MATERIAL GAS FEED TIME (min)
MATERIAL FEED RATE 2.0Nm/h
CHLORINE CONCENTRATION IN MATERIAL 15vol%
TEMPERATURE MEASUREMENT POSITION ,
A POSITON SEPARATED AS MUCH AS 1/10 (20cm)
FROM THE UPPER PORTION OF AN ADSORBING TOWER
METHOD AND APPARATUS FOR
CONCENTRATING CHLORINE GAS
BACKGROUND OF THE INVENTION 5
(i) Field of the Invention
The present invention relates to a method for concentrating (purifying) a chlorine 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 a chlorine containing gas so as to lower the chlorine concentration in a treated exhaust or residue gas and increase the purity of a (concentrated) chlorine gas prod- ^ uct.
(ii) Description of the Related Art
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-con- 2Q taining 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 (U.S. Pat. No. 1,617,305); and other tech- 25 niques. However the method using a chlorine-containing solvent is not preferable, because the chlorine solvent has an influence on process parameters and hence the use of chlorine as a solvent will be restricted. On the other hand, the method which comprises pressurizing- 30 /cooling the chlorine gas to liquefy it requires a compressor and a freezer and therefore this method is not 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 35 effective technique. Therefore, this method has not been utilized so far. In particular, there have not been any industrially effective method and apparatus by which gases having low chlorine concentration can be converted to gases with a high concentration are con- 40 centrated without leaking the chlorine gas into an exhaust or residue gas.
As is apparent from the foregoing, a method and apparatus have not been developed thus far which provides an industrially effective process for concen- 45 trating/purifying a chlorine gas which contains impurities.
Furthermore, there has not existed a method for separating chlorine from a mixed gas containing the chlorine gas, particularly from a gas having a relatively low 50 chlorine concentration and containing moisture to concentrate the chlorine.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an 55 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 60 gas can be discharged.
Another object of the present invention is to provide a pressure swing adsorption (PSA) method and apparatus which comprise separating chlorine from a gas containing chlorine to concentrate the chlorine and to 65 thereby decrease the chlorine content in the remaining gas (treated waste gas) to substantially zero, the method being characterized in that operation conditions for
2
decreasing a chlorine concentration in the remaining gas to substantially zero ar automatically controlled.
Usually, in the PSA system, the operation conditions are selected 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, the adsorption performance of the towers, the amount of adsorbed components in the treated gas, the amount of the gas and the 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, selecting treatment conditions which will decrease the specific gas concentration in the residue 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 residue 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). For this reason, a long operation which will decrease the chlorine gas concentration in the remaining gas to substantially zero is very difficult to achieve.
Still another object of the present invention is to 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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are diagrams of examples of PSA apparatuses by which the present invention is practiced.
FIG. 3 is a graph showing the change time of the temperature rise of an adsorbent at positions separated as much as 1/10 and I 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.
FIG. 4 is a graph showing the relation of a temperature rise to the chlorine concentration in a remaining gas.
The symbols in FIG. 1 mean: 0, 1 . . . Material gas receiving pipe 2 , . . Compressor for a material gas 3a, b, c, d... Switch valve for receiving the material gas
Aa, b, c, d . . . Chlorine gas adsorbing tower 5a, b, c, d . . . Switch valve for discharging a treated gas
6a, b, c, d . . . Switch valve for receiving the treated gas
la, 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) .. . Adsorption 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
DETAILED DESCRIPTION OF THE
INVENTION
The present invention is directed to a method for concentrating a chlorine gas which comprises repeating an adsorbing operation, in which a chlorine gas containing impurities is fed to an adsorption 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 and a residue gas; the method comprises the treated gas coming from a first adsorption tower is introduced into a second adsorption tower to remove most of chlorine from the exhaust gas and, prior to the desorbing operation, the product chlorine gas is introduced into the adsorbing tower to purge a gas residue from the system, thereby heightening the purity of the product chlorine gas.
The first adsorption tower can be directly connected to the second adsorption tower to feed a material gas (a gas to be treated) to the first adsorption tower until 45 breakthough of chlorine gas from the adsorbent occurs. In the second adsorption tower, the chlorine gas leaked from the first adsorption tower can be completely adsorbed.
Furthermore, the product chlorine gas can be intro- 50 duced into the adsorption tower prior to the desorbing operation to purge the residue gas having a low purity from 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 dis- 55 charged.
Intensive investigation has been made of 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 adsorption tower, and the present invention has been attained on the basis of this knowledge.
That is, the present invention is directed to a method for adsorbing/removing chlorine from a gas stream by the use of the pressure swing adsorption method in which a plurality of adsorption towers packed with an adsorbent capable of adsorbing chlorine are employed, the method comprising the steps of continuously detecting chlorine by a sensor for detecting the presence of the chlorine gas to recognize the completion of the adsorbing operation, whereupon the feed of the chlo-. line gas containing impurities to the adsorption tower is stopped and the adsorption 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 adsorption 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-described method.
The gas containing chlorine which can be used in the method of the present invention can contain gases such as oxygen, nitrogen, carbon dioxide, carbon monoxide, argon and methane, in addition to chlorine. In order to separate chlorine from these gases by the PSA method, it is necessary to select an adsorbent which has a sufficient gap between its adsorption affinity for these gases and its adsorption affinity for chlorine and which adsorbs the chlorine with a heat of adsorption, which exceeds 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.
There is no particular restriction is 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 accordance with the present invention, it is preferred that the temperature of the adsorbent is measured at a position of i 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, when the adsorption/separation of a specific chlorine gas is carried out in accordance with the PSA method, it is known that the adsorption heat which is generated raises the temperature of the adsorbent. The present inventors have measured the temperature change of the adsorbent which occurs as a result of the gas containing chlorine being adsorbed on 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 heat of adsorption 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 an abrupt temperature rise in the adsorbent occurs approximately corresponds to a site where the adsorption of the chlorine gas is occurring.
The rate of temperature change of the heat of adsorbent due to the adsorption of the gas containing chlorine depends upon the chlorine concentration in the material gas (the gas to be treated), i.e., the higher the chlorine concentration, the larger per unit time is the
5 6
temperature rise as a result of its adsorption. Therefore,
the leakage of chlorine into the residue gas can be inhib- EXAMPLE 1
ited by appropriate selection of the differential value of FIG. 1 shows one example of a chlorine gas purifying
the temperature change of the adsorbent due to the heat device of the present invention.
of adsorption at the certain chlorine concentration in 5 A pipe 1 is employed as a material gas introducing
the material gas and the detection site for measuring the pipe, and a gas having a relatively low chlorine purity is
temperature change. introduced into pipe 1, pressurized to a predetermined
FIG. 4 shows the relation between the temperature pressure by means of a compressor 2, and then introchange of the adsorption heat in the case that the mate- duced into a first tower 4a of four adsorption towers 4a, rial gas having a chlorine concentration of 15% by 10 4ft, 4c and Ad via a switch valve 3a. volume is used and the chlorine concentration in the The four towers 4a, Ab, 4c and Ad are packed with an residue gas. As is apparent from FIG. 4, in order to adsorbent for predominantly adsorbing chlorine, for prevent chlorine from leaking from the adsorbent back example, a synthetic or natural zeolite, a non-zeolite into the residue gas in the case of a material gas having type porous acidic oxide, active carbon or molecular a chlorine concentration of 15% by volume and the 15 sieve carbon, and chlorine in the material gas intromeasurement site of the temperature change is specified, duced while pressured is predominantly adsorbed by a gas introducing valve should be switched at the time the adsorbent. After a predetermined amount of the when the differential value of the temperature change material gas has been mtroduced, the product chlorine due to the adsorption heat has reached 1.5 C./minute (a gas pressurized by a compreSsor 14 is introduced into predetermined value). 20 tower 4a by switching switch valves la, 16 to purge the
Accordingly, the temperature change of the adsor- material gas off
tower Aa.
bent in the upper port.on of the adsorbing tower can be The stream havm a ,owered cWorine concentra.
continuously measured (monitored), and immediately ^ comi from ^ om,et of tower ^ fa introduced
when the adsorption heat of chlonne has been detected, ^ tQwer Ab yh ... ... 5ft 6[> The
i.e., immediately when the differential value of the tem- 25 . . , f , , , ,„ • . ,
, J , . ,Ji treated gas from which residual chlonne has been re
perature change has reached the predetermined value, Jvj j» • j- * .
r . °, , . f- ii v r. J r moved by adsorption in second tower Ab is discharged
an automatic valve can be automatically switched from ,' .. ,* , _, ..... .. . r,.
, ^ i.i- i_i through a switch valve 76. At this time, a part of the gas
the adsorbing operation to completely inhibit chlonne ,. , 6 , . . , , , .6
f,„„ . „t; • t, „„„:„: „„ *ru„*♦»„ discharged from the second tower Ab is mtroduced into
rrom leaking into the remaining gas. That is, if the tem- ... „ j • •
perature change is measured by a thermocouple or an- 30 th*rd towe* * ,vla f rate adjusting mechanism 11 other means disposed in the adsorbent in the upper and a ^lteh valve 10c" f a Pressure fi»mg steP,s then portion of the adsorbing tower, the adsorption of chlo- carried out to increase the pressure in this tower rine can be easily detected. This detection can be car- ^ the fourth tower Ad, desorption is carried out ried out on the basis of the differential change of the under atmosphenc pressure by switching a switch valve temperature so as not to be affected by the inlet temper- 35 8d Fourth tower Ad is further connected to a vacuum ature of the gas containing the chlorine gas and the like, PumP 12 VIa a switch valve 9b- TM& m fourth tower Adand suitable means can be employed in accordance with the concentrated chlonne gas is recovered and the adthe above-mentioned result to determine the time of sorbent 1S regenerated (desorbed). After tower 4a has termination of the adsorbing operation. Simultaneously, adsorbed a predetermined amount of chlonne and has the result can be converted into an electrical output to 40 been purged with the product gas, the introduction of switch the automatic valve for changing the operation the 8as thereinto is stopped by switching a switch valve of the adsorbing tower. 3°' and the pressure in the tower is reduced by switchThat is, according to the present invention, the tem- in8 a switch valve 8a and the tower is further evacuated perature change of the adsorbent in the upper portion of t0 a reduced pressure state by a vacuum pump 12, the adsorbing tower can be continuously measured 45 whereby chlorine is desorbed from the adsorbent to (monitored) by an inexpensive temperature measuring regenerate the same. In this reproduction step, the chlomeans, and when the differential value of the tempera- rine gas product having a high purity can be stored in a ture rise has reached a predetermined value, the adsorb- buffer tank 13, pressurized by the compressor 14, and ing operation is automatically switched, whereby a then obtained through a product discharge pipe 15. chlorine concentration in the residue gas can be reduced 50 At this time, the material gas is introduced into seeto zero. Since the chlorine concentration in the residue ond tower Ab via a switch valve 3b, and chlorine in the gas is substantially zero, measures for avoiding the ad- material gas is predominantly adsorbed. The gas having verse effects of chlorine can be simplified. Therefore, a low chlorine concentration is obtained through the the present invention is very useful from an industrial outlet of tower Ab and is then introduced into tower 4c viewpoint. 55 via switch valves 56, 6c. In tower 4c, residual chlorine is In the present invention, a dehydrator may be adsorbed, and the chlorine-free gas is then discharged equipped in order to remove moisture from a starting through a switch valve 7c. After the adsorption of the mixed gas containing the chlorine gas on the upstream material gas has been computed, the product chlorine side of the adsorbing tower in a chlorine gas concen- gas pressurized by compressor 14 is introduced into the trating/purifying apparatus in which the above-men- 60 tower Ab by switching the switch valves la, lb to purge tioned PSA method is utilized. the material gas off tower Ab, and the purged gas is When the dehydrator is equipped in order to remove discharged through the switch valves 56, 6c and then moisture from the mixed gas containing the chlorine introduced into third tower 4c. In third tower 4c, chlogas, the life of the adsorbent can be prolonged and rine is adsorbed, and the gas is then discharged through corrosion of the apparatus can be also prevented. 65 the switch valve 7c.
Now, the present invention will be described in detail Furthermore, a part of the gas discharged from third
in reference to examples, but the scope of the present tower 4c is introduced into fourth tower 4a" via the flow
invention should not be limited to these examples. rate adjusting mechanism 11 and a switch valve 10^,
7
and a pressure filling step is then carried out to increase the pressure in this tower.
Afterward, the material gas is introduced into third tower 4c via a switch val ve 3c, and then chlorine in the material gas is predominantly adsorbed. At the outlet of 5 tower 4c, the gas having a low chlorine concentration is obtained, and then introduced into tower 4d via switch valves 5c, 6d.
Simultaneously, a part of the gas discharged from the fourth tower 4c? is introduced into the first adsorbing 10 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 second tower 4b is stopped by switching the 15 switch valve 3b, and the pressure in this tower is then reduced by switching a switch valve Sb. Furthermore, second 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 20 adsorbent to recover it.
Similarly, the above-mentioned serial operation is repeated alternately by the use of four towers 4a, 4b, 4c and 4a", whereby high-purity chlorine can be continuously prepared from the chlorine gas containing impuri- 25 ties.
EXAMPLE 2
One example of an apparatus by which the method of the present invention can be carried out will be de- 30 scribed in reference to drawings as follows.
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 35 for desorption/regeneration, and (6) to (11) are switch valves.
Assuming that an adsorption operation is carried out in adsorbing tower (1) and a desorption/regeneration operation is performed in adsorption tower (2), the 40 switch valves (7), (8) and (10) are opened and switch valves (6), (9) and (11) are closed. A gas containing chlorine from compressor (3) is fed to adsorbing tower (1) through the bottom thereof via switch valve (8), and chlorine is mainly adsorbed therein. The chlorine-free 45 gas is then discharged as a residue gas from adsorbing tower (1) through the upper portion thereof and further eliminated from the system through switch valve (10).
On the other hand, in adsorbing tower (2), the adsorbed components, of which chlorine is the main com- 50 ponent, are desorbed from the adsorbent by the use of a vacuum pump, and the chlorine gas is then discharged from adsorbing tower (2) through the bottom thereof and thereafter from the system through switch valve (7) by vacuum pump (5). This discharged gas is the gas 55 containing concentrated chlorine, and it can 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 other means, but when chlorine concentration, tem- 60 perature, pressure and the like of a gas containing chlorine change, it is difficult to conduct such an operation as to decrease a chlorine concentration in a residue gas to substantially zero over a long period of time.
On the contrary, in the present invention, the temper- 65 ature of the adsorbent in the adsorption tower can be continuously measured (monitored) to detect the differential change at an optional temperature, whereby an
8
optional adsorption time can be selected and after the adsorption, the adsorption 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 adsorption towers (1), (2) of the above-mentioned apparatus, and the temperature change of the adsorbent is continuously detected so as 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 which more than a predetermined differential change at a selected temperature is detected by 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 switch valves (6) to (11).
The gist of the present invention resides in that the adsorbing step in the PSA method is monitored so that the chlorine concentration in the remaining gas is always substantially zero, even if a gas treatment conditions for the gas containing chlorine (composition, flow rate, pressure temperature) changes.
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 the heat of, adsorption, and in the tower, a temperature rise zone advances along the tower substantially proportionately to the advancement of the chlorine adsorption zone in the tower. Therefore, when this temperature rise zone is detected in the vicinity of the outlet of the tower, the detected values can be utilized to correctly predict the chlorine breakthrough time, which permits monitoring the set switch time of the tower. However, the mere detection of the temperature rise is an insufficient monitor owing to a dependent relationship which it has with the inlet temperature of the gas containing chlorine. The accurate monitor for determining the time of termination of the adsorbing step in a tower can be achieved by determining the differential change in the temperature.
The present invention is effective in the case that the heat of adsorption by the adsorption of the chlorine gas by the adsorbent is much larger than the heat of 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 over a long period of time. In addition, the monitoring can be carried out so as to prevent chlorine from leaking into the residue gas. Therefore, the present invention has high reliability and is very effective from an industrial viewpoint.
EXAMPLE 3
In a dual-adsorption type PSA apparatus for separating chlorine from a gas containing chlorine used in the example to concentrate chlorine, each adsorption 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 adsorption tower is packed with 3 kg of the adsor
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