Method for separating vinyl chloride from a thermally decomposed product of 1,2-dichloroethane

A method for separating vinyl chloride, which comprises cooling a cracked gas obtained by cracking 1,2-dichloroethane by a thermal cracking furnace, firstly in a heat exchanger, then further cooling it in a quenching tower and then distilling it, wherein the cracked gas is cooled in the heat exchanger to at least 350.degree. C., the quenching tower is controlled so that from 80 to 98 wt % of the cracked gas introduced is withdrawn as an overhead product and the rest of from 20 to 2 wt % of the cracked gas is withdrawn as a bottom effluent, and they are respectively sent to the subsequent steps, and formed coke is discharged together with the bottom effluent.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for separating vinyl chloride 
(hereinafter referred to simply as VCM) via cooling and a purification 
step such as distillation, from a gas formed by thermal cracking of 
1,2-dichloroethane (hereinafter referred to simply as EDC). Particularly, 
the present invention relates to a method wherein the gas formed by 
thermal cracking of EDC is cooled in specific steps, and formed coke is 
efficiently separated to prevent a trouble in the subsequent step for 
separating VCM. 
2. Discussion of Background 
Heretofore, as a method for producing VCM, a method is known wherein EDC is 
thermally cracked at a temperature of from 450 to 550.degree. C. in a 
thermal cracking furnace, and the formed cracked gas is cooled and then 
VCM is separated by distillation. The high temperature cracked gas 
discharged from the cracking furnace contains VCM and hydrogen chloride as 
cracked products, and noncracked EDC mainly, and it is supplied to a 
quenching tower as it is, or it is indirectly cooled by a heat exchanger 
and then supplied to a quenching tower, where the heat of the high 
temperature cracked gas is recovered. 
In such a method, it is known that coke will deposit in the cooling pipe of 
the heat exchanger used for cooling the cracked gas to cause deterioration 
of the performance, increase of the pressure loss and clogging of the 
pipe, and it is impossible to conduct a continuous operation over a long 
period of time. 
Therefore, JP-B-6-92328 proposes to carry out heat exchange of EDC to be 
supplied to the thermal cracking furnace with a high temperature cracked 
gas discharged from the cracking furnace by permitting the cracked 
effluent gas to flow through a pipe at a flow rate of at least 5 m/sec and 
less than 20 m/sec and cooling the cracked effluent gas to a temperature 
of from 180 to 350.degree. C., whereby it is said to be possible to 
prolong the continuous operation period of the thermal cracking furnace. 
However, in this method, the cracked gas cooled in a heat exchanger is 
introduced as it is to the conventional quenching tower, then quenched to 
e.g. 80.degree. C. and then sent to the subsequent step, whereby the 
bottom liquid is withdrawn in a large amount, and it is difficult to 
sufficiently separate coke from the bottom liquid before supplying the 
bottom liquid to a hydrogen chloride tower. Further, JP-A-6-219977 
discloses a method in which a specific cooler which is single pipe of from 
150 to 250 mm and which employs the bottom liquid of an EDC distillation 
tower as a cooling medium, is used for indirectly cooling the cracked gas, 
and the cracked gas is permitted to flow in the single pipe of the cooler 
under a pressure of from 0.9 to 1.4 MPa and cooled at an average cooling 
rate of from 15 to 45.degree. C./sec until the cracked gas temperature at 
the outlet of the cooler becomes from 250 to 350.degree. C., and EDC 
thermally recovered by a cooling medium in the cooler is directly refluxed 
to the EDC distillation tower, whereby the cracked gas withdrawn from the 
cooler is quenched to a level of from 40 to 150.degree. C. in a quenching 
tower, and VCM is recovered in the subsequent step, but the bottom 
effluent withdrawn from the bottom of the quenching tower is separately 
treated without passing through a distillation purification step. 
This method has a merit such that the continuous operation periods of the 
thermal cracking furnace and the cooler can be prolonged equally to sixth 
months. However, the effluent from the bottom of the quenching tower is 
separately treated, thus leading to a disadvantage such as a loss of EDC 
or VCM in the effluent. 
SUMMARY OF THE INVENTION 
In the production of VCM by thermal cracking of EDC, formation of coke in 
the step of cooling the cracked gas brings about a serious hindrance to 
maintain continuous operation. Accordingly, it has been attempted to avoid 
such formation of coke, but such attempts have been mainly for the purpose 
of suppressing the formation by changing the operation conditions of the 
thermal cracking furnace. However, it is difficult to completely suppress 
formation of coke, and it is desired to efficiently treat formed coke to 
avoid a trouble against the continuous operation. 
The present inventors have conducted an extensive study on the mechanism 
for the formation of coke in each step from cooling of the cracked gas of 
EDC to separation of VCM and as a result, have found that the nature of 
formed coke differs depending upon the temperature for treatment of the 
cracked gas, and the handling efficiency of coke differs depending upon 
the nature. It has also been found that coke formed under certain specific 
conditions can be removed by an extremely simple separation operation, and 
as a result, constant operation for a long period of time can be made 
possible. The present invention has been accomplished on the basis of 
these discoveries. 
The present invention is directed to a method for separating VCM which 
comprises cooling a cracked gas discharged from a thermal cracking furnace 
of EDC by indirect cooling to a predetermined temperature, and then 
cooling it in a quenching tower, wherein the conditions for indirect 
cooling and operation of the quenching tower, are controlled, so that coke 
is efficiently separated thereby to prevent a trouble in the subsequent 
distillation step. 
Namely, the present invention provides a method for separating vinyl 
chloride, which comprises cooling a cracked gas obtained by cracking 
1,2-dichloroethane by a thermal cracking furnace, firstly in a heat 
exchanger, then further cooling it in a quenching tower and then 
distilling it, wherein the cracked gas is cooled in the heat exchanger to 
at least 350.degree. C., the quenching tower is controlled so that from 80 
to 98 wt % of the cracked gas introduced is withdrawn as an overhead 
product and the rest of from 20 to 2 wt % of the cracked gas is withdrawn 
as a bottom effluent, and they are respectively sent to the subsequent 
steps, and formed coke is discharged together with the bottom effluent.

EXAMPLE 1 
By the apparatus as shown in FIG. 1, thermal cracking of EDC was carried 
out to produce VCM. 
Gaseous EDC preliminarily heated to 230.degree. C., was supplied to the 
thermal cracking furnace (1) at a flow rate of 107.86 parts/hr. The 
cracking temperature of the cracking furnace was 490.degree. C., and the 
cracking pressure was 19/cm.sup.2 G, whereby about 53% of supplied EDC was 
cracked. 
The cracked gas (490.degree. C.) discharged from the thermal cracking 
furnace was introduced to the heat exchanger (2). The heat exchanger was a 
single pipe vertical type, and as a cooling liquid medium, EDC 
(220.degree. C.) was supplied. The temperature of the cracked gas 
discharged from the outlet of the heat exchanger was 300.degree. C. The 
cooled cracked gas was supplied to the quenching tower (3), and the 
quenching tower was operated at a tower bottom temperature of 
167.9.degree. C., at a tower top temperature of 165.8.degree. C. under a 
tower top pressure of 18.5 kg/cm.sup.2 G, whereby from the tower bottom, 
the bottom effluent was withdrawn at a rate of 5.35 parts/hr, and the rest 
was withdrawn from the tower top as the overhead product. Here, the weight 
ratio of the overhead product to the bottom effluent was 95.0:5.0. 
In the effluent from the tower bottom, coke was contained in addition to 
VCM, EDC, high boiling substances, etc. Therefore, after removing coke by 
the separating apparatus (4), the effluent was sent to the hydrogen 
chloride tower and treated together with the overhead product. On the 
other hand, the overhead product was supplied to the hydrogen chloride 
tower, and by distillation, HCl was distilled and recovered from the tower 
top, and the tower bottom liquid was rectified by a VCM tower (not shown) 
to recover VCM. EDC gasified by the heat exchanger was supplied as 
starting material EDC to the thermal cracking furnace. 
Under these conditions, continuous operation was carried out, and each of 
the cracking furnace, the quenching tower and the hydrogen chloride tower 
was operated at least 11 months, and thus it was possible to prolong the 
operation period substantially. Further, scatter of coke was not observed, 
and no cleaning operation of equipments was required. 
EXAMPLE 2 
In the apparatus as shown in FIG. 1, boiler water (pure water) was employed 
instead of EDC, as a cooling liquid medium for the heat exchanger (2). The 
thermal cracking was carried out under the same conditions as in Example 
1, except that gaseous EDC was supplied at a flow rate of 116.64 parts/hr. 
The cracked gas (490.degree. C.) discharged from the thermal cracking 
furnace was introduced into the heat exchanger (2). The temperature of the 
cracked gas discharged from the outlet of the heat exchanger was 
280.degree. C. The cooled cracked gas was supplied to the quenching tower 
(3). The quenching tower was operated at a tower bottom temperature of 
165.4.degree. C., at a tower top temperature of 157.5.degree. C. under a 
tower top pressure of 16.85 kg/cm.sup.2 G, whereby an overhead product was 
withdrawn at a rate of 110.69 parts/hr from the tower top, and a bottom 
effluent was withdrawn at a flow rate of 5.95 parts/hr from the tower 
bottom. Here, the weight ratio of the overhead product to the bottom 
effluent was 94.9:5.1. 
The respective effluents withdrawn from the quenching tower were treated in 
the same manner as in Example 1. By the operation under the 
above-described conditions, it was possible to continuously operate each 
of the cracking furnace, the quenching tower and the hydrogen chloride 
tower constantly for at least 11 months. Further, scatter of coke was not 
observed, and no cleaning operation of equipments was required. 
Comparative Example 1 
By the apparatus shown in FIG. 2, thermal cracking of EDC was carried out 
to produce VCM. Liquid EDC preliminarily heated to 205.degree. C., was 
supplied to the thermal cracking furnace (1) at a flow rate of 96.5 
parts/hr. The cracking temperature of the cracking furnace was 510.degree. 
C., the cracking pressure was 32 kg/cm.sup.2 G, and the operation was 
carried out at a cracking rate of supplied EDC of about 52%. 
The cracked gas (510.degree. C.) discharged from the thermal cracking 
furnace was supplied directly to the quenching tower (3) and quenched. The 
quenching tower was operated at a tower bottom temperature of 89.degree. 
C., at a tower top temperature of 85.degree. C. under a tower top pressure 
of 18 kg/cm.sup.2 G, whereby the overhead product was withdrawn at a flow 
rate of 41.8 parts/hr from the tower top, and the bottom effluent was 
withdrawn at a flow rate of 54.7 parts/hr from the tower bottom. Here, the 
weight ratio of the overhead product to the bottom effluent was 43.3:56.7. 
Under these conditions, continuous operation was carried out. As a result, 
the performance of the cooler for the tower bottom liquid of the quenching 
tower decreased by coke, and cleaning operations were required at a 
frequency of about once a month. Further, soft coke discharged together 
with the overhead product was transported to the hydrogen chloride tower 
and fouled the equipments. 
Comparative Example 2 
By the apparatus shown in FIG. 3, thermal cracking of EDC was carried out 
to produce VCM. Gaseous EDC preliminarily heated to 230.degree. C., was 
supplied to the thermal cracking furnace (1) at a flow rate of 96.5 
parts/hr. The cracking temperature of the cracking furnace was 487.degree. 
C., the cracking pressure was 18.5 kg/cm.sup.2 G, and the operation was 
carried out at a cracking rate of supplied EDC of about 52%. 
The cracked gas (487.degree. C.) discharged from the thermal cracking 
furnace was introduced into the heat exchanger (2). To the heat exchanger, 
EDC (220.degree. C.) was supplied as a cooling liquid medium. The 
temperature of the cracked gas discharged from the outlet of the heat 
exchanger was 200.degree. C. The cooled cracked gas was supplied to the 
quenching tower (3) and cooled. The quenching tower was operated at a 
tower bottom temperature of 89.degree. C., at a tower top temperature of 
85.degree. C. under a tower top pressure of 18 kg/cm.sup.2 G, whereby the 
overhead product was withdrawn at a flow rate of 41.8 parts/hr from the 
tower top, and the bottom effluent was withdrawn at a flow rate of 54.7 
parts/hr from the tower bottom. Here, the weight ratio of the overhead 
product to the bottom effluent was 43.3:56.7. 
Under these conditions, continuous operation was carried out, and as a 
result, the efficiency of the cooler for the tower bottom liquid of the 
quenching tower decreased by coke, and cleaning operations were required 
at a frequency of once per 6 months. 
Comparative Example 3 
By the same apparatus as in FIG. 1 except that no heat exchanger (2) for 
indirect cooling of the cracked gas is provided, thermal cracking of EDC 
was carried out to produce VCM. 
Liquid EDC preliminarily heated to 143.degree. C. was supplied to the 
thermal cracking furnace (1) at a flow rate of 94.4 parts/hr. The cracking 
temperature of the cracking furnace was 498.degree. C., the cracking 
pressure was 26.4 kg/cm.sup.2 G, and the operation was carried out at a 
cracking rate of supplied EDC of about 52%. 
The cracked gas (498.degree. C.) discharged from the thermal decomposition 
furnace was supplied directly to the quenching tower (3) and quenched. The 
quenching tower was operated at a tower bottom temperature of 
192.1.degree. C., at a tower top temperature of 185.6.degree. C., under 
the tower top pressure of 21.8 kg/cm.sup.2 G, whereby the overhead product 
was withdrawn at a flow rate of 90.37 parts/hr from the tower top, and the 
bottom effluent was withdrawn at a flow rate of 4.03 parts/hr from the 
tower bottom. Here, the weight ratio of the overhead product to the bottom 
effluent was 95.7:4.3. 
The bottom effluent and the overhead product were treated in the same 
manner as in Example 1. 
Under these conditions, continuous operation was carried out, whereby the 
quenching tower and equipments coke, and cleaning operations were required 
at a frequency of once per 3 months. 
As described in the foregoing, according to the method of the present 
invention, the crack ed gas of EDC is cooled by indirect cooling to a 
predetermined temperature and then cooled in a quenching tower under the 
predetermined conditions, whereby hard coke is selectively formed, and 
formed coke is withdrawn together with the bottom effluent from the 
quenching tower, whereby it is possible to prevent fouling by coke of the 
quenching tower and the subsequent equipments and to carry out the 
operation constantly for a long period of time. Further, the bottom 
effluent from the quenching tower is in a small amount, whereby reduction 
of the installation becomes possible. Thus, the method of the present 
invention is economically advantageous.