Method and apparatus for concentrating chlorine gas

Abstract: A method and an apparatus for concentrating/purifying a chlorine gas are here disclosed in which a pressure swing adsorption method for alternately repeating an adsorbing operation and a desorbing operation is applied to the concentration/purification of the chlorine gas and novel improvements are added thereto. According to the present invention, one or more adsorbing towers packed with an adsorbent for adsorbing the chlorine gas are used to obtain a high-purity concentrated/purified chlorine gas and a treated exhaust gas scarcely containing the chlorine gas from the chlorine gas containing impurities.

Description: The present invention relates to a method for concentrating (purifying) chlorine in a raw material gas by the utilization of a pressure swing adsorption method (hereinafter referred to as "PSA method"), and particularly, it relates to a method and an apparatus for concentrating/purifying chlorine which is constituted so as to lower a chlorine concentration in a treated exhaust gas and so as to heighten the purity of a (concentrated) chlorine gas product. As methods for concentrating a chlorine gas which have been heretofore known, there are a method which comprises absorbing the chlorine gas by a chlorine-containing organic solvent, and then vaporizing the chlorine gas; a method which comprises pressurizing/cooling a gas, and then separating the resultant liquid chlorine; a method which comprises adsorbing chlorine by silica gel (USP 1,617,305); and other techniques. However, the method using the chlorine solvent is not preferable, because the chlorine solvent has environmental effects, and hence there is the tendency that the use of the chlorine solvent will be restricted. On the other hand, the method which comprises pressurizing/cooling the chlorine gas to liquefy it requires a compressor and a freezer, and therefore this method is scarcely considered to be an advantageous technique. The method which comprises adsorbing chlorine by silica gel is also poor in efficiency and it is not an industrially effective technique, and this method has not been utilized so far. In particular, there have not been any industrially effective method and apparatus by which chlorine gases having low concentrations can be raised to high concentrations without leaking the chlorine gas into an exhaust gas. As is apparent from the foregoing, no method and apparatus have been provided so far which utilize an industrially effective process in concentrating/purifying chlorine containing impurities. Furthermore, there has not been a pressure swing adsorption method for separating chlorine from a mixed gas, particularly from a gas having a relatively low chlorine concentration and containing moisture. Chemical Abstracts 114 (4) 26668W (CN-A-1043 111) discloses a non-PSA method for purifying industrial-grade chlorine by passing it first through silica gel to dry it and then through zeolite to remove other impurities. In a first aspect the present invention provides a method for concentrating/purifying a chlorine gas in accordance with a pressure swing adsorption system employing at least three adsorbing towers packed with an adsorbent for adsorbing chlorine gas, which method comprising cyclically repeating an adsorbing operation in which the chlorine gas containing impurities is fed to a first one of said towers acting as a first adsorption tower to adsorb the chlorine gas; and a treated exhaust gas discharged from the first tower during the adsorbing operation is introduced into a second one of said towers acting as a second adsorption tower for adsorbing remaining chlorine so that the adsorbing operation is effected in at least two steps; and a desorbing operation applied to a third one of said towers acting as a desorption tower in which operation the chlorine gas previous adsorbed by the third tower is desorbed; and wherein in the subsequent phase of operation, the tower which was acting as the first adsorption tower acts as the desorption tower; the tower which was acting as the second adsorption tower acts as the first adsorption tower; and the tower which was acting as the desorption tower acts as an adsorption tower; whereby the chlorine gas containing impurities is concentrated/purified to a high purity. In a second aspect the invention provides apparatus for carrying out such a method. Preferred embodiments of the present invention may enable one to provide an industrial method and apparatus by which a chlorine gas containing impurities can be concentrated/purified to obtain high-purity chlorine without using any solvent and without requiring the liquefaction of chlorine and by which a substantially chlorine-free treated exhaust gas can be discharged; and/or to provide a pressure swing adsorption (PSA) method and apparatus which comprise separating chlorine from a gas containing chlorine to concentrate chlorine and to thereby decrease the chlorine content in the remaining gas (treated waste gas) to substantially zero. Operation conditions for decreasing a chlorine concentration in the remaining gas to substantially zero are desirably automatically controlled. Usually, in the PSA system, the operation conditions are decided by calculating an adsorption cycle and a desorption cycle on the basis of the amount of an adsorbent with which plural adsorption towers are packed, an adsorption performance, the amount of adsorbed components in the treated gas, the amount of the gas and an operating time. In this method, however, the actual operation conditions deviate from the optimum operation conditions owing to the deterioration of the adsorbent with time and the alteration of treatment conditions (temperature, pressure, flow rate and the like). In particular, such an operation as to decrease the specific gas concentration in the remaining gas to substantially zero has been difficult. The present inventors have invented a method for separating chlorine from a gas containing chlorine to concentrate it by the PSA method. When the operation conditions are suitably selected in this method, chlorine can be separated from the gas containing chlorine and concentrated, so that the chlorine concentration in the remaining gas can be reduced to substantially zero. However, also in this method, the actual operation conditions deviate from the optimum operation conditions owing to the deterioration of the adsorbent with time and the alteration of treatment conditions (temperature, pressure, flow rate and the like), and for this reason, over a long operation, to decrease the chlorine gas concentration in the remaining gas to substantially zero is very difficult. Preferred embodiments of the present invention may provide an industrial apparatus which can maintain the life of the adsorbent for a long period of time and which can prevent the corrosion of the apparatus itself without using any solvent and without requiring the liquefaction of chlorine in the case that chlorine is separated from a gas having a relatively low chlorine concentration and containing moisture. Some embodiments of the invention will now be described in more detail with reference to the accompanying drawings in which: Fig. 1 is a diagram of a PSA apparatus by which the present invention may be practiced. Fig. 2 shows an apparatus not according to the present invention. Fig. 3 is changes with time of the temperature rise of an adsorbent at positions separated as much as 1/10 and 1/3 of the total height of the adsorbent from the tip of the adsorbent at a gas outlet during an adsorbing operation under conditions in Table 1. The ordinate axis denotes a temperature (°C) and the abscissa axis denotes an adsorbing time. Fig. 4 is a graph showing the relation of a temperature rise to the chlorine concentration in a remaining gas. Explanation of the symbols in Fig. 1. 0, 1 Raw Material gas receiving pipe 2 Compressor for a material gas 3a, b, c, d Switch valve for receiving the material gas 4a, b, c, d Chlorine gas absorbing tower 5a, b, c, d Switch valve for discharging a treated gas 6a, b, c, d Switch valve for receiving the treated gas 7a, b, c, d Switch valve for discharging the treated gas 8a, b, c, d Switch valve for discharging a product gas 9a, b Switch valve for discharging the product gas 10a, b, c, d Switch valve for receiving a pressurized gas 11 Pressure adjusting mechanism 12 Vacuum pump 13 Product tank 14 Compressor for the product gas 15 Product gas discharging pipe 16 Buffer tank 17 Treated gas discharging pipe 18 Dehydrating tower 19 Pump 20 Sulfuric acid feeding pipe 21 Sulfuric acid discharging pipe Explanation of symbols in Fig. 2. (1), (2) Adsorbing tower (3) Compressor for a gas containing chlorine (4) Adsorption pressure controlling device (5) Vacuum pump (6), (7), (8), (9), (10), (11) Switch valve (12), (13) Thermo-couple (14) Amplifier (15) Calculation controller (16) Output portion (17) Electromagnetic valve The present invention may employ a method for concentrating a chlorine gas which comprises repeating an adsorbing operation in which a chlorine gas containing impurities is fed to an adsorbing tower packed with an adsorbent for adsorbing the chlorine gas, and a desorbing operation in which the chlorine gas adsorbed by the adsorbent is desorbed, to remove the impurities from the chlorine gas containing the impurities fed to the adsorbing tower and to thereby obtain a high-purity chlorine gas. The treated gas coming from the above-mentioned first adsorbing tower is desirably introduced into the next second tower to remove most of chlorine from the exhaust gas. Prior to the desorbing operation, the product chlorine gas is desirably introduced into the adsorbing tower to purge a remaining gas off the system, thereby heightening the purity of the product chlorine gas. The first adsorbing tower can be directly connected to the second adsorbing tower to feed a material gas (a gas to be treated) to the first adsorbing tower until the breakthough of the adsorbent occurs. In the second adsorbing tower, the chlorine gas leaked from the first adsorbing tower can be completely adsorbed. Furthermore, the product chlorine gas can be introduced into the adsorbing tower prior to the desorbing operation to purge the remaining material gas having a low purity off the adsorbing tower. As a result, the purity of the product chlorine gas can be heightened, and a substantially chlorine-free treated exhaust gas can be discharged. Intensive investigation has been made about the technique of automatically setting operating conditions under which the chlorine concentration in a remaining gas can be reduced to substantially zero in a PSA method for separating chlorine from a gas containing chlorine to concentrate chlorine. As a result, it has been found that the automatic control can be achieved by continuously measuring a temperature change (a temperature rise) of an adsorbent (an adsorbent-packed layer) in the vicinity of the outlet of an adsorbing tower. Thus a preferred embodiment of the present invention is directed to a method for adsorbing/removing chlorine in a gas to be treated by the use of the pressure swing adsorption method in which a plurality of adsorbing towers packed with an adsorbent capable of adsorbing chlorine are employed, the method being characterized by comprising the steps of continuously being detected by a sensor for detecting the presence of the chlorine gas to recognize the completion of the adsorbing operation, whereby the feed of the chlorine gas containing impurities to the adsorbing tower is stopped and the adsorbing tower is changed from the adsorbing operation to the desorbing operation, and especially measuring a temperature change of the adsorbent (an adsorbent-packed layer) in the vicinity of the outlet of the treated exhaust gas in the adsorbing tower, stopping the feed of the gas to be treated to the adsorbing tower, when a differential value of the adsorbent temperature has reached a predetermined value, and then beginning a desorbing/regeneration operation of the adsorbing tower. In addition, the present invention is also directed to an adsorption apparatus for the practice of the above-mentioned method. The gas containing chlorine which can be used in the method of the present invention may for example, contain gases such as oxygen, nitrogen, a carbon dioxide gas, a carbon monoxide gas, argon and methane in addition to chlorine. In order to separate chlorine from these gases by the PSA method, it is necessary to select the adsorbent which has a sufficient gap between an adsorption affinity for these gases and an adsorption affinity for chlorine and which permits the adsorption heat of chlorine to exceed that of each of the other gases. Thus, examples of the adsorbent for chlorine which can be used in the present invention include zeolites, non-zeolite type porous acidic oxides, active carbon and molecular sieve carbon. No particular restriction is put on the chlorine concentration of the gas containing chlorine which can be used in the method of the present invention, but it is usually from 5 to 98% by volume. In order to measure the temperature change of the adsorbent in the present invention, it is preferred that the temperature of the adsorbent is measured at a position of 1/2 or less of the total height of the packed adsorbent from the tip of the adsorbent present at the gas outlet of the treated exhaust gas during the adsorbing operation. In general, in the case that the adsorption/separation of a specific gas is carried out in accordance with the PSA method, it is known that the adsorption heat is generated to raise the temperature of the adsorbent. The present inventors have measured the temperature change of the adsorbent in adsorbing the gas containing chlorine by the use of the zeolite, non-zeolite type porous acidic oxide, active carbon or molecular sieve carbon as the adsorbent, and as a result, it has been confirmed that the adsorption heat of chlorine is much larger than that of each of the other gas components. Furthermore, it has been also confirmed that the site of the adsorbent-packed layer where the abrupt temperature change of the adsorbent occurs approximately corresponds to a site where the adsorption of the chlorine gas occurs. The temperature change of the adsorbent due to the adsorption heat of the gas containing chlorine depends upon the chlorine concentration in the material gas (the gas to be treated), and the higher the chlorine concentration is, the larger the temperature change attributed to the adsorption is. Therefore, the leakage of chlorine into the remaining gas can be inhibited by specifying a differential value of the temperature change of the adsorbent due to the adsorption heat at the certain chlorine concentration in the material gas and the detection site of the temperature change. Fig. 4 shows a relation between the temperature change of the adsorption heat in the case that the material gas having a chlorine concentration of 15% by volume is used and the chlorine concentration in the remaining gas. As is apparent from Fig. 4, in order to prevent chlorine from leaking into the remaining gas in the case that the material gas having a chlorine concentration of 15% by volume is used and the measurement site of the temperature change is specified, a gas introducing valve should be switched at the time when the differential value of the temperature change due to the adsorption heat has reached 1.5°C/minute (a predetermined value). Accordingly, the temperature change of the adsorbent in the upper portion of the adsorbing tower can be continuously measured, and immediately when the adsorption heat of chlorine has been detected, i.e., immediately when the differential value of the temperature change has reached the predetermined value, an automatic valve can be automatically switched from the adsorbing operation to completely inhibit chlorine from leaking into the remaining gas. That is, if the temperature change is measured by a thermocouple or another means disposed in the adsorbent in the upper portion of the adsorbing tower, the adsorption of chlorine can be easily detected. This detection can be carried out on the basis of the differential change of the temperature so as not to be affected by an inlet temperature of the gas containing the chlorine gas and the like, and a suitable means can be taken in accordance with the above-mentioned result to determine the termination of the adsorbing operation. Simultaneously, the result can be converted into an electrical output to switch the automatic valve for changing the operation of the adsorbing tower. That is, in a preferred embodiment, the temperature change of the adsorbent in the upper portion of the adsorbing tower can be continuously measured by an inexpensive temperature measuring means, and when the differential value of the temperature rise has reached a predetermined value, the adsorbing operation is automatically switched, whereby a chlorine concentration in the remaining gas can be reduced to zero. Since the chlorine concentration in the remaining gas is substantially zero, measures for removing ill effects of chlorine can be simplified. Therefore, the present invention is very useful from an industrial viewpoint. In the present invention, a dehydrator may be equipped in order to remove moisture from a mixed gas containing the chlorine gas on the upstream side of the adsorbing tower in a chlorine gas concentrating/purifying apparatus in which the above-mentioned PSA method is utilized. When the dehydrator is equipped in order to remove moisture from the mixed gas containing the chlorine gas, the life of the adsorbent can be prolonged and the corrosion of the apparatus can be also prevented. Now, the present invention will be described in detail in reference to examples, but the scope of the present invention should not be limited to these examples. Example 1 Fig. 1 shows one example of a chlorine gas purifying device of the present invention. A pipe 1 is a raw material gas introducing pipe, and a gas having a relatively low chlorine purity is introduced into the pipe 1, pressurized to a predetermined pressure by means of a compressor 2, and then introduced into a first tower 4a of four adsorbing towers 4a, 4b, 4c and 4d via a switch valve 3a. The four adsorbing towers 4a, 4b, 4c and 4d are packed with an adsorbent for predominantly adsorbing chlorine, for example, a synthetic or natural zeolite, a non-zeolite type porous acidic oxide, active carbon or molecular sieve carbon. After a predetermined amount of the gas has been introduced, the product chlorine gas pressurized by a compressor 14 is introduced into the adsorbing tower 4a by switching switch valves 1a, 1b to purge the raw material gas off the adsorbing tower 4a. The gas having a lowered chlorine concentration coming from the outlet of the adsorbing tower 4a is introduced into the second tower 4b via switch valves 5a, 6b. The treated gas from which residual chlorine has been removed by adsorption in the second tower 4b is discharged through a switch valve 7b. At this time, a part of the gas discharged from the second tower 4b is introduced into the third adsorbing tower 4c via a flow rate adjusting mechanism 11 and a switch valve 10c, and a pressure filling step is then carried out to increase the pressure in this tower. In the fourth adsorbing tower 4d, desorption is initially carried out under atmospheric pressure by switching a switch valve 8d. The fourth adsorbing tower 4d is thereafter further connected to a vacuum pump 12 via a switch valve 9b, and in the fourth adsorbing tower 4d, the concentrated chlorine gas is recovered and the adsorbent is regenerated (desorbed). After the adsorbing tower 4a has adsorbed a predetermined amount of chlorine and has been purged with the product gas, the introduction of the gas thereinto is stopped by switching a switch valve 3a, and the pressure in the tower is reduced by switching a switch valve 8a and the tower is further evacuated to a reduced pressure state by a vacuum pump 12, whereby chlorine is desorbed from the adsorbent to regenerate the same. In this reproduction step, the chlorine gas product having a high purity can be stored in a buffer tank 13, pressurized by the compressor 14, and then obtained through a product discharge pipe 15. At this time, the raw material gas is introduced into the second adsorbing tower 4b via a switch valve 3b, and chlorine in the material gas is predominantly adsorbed. The gas having a low chlorine concentration is obtained through the outlet of the adsorbing tower 4b, and it is then introduced into the adsorbing tower 4c via switch valves 5b, 6c. In this tower 4c, residual chlorine is adsorbed, and the chlorine-free gas is then discharged through a switch valve 7c. After the adsorption of the raw material gas has finished, the product chlorine gas pressurized by the compressor 14 is introduced into the adsorbing tower 4b by switching the switch valves 1a, 1b to purge the raw material gas off the adsorbing tower 4b, and the purged gas is discharged through the switch valves 5b, 6c and then introduced into the third adsorbing tower 4c. In this third adsorbing tower 4c, chlorine is adsorbed, and the gas is then discharged through the switch valve 7c. Furthermore, a part of the gas discharged from the third tower 4c is introduced into the fourth adsorbing tower 4d via the flow rate adjusting mechanism 11 and a switch valve 10d, and a pressure filling step is then carried out to increase the pressure in this tower. Afterward, the raw material gas is introduced into the third adsorbing tower 4c via a switch valve 3c, and then chlorine in the gas is predominantly adsorbed. At the outlet of the adsorbing tower 4c, the gas having a low chlorine concentration is obtained, and then introduced into the adsorbing tower 4d via switch valves 5c, 6d. Simultaneously, a part of the gas discharged from the fourth tower 4d is introduced into the first adsorbing tower 4a via the flow rate adjusting mechanism 11 and a switch valve 7a, and a pressure filling step is then carried out to increase the pressure in this tower. The introduction of the material gas and the purged gas into the second adsorbing tower 4b is stopped by switching the switch valve 3b, and the pressure in this tower is then reduced by switching a switch valve 8b. Furthermore, the second adsorbing tower 4b is evacuated to a reduced pressure state by switching a switch valve 9b and by means of a vacuum pump 12, whereby chlorine is desorbed from the adsorbent to reproduce the same. Similarly, the above-mentioned serial operation is repeated alternately by the use of the four adsorbing towers 4a, 4b, 4c and 4d, whereby high-purity chlorine can be continuously prepared from the chlorine gas containing impurities. Control by temperature monitoring The apparatus shown in Figure 2 is not an embodiment of the present invention but embodies temperature monitoring techniques that may be applied to embodiments of the invention. In Fig. 2, reference numerals (1) and (2) are adsorbing towers packed with an adsorbent, numeral (3) is a compressor for a gas containing chlorine, (4) is an adsorption pressure control device, (5) is a vacuum pump for desorption/regeneration, and (6) to (11) are switch valves. Assuming that an adsorption operation is carried out in the adsorbing tower (1) and a desorption/regeneration operation is performed in the adsorption tower (2), the switch valves (7), (8) and (10) are opened and the switch valves (6), (9) and (11) are closed. A gas containing chlorine from the compressor (3) is fed to the adsorbing tower (1) through the bottom thereof via the switch valve (8), and chlorine is mainly adsorbed therein. The chlorine-free gas is then discharged as a remaining gas from the adsorbing tower (1) through the upper portion thereof and further eliminated from the system through the switch valve (10). On the other hand, in the adsorbing tower (2), the adsorbed components in which chlorine is the main component are desorbed from the adsorbent by the use of a vacuum pump, and the chlorine gas is then discharged from the adsorbing tower (2) through the bottom thereof and further from the system through the switch valve (7) by the vacuum pump (5). This discharged gas is the gas containing concentrated chlorine, and it will be treated in another step or the like. In a conventional apparatus, switch valves are switched in accordance with a given design by a timer or another means, but in the case that chlorine concentration, temperature, pressure and the like of a gas containing chlorine change, it is difficult to maintain conditions so that the chlorine concentration in the discharged gas stays at substantially zero over a long period of time. However we have found that the temperature of the adsorbent in the adsorbing tower can be continuously measured to detect the differential change at an optional temperature, whereby an adsorption time can be selected and after the adsorption, the adsorbing tower can be switched to the desorbing operation. Thus, thermocouples (12), (13) are disposed in the upper portion of the adsorbing towers (1), (2) of the above-mentioned apparatus, and the temperature change of the adsorbent is continuously detected to output an electrical signal. This output electrical signal is input into an amplifier (14), and the amplified electrical signal is then input into a calculation controller (15), in which a differential change of the temperature is calculated. When the differential change of the temperature is more than a predetermined differential change at an optional temperature in the calculator (15), the above-mentioned electrical signal is input into an output portion (16), which controls the switch of an electromagnetic valve (17) to switch the switch valves (6) to (11). In short, in preferred embodiments of the present invention, a set time of the adsorbing step in the PSA method is monitored so that the chlorine concentration in the remaining gas may be always substantially zero, even if gas treatment conditions for the gas containing chlorine (composition, flow rate, pressure and temperature) change. That is, when the gas containing chlorine is passed through the adsorbing tower, the temperature of the adsorbent rises owing to adsorption heat. In the tower, a temperature rise zone moves in substantial proportion to the advancement of a chlorine adsorption zone. Therefore, if the temperature is continuously detected in the vicinity of the outlet of the tower, the detected values can be utilized to correctly foresee a breakthrough time of chlorine, which permits monitoring the set switch time of the tower. However, the mere detection of the temperature is insufficient owing to a relation with the inlet temperature of the gas containing chlorine. The accurate monitoring of the set time in the adsorbing step can be achieved by always checking the differential change of the temperature. The present invention is effective provided that the adsorption heat for chlorine gas is much larger than the heat of adsorption of another gas by the adsorbent. Examples of the adsorbent having such characteristics include zeolites, non-zeolite type porous acidic oxides, active carbon and molecular sieve carbon. A PSA method for recovering chlorine from a gas containing chlorine embodying the present invention enables the chlorine concentration in the remaining gas to be reduced to substantially zero for a long period of time. In addition, the monitoring can be carried out so as to prevent chlorine from leaking into the remaining gas. Therefore, the present invention has high reliability, and it is a very effective means from an industrial viewpoint. In a practical example of a dual-adsorption type PSA apparatus as shown in Fig. 2, each adsorbing tower is made of a pressure-resistant steel pipe having a diameter of 50 mm and a height of 2000 mm, and a Y type zeolite (Zeochem Co., Ltd.) is used as an adsorbent and each adsorbing tower is packed with 3 kg of the adsorbent. The gas containing chlorine used in the example is composed of 15% by volume of chlorine, 50% by weight of oxygen, 15% by volume of nitrogen and 20% by volume of carbon dioxide, and this gas is fed at a flow rate of 2 Nm 3 /h by the use of a compressor (3) shown in Fig. 2. In this case, adsorption pressure can be optionally set in the range of 1 to 7 kg/cm 2 gauge pressure by a pressure control device (4). For the accomplishment of the adsorption pressure, a vacuum pump (5) having a maximum power by which a vacuum degree of 100 torr can be reached in 3 minutes is selected and used. In order to obtain basic data necessary for the operation of the above-mentioned PSA apparatus, a temperature change of the adsorbent which occurs at the time of the adsorption of chlorine is measured. Fig. 3 shows temperature changes with time of the adsorbent at positions separated as much as 1/10 and 1/3 of the total height of the packed adsorbent from the tip of the adsorbent at a gas outlet during an adsorbing operation. As a result, it has been confirmed that when the adsorption of chlorine is started, the temperature change of the adsorbent is a temperature rise of 2°C/minute, which is much larger as compared with a temperature change of 0.5°C/minute at the time of the passage (adsorption) of the other gas components. This result is input into a computer, and programing is made so that the adsorbing operation might be stopped and the adsorbing tower right be switched, immediately when the temperature change of the adsorbent is 1.5°C or more. Operation is continuously carried out for 2 weeks under operating conditions shown in Table 1, but chlorine in a remaining gas is not detected during this period. The chlorine concentration in the concentrated gas is constant at a level of 60% by volume. In this connection, the chlorine concentrations in the concentrated gas and the remaining gas are determined by gas chromatography. 1 Adsorption Time 5 minutes (when a differential temperature of 1.5°C/minute is detected, the adsorption is stopped) 2 Pressure Equalization Time 1 minute (which is set by a fixed timer) 3 Desorption Time 5 minutes (which is set by a fixed timer) 4 Pressure Equalization 1 minute (which is set by a fixed timer) Temperature measurement position ... A position separated as much as 1/10 of the total height of the adsorbent from the tip of the adsorbent at a gas outlet during an adsorbing operation. Adsorption Pressure: 7 kg/cm 2 (7x10 5 Pa) Desorption Pressure: 100 torr (1x10 4 Pa) Temperature of the treated gas at the inlet of the adsorbing tower: 15°C Example 2 Now, one example of the present invention will be described. Fig. 1 shows one example of a chlorine gas concentrating apparatus of the present invention. The present example uses only three towers 4a, 4b, 4c thereof. A pipe 0 is a material gas introducing pipe, and a gas having a relatively low chlorine concentration and containing moisture is introduced into the pipe 0. Reference numeral 18 is a dehydrating tower, through which concentrated sulfuric acid is circulated by means of a pump 19. A pipe 20 is a feed pipe for fresh concentrated sulfuric acid, and a pipe 21 is a discharge pipe for relatively dilute sulfuric acid. The gas containing moisture is fed to the dehydrating tower 18, in which moisture is removed therefrom, and the gas is then pressurized up to a predetermined pressure by a compressor 2. Afterward, the gas is introduced into a first tower 4a of the three adsorbing towers 4a, 4b and 4c via a switch valve 3. The three towers 4a, 4b and 4c are packed with an adsorbent such as a zeolite or active carbon for predominantly adsorbing chlorine, and chlorine in the material gas introduced in a pressurized state is predominantly adsorbed by the adsorbent. At the outlet of the adsorbing tower 4a, the gas scarcely containing chlorine is obtained, and this gas is then discharged through a switch valve 7a. At this time, a part of the gas discharged from the first tower 4a is introduced into the second adsorbing tower 4b via an adjusting mechanism 11 and a switch valve 10b, and a pressure filling step is then carried out to increase the pressure in this tower. Furthermore, the third absorbing tower 4c is connected with a vacuum pump 12 via switch valves 8c, 9b, and in this third absorbing tower 4c, the concentrated chlorine gas is recovered and a desorbing step is carried out in which the adsorbent in this tower are regenerated. The introduction of the material gas into the adsorbing tower 4a in which a predetermined amount of chlorine has been adsorbed is stopped by switching a switch valve 3a, and a switch valve 8a is then switched. As a result, this tower is evacuated by the vacuum pump 12 to become a reduced pressure state, so that chlorine adsorbed by the adsorbent is desorbed and consequently the adsorbent is regenerated. In this desorbing step, the concentrated chlorine gas can be obtained as a product through the outlet of the vacuum pump 12. At this time, the material gas is introduced into the second adsorbing tower 4b via a switch valve 3b, and in this tower 4b, chlorine in the material gas is predominantly adsorbed. At the outlet of the adsorbing tower 4b, the gas scarcely containing chlorine is obtained and then discharged through a switch valve 7a. Moreover, a part of the gas discharged through the second tower 4b is introduced into the third adsorbing tower 4c via the pressure adjusting mechanism 11 and a switch valve 10c, and a pressure filling step is then carried out to increase the pressure in this tower. Afterward, the material gas is introduced into the third adsorbing tower 4c via a switch valve 3c, and in this tower 4c, chlorine in the material gas is predominantly adsorbed. At the outlet of the adsorbing tower 4c, the gas scarcely containing chlorine is obtained and then discharged through a switch valve 7c. Simultaneously, a part of the gas discharged through the third tower 4c is introduced into the first adsorbing tower 4a via the pressure adjusting mechanism 11 and a switch valve 10a, and a pressure filling step is then carried out to increase the pressure in this tower. The introduction of the material gas into the second adsorbing tower 4b is stopped by switching a switch valve 3b, and a switch valve 8a is then switched. As a result, this tower is evacuated by the vacuum pump 12 to become a reduced pressure state, so that chlorine adsorbed by the adsorbent is desorbed and consequently the adsorbent is regenerated. According to the system established by the present invention, this serial operation is afterward similarly repeated by the use of the three adsorbing towers 4a, 4b and 4c, whereby chlorine can be continuously separated from the material gas containing chlorine and concentrated. In this case, materials for equipments constituting this apparatus are not required to be expensive anti-corrosive materials, and relatively inexpensive materials may be acceptable. As described above, according to the present invention, a chlorine gas containing impurities can be purified by the PSA method without leaking chlorine into an exhaust gas, to obtain a high-purity chlorine product. Furthermore, in the present invention, a temperature change of an adsorbent in the vicinity of the gas outlet in an adsorbing tower is continuously measured, and when a differential value of this adsorbent temperature has reached a predetermined value, the introduction of the gas into the adsorbing tower is stopped and the desorbing operation of the adsorbing tower is started, whereby even if gas treatment conditions fluctuate, a chlorine concentration in a remaining gas can always be maintained at substantially zero. Therefore, it is fair to say that the present invention is an industrially extremely effective control means. In addition, the present invention can provide a relatively inexpensive apparatus which can separate chlorine from a gas containing chlorine and moisture to concentrate chlorine.

Claims: A method for concentrating/purifying a chlorine gas in accordance with a pressure swing adsorption system employing at least three adsorbing towers packed with an adsorbent for adsorbing chlorine gas, which method comprising cyclically repeating an adsorbing operation in which the chlorine gas containing impurities is fed to a first one of said towers acting as a first adsorption tower to adsorb the chlorine gas; and a treated exhaust gas discharged from the first tower during the adsorbing operation is introduced into a second one of said towers acting as a second adsorption tower for adsorbing remaining chlorine so that the adsorbing operation is effected in at least two steps; and a desorbing operation applied to a third one of said towers acting as a desorption tower in which operation the chlorine gas previous adsorbed by the third tower is desorbed; and wherein in the subsequent phase of operation, the tower which was acting as the first adsorption tower acts as the desorption tower; the tower which was acting as the second adsorption tower acts as the first adsorption tower; and the tower which was acting as the desorption tower acts as an adsorption tower; whereby the chlorine gas containing impurities is concentrated/purified to a high purity. The method for concentrating/purifying a chlorine gas according to Claim 1 wherein prior to the start of the desorbing operation of a tower which was acting as the first adsorption tower, high-purity chlorine gas is introduced into that tower to purge the chlorine gas containing impurities from that tower. The method for concentrating/purifying a chlorine gas according to Claim 1 or 2 wherein the impurity contained in the chlorine gas is or includes oxygen gas. The method for concentrating/purifying a chlorine gas according to Claim 1, 2 or 3 wherein the adsorption state of the adsorbent-packed layer in an adsorbing tower during the adsorbing operation is detected by a sensor for detecting the presence of the chlorine gas to recognize the completion of the adsorbing operation, whereby the feed of the chlorine gas containing impurities to the adsorbing tower is stopped and the adsorbing tower is changed from the adsorbing operation to the desorbing operation. The method for concentrating/purifying a chlorine gas according to Claim 4 wherein the sensor for detecting the presence of the chlorine gas is a thermometer. The method for concentrating/purifying a chlorine gas according to Claim 5 wherein the temperature rise of the adsorbing tower during the adsorbing operation is detected at a position in the adsorbent separated as much as 1/2 or less of the total height of the packed adsorbent from the tip of the adsorbent adjacent the treated exhaust gas outlet. The method of Claim 5 or Claim 6 wherein the thermometer is used to detect a predetermined value of temperature rise of 1.5° per minute. The method for concentrating/purifying a chlorine gas according to any preceding claim wherein the adsorbent for adsorbing the chlorine gas is one selected from zeolites, non-zeolite type porous acidic oxides, active carbon and molecular sieve carbon. The method for concentrating/purifying a chlorine gas according to Claim 8 wherein the adsorbent for adsorbing the chlorine gas is a Y type zeolite. Apparatus for carrying out the method of any preceding claim comprising at least three adsorbing towers packed with an adsorbent for adsorbing chlorine, and a mechanism whereby the apparatus is adapted to carry out an operational cycle such that in one phase raw material chlorine gas containing impurities is fed to a first one of said towers which acts as a first adsorption tower; the outlet of said first tower being coupled to a second said tower to pass a treated exhaust gas to it so that said second tower acts as a second adsorption tower; there being desorption means which in said phase are coupled to the third tower which thus acts as a desorption tower in which previously-adsorbed chlorine is desorbed; said mechanism being adapted to exchange the roles of the towers so that in a subsequent phase the first adsorption tower becomes the desorption tower, the second adsorption tower becomes the first adsorption tower, and the desorption tower becomes an adsorption tower. The apparatus for concentrating/purifying a chlorine gas according to Claim 10 including means whereby prior to the start of the desorbing operation, the concentrated high-purity chlorine gas is refluxed through the adsorbing tower in which the adsorbing operation has been finished, to replace the chlorine gas containing impurities in the system with the concentrated high-purity chlorine gas. The apparatus for concentrating/purifying a chlorine gas according to Claim 10 or 11 which has the following members (a) to (f): (a) at least three of said adsorbing towers each provided with an inlet for the gas to be treated and an outlet for the treated exhaust gas, and packed with an adsorbent for adsorbing the chlorine gas, (b) a pipe connecting with an inlet for the gas to be treated of each adsorbing tower, a pipe connecting with an outlet for the treated exhaust gas of each adsorbing tower, a pipe connecting with an inlet for the gas to be treated for each adsorbing tower which is disposed to take out the concentrated chlorine gas from each adsorbing tower by desorption, a pipe connecting the outlet for the treated exhaust gas of one adsorbing tower with the inlet for the gas to be treated of another adsorbing tower, and a pipe for connecting an inlet for the gas to be treated of each adsorbing tower disposed to reflux the concentrated chlorine gas through the adsorbing tower with an exhaust vent of a product gas tank or a compressor for a concentrated/purified chlorine gas, (c) a switch valve for switching a specific pipe of these pipes so that a specific gas may flow at any time through the specific pipe, (d) a vacuum pump for reducing the pressure in the system for the sake of the desorbing operation of the chlorine gas, (e) a product tank for uniforming the pressure of the concentrated/purified chlorine gas, and (f) a compressor for raising the pressure of the concentrated/purified chlorine gas. The apparatus for concentrating/purifying a chlorine gas according to Claim 10, 11 or 12 wherein a sensor for detecting the presence of the chlorine gas is disposed in the upper portion of an adsorbent-packed layer in the adsorbing tower, whereby the completion of the adsorbing operation of the adsorbent-packed layer during the adsorbing operation is detected, and the feed of the chlorine gas containing impurities to the adsorbing tower is stopped and the adsorbing tower is changed from the adsorbing operation to the desorbing operation. The apparatus for concentrating/purifying a chlorine gas according to claim 13 wherein the sensor for detecting the presence of the chlorine gas is a thermometer. The apparatus for concentrating/purifying a chlorine gas according to Claim 13 or 14 which is characterized by having the following members (a) to (f): (a) at least three of said adsorbing towers each provided with an inlet for the gas to be treated and an outlet for the treated exhaust gas, and equipped with a thermometer in the upper portion of an adsorbent-packed layer packed with an adsorbent for adsorbing the chlorine gas, (b) a pipe connecting with an inlet for the gas to be treated of each adsorbing tower, a pipe connecting with an outlet for the treated exhaust gas of each adsorbing tower, a pipe connecting with an inlet for the gas to be treated of each adsorbing tower which is disposed to take out the concentrated chlorine gas from each adsorbing tower by desorption, a pipe connecting the outlet for the treated exhaust gas of one adsorbing tower with the inlet for the gas to be treated of another adsorbing tower, and a pipe for connecting an inlet for the gas to be treated of each adsorbing tower disposed to reflux the concentrated chlorine gas through the adsorbing tower with an exhaust vent of a product gas tank or a compressor for a concentrated/purified chlorine gas, (c) a switch valve for switching a specific pipe of these pipes so that a specific gas may flow at any time through the specific pipe, (d) a vacuum pump for reducing the pressure in the system for the sake of the desorbing operation of the chlorine gas, (e) a detector for detecting the temperature rise of the adsorbent-packed layer in each adsorbing tower during the adsorbing operation, and (f) an adjusting mechanism by which a switch valve is operated so as to stop the introduction of the gas to be treated, when the temperature rise is in excess of a predetermined value. The apparatus for concentrating/purifying a chlorine gas according to Claim 15 wherein the temperature detector for detecting the temperature rise of the adsorbing tower during the adsorbing operation is disposed at a position in the adsorbent separated as much as 1/2 or less of the total height of the packed adsorbent from the tip of the adsorbent adjacent the treated exhaust gas outlet. The apparatus for concentrating/purifying a chlorine gas according to Claim 13, 14 or 15 having means for detecting and responding to a predetermined value of temperature rise of 1.5°C per minute. The apparatus for concentrating/purifying a chlorine gas according to any of Claims 10-17 which is equipped with a dehydrator on the upstream side of the adsorbing tower in order to remove moisture from the chlorine gas containing impurities. The apparatus for concentrating/purifying a chlorine gas according to Claim 18 wherein the dehydrator is a dehydrating tower in which concentrated sulfuric acid is circulated to bring sulfuric acid into contact with the gas to be treated.

Inventors: ITOH HIROYUKI, KONO YOSHITSUGU, TSURUDA SATOSHI, URA MASAAKI, ITOH, HIROYUKI, KONO, YOSHITSUGU, TSURUDA, SATOSHI, URA, MASAAKI
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1992
Main classification: C01B7
IPC codes: B01D53, C01B7