Method and apparatus for separating chlorine gas

Abstract: A method and an apparatus for concentrating/purifying chlorine gas uses a pressure swing adsorption method. Adsorbing towers packed with an adsorbent for adsorbing the chlorine gas are cycled between adsorbing and desorbing modes. During adsorbing, the state of the adsorbent is monitored, preferably by a thermometer in the adsorbent in the region of the gas exit. The mode of operation is switched in response to a rise in temperature.

Description: The present invention relates to a method and apparatus for separating chlorine from a mixed gas by the utilization of a pressure swing adsorption method (hereinafter referred to as "PSA method"). More specifically the present inventions provides a method of removing chlorine from a chlorine-containing gas by a pressure swing adsorption method in which a plurality of adsorbing towers packed with an adsorbent capable of adsorbing chlorine are employed and cycled between adsorbing and desorbing operations, the method being characterised by continuously monitoring the adsorption state of the adsorbent in each tower when engaged in adsorption operation by means of a respective sensor for detecting the presence of chlorine gas to recognise the completion of the adsorbing operation, whereupon the feed of the chlorine-containing gas to the adsorbing tower is stopped and the absorbing tower is changed from the adsorbing operation to the desorbing operation; wherein the adsorbent is selected such that the adsorption heat of chlorine substantially exceeds that of each of the other components of said gas, and wherein each said sensor is a temperature sensor and is arranged to measure a temperature change of the adsorbent. Preferred embodiments of the present invention may enable one 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, the method being characterized in that operation conditions for decreasing a chlorine concentration in the remaining gas to substantially zero are 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, such a long operation as 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 an example of PSA apparatus by which the present invention is practised. Fig. 2 is a graph of 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. 3 is a graph showing the relation of a temperature rise to the chlorine concentration in a remaining gas. 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. 3 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. 3 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 One example of an apparatus by which the method of the present invention can be carried out will be described in reference to drawings as follows. In Fig. 1, 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 do such an operation as to decrease a chlorine concentration in a remaining gas to substantially zero for a long period of time. On the contrary, in the present invention, the temperature of the adsorbent in the adsorbing tower can be continuously measured to detect the differential change at an optional temperature, whereby an optional adsorption time can be selected and after the adsorption, the adsorbing tower can be switched to the desorbing operation. In the practice of the present invention, 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, the gist of the present invention resides in that 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 to adsorb chlorine by the adsorbent, the temperature of the adsorbent rises owing to adsorption heat, and in the tower, a temperature rise belt section moves in substantial proportion to the advancement of an chlorine adsorption belt. Therefore, if this temperature rise belt section 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 results in the insufficient monitor owing to a relation with the inlet temperature of the gas containing chlorine. The accurate monitor 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 in the case that the adsorption heat at the adsorption of the chlorine gas by the absorbent is much larger than an adsorption heat at the adsorption of another gas thereby. Examples of the adsorbent having such characteristics include zeolites, non-zeolite type porous acidic oxides, active carbon and molecular sieve carbon. According to a PSA method for recovering chlorine from the gas containing chlorine to concentrate chlorine which is concerned with the method of the present invention, the operation by which the chlorine concentration in the remaining gas is always reduced to substantially zero can be achieved for a long period of time. In addition, the monitor 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. Example 2 In a dual-adsorption type PSA apparatus for separating chlorine from a gas containing chlorine used in the example to concentrate chlorine, 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. 1. 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. 2 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 might 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 Desorption Pressure: 100 torr Temperature of the treated gas at the inlet of the adsorbing tower: 15°C Example 3 A PSA method for separating chlorine from a gas containing chlorine to concentrate chlorine is started under the same conditions as in Example 2 and a temperature of the treated gas at the inlet of an adsorbing tower is changed from 15°C to 40°C during this experiment. When the temperature of the treated gas at the inlet of the adsorbing tower is raised, an adsorption time is shortened from 5 minutes of an initial value to 4 minutes 50 seconds, but chlorine in a remaining gas is not detected, and the chlorine concentration in the concentrated gas is constant at a level of 60% by volume. Example 4 A PSA method for separating chlorine from a gas containing chlorine to concentrate chlorine is started under the same conditions as in Example 2, and adsorption pressure is lowered from 7 kg/cm 2 to 3 kg/cm 2 during this experiment. In this case, an adsorption time is shortened from 5 minutes of an initial value to 4 minutes 20 seconds, but chlorine is not detected in a remaining gas, and the concentration of a purified chlorine gas is 60% by volume which is not so much different from the result of Example 3. Example 5 A PSA method for separating chlorine from a gas containing chlorine to concentrate chlorine is started under the same conditions as in Example 2, and the flow rate of the gas containing chlorine is changed from 2 Nm 3 /h to 3 Nm 3 /h. In this case, an adsorption time is shortened from 5 minutes of an initial value to 4 minutes 00 second. At this time, a chlorine concentration in a remaining gas is several hundreds ppm. This would result from the fact that the linear velocity of the treated gas increases, and consequently the chlorine gas ascends to the upper portion of an adsorbing tower, while a differential value of 1.5°C/minute which is indicative of the temperature change of the adsorbent is detected. Thus, a temperature measurement position is changed to a position separated as much as 1/5 of the total height of the packed adsorbent from the tip of the adsorbent present a gas outlet during an adsorbing operation, and an experiment is similarly carried out. As a result, an adsorbing time is shortened from 5 minutes of an initial value to 4 minutes and 30 seconds, but chlorine is not detected in the remaining gas. However, the concentration of the purified chlorine gas is 55% by volume. 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 of removing chlorine from a chlorine-containing gas by a pressure swing adsorption method in which a plurality of adsorbing towers packed with an adsorbent capable of adsorbing chlorine are employed and cycled between adsorbing and desorbing operations, the method being characterised by continuously monitoring the adsorption state of the adsorbent in each tower when engaged in adsorption operation by means of a respective sensor for detecting the presence of chlorine gas to recognise the completion of the adsorbing operation, whereupon the feed of the chlorine-containing gas to the adsorbing tower is stopped and the absorbing tower is changed from the adsorbing operation to the desorbing operation; wherein the adsorbent is selected such that the adsorption heat of chlorine substantially exceeds that of each of the other components of said gas, and wherein each said sensor is a temperature sensor and is arranged to measure a temperature change of the adsorbent. A method according to claim 1 wherein the adsorbent is selected from zeolite, non-zeolite type porous acidic oxide, active carbon or molecular sieve carbon. A method according to claim 1 or claim 2 wherein a gas introducing valve is switched to stop the feed of gas to an adsorbing tower when the sensor detects that the differential value of the temperature change has reached 1.5°C/minute. A method according to any preceding claim wherein each said sensor is arranged to detect the temperature rise of the adsorbent in a tower at a position separated by not more than half of the total height of the packed adsorbent from the tip of the adsorbent present at the gas outlet of the treated exhaust during the adsorbing operation. Apparatus for carrying out the method of claim 4 comprising said plurality of adsorbing towers packed with said adsorbent capable of absorbing chlorine, and each including said temperature sensor for detecting the presence of chlorine gas, whereby the completion of the adsorbing operation of the adsorbent-packed layer during the adsorption operation is detectable; and means for switching a tower from the adsorbing operation to the desorbing operation in response to the detection of the completion of the adsorbing operation.

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