WO2000056686A1

WO2000056686A1 - Method and unit for producing vinyl chloride by thermal cracking of 1,2-dichloroethane

Info

Publication number: WO2000056686A1
Application number: PCT/FR2000/000655
Authority: WO
Inventors: Bruno Marciano; Reinier Westerink
Original assignee: Technip France
Priority date: 1999-03-19
Filing date: 2000-03-17
Publication date: 2000-09-28
Also published as: FR2791055B1; CN1349483A; EP1163197A1; AU3437300A; FR2791055A1; EA200100988A1

Abstract

The invention concerns a method and a unit, both improved, for producing vinyl chloride (monomer: VCM) by thermal cracking of 1,2-dichloroethane (DCE). Said improved method is implemented with recovery of calories on the gaseous effluent (G) derived from cracking. The method is characterised in that said gaseous effluent (G) derived from cracking is transferred via at least a transfer line without decrease in its mass rate and said calories are recovered by at least a fluid cooler (L) circulating around the transfer line and around at least one of said transfer lines of said gaseous effluent derived from cracking, in a space provided between said transfer line(s) and a sleeve (11) arranged around the latter, said recovery of calories being carried out in conditions wherein the fluid cooler (L) remains liquid and wherein said gaseous effluent (G) remains substantially in gas form.

Description

Process and unit for producing vinyl chloride by thermal cracking of 1, 2-dichloroethane

The present invention relates to improved processes and units for the production of vinyl chloride (monomer: VCM) by thermal cracking of 1,2-dichloroethane (DCE).

Said cracking of DCE, to produce VCM, is a per se known method, implemented on several industrial sites. In the context of the present invention, the Applicant proposes to provide an improvement in the recovery of calories from the gaseous effluent from cracking.

In order to situate the context of the invention, it is proposed, below, to give, firstly, some general indications on said cracking process of DCE and, secondly, details on this subject. , with reference to FIGS. 1 and 2 annexed to the present text, figures which schematically illustrate, respectively, a process for cracking low pressure DCE (6 to 12 bar eff. at the outlet of the oven) and a process for cracking high pressure DCE (> 12 bar eff. At the exit of the oven) (processes implemented without recovery of calories on the gaseous effluent directly from cracking).

We have, in said figures 1 and 2 appended as well as in the other figures 3 to 7 also appended and commented further in the present text.

(Figures 3 to 7 which illustrate the invention), referenced from 1 to 7 the seven successive stages of the method and their associated device, specifying by an index the illustrated embodiment.

The vinyl chloride monomer (VCM) is thus produced on a large scale by thermal cracking of 1,2-dichloroethane (DCE); the cracking process comprising the following steps:

1. pressurization of the liquid DCE (6-30 bar) by a pump;

2. preheating of the liquid DCE;

3. vaporization of the preheated liquid DCE; 4. overheating of the vaporized DCE;

5. partial conversion of DCE (45-65%) into VCM according to the reaction: DCE - »VCM + HC1, by thermal cracking at temperatures between 400 and 500 ° C; 6. cooling and washing of the cracked gas (containing VCM, HC1, DCE) by direct injection of a mixture of liquid VCM + DCE and / or by spraying with such a mixture. The purpose of washing the cracked gas is to eliminate the particles and heavy products contained in said gas and thus to minimize the fouling of downstream equipment. The temperature of the gas, at the end of this operation, is variable, depending on the operating pressure;

7. additional cooling of the washed gas (containing VCM, HC1, DCE) to a temperature, depending on the operating pressure. A fraction of the VCM + DCE condensed at this stage is used for the injection / watering of the previous step 6.

The HC1, VCM, DCE mixture thus produced in the cracking section is then separated into its components (HC1, VCM and DCE) by means of multiple distillations with:

- recovery of HC1, which can be used for the production of DCE by oxychlorination,

- recovery of DCE (not cracked), for recycling as a raw material,

- and recovery of the VCM produced.

The means or devices, most commonly used, to implement the seven steps of the process listed above are specified below. Said means are referenced nj in the figures, with n which identifies step 1 to 7 concerned and i the variant illustrated.

Thus, step 2 of preheating the liquid DCE (pressurized by the pump 1) can it be implemented in three types of device a, b and c, referenced respectively 2a (see FIG. 2), 2b (see Figure 1) and 2c (not shown). What is stated:

2. preheating of the liquid DCE a - exchanger supplied with low pressure steam or other heat transfer fluid, b - exchanger heated by an intermediate current in the cooling / distillation section, c - bundle in the convection zone of the cracking oven (heat recovery from fumes), In the same way, we have:

3. vaporization of the preheated liquid DCE a - kettle type reboiler supplied with water vapor or other heat transfer fluid, b - vaporization column with kettle 3b 'reboiler (s) or thermosiphon supplied with water vapor or other heat transfer fluid, c - bundle in the convection zone of the cracking furnace, d - independent furnace, 4. overheating of the vaporized DCE a - bundle in the convection zone of the cracking furnace, b - bundle in the direct heating zone ( "radiation") from the cracking oven,

5. partial conversion of the DCE to - one or more tube bundles placed in the direct heating ("radiation") zone of the cracking oven; the heat necessary for the reaction is supplied by burners located either in the walls or in the bottom of the oven,

6. cooling and washing of the cracked gas a - T of mixture with direct injection of a liquid VCM + DCE stream into the hot effluent followed by a separation column, b - sprinkler column using a liquid VCM + DCE stream , c - T of mixture combined with a watering column,

7. cooling of the washed gas a - exchanger supplied with a process current (see 2b), b - water cooler, c - air cooler.

The low temperature level of the washed gas does not allow considerable heat recovery at this stage (step 7). Most of said heat is lost to air and / or cooling water.

According to the prior art, the cracking of DCE has therefore generally been implemented, as described above, without recovery of calories from the gaseous effluent directly obtained from cracking. However, it was also recommended, according to said prior art, the intervention of different types of device to recover such calories.

Thus, in application EP-A-014 920, it is described the intervention of a tubular exchanger to cool the cracked gas and the use of the heat recovered, in the process, for:

- heat / evaporate the bottoms of the distillation section directly,

- heating a heat transfer fluid in order to supply the reboilers in the distillation section, - generating steam in order to supply the reboilers in the distillation section,

- preheat or spray the charge DCE.

The recommended exchanger consists of a single tubular bundle, in the form of a coil, placed in a cylindrical container. The cracked gas circulates inside the tube, the coolant outside of said tube, inside said container.

An exchanger of this same type operates according to the teaching of GB-A-2 179 938. The refrigerant used is a gas circulating in a flue. The heat recovered by said refrigerant is used for the partial vaporization of the charge, within the cracking oven.

In application EP-A-264 065, it is also proposed to use a heat exchanger of this type, for the sole purpose of vaporizing the charge DCE. Said exchanger is equipped with a charge tank which ensures both natural circulation through the shell of the exchanger and a separation of the vapor / liquid mixture coming from said exchanger.

The devices recommended in these documents of the prior art are not fully satisfactory. The volume of the cylindrical container necessary to accommodate the serpentine beam is very large and limits the practical use of this type of exchanger. Indeed: - in the event of the intervention of a liquid as refrigerant, the flow required to ensure correct heat transfer on the ferrule side is approximately 20 times the flow circulating inside the bundle: the flow required which results is incompatible with the available currents whose flows are either fixed by the process (liquid DCE to be preheated), or subjected to other constraints (coolant). The use of the exchanger is therefore limited to vaporization systems (DCE vaporization or generation of water vapor) in which the heat transfer on the shell side is ensured either by the system's own turbulence (liquid in boiling state) ), or by installing a recycling system (forced or natural circulation);

- in the case of the use of the exchanger as a DCE vaporizer according to EP-A-264 065, (natural) circulation through the exchanger is ensured by the charging tank; however, said charging tank only increases the volume of the assembly to give a residence time on the DCE side of the order of 50 minutes. This high residence time causes more or less significant thermal degradation depending on the operating temperature on the DCE side (between 150 and 250 ° C. depending on the operating pressure) and requires a significant purging rate to maintain the content of degraded products at an acceptable level. In US-A-4,324,932, it is proposed to recover calories from the gaseous effluent from cracking, while minimizing the formation of coke. The heat exchangers in issue are not specifically described. Mention is made of plates heat exchangers ^and tube exchangers with a single tube. Within them, it is planned to partially condense the gaseous effluent. It is in fact taught, in said patent US-A-4,324,932, to implement rapid cooling, of at least 1/10 ^th of the inlet temperature of the effluent in question, expressed in degrees Celsius, by second. Said rapid cooling can be implemented directly and / or indirectly, with a liquid and / or gaseous refrigerant. It is advantageously implemented in several stages, in particular directly and indirectly. The speed - rapid - of said cooling is recommended to minimize the formation of coke.

In patent US-A-5,728,906, heat exchange is implemented, as in GB-A-2,179,938 and DE-A-197 27,659, between the preheating and cracking zones of the oven cracking. The calories given off by the cracking effluent allow the charge to be vaporized, at least in part. The heat exchange is implemented in a double tube type exchanger. A single variant is possible. In fact, within said double tube exchanger, one can fear fouling in each of the two tubes. In such a context of VCM production by DCE cracking, the Applicant proposes an original process - more efficient and easier to implement than those mentioned above - to recover calories from the gaseous effluent from cracking . According to its first object, the invention therefore relates to a process for producing VCM by thermal cracking of DCE, implemented with recovery of calories from the gaseous effluent resulting from cracking. Typically, in the context of said process:

- the gaseous effluent from cracking is transferred via at least one transfer line (a single transfer line generally intervenes but it is not at all excluded to involve several transfer lines. One can in particular have two transfer lines in the context of the "low pressure" process where the ovens used generally contain two cracking beams, said two beams can be connected to a single transfer line but generally each of said two beams is connected independently to a transfer line. conceives that one can thus have, in absolute terms, n transfer lines each connected to a cracking beam of the n beams that the cracking oven contains) without consequent reduction in its mass speed; and

- the calories of said gaseous effluent are recovered by at least one liquid refrigerant circulated, around said transfer line or at least one of said transfer lines, in a space provided between said (said) line (s) transfer and a shirt arranged around the latter; said recovery of said calories being implemented under conditions where said liquid refrigerant remains liquid and where said gaseous effluent remains substantially gaseous. It has been found, unexpectedly, that the installation of a simple jacket, around the transfer line (s), at the outlet of the cracking oven (generally between said outlet of the cracking oven and the injection / sprinkler cooling system (quench)), makes it possible to constitute one (or more) exchanger (s) of the "double tube" type, the characteristics of which are suitable for the use of a liquid refrigerant; particularly efficient exchanger (s) insofar as:

- the problems of fouling, by deposit of particles, are minimized in its (their) breast: a) indeed, we are dealing with a single line (s) of transfer, which does not contain any disturbing elements, such as bifurcations, and with a simple lining around that (s) )-this ; b) the gaseous effluent is maintained in the gaseous state. We aim to avoid any condensation of it, to maintain it at a temperature higher than its dew point temperature. To do this, a liquid coolant is advantageously used at a temperature (inlet temperature of said liquid coolant) higher than said dew point temperature of the cracked gas; said gaseous effluent is generally maintained at more than 250 ° C; c) said gaseous effluent enters said circulating transfer line (s) and circulates there without consequent reduction in its mass speed. Advantageously, said gaseous effluent is maintained at a mass speed equal to (close to) that which it had in the cracking beam (s) from which it originated. It is conceivable to increase said mass speed, but in no case to reduce it substantially (we would then observe (as in some exchangers of the prior art) a significant deposit of coke particles on the walls of the transfer line in question ). Advantageously, as indicated, said mass speed is kept (substantially) constant; d) the refrigerant being maintained in the liquid state, no coke is formed therein.

In the context of the process of the invention, any coke formation in the intervening coolant is avoided and the deposition of coke in the effluent transfer line (s) is minimized. The coke present in said effluent is entrained by it. The Applicant has established that the fouling products ("coke" particles) are present in the cracked effluent and are not formed, appreciably, within the exchanger, whatever the rate of cooling carried out within said exchanger;

- the implementation of cooling is perfectly controlled with the intervention of a liquid refrigerant, maintained in the liquid state. The term tube used here does not imply any limitation as to the exact geometry of the couple transfer line (s) and jacket (s) arranged around it (s). Advantageously, said term tube is read, however, according to its usual connotation of hollow cylinder (of circular section). The inner tube is therefore generally formed by the transfer line which brings together the effluent from one or more cracking beams. The passage section of said inner tube generally corresponds to the total passage section of the connected cracking beam (s); this ensures sufficient gas velocity to obtain correct heat transfer and minimize fouling. The outer tube is therefore formed by the jacket. The liquid refrigerant circulates, co-current or counter-current, advantageously against the current (of the gaseous effluent), in the annular space (if, really, two tubes intervene) formed by the transfer line and the jacket . The inner diameter of the jacket is determined according to the flow rate and the characteristics of the refrigerant, in order to optimize the heat transfer.

The jacketed length determines the available exchange surface and can be adapted according to the amount of heat to be evacuated / recovered. It can be carried out in one or more segments connected in series, gas side. We can already specify here, with reference to the device aspect of the invention, that:

- cracked gas side, the segments can be connected either by straight sections or by elbows; the use of flanges to connect the segments facilitating possible mechanical cleaning of the transfer line;

- refrigerant side, the segments can be connected in series or in parallel, either by standard piping elements, or by elbows or return boxes specifically adapted to the dimensions of the jacket.

The use of several segments also allows the use of different refrigerants or the use of the same type of refrigerant under different conditions (temperature, pressure, flow). The double-tube exchanger (s), within the meaning of the invention, can be declined according to numerous variant embodiments.

The recovery of calories, implemented according to the invention in a very simple way by means of a very simple device, can be done with any type of liquid refrigerant. It is generally implemented with a view to using said calories and advantageously with a view to such use in the process for producing VCM. In this perspective of recovering the calories recovered, it is recommended to use refrigerants such as: - boiler feed water,

- the load of liquid DCE,

- a heat transfer fluid, for example pressurized water or a commercial fluid (mineral oil, synthetic oil, etc.). We can, in fact, implement:

- recovery / recovery of calories, "indirect", through a heat transfer fluid (see above). (The operating temperatures of the various heat consumers in a VCM production unit (liquid DCE preheater, reboilers in the distillation section, charge DCE evaporator) range from 100 to 210 ° C for a "low pressure" unit. "and 100 to 260 ° C for a" high pressure "unit. The use of a commercial heat transfer fluid that can operate at low pressure (0 - 2 bar) and at temperatures from 150 to 350 ° C is suitable for cooling the cracked gas and then allows the absorbed heat to be transferred to users operating in a temperature range from 100 to 260 ° C, thus covering both "low pressure" and "high pressure" processes);

- recovery / recovery of calories, "direct", through a process fluid (fluid from the VCM production process (the load of liquid DCE, for example) or fluid from another process, works nearby, (boiler feed water, for example)).

Within the framework of the process of the invention, an indirect recovery (via a heat-transfer liquid) of the calories is advantageously implemented.

As already indicated, care is taken to ensure that the minimum temperature of the refrigerant is above the dew point temperature of the cracked gas; in order to avoid the concentration of solid particles, possibly present, in the (small) amount of condensed liquid.

We now propose to describe, in general terms, the second object of the present invention, namely - the device associated with its first object - a VCM production unit by DCE cracking. Such a unit comprises: - means for conditioning the pressure and temperature of the liquid DCE feedstock;

- a cracking oven of said conditioned DCE; said oven containing at least one cracking beam; - Means for washing, cooling and condensing the gaseous effluent from said cracking oven; said conditioning means and said oven, on the one hand, said oven and said means for washing, cooling and condensing, on the other hand, being interconnected by suitable lines for transferring the treated charge;

- as well as means for recovering calories from the gaseous effluent from said cracking oven.

Typically, within such a unit:

- Said means for recovering said calories include: + a simple lining of the transfer line or at least one of the transfer lines of the gaseous effluent from said cracking oven; said liner advantageously constituting a “double tube” type exchanger; and + means for circulating, in the space thus arranged between said transfer line (s) and the jacket arranged around, a liquid refrigerant, and

each transfer line thus lined with the gaseous effluent from said cracking furnace has a passage section, for said gaseous effluent, less than or equal to the total passage section of the cracking bundle (s) from said kiln cracking, leading into said transfer line.

The conditions required on the passage section (s) of the jacketed transfer line (s) involved are obviously with reference to the problem of maintaining the mass speed of the gaseous effluent, to minimize the deposition of fouling particles. Advantageously, each lined transfer line (inner tube) has fins (longitudinal) on the outside, in order to improve the heat transfer on the refrigerant side.

We have seen above that the jacketed length of said transfer line can be made in one or more segments, arranged in various ways. Advantageously, the particular means involved, according to the invention, for recovering calories from the gaseous effluent from cracking, are arranged in a circuit for recovering (therefore) and distributing said calories, integrated in the production unit from VCM. In the context of this advantageous variant, said circuit can be arranged to distribute the calories recovered, at the level of the means for conditioning the feedstock of liquid DCE. Said circuit also contains, according to a preferred variant, an additional device for supplying heat (an oven for example) and / or an additional device for removing heat (an (aero) refrigerant, for example). The intervention of this type of device increases the flexibility of implementation of the method.

The intervention of an oven or any other equivalent means of heat supply makes it possible to increase the capacity of the circuit beyond the quantity of heat recovered in the process and possibly cover all of the heat requirements of the 'VCM production unit. It is also useful to facilitate / speed up start-up operations.

The intervention of an (aero) refrigerant or any other equivalent means of heat extraction increases the flexibility of the system during the starting, stopping and decoking operations. It is incidentally noted here that the calories recovered according to the invention can also quite be distributed, by means of an adequate circuit, in another unit.

It is also noted incidentally here that the invention has been described above and is claimed below, in the hypotheses of the intervention of a single or several transfer lines, implemented on said single transfer line or on at least one of said several transfer lines. For an optimized recovery of calories, it is obviously implemented on each of said several transfer lines. This corresponds to the most logical variant, the most advantageous, when at least two transfer lines are involved.

We now propose to describe the invention, under its two aspects of process and device, with reference to the appended figures.

Said figures are diagrams.

Figures 1 and 2 illustrate the technique of the prior art. Figure 1 illustrates a low pressure cracking process while Figure 2 illustrates a high pressure cracking process. Said figures 1 and 2 have been commented on in the introduction to the present text. Figures 3 A to 3D schematically illustrate, without limitation, different types of liner, which can be arranged, according to the invention, around the transfer line of the effluent from cracking.

FIG. 4 generally illustrates the invention. Figures 5, 6 and 7 illustrate particular embodiments of the invention:

- Figure 5 shows a modification according to the invention (with the intervention of a coolant as refrigerant) of the technology of the prior art according to Figure 1 (low pressure technology), - Figure 6 shows another modification according to l invention (with intervention of the DCE feedstock as refrigerant) of the technology of the prior art according to FIG. 1 (low pressure technology),

- Figure 7 shows a modification according to the invention (with the intervention of a coolant as refrigerant) of the high pressure technology of the prior art (modification implemented in a unit different from that shown in Figure 2).

Said Figures 5, 6 and 7 are commented, respectively, in the text of Examples 1, 2 and 3 below.

In all the appended figures, FIGS. 1 to 7, the same logic is found at the level of the references.

With reference to FIGS. 3A to 3D and to FIG. 4, the following can be specified.

. Figure 3A schematically shows the basic apparatus which can typically be used in the context of the present invention. Around the transfer line of the gaseous effluent from cracking - transfer line in a single straight segment - a jacket 1a (in a single straight segment) has been arranged. In the variant shown, provision is made to circulate the liquid refrigerant L against the current, in the annular space, thus formed between said transfer line and said liner 11a. Figures 3B to 3D show variants of such a device, more sophisticated. Circulation has always been planned - gaseous effluent G, liquid refrigerant L - against the current. In FIGS. 3A and 3B, the reference 20 represents expansion compensators. On, respectively, FIGS. 3B and 3C, the jacketed length is in several segments:

- connected by straight sections (folder 11b); - connected by elbows (1 le shirt).

In Figure 3D, there is shown an exchanger in 8 segments (1 ld jacket), which offers two passes to the refrigerant L.

. FIG. 4 schematically shows the integration, within the meaning of the invention, of a heat recovery and distribution circuit (circuit shown in bold), based on the use of a commercial heat transfer fluid in a VCM unit .

The different "producers" and "consumers" are connected in series according to their respective operating temperatures but other configurations are possible. We find in said FIG. 4 (as well as in FIGS. 5 to 7) the references of the type ni, with n identifying one of the steps 1 to 7, as specified in the introduction to the present text, and i one of its variants of implementation in such or such device (i = a, b or c, generally).

There are, in addition, stages or means characteristic of the invention, namely, by:

1 1. heating of the heat transfer fluid in one or more of the following devices a. exchanger on transfer line 11 '. bundle in the convection zone of the cracking furnace 12. cooling of the heat transfer fluid to its initial temperature by transferring its heat in one or more of the following devices a. a reboiler used to vaporize part or all of the load DCE b. a load DCE preheater c. one or more reboilers in the distillation section d. one or more exchangers heating other fluids (boiler feed water, heating water, etc.) depending on their presence near the VCM production unit 13. the circuit is completed by a. an expansion tank b. a circulation pump

14. and advantageously comprises a. an independent oven allowing heat to be supplied in the event of a deficit b. an (aero) refrigerant to dissipate excess heat.

The means 12d above (underlined) are not shown in FIG. 4.

We now come to the description of three examples of implementation of the invention.

Example 1. Figure 5: Low pressure process - Heat transfer fluid as refrigerant - Heat transfer circuit grafted onto an existing unit, according to Figure 1.

The original unit (of Figure 5) is identical to that shown in Figure 1. It includes, as already indicated with reference to said Figure 1, the following equipment:

2. Preheating of the liquid DCE b. exchanger heated by an intermediate current in the cooling section 3. spraying of the DCE b. vaporization column with reboiler 3b 'thermosiphon type supplied with steam

4. overheating of the sprayed DCE a. beam in the heat recovery zone on smoke from the cracking oven

5. partial conversion of the DCE a. two tubular bundles (Di = 154 mm) placed in the direct heating zone ("radiation") of the cracking oven

6. cooling and washing of cracked gas c. T of mixture 6a combined with a watering column 6b

7. cooling of the washed gas a. exchanger supplied with process current (see 2b) b. water cooler. The main operating parameters are indicated below:

DCE load flow rates 35 660 kg / h DCE purge spray column 825

VCM produces 12,550 temperature DCE preheater inlet 30 ° C

DCE preheater output 110

DCE vaporization column outlet 195 cracked gas oven outlet 485 cracked gas T quench inlet 485 oven outlet pressure 10 bar eff

steam consumption:

- DCE 7240 kg / h reboiling reboiler DCE residence time:

- the residence time (calculated) of the liquid DCE in the vaporization system (column + reboiler + connecting piping) is 10 minutes.

Said original unit (according to Figure 1) has been modified, as shown in Figure 5, to incorporate a simple heat transfer circuit, comprising: 1 lb. an exchanger on a transfer line

12a. a reboiler used to vaporize part of the load DCE 13 a. an expansion tank 13b. a circulation pump.

The heat transfer fluid crosses the 1 lb exchanger on the transfer line, cooling the cracked gas; the heat absorbed by the fluid is then used to vaporize part of the charge DCE in the additional reboiler

12a, thereby reducing the heat exchanged in the (the duty of) reboiler 3b 'supplied with steam.

The heat exchanger on the transfer line is obtained by installing a jacket (Di = 255 mm), around the transfer line (D _ext = 219 mm;

Dj _nt = 202 mm), in several segments over a total length of 70 m. The annular space is supplied (in two passes) with 148,000 kg / h of heat transfer fluid of the "synthetic oil" type at a temperature of 220 ° C.

The regulation of the system remains simple and flexible: - the flow DCE vaporized towards the cracking furnace is regulated by the addition of steam to the existing reboiler 3b ',

- the temperatures of the heat transfer circuit are maintained by a bypass on the additional reboiler 12a. For production conditions (charge rate, conversion) identical to those of origin, we observe:

- a decrease in the temperature of the cracked gas by 185 ° C through the 1 lb exchanger on the transfer line;

- An increase in the temperature of the heat transfer fluid of 22 ° C through this same exchanger 11b;

- a reduction in steam consumption by the original reboiler 3b 'from 7240 to 3195 kg / h, ie a reduction of 4045 kg / h;

- a residence time (calculated) of the liquid DCE in the vaporization system slightly increasing (12 minutes compared to 10 minutes at the origin) due to the volume of the additional reboiler 12a and its connecting piping. For all practical purposes, it can be specified here: - that the cooling rate implemented according to the invention on the gaseous effluent was 59 ° C / s (i.e. 1/8 of the inlet temperature of said effluent ( 485 ° C)); - that no variation in the fouling rate has been observed, by reducing said speed to 35 ° C / s (ie 1/14 of said inlet temperature); the reduction in said speed being obtained by increasing the inlet temperature of the refrigerant to 340 ° C.

Example 2. Figure 6: Low pressure process - Fluid process as refrigerant (preheating and vaporization of the DCE of charge) - Heat transfer circuit grafted on an existing unit, according to figure 1.

The original unit (in Figure 6) is identical to that shown in Figure 1. It operates as described above. The main operating parameters are indicated below: load DCE flow rates 35660 kg / h purge DCE spray column 825

VCM produces 12550 temperature DCE preheater inlet 30 ° C

DCE preheater output 110

DCE vaporization column outlet 195 cracked gas oven outlet 485 cracked gas T quench inlet 485 oven outlet pressure 10 bar e steam consumption:

- DCE 7240 kg / h reboiling reboiler DCE residence time:

Said original unit (according to FIG. 1) has been modified, as shown in FIG. 6, by the addition of a heating circuit of liquid DCE, comprising:

1 lb. an exchanger on transfer line 15a. a DCE circulation pump.

The refrigerant is made up of cold DCE and a flow of recycled DCE coming from the vaporization column 3b. The mixture passes through the exchanger 11b on the transfer line at a pressure slightly higher than the pressure of the vaporization column 3b in order to maintain the fluid in the liquid state. Thereafter, the fluid is expanded at the inlet of column 3b and a fraction of the DCE vaporizes under the effect of expansion. The heat absorbed by the DCE comes directly as a reduction in the heat exchanged in the (duty of) reboiler 3b 'supplied with steam.

The heat exchanger 1 1b on the transfer line is obtained by installing a jacket (Di = 255 mm) around the transfer line (D _ext = 219 mm; Dj _m = 202 mm) in several segments over a length total of 70 m. The annular space is supplied (in two passes) with 129,000 kg / h of liquid DCE at a pressure of 15 bar eff; the temperature of the current at the inlet of the exchanger is The regulation of the system remains simple:

- the flow of DCE vaporized towards the cracking furnace is regulated by the addition of water vapor to the existing reboiler 3b ',

- the flow of recycled DCE remains fixed and its temperature does not require any regulation.

For production conditions (charge rate, conversion) identical to those of origin, we observe:

- a decrease in the temperature of the cracked gas by 185 ° C through the 1 lb exchanger on the transfer line, - an increase in the temperature of the DCE by 37 ° C through this same exchanger 11b; after expansion at the inlet of column 3b, the temperature drops to 195 ° C under the effect of the vaporization of part of the WFD (approximately 10% - calculated value),

- a reduction in steam consumption by the original reboiler 3b 'from 7240 to 3195 kg / h, i.e. a reduction of 4045 kg / h,

- a residence time (calculated) of the liquid DCE in the vaporization system slightly increasing (12 minutes compared to 10 minutes originally) due to the volume of the exchanger 11b on the transfer line and the connecting piping.

Example 3. Figure 7: High pressure process - Heat transfer fluid as refrigerant - Heat transfer circuit in a new unit.

The new unit is shown in Figure 7 and has, side

DCE, the following equipment: 2. preheating of the liquid DCE a. exchanger supplied with low pressure water vapor 3. spraying of DCE b. vaporization column with reboiler type thermosiphon supplied with heat transfer fluid 4. overheating of the vaporized DCE a. beam in the heat recovery zone on smoke from the cracking oven 5. partial conversion of the DCE a. a tube bundle (Di = 162 mm) placed in the direct heating ("radiation") area of the cracking oven

1 on. an exchanger on a transfer line 6. cooling and washing of the cracked gas c. T of mixture 6a combined with a watering column 6b 7. cooling of the washed gas b. water cooler

The heat transfer circuit of the invention comprises the following equipment: l ie. the exchanger on transfer line 11 '. bundle in convection zone of cracking oven 14a. an independent oven allowing a possible supply of heat 12a. a reboiler serving to vaporize all of the charge DCE 14b. an air cooler to dissipate any excess heat

13a. an expansion tank 13b. a circulation pump.

The main operating parameters are as follows: DCE load flow rates 55,500 kg / h purge DCE vaporization column 2,100 VCM produced 18,550 DCE temperature preheater inlet 30 ° C

DCE preheater outlet 145 DCE vaporization column outlet 250 DCE convection beam outlet 300 cracked gas oven outlet 485 cracked gas T quench inlet 350 pressure oven outlet 21 bar e DCE residence time:

The heat transfer fluid is heated from 300 to 315 ° C in the heat exchanger l ie on the transfer line by cooling the cracked gas, then heated to

335 ° C in the convection of the 11 'oven. The heat absorbed by the fluid is then used to vaporize all of the feed DCE in the reboiler 12a of the vaporization column 3b.

The heat exchanger l ie on transfer line is obtained by installing a jacket (Di = 255 mm) around the transfer line (D _ext = 194 mm; D _ιnt = 162 mm) in several segments over a length total of 100 m. The annular space is supplied (in two passes) with 220,000 kg / h of heat transfer fluid of the "synthetic oil" type at a temperature of 300 ° C.

For all practical purposes, it can be specified here that the cooling rate implemented according to the invention on the gaseous effluent was 32 ° C / s (i.e. 1/15 of the inlet temperature of said effluent (485 ° VS)).

In normal operation, the thermal balance of the system is balanced by adjusting the temperature of the liquid DCE towards the vaporization column 3b. The auxiliary oven 14a is out of service and the air cooler 14b is bypassed, in normal operation: their presence only serves to facilitate / accelerate the start-up and shutdown operations of the installation.

Compared to a "conventional" unit operating at high pressure (unit as shown diagrammatically in FIG. 2), the following advantages are noted:

- The use of a heat transfer circuit allows the use of a vaporization column 3b with reboiler 12a; degradation products (especially solids) can be purged from the system instead of passing through all of the beams in convection and radiation as in the conventional system. The reduction in fouling allows a longer operating time and a higher cracking rate,

- the energy gain (fuel consumption) compared to a conventional unit with the same VCM production capacity is around

4650 therms / h. The gain directly attributable to the installation of the heat exchanger on the transfer line is 2300 therm / h (calculated on the basis of the use of the auxiliary oven instead of the heat exchanger), the rest comes from the installation of the heat transfer circuit (higher cracking rate, better use of the heat available in the convection zone).

In order not to burden the claims below, the devices which exist according to different variants i have been referenced by the single reference n, in place of the different m.

Claims

claims

1. Process for the production of vinyl chloride (monomer: VCM) by thermal cracking of 1, 2-dichloroethane (DCE), implemented with recovery of calories on the gaseous effluent (G) resulting from cracking, characterized in that said gaseous effluent (G) from cracking is transferred via at least one transfer line without consequent reduction in its mass speed and in that said calories are recovered by at least one liquid refrigerant (L) put into circulation, around said line transfer or at least one of said transfer lines, in a space arranged between said (said) transfer line (s) and a folder (11) arranged around the latter; said heat recovery being implemented under conditions where said liquid refrigerant (L) remains liquid and where said gaseous effluent (G) remains substantially gaseous.

2. Method according to claim 1, characterized in that said liquid coolant (L) is circulated against the flow of the gaseous effluent (G).

3. Method according to one of claims 1 or 2, characterized in that said recovered calories are used in said VCM production process.

4. Method according to any one of claims 1 to 3, characterized in that said liquid refrigerant (L) is a heat transfer fluid, in particular chosen from pressurized water and a commercial heat transfer fluid of oil type.

5. Method according to any one of claims 1 to 3, characterized in that said liquid refrigerant (L) is a process fluid, in particular chosen from a boiler feed water and the liquid DCE feed charge.

6. Method according to any one of claims 3 to 5, characterized in that said recovered calories are used to heat and vaporize, advantageously only partially, the feedstock of liquid DCE.

7. Unit for the production of vinyl chloride (monomer: VCM) by thermal cracking of 1,2-dichloroethane (DCE), comprising:

- means for conditioning pressure and temperature (1, 2, 3, 4) of the liquid DCE feedstock;

- a cracking oven (5) of said conditioned DCE, said oven containing at least one cracking beam;

- Means (6, 7) for washing, cooling and condensing the gaseous effluent from said cracking oven (5); said conditioning means (1, 2, 3, 4) and said oven (5) on the one hand, said furnace (5) and said means (6,7) for washing, cooling and condensing, on ^* the other hand, being linked together by suitable transfer lines of the treated charge;

- as well as means for recovering calories from the gaseous effluent from said cracking oven (5); characterized in that

said means for recovering said calories include:

+ a simple lining (11) of the transfer line or at least one of said lines for transferring the gaseous effluent from said cracking furnace (5); said liner advantageously constituting a “double tube” type exchanger; and

+ means (13b, 15a) for circulating, in the space thus arranged between said (said) transfer line (s) and the jacket (11) arranged around, a liquid refrigerant (L); and in that - each jacketed transfer line of the gaseous effluent from said cracking oven (5) has a passage section, for said gaseous effluent, less than or equal to the total passage section of the bundle (s) (x ) cracking said cracking oven (5), opening into said transfer line.

8. Unit according to claim 7, characterized in that said jacketed transfer line comprises fins.

9. Unit according to one of claims 7 or 8, characterized in that the jacketed length of said transfer line is made in one or more segments.

10. Unit according to any one of claims 7 to 9, characterized in that said means for recovering said calories are arranged in a circuit for recovering and distributing calories, integrated in said VCM production unit.

11. Unit according to claim 10, characterized in that said circuit is suitable for the distribution of the calories recovered, at the level of the means (1, 2, 3, 4) for conditioning the feed load of liquid DCE.

12. Unit according to one of claims 10 or 11, characterized in that said circuit contains a heat supply device (14a) and / or a heat removal device (14b).