Process for the manufacture of vinyl chloride monomer (VCM) and of
polyvinyl chloride (PVC)
The present invention relates to a process for the manufacture of vinyl
chloride monomer (VCM) and of polyvinyl chloride (PVC).
For producing VCM, two methods generally are employed: the
hydrochlorination of acetylene and the dehydrochlorination of ethylene
dichloride ( 1,2-dichloroethane) 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 (pyrolysis) of EDC to VCM:
C2H4C12 C2H3Cl+HCl-71kJ/mol
3. Recovery of HC1 and oxychlorination of ethylene to EDC:
C2H4+2HC1+0.50 2 C2H4C12+H20+23 8kJ/mol
Hence, an ideal balanced process can be described by the overall equation:
C2H4+0.5C12+0.25O2 C2H3Cl+0.5H2O+192.5kJ/mol
As set forth above, the reaction product of the pyrolysis reaction is a
gaseous mixture of VCM and HC1 and since this reaction is in fact not entirely
completed, unconverted EDC is also present in said mixture. This gaseous
mixture, which generally is at a high temperature (about 500°C), is rapidly
cooled by quenching and then condensed and the gas+liquid mixture so obtained
is then subjected to separation, generally by distillation and generally using at
least two steps/columns:
Column 1 (or HC1 column): feed: (VCM + HC1 + EDC) from cracker/top: HC1
(+C2H2)/bottom: (VCM + EDC)
Column 2 (or VCM column): feed: (VCM + EDC) from column 1/top: crude
VCM/bottom: unconverted EDC.
This same document sets forth, namely in sub-chapter 7.7, several ways of
saving energy in a "balanced" process as described above. One of these ways
consists in using the enthalpy of the cracker outlet stream after it has been
quenched (in order to prevent decomposition of the VCM produced and to
remove coke and other impurities) for ensuring the reboiler duty of column 2.
This is possible in the process detailed in that document because of the
respective temperatures at the outlet stream (139°C) and at reboiler (129°C).
However, in many industrial processes, column 2 operates at higher temperature
(and pressure) so that this solution cannot be applied.
Patent application CA 1127669 also discloses using the enthalpy of the
cracker outlet stream for ensuring the reboiler duty of column 2, but before said
stream has been quenched in order to have a sufficient heat (temperature)
available for ensuring said duty, considering the fact that said reboiler operates at
a temperature of at least 200°C.
The present invention aims at providing a new route for energy saving in a
VCM manufacturing process, which also focuses on this VCM column energy
consumption, but allows using a stream of lower thermal content.
To this effect, the invention relates to a process for the manufacture of
vinyl chloride monomer (VCM), comprising the steps of:
1. subjecting 1,2-dichloroethane (EDC) to pyrolysis in order to generate a gas
mixture comprising VCM, HC1 and EDC
2. quenching and eventually further cooling and/or condensing said gas mixture
to a liquid+gas mixture
3. subjecting said liquid+gas mixture to a first separation step to remove
substantially all the HC1 there from so as to leave a stream consisting
substantially of VCM and EDC
4. subjecting said VCM+EDC stream to a second separation step so as to get a
stream of substantially pure VCM and a stream of unconverted EDC,
according to which a heat exchanger is used to heat up the VCM+EDC stream
prior to being fed to a distillation column in step 4, said heat exchanger being
powered by a stream of hot fluid available in any one of steps 2 to 4 of the
process but after the quenching of step 2.
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. As to the terms "a stream of hot fluid available in any one of steps 1 to 4
of the process", they tend to designate any stream of fluid (gas and/or gas+liquid
mixture) entering, being inside or leaving any of said steps.
In a first embodiment of the invention, the heat exchanger is powered by at
least part of the stream of unconverted EDC obtained in step 4.
In a second embodiment of the invention, the heat exchanger is powered by
a stream of hot mixture comprising VCM, HCl and EDC available in step 2 after
the quenching.
In step 1 of the process according to the invention, the conditions under
which the pyrolysis may be carried out are known to persons skilled in the art.
This pyrolysis is advantageously obtained by a reaction in the gaseous phase in a
tubular oven. The usual pyrolysis 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 by-products and the fouling of the tubes of the
oven. Typically, the gas mixture coming from the pyrolysis is at a pressure from
10 to 25 barg.
In step 2 of the process according to the invention, this gas mixture is first
cooled down in a quench device (tower generally) and thereafter, generally
partially condensed using at least one condenser but preferably, at least 2 or even
more preferably: a train of successive condensers. As used herein, "quench
device" is a device for removing some components of the gases (namely coke
particles that are generally generated during pyrolysis) there from by putting a
sufficient quantity of liquid quench medium (generally a liquid mixture of
VCM+HC1+EDC recycled from downstream condensation step) in contact with
them. After quenching, the temperature of the gases is generally below 200°C,
preferably below 180°C and even more preferably, below 150°C. Such a low
thermal content would not allow ensuring the thermal duty of the VCM column
but it is sufficient to heat up the entry (feed) of said column according to the
present invention.
At the end of step 2, when said step also comprises partial condensing, the
temperature of the gases is generally comprised between 25 and 50°C and the
pressure is adapted between the pressure of step 1 and the operating pressure of
the first separation step 3.
In a preferred embodiment of the invention, at least 2 condensers are used
having each an inlet and an outlet stream and the heat exchanger is powered by
at least part of the inlet stream of the last condenser.
Preferably, the first separation step 3 of the process according to the
invention involves a distillation column that separates HC1 on top from VCM
and EDC at the bottom. This column is preferably operated under a pressure of
from 9 to 14 barg. The HC1 separated on top can be used in an oxychlorination
unit (for instance for making EDC from ethylene) or for any other purpose. A
refrigeration unit is preferably used on top of this column to liquefy the HC1
required for the reflux of the column. Sieve trays or valves trays can be used in
this column.
Preferably, the second separation step 4 of the process according to the
invention involves a distillation column that separates VCM on top while
unconverted EDC is purged at the bottom. This column is preferably operated
under a pressure of from 4 to 8 barg depending on the temperature of the cooling
fluid (usually cooling water) available for the condensation on top of the column.
VCM required for the reflux of the column and produced VCM are condensed.
Sieve trays or valves trays can be used in this column.
According to the invention, the heat exchanger may be of any type. It
preferably is a multi-tubular heat exchanger, a spiral heat exchanger or a
Compabloc® heat exchanger. Multi-tubular heat exchangers are more
particularly preferred.
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 3 attached, which show some preferred embodiments thereof. In these
figures, identical reference numbers designate identical or similar items.
Figure 1 shows a typical arrangement of HC1 and VCM columns according
to prior art, and figures 2 and 3 show two different embodiments of arrangements
according to the invention.
As can be seen from figure 1, a gaseous mixture (4) of (HC1 + VCM +
EDC) coming from an EDC pyrolysis section and its downstream quench unit
(not shown) is first condensed in 2 condensers (3 and 3'), then separated in 2
steps:
- HC1 (5) is separated on top of the HC1 column (1), and a mixture of (VCM +
EDC) (6) is directed to the VCM column (2);
- VCM (7) is separated on top of the VCM column (2);
- unconverted EDC (8) is separated at the bottom of the VCM column and
recycled to an EDC purification section (not shown).
In this typical arrangement, the (VCM + EDC) mixture (6) is directly sent
to the VCM column (2), which has a VCM condenser (12) and a reflux drum (9)
on the VCM stream, and a reboiler (10) at the bottom.
In a first embodiment of the invention, illustrated in figure 2, a heat
exchanger ( 11) is installed between the feed (6) and the bottom of the VCM
column (2). This arrangement leads to a reduction of the energy consumption of
the reboiler (10), with a very low impact on the heat duty of the VCM condenser
(12).
In a second embodiment of the invention, illustrated in figure 3, a heat
exchanger ( 11) is installed between the feed (6) of the VCM column (2) and the
HC1/VCM/EDC mixture coming from the pyrolysis (4) and its downstream
quench unit (not shown), right before said mixture enters the second condenser
(3'). As can be seen on this figure, not all the mixture coming from the pyrolysis
passes through this heat exchanger ( 11) but instead, some of is by-passed. This
arrangement also leads to a reduction of the energy consumption of the reboiler
(10).
The embodiments described above have been the object of numerical
simulations using ASPEN software, of which you will find the results in tables 1
and 2 below.
Table 1 is the result of a numerical simulation using version V7.2 of the
Aspen software and comparing the classical layout (represented in Figure 1) and
the layout of Figure 2 using the conditions set forth in said Table 1.
Table 2 is the result of a numerical simulation using version 2004.1 of the
Aspen software and comparing the classical layout (represented in Figure 1) and
the layout of Figure 3 using the conditions set forth in said Table 2.
As can be seen on these Tables, both the layout of Figure 2 and the one of
Figure 3 lead to a substantial reduction of the duty (energy consumption) of the
reboiler (10).
Table 1
Table 2
C L A I M S
1 - Process for the manufacture of vinyl chloride monomer (VCM),
comprising the steps of:
1. subjecting 1,2-dichloroethane (EDC) to pyrolysis in order to generate a gas
mixture comprising VCM, HCl and EDC
2. quenching and eventually further cooling and/or condensing said gas mixture
to a liquid+gas mixture
3. subjecting said liquid+gas mixture to a first separation step to remove
substantially all the HCl there from so as to leave a stream consisting
substantially of VCM and EDC
4. subjecting said VCM+EDC stream to a second separation step so as to get a
stream of substantially pure VCM and a stream of unconverted EDC,
according to which a heat exchanger is used to heat up the VCM+EDC stream
prior to being fed to a distillation column in step 4, said heat exchanger being
powered by a stream of hot fluid available in any one of steps 2 to 4 of the
process but after the quenching of step 2.
2 - Process according to claim 1, wherein the heat exchanger is powered by
at least part of the stream of unconverted EDC obtained in step 4.
3 - Process according to claim 1, wherein the heat exchanger is powered by
a stream of hot mixture comprising VCM, HCl and EDC available in step 2 after
the quenching.
4 - Process according to any of the preceding claims, wherein in step 2, the
gas mixture leaving step 1 is first cooled down in a quench device and thereafter,
partially condensed using at least one condenser.
5 - Process according to the preceding claim, wherein the quench device
uses a liquid quench medium which is a liquid mixture of VCM+HC1+EDC
recycled from a downstream condensation step.
6 - Process according to claim 4 or 5, wherein at least 2 condensers are used
having each an inlet and an outlet stream and wherein the heat exchanger is
powered by at least part of the inlet stream of the last condenser.
7 - Process according to any of the preceding claims, wherein the first
separation step 3 involves a distillation column that separates HCl on top from
VCM and EDC at the bottom.
8 - Process according to any of the preceding claims, wherein the second
separation step 4 involves a distillation column that separates VCM on top while
unconverted EDC is purged at the bottom.
9 - Process according to any of the preceding claims, wherein the heat
exchanger is a multi-tubular heat exchanger, a spiral heat exchanger or a
Compabloc® heat exchanger.
10 - Process according to claim 9, wherein the heat exchanger is a multi
tubular heat exchanger.
11 - Process for the manufacture of PVC by polymerization of the VCM
obtained by a process according to any of the preceding claims.