WO2014095440A1 - Process for the purification of ethylene dichloride (edc), and process for the manufacture of vinyl chloride monomer (vcm) and of polyvinyl chloride (pvc) - Google Patents

Abstract

Process for the purification of an EDC stream, comprising the steps of : eventually removing the lower boiling impurities from the EDC stream so as to generate a stream of EDC substantially free from lower boiling impurities; feeding this EDC stream to a heavy ends distillation column so as to get a top stream of substantially pure EDC and a bottom stream comprising higher boiling impurities with a low content of EDC, said heavy ends distillation column comprising an upper part and a lower part which may either be combined in a single column or may be 2 separate parts (columns) but with gas phase and liquid phase communication between them, and with a side reboiler at the bottom of the upper part.

Description

Process for the purification of ethylene dichloride (EDC), and process for the manufacture of vinyl chloride monomer (VCM) and of polyvinyl chloride (PVC)

This application claims priority to European application No. 12198579.0 filed December 20, 2012, the whole content of this application being

incorporated herein by reference for all purposes.

The present invention relates to a process for the purification of ethylene dichloride (EDC), and to a process for the manufacture of vinyl chloride monomer (VCM) and of polyvinyl chloride (PVC).

For producing VCM, two methods are generally employed: the

hydrochlorination of acetylene and the dehydrochlorination of ethylene dichloride (1,2-dichloro ethane) or EDC. The latter generally happens by thermal cracking and the EDC used therefore is generally obtained by direct chlorination and/or oxychlorination of ethylene.

As namely explained in "Chemical Process Design: Computer-Aided Case Studies", Alexandre C. Dimian and Costin Sorin Bildea, Copyright © 2008 WILEY- VCH Verlag GmbH & Co. KGaA, Weinheim, ISBN: 978-3-527-31403- 4, Chapter 7 entitled: "Vinyl Chloride Monomer Process", to date, most of the VCM technologies are based on "balanced" processes.

By this is meant that all intermediates and by-products are recycled in a way that ensures a tight closure of the material balance to only VCM as the final product, starting from ethylene, chlorine and oxygen. The main chemical steps involved are:

1. Direct chlorination of ethylene to 1,2 - ethylene dichloride (EDC):

C2H4+C12→C2H4C12+218kJ/mol

2. Thermal cracking (pyro lysis) of EDC to VCM:

C2H4C12→C2H3Cl+HCl-71kJ/moI

3. Recovery of HC1 and oxychlorination of ethylene to EDC:

C2H4+2HCl+0.5O2→C2H4C12+H2O+238kJ/mol

Hence, an ideal balanced process can be described by the overall equation: C2H4+0.5C12+0.25O2→C2H3Cl+0.5H2O+192.5kJ/mol

This document gives, namely in sub-chapter 7.6, a flow chart (Figure 7.8) showing how the EDC (both "fresh" EDC coming from the (oxy)chlorination and recycled EDC, which has not been cracked and has been separated from the pyro lysis reaction products (VCM and the HC1)) is purified prior to being fed to the pyrolysis reactor. Such a purification is generally carried out in at least 3 steps: first, a purification in "light" impurities (having a boiling point below the one of EDC), generally using a distillation column; then, a purification in "heavy" impurities (having a boiling point above the one of EDC), generally also using a distillation column and finally, on the bottom products of the latter, which still contains some EDC in order to allow advanced removal of impurities, a concentration step, also using a distillation column. In other words: this document teaches the use of 2 separate columns for the removal of heavy ends (impurities): the heavy end column and the heavy end concentration column.

This same document sets forth, namely in sub-chapter 7.7, several ways of saving energy in a "balanced" process as described above. However, none of them involves energy savings on the heavy ends column.

US 4,788,357 discloses a process involving an energy saving on this very column by adiabatically condensing the top product and to use the heat of condensation so obtained to reboil the heavy ends column. This document however does not disclose the further concentration of said heavy ends. It only teaches to drain out said heavy ends (at 24) but does not tell anything about their further treatment.

US 7,182,840 also discloses a process involving an energy saving on this very column, but it does so by recovering the reaction heat of the direct chlorination of ethylene into EDC.

The present invention aims at providing a new route for energy and investment savings in an EDC purification process.

To this end, the invention relates to a process for the purification of an

EDC stream, comprising the steps of:

- eventually removing the lower boiling impurities from the EDC stream so as to generate a stream of EDC substantially free from lower boiling impurities;

- feeding this EDC stream to a heavy ends distillation column so as to get a top stream of substantially pure EDC and a bottom stream comprising higher boiling impurities with a low content of EDC, said heavy ends distillation column comprising an upper part and a lower part which may either be combined in a single column or may be 2 separate parts (columns) but with gas phase and liquid phase communication between them, and with a side reboiler at the bottom of the upper part.

By doing so, the above mentioned heavy ends concentration column can be spared, which allows sparing energy and investments (see below for further details).

In order to further save energy, said process preferably comprises the additional steps of:

- using a Vapour Recompression (VR) device to increase the pressure of part of the top stream of substantially pure EDC and to generate heat; and

- using the heat so generated to heat up the side reboiler.

In the above, the term « substantially » means in fact that there only remains a limited amount of impurities (typically: a few w% or less) in said streams.

The terms "EDC stream" intend to designate a fluid stream, gaseous or liquid, but generally liquid, comprising preferably more than 90 % and more preferably more than 95 % and even more preferably more than 97 % of EDC, the remainder being lower or higher boiling impurities i.e. compounds having a boiling point lower or a higher than the one of EDC. Typical impurities are VCM, ethylchloride, 1 1EDC, chloroprene (a and β), carbon trichloride, carbon tetrachloride, trichlorethane, perchlorethylene, tetrachlorethane, dichlorobutane, dichlorobutene, tars.

Lower and higher boiling impurities, generally called light ends and heavy ends, are preferably both separated in order to obtain a purified EDC that can be used to feed the furnaces of the cracking section of a VCM plant or that can be sold on the market. Hence, in a preferred embodiment of the invention, light ends are separated in a first step, preferably in a distillation column. This column can also be used simultaneously as dewatering column. Depending on the plant, this column can be dedicated to one source of EDC (in this case several light ends columns are generally installed) or common for different EDC sources.

Depending on the efficiency of the heavy end column, a heavy end concentration column may be added to concentrate the heavy ends, purged in the bottom, and to recover EDC content. However, the drawback of such a column is the fact that the recovered EDC must be condensed, which is energy consuming. As explained above, the present invention allows to avoid the recourse to such a dedicated and separated column and hence, saving energy and investment.

The distillation column(s) used in the process of the invention is a f actionating column, device widely used in the chemical process industries where a multi-component mixture must be separated in groups of compounds within a relatively small range of boiling points, also called fractions. The

"lightest" products with the lowest boiling points exit from the top of the column and the "heaviest" products with the highest boiling points exit from the bottom.

Inside the column, the down flowing reflux liquid provides cooling and condensation of up flowing vapors thereby increasing the efficacy of the distillation column. The more reflux and/or more trays provided, the better is the separation of lower boiling materials from higher boiling materials.

Bubble-cap "trays" or sieve "trays" or valve "trays" are one of the types of physical devices which can be used to provide good contact between the up flowing vapor and the down flowing liquid inside an industrial f actionating column.

However, in the frame of the invention, it might be advantageous to use a packing material instead of trays, because it allows a lower pressure drop across the column. This packing material can either be random dumped packing such as Pall, CMR or Raschig rings (1-3" wide) or structured sheet metal. The advantage of this lower pressure drop namely is a reduction of the pressure increase of the top gas stream in the VR device, when such a device is present, since said lower pressure drop is associated with a corresponding lower temperature drop.

According to the invention, the heavy ends distillation column is divided in two parts which may either be combined in a single column or may be 2 separate parts (columns) but with gas phase and liquid phase communication between them. Advantages compared with the typical arrangement involving two separate columns without this communication are:

- lower investments because a condenser and a reflux drum can be spared;

- lower energy consumption for the same EDC content in the bottom heavy end purge;

- additional energy saving thanks to the use of the VR device, made

possible/easier through this design.

The invention can be applied to heavy end distillation columns operated under pressure or under vacuum. The latter may be preferred in order to lower the fouling of the main reboiler i.e. the one at the bottom of the column.

According to the invention, a side reboiler is installed between the 2 parts of the column. Reboilers are heat exchangers typically used to provide heat to (generally the bottom of) industrial distillation columns. They boil the liquid inside the distillation column to generate vapors which are returned to the column to drive the distillation separation. According to the invention, the side reboiler preferably is of a multitubular thermosyphon type or of a falling film evaporator type. Thermo syphon reboilers are generally heat exchangers used to provide vapour boil-up to a distillation column and comprising tubes and a shell. They can be provided in either a vertical position with the boiling fluid in the tubes or in the horizontal position with the boiling fluid in the shell side.

Falling film evaporators are generally heat exchangers with vertical tubes, where a fluid is partially evaporated while flowing as a thin film downward inside the heated tube-walls. Falling film reboilers are preferred because they allow a lower difference between the temperature of the heating medium on shell side and the temperature of the fluid evaporating inside the tubes. This lower difference in temperature leads to more favourable operating conditions of the VR device.

According to a preferred embodiment of the invention, the side reboiler is heated by a VR device. If vapour compression is performed by a mechanically- driven compressor or blower, this evaporation process is usually referred to as MVR (Mechanical Vapour Recompression). In case of compression performed by high pressure motive vapour ejectors, the process is usually called TVR (Thermal Vapour Recompression) or thermocompression or Vapour

Compression.

When the former (MVR) is used, part of top gas stream of the heavy end distillation column (substantially purified EDC) is compressed and used as heating fluid in the side reboiler. The amount of compressed EDC is fixed by the thermal duty of the side reboiler. With this system, all the thermal energy required by the side reboiler can be saved but mechanical energy (delivered for example by an electrical motor) is required to drive the compressor of the MVR device.

When the latter (TVR) is selected, high pressure vapour EDC is injected in the ejectors to compress part of top gas stream of the heavy end distillation column. At the outlet of the ejector, the compressed EDC vapour is used as heating fluid of the side reboiler. The amount of compressed EDC is fixed by the thermal duy of the side reboiler. High pressure vapour EDC is obtained by vaporization under pressure of liquid EDC. For this purpose, liquid EDC can be pumped from a reflux drum on top of the heavy end distillation column. TVR avoids the mechanical energy requirement of the compressor of a MVR device but thermal energy is required instead to vaporize liquid EDC under pressure.

The invention also relates to a process for the manufacture of vinyl chloride monomer (VCM) by pyro lysis of a purified EDC obtained by a process as described above.

The conditions under which the pyro lysis may be carried out are known to persons skilled in the art. This pyro lysis is advantageously obtained by a reaction in the gaseous phase in a tubular oven. The usual pyro lysis temperatures are between 400 and 600°C with a preference for the range between 480°C and 540°C. The residence time is advantageously between 1 and 60 s with a preference for the range from 5 to 25 s. The rate of conversion of the EDC is advantageously limited to 45 to 75 % in order to limit the formation of byproducts and the fouling of the tubes of the oven.

The present invention also relates to a process for the manufacture of PVC.

To this effect, the invention relates to a process for the manufacture of PVC by polymerization of the VCM obtained by a process as described above.

The process for the manufacture of PVC may be a mass, solution or aqueous dispersion polymerization process; preferably, it is an aqueous dispersion polymerization process.

The expression "aqueous dispersion polymerization" is understood to mean free radical polymerization in aqueous suspension as well as free radical polymerization in aqueous emulsion and polymerization in aqueous

microsuspension.

The expression "free radical polymerization in aqueous suspension" is understood to mean any free radical polymerization process performed in aqueous medium in the presence of dispersing agents and oil-soluble free radical initiators.

The expression "free radical polymerization in aqueous emulsion" is understood to mean any free radical polymerization process performed in aqueous medium in the presence of emulsifying agents and water-soluble free radical initiators.

The expression "aqueous microsuspension polymerization", also called polymerization in homogenized aqueous dispersion, is understood to mean any free radical polymerization process in which oil-soluble initiators are used and an emulsion of droplets of monomers is prepared by virtue of a powerful mechanical stirring and the presence of emulsifying agents.

The present invention is illustrated in a non limitative way by figures 1 to 7 attached, which show some preferred embodiments thereof by

comparison with prior art. In these figures, identical reference numbers designate identical or similar items. Figures 1 and 2 show typical arrangements for EDC purification columns and figures 3 to 7 show five different embodiments of arrangements according to the invention. More precisely:

- Figure 1 shows the disposal of typical EDC purification columns

- Figure 2 details a heavy ends separation system made of two separate independent columns: the heavy ends column and the heavy ends

concentration column.

- Figures 3 shows an embodiment of the invention using a single heavy ends column divided in 2 combined parts with a side reboiler.

- Figures 3bis shows an embodiment of the invention using a heavy ends column divided in 2 separated parts with a side reboiler.

- Figure 4 shows an embodiment of the invention using a single heavy ends column dived in 2 combined parts with a side reboiler and a MVR device

- Figure 5 shows an embodiment of the invention using a single heavy ends column divided in 2 combined parts with a side reboiler and a TVR device

- Figure 6 shows an embodiment of the invention using a heavy ends column divided in two separate parts, with a side reboiler on the first part and a MVR device

- Figure 7 shows an embodiment of the invention using a heavy ends column divided in two separate parts, with a side reboiler on the first part and a TVR.

In these figures, similar devices have similar or identical reference numbers.

As can be seen from figure 1, in a typical EDC purification process, a feed of impure EDC (4) is separated in a first distillation column (1) (or light ends column) in light ends at the top (5) and in a stream (6) of EDC containing heavy impurities at the bottom. This stream is fed to a second distillation column (2) (or heavy ends column), where it is separated again in a top stream of substantially pure EDC (7) and a bottom stream of EDC enriched in heavy ends (8). This bottom stream is sent to a heavy ends concentration column (3) where it is separated in a stream of substantially pure EDC (9) which is sent back in column (2), and to heavy ends (10) which can for instance be further treated or eliminated (for instance by incineration).

In this typical arrangement, the heavy end concentration column (3) is a classical distillation column with reboiler, condenser...

As can be seen from figure 2, the top stream of substantially pure EDC (7) leaving column (2) is first condensed in a condenser (13) and then, sent to a reflux drum (14) from which vent gases (16) are separated, eventually using a vacuum system (15), from a liquid stream of purified EDC (71) which is partly recycled as reflux to column (2) using a pump (121). The other part of purified EDC is send to downstream application (70), like a pyrolis device for making VCM.

The bottom of the heavy end column (2) is heated with a reboiler (1 1 ). The stream (8) removed through pump (12) is directed to a column (22). A top stream of substantially pure EDC (72) is first condensed in a condenser (132) and then, sent to a reflux drum (142) from which vent gases (162) are separated, eventually using a vacuum system (152), from a liquid stream of purified EDC (73) which is recycled partly to column (22) as reflux and partly to column (2) using a pump (122). Also, a reboiler (1 12) is used to heat the bottom of column (22) and the bottom stream of heavy ends (82) is removed from column (22) using a pump (123).

A first embodiment of the invention is illustrated in figures 3 and 3bis.

Heavy end separation is based on a system using only one heavy ends column (2) divided in 2 combined parts (figure 3) or in 2 separated parts (figure 3bis). Top part of the column is the same as for figure 2. A side reboiler (1 14) is installed below the upper part of column (2) and delivers most of the heat required by the column to separate substantially pure EDC on top of the column. In the bottom part of the column (2), heavy ends are concentrated. Bottom reboiler (1 12) gives the heat for this separation. Heavy ends are removed by pump (123). Compared to figure 2, arrangement of figure 3 or 3bis allow a reduction of investment (namely: condenser (132), reflux drum (142) and pump (122) can be spared) and energy saving : see below.

In a second embodiment of the invention, illustrated in figure 4 and using only one heavy ends column (2) divided in 2 combined parts like in figure 3, one part (7') of stream (7) is condensed in a condenser (13) and sent to a reflux drum (14), while the rest (7") of said stream (7) is sent to a MVR (17) which generates heat used to heat up a side reboiler (113) of column (2). After condensation in side reboiler (1 13), liquid stream (7" ') is sent to the reflux drum (14). Column (2) also has a back up side reboiler (1 14) for start up of the column or when MVR device is not available.

Figure 5 shows a third embodiment of the invention, identical to the one of Figure 4 except for the MVR (17) which is replaced by a TVR (18) receiving high pressure vapour from an evaporator (19) fed with a liquid EDC stream (71 ') pumped from the reflux drum (14) eventually through an additional dedicated high pressure pump (not shown).

A fourth embodiment of the invention is shown in Figure 6, which has a layout quite similar to the one of Figure 4 but where the heavy ends column is built as two separate parts (2, 22). Compared to this figure 2, the layout of Figure 6 does not only allow energy savings but besides, it allows saving equipment, namely: condenser 132, reflux drum 142.

And Figure 7 shows a fifth embodiment of the invention, identical to the one of Figure 6 except for the MVR (17) which is replaced by a TVR (18) or ejector.

Table 1 hereafter shows the results of simulation/calculations made using the 7.2 release of the AspenONE® software.

These results are based on the layouts of Figures 2-2, 3 bis-2, 6-2 and 7-2 attached (which are slight modified versions of corresponding Figures 2, 3 bis, 6 and 7 described above), using the working conditions set forth in Tables 2 to 5 (Table 2 giving the working conditions of Figure 2-2, Table 3, those of Figure 3 bis-2, Table 4, those of Figure 6-2 and Table 5, those of Figure 7-2) and the rate of the injector (18) of Figure 7-2 being set on 1/1.

The new layout of the Figures incorporates the flows from Tables 2 to 5, which are all put into squares (to avoid confusion with device features). It also suppresses optional features from the parent figures and when required (namely: for the embodiments of Figures 6-2 and 7-2), takes into account the farther treatment of streams 7' (renamed as flow 7 A), 7" (renamed as flow 7B) and 7" ' (renamed as flow 7C). This further treatment does not involve any external energy consumption but only some hardware investment.

The energy savings indicated in Table 1 (as negative figures, the positive ones concerning new energy consumptions) for the layouts of Figures 3 bis-2, 6- 2 and 7-2 are all calculated versus the energy consumption of the layout of Figure 2-2.

The embodiments of Figures 3 bis-2, 6-2 and 7-2 all allow sparing a condenser and those of Figures 6-2 and 7-2 additionally allow sparing a boiler.

The embodiment of Figure 3 bis-2 leads to the less energy savings, but the investments are minimal. The embodiment of Figure 6-2 is the most interesting in terms of energy savings but it implies the addition of a MVR which implies rather high equipment investment and electricity consumption. The embodiment of Figure 7-2 is in between both other embodiments as far as energy savings and investments are concerned.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

Table 1

LPS = Low Pressure Steam

MPS = Medium Pressure Steam

CD = Condenser

Table 2

Table 3

Table 4

Table 5

Claims

C L A I M S

1 - Process for the purification of an EDC stream, comprising the steps of:

- eventually removing the lower boiling impurities from the EDC stream so as to generate a stream of EDC substantially free from lower boiling impurities; - feeding this EDC stream to a heavy ends distillation column so as to get a top stream of substantially pure EDC and a bottom stream comprising higher boiling impurities with a low content of EDC, said heavy ends distillation column comprising an upper part and a lower part which may either be combined in a single column or may be 2 separate parts (columns) but with gas phase and liquid phase communication between them, and with a side reboiler at the bottom of the upper part.

2 - Process according to claim 1 , comprising the additional steps of:

- using a Vapour Recompression (VR) device to increase the pressure of part of the top stream of substantially pure EDC and to generate heat; and - using the heat so generated to heat up the side reboiler.

3 - Process according to claim 1 or 2, wherein the heavy ends distillation column is a fractionating column comprising packing material.

4 - Process according to any of the preceding claims, wherein the heavy ends distillation column operates under vacuum. 5 - Process according to any of the preceding claims, wherein the side reboiler is of a multitubular thermosyphon type or of a falling film evaporator type.

6 - Process according to the preceding claim, wherein the side reboiler is a falling film evaporator. 7 - Process according to any one of claims 2 to 6, wherein the VR device is a MVR or Mechanical Vapour Recompression device. 8 - Process according to any of claims 2 to 6, wherein the VR device is a TVR or Thermal Vapour Recompression device.

9 - Process for the manufacture of vinyl chloride monomer or VCM by pyrolysis of a purified EDC obtained by a process according to any of the preceding claims.

10 - Process for the manufacture of PVC by polymerization of the VCM obtained by a process according to the preceding claim.