Method and Apparatus for Separating of Product Gas From Raw Gas

1. Field of the Invention

The present invention relates to a gas separating method far of separating valuable gas components such as nitrogen, oxygen, argon or the like from raw gas so as to extract these components by a cryogenic process and is also concerned with a gas separating apparatus suitable for the above-described gas separating method.

2. Description of the Prior Art

There is broadly known a conventional method wherein offgas separated by means of a gas separating apparatus is introduced into a heat exchanger in which the offgas is subjected to a heat exchange with raw air in order to cool down the raw air and at the same time the offgas is made to flow in an expansion turbine, thereby generating a cryogenic effect. Namely, a cryogenic process which utilizes the above-described low temperature offgas has been disclosed in the specification of Japanese Patent Laid-Open No. 79972 in 1980. The method disclosed therein is such that the low temperature offgas discharged from a nitrogen condenser of an air separating apparatus is fed into a heat exchanger wherein the offgas is thermally restored to an intermediate temperature and is then made to flow in the expansion turbine so as to expand in an adiabatic manner, thereby generating the cryogenic effect; and the low temperature offgas employed for generating the cryogenic effect is again led into the heat exchanger for the purpose of returning it to the normal temperature. With this method, the low temperature offgas discharged from the air separating apparatus is transferred intact to the expansion turbine via the heat exchanger. The pressure at an inlet of the expansion turbine is determined by the pressure of the low temperature offgas which is released from the air separating apparatus so that the former never exceeds the latter. For this reason, there exists a limitation on the extent to which the cryogenic effect can be generated per unit processing gas quantity.

Inasmuch as the extent of the cryogenic effect generated per unit processing gas quantity is small, it is necessary to employ a large amount of gas in order to generate a corresponding cryogenic effect which the gas separating apparatus requires. In the case of, for instance, a plant which extracts nitrogen and oxygen from air serving as raw gas by effecting a separating operation, this plant necessitates a step wherein a great amount of air is pressurized and is then supplied to the plant. Such being the case, it is necessary to prepare a large booster for pressurizing a large amount of raw air; and as a result, the consumption in energy (electricity in ordinary cases) whereby the booster is driven increases. The large sized equipment and the increased consumption in energy disadvantageously boost the costs in production of product gases such as nitrogen, oxygen, argon or the like which are produced (extracted) by means of the gas separating apparatus.

Accordingly, it is a primary object of the present invention to provide a gas separating method and an apparatus pertaining thereto which are capable of augmenting the cryogenic effect generated per unit gas flow rate when low temperature gas within a process is utilized with a view to generating the cryogenic effect.

To this end, according to one aspect of the invention, there are provided a gas separating method and a gas separating apparatus which is small and of an energy-saving type.

According to another aspect of the invention, there are provided a gas separating method and a gas separating apparatus which are capable of extracting product gases at low cost.

A gas separating method according to the present invention comprises the steps of: thermally exchanging low temperature gas within a process with raw gas by means of a heat exchanger in order to restore the same gas in terms of temperature; causing the thermally restored gas to flow in a booster which is energized by an expansion turbine to pressurize the above-described gas; cooling down the thus pressurized gas to a low temperature; causing the gas cooled down to the low temperature to flow in the foregoing expansion turbine so as to expand such gas in an adiabetic manner; and generating a cryogenic effect.

Other objects and features of the present invention apart from the above will become evident from the following description with reference to the accompanying drawings.

FIG. 1 is a flowchart illustrating one embodiment according to the present invention;

FIG. 2 is a flowchart illustrating another embodiment according to the present invention;

FIG. 3 is a flowchart illustrating still another embodiment according to the present invention; and

FIG. 4 is a flowchart illustrating a further embodiment according to the present invention.

The present invention will hereinafter be described in detail through the following embodiments thereof.

Referring now to FIG. 1, there is shown a system flowchart which illustrates a case where the present invention is applied to a plant designed for extracting nitrogen. Raw gas employed in the preferred embodiments according to the present invention involves air. The raw air is made to flow into a heat exchanger 1 via a conduit 10 at a pressure of approximately 8 kg/cm.sup.2 G. It is to be noted that moisture content and CO.sub.2 which are contained in the raw gas are eliminated by means of a pre-treater (not illustrated). The raw air fed in the heat exchanger 1 is subjected to a thermal exchange with returning product nitrogen and low temperature offgas and is then cooled down to the saturation temperature, so that part of it is liquified and supplied via a conduit 11 to a fractionating tower 2. In the fractionating tower 2, the raw air is separated into product nitrogen and liquid air. The thus separated product nitrogen is drawn out from the upper portion of the fractionating tower 2 and is then led via a conduit 20 to the heat exchanger 1 wherein the product nitrogen is thermally returned to the normal temperature. Thereafter, the product nitrogen is sent outside the system through a conduit 21. The pressure at which the product nitrogen is transferred via the conduit 21 is 7 kg/cm.sup.2 G. The liquid air is drawn out from the lower portion of the fractionating tower 2 and passes through a conduit 18. The thus drawn-out liquid air is swollen up to a pressure of about 3 kg/cm.sup.2 G by use of a valve V.sub.1 and is then fed via a conduit 19 to a nitrogen condenser 3. In the nitrogen condenser 3, the ascending nitrogen in the fractionating tower 2 is liquified, while at the same time, the liquid air is gasified. The thus gasified low temperature offgas is drawn out through a conduit 12. The low temperature offgas transferred via the conduit 12 is restored to the normal temperature in the heat exchanger 1. Thereupon, such offgas flows via a conduit 13 in a booster 4 which is linked with an expansion turbine 5. The booster 4 carries out such a step that the energy equivalent to that of the cryogenic effect generated in the turbine 5 is imparted to the offgas as a result of increases in pressure and temperature. So far as the specification hereof is concerned, the booster involves a compressor, blower or the like which pressurizes gas. In this embodiment, the offgas on the output-side of the booster 4 is subjected to a pressure of 5 kg/cm.sup.2 G; and the temperature thereof ranges from 70.degree. C. to 80.degree. C. inclusive. The offgas forwarded from the booster is cooled down to the normal temperature in an after-cooler 9. A water-cooler, an air-cooler or the like can be employed as the after-cooler. The cooled offgas with a pressure of approximately 5 kg/cm.sup.2 G again enters the heat exchanger 1 via a conduit 14, where the offgas is further cooled to -120.degree. C. Then, the offgas is drawn out through a conduit 15. The low temperature offgas sent via the conduit 15 flows in the expansion turbine 5 in which the offgas is adiabatically expanded up to a pressure of 0.3 kg/cm.sup.2 G, thereby to generate the cryogenic effect required for the apparatus. The offgas which has its temperature further lowered passes through a conduit 16 and enters the heat exchanger 1 wherein the thus cooled offgas is returned to the normal temperature. Thereafter, the offgas is discharged outside the system through a conduit 17. The product liquid nitrogen with a pressure of approximately 7 kg/cm.sup.2 G and also the saturation temperature is drawn out from the upper portion of the fractionating tower 2 and is then forwarded outside the system through a conduit 22. In the Figure, the reference numeral 8 stands for a low-temperature insulation tank.

According to this embodiment, the arrangement is not such that the low temperature offgas discharged from the nitrogen condenser 3 is made to directly flow in the expansion turbine, but such that the same offgas is made to flow therein after it has been pressurized by means of the booster. Consequently, it is possible that the pressure impressed on the offgas at the inlet of the expansion turbine exceeds the pressure applied to the offgas which is discharged from the nitrogen condenser. With respect to the expansion turbine, as the pressure at the inlet thereof increases, the pressure at the outlet thereof decreases, the cryogenic effect generated per unit gas flow rate is enlarged. Hence, a still greater amount of gelidity can be produced as compared with the prior art. With the gelidity increased, a stable operation can be realized, and concurrently, the amount of product liquid nitrogen to be extracted is increased. In case where a blower of the expansion turbine serves as the booster, the installation of a filter is unnecessary since the purified offgas is supplied to the blower. Furthermore, the blower eliminates the need for an air draw-out silencer, since the offgas never emanates in the atmosphere on the output-side of the blower. The constitution can therefore be simplified on the whole. Inasmuch as the offgas has virtually is or CO.sub.2, corrosion of the blower of the expansion turbine is markedly reduced so that the lifetime thereof can be lengthened.

One embodiment of a plant suitable for extracting nitrogen is described above, however, the present invention is not confined thereto. The present invention has a wide variety of adoptability relative to plants necessary therefor; namely, a plant designed for extracted oxygen as well as a plant for extracting oxygen and nitrogen can, as a matter of course, be employed. Other plants are also usable. The gas to be employed for the cryogenic process may include not only offgas but also product gas or raw gas.

Referring to FIG. 2, there is described another embodiment according to the present invention. FIG. 2 is a system flowchart of a plant designed for extracting nitrogen from air by conducting a separating operation in the same manner as that shown in FIG. 1. A great difference between the embodiment shown in FIG. 1 and that in FIG. 2 is that nitrogen (N.sub.2) defined as the product gas is utilized to generate the cryogenic effect when the low temperature gas within the process is to be employed for producing the cryogenic effect. That is, N.sub.2 (product gas) with a pressure of some 2.2 kg/cm.sup.2 G which is drawn out via the conduit 20 from the fractionating tower 2 passes through a path provided within the heat exchanger 1 and is thermally restored to the normal temperature. Thereafter, the N.sub.2 is transferred via a conduit 13 to the booster 4, where it is pressurized up to 3.5 kg/cm.sup.2 G. The N.sub.2 which has risen in temperature due to the above-described pressurization is cooled down to the normal temperature by means of the after-cooler 9 and is then fed via a path constituted by a conduit 14 to the heat exchanger 1 wherein the N.sub.2 gas is again cooled down to a still lower temperature (approximately -120.degree. C.). The thus cooled N.sub.2 is sent via a conduit 15 to the expansion turbine 5 wherein the N.sub.2 is adiabatically expanded to a pressure of some 0.3 kg/cm.sup.2 G, whereby the cryogenic effect is generated. The thermally lowered N.sub.2 is supplied via a conduit 16 to the heat exchanger 1 and passes through a path provided therein, thus returning to the normal temperature. The N.sub.2 which has been thermally restored is fed via a conduit 21 to the destination of demand thereof. In the embodiment shown in FIG. 2, it can be seen that, if the cryogenic effect is greater than is required, a conduit 23 and valves V.sub.2, V.sub.3 which are combined into a bypass need to be provided therein with a view to preventing the plant as a whole from being unstable; and the pressure of the N.sub.2 to be fed to the expansion turbine 5 is arranged to be controlled by means of a pressure controller (PC) 30.

According to the embodiment described in FIG. 2, the pressure of air serving as raw gas may be minimum, viz., 3 kg/cm.sup.2 G or thereabouts in ordinary cases, this minimum value being necessary for the fractionation thereof. In addition, the minimum pressure can lead to a saving in the energy required for pressurizing the raw air. The pressure controller provided with a bypass line makes it possible to operate in an extremely stable manner.

Still another embodiment according to the present invention will hereinafter be described with reference to FIG. 3. FIG. 3 is a system flowchart of a plant designed for extracting nitrogen and oxygen from air by effecting a separating operation. The method adopted in the embodiment shown in FIG. 3 in much the same as those described in FIGS. 1, 2. The arrangement is therefore such that the low temperature gas within the process is made to become normal in temperature through the intermediary of the heat exchanger 1; the thermally normalized gas is pressurized by means of the booster 4 and is then cooled to the normal temperature by the after-cooler 9; the thus cooled gas is fed via a conduit 14 to the heat exchanger 1 wherein the gas is cooled to an even lower temperature; thereafter, the gas flows in the expansion turbine 5 through the conduit 15; and cryogenic effect can be generated adiabatically in the expansion turbine 5. However, an outstanding difference between the embodiment shown in FIG. 2 and that in FIG. 3 is such that a part of raw gas is transferred via a conduit 27 to the heat exchanger 1 wherein the gas is subjected to the thermal exchange and thus exchanged gas is then fed to the booster 4, the raw gas being utilized as the low temperature gas within the process to generate the cryogenic effect. The low temperature raw gas, after the cryogenic effect has been produced in the expansion turbine 5, is arranged to be supplied via a conduit 28 to a low-pressure tower (upper tower) of a duplex fractionating tower 50. The duplex fractionating tower 50 extracts nitrogen and oxygen from air defined as raw gas, each of which is drawn out through conduit 20, 25. Thereafter, the nitrogen and oxygen are thermally exchanged with the raw air in the heat exchanger 1 so as to be restored in terms of temperature; and the thus restored oxygen is fed via a conduit 26 to the destination of demand thereof. The nitrogen too is supplied via a conduit 21 to the destination of demand thereof. The fractionating processes within the duplex fractionating tower 50 are, however, broadly known, so that the detailed description relative thereto is herein omitted.

According to the embodiment shown in FIG. 3, a part of the raw air is fed out in order to be pressurized by means of the booster; and the pressurized raw air is utilized for generating the cryogenic effect, so that the amount of gelidity generated per unit gas flow rate is increased. Owing to this advantages, it is feasible to reduce the quantity of raw air needed for the generation of the cryogenic effect and the costs of power as well.

With reference to FIG. 4, there is described in detail a further embodiment according to the present invention. The embodiment shown in FIG. 4 is fundamentally identical with those shown in FIGS. 1 to 3 inclusive. A modified arrangement with respect to the embodiment shown in FIG. 4, as compared with the previous embodiments, is not such that, when the low temperature gas within the process which has been discharged from a separator 60 is employed for the generation of the cryogenic effect, the gas, which has its temperature increased by the principal heat exchanger 1 is supplied to the booster 4, but such that the gas which is heightened in temperature by use of an auxiliary heat exchanger 70 otherwise provided is fed to the booster 4. Moreover, another different step must be added wherein the auxiliary heat exchanger 70 is employed to cool the gas, such gas being pressurized in advance by the booster 4.

Concerning the embodiment in FIG. 4, there is utilized the offgas which is used as the low temperature gas within the process to generate the cryogenic effect. It is, however, practicable to make use of product gas (N.sub.2 or O.sub.2) and raw gas defined as another low temperature gas within the process which is useful for the generator of gelidity. In such a case, the piping is carried out in the same way as those shown in FIGS. 2, 3 so that a description with a view pertaining thereto is omitted. As the separator 60, the fractionating tower shown in FIGS. 1 to 3 inclusive can be employed, which separator is not, however, confined thereto. An absorption type separator, for instance, may be adopted, such separator separating the gas with the aid of an absorbent such as zeolite or the like.

According to the present invention, as described above, it is possible to increase the amount of cryogenic effect generated per unit flow rate of low temperature gas by a cryogenic process wherein the low temperature gas within the process is utilized, this crucially contributing to the saving in energy and the miniaturization of the apparatus.