterized in that the gas led out of said pressure raising line is separated to a first permeate gas and a first retentate gas by a first gas separation membrane, and then the first retentate gas is separated to a second permeate gas and a second retentate gas by a second gas separation membrane, and said first permeate gas and said second permeate gas are introduced into the pressure raising line separately or after they are joined, and the pressure ratio (the feed side/the permeation side) on the feed side and permeation side in the second gas separation membrane is made larger than the pressure ratio (the feed side/the permeation side) on the feed side and permeation side in the first gas separation membrane.
In the aforementioned construction, it is preferable that said gas separation membrane is composed of three or more stages, where a retentate gas of the preceding stage is separated in turn by a gas separation membrane of the succeeding stage, and permeate gases in the respective stages are introduced into said pressure raising line separately or after they are joined, and that the pressure ratio (the feed side/the permeation side) on the feed side and permeation side is made larger as in the gas separation membran
Greene Jason M.
L'Air Liquide, Societe Anonyme a Directoire et Conseil de S
Smith Duane
Young & Thompson
LandOfFree
Mixed gas concentration regulating method and concentration... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Mixed gas concentration regulating method and concentration..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Mixed gas concentration regulating method and concentration... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2901401