A Review on Enhanced Productivity Approaches in VCM Suspension Polymerization-Juniper Publishers
Authored
by Darvishi R
Abstract
VCM suspension polymerization is performed as the
batch-wise process. For this, the productivity of this high demand
product is low. As VCM suspension polymerization is an exothermic
process, the full capacity of the cooling system remains idle most of
the time during the course of the polymerization.There are three
strategies for keeping the reaction rate constant during the entire
polymerization process. fast initiator dosage during reaction, cocktail
initiator system and the temperature trajectory are the strategies that
one can utilize to maintain the polymerization reaction rate at a
constant value close to the allowed maximum rate. There are several
reports that investigated separately the PVC resin properties obtained
by these modes of polymerization. However, no report is published to
compare the specifications of each mode together and also the PVC resin
properties prepared by these policies. Letreture review shows that to
achieve an enhanced productivity accompanied with the equivalent
properties of a conventional VCM suspension polymerization, in terms of
molecular and morphological characteristics, the best approach is using
cocktail initiator system.
Keywords: VCM
suspension polymerization is performed as the batch-wise process. For
this, the productivity of this high demand product is low. As VCM
suspension polymerization is an exothermic process, the full capacity of
the cooling system remains idle most of the time during the course of
the polymerization.There are three strategies for keeping the reaction
rate constant during the entire polymerization process. fast initiator
dosage during reaction, cocktail initiator system and the temperature
trajectory are the strategies that one can utilize to maintain the
polymerization reaction rate at a constant value close to the allowed
maximum rate. There are several reports that investigated separately the
PVC resin properties obtained by these modes of polymerization.
However, no report is published to compare the specifications of each
mode together and also the PVC resin properties prepared by these
policies. Letreture review shows that to achieve an enhanced
productivity accompanied with the equivalent properties of a
conventional VCM suspension polymerization, in terms of molecular and
morphological characteristics, the best approach is using cocktail
initiator system.
Introduction
Suspension polymerization is a process used for
producing many commercial resins, including poly(vinyl chloride) (PVC).
The suspension polymerization process is typically carried out in
well-stirred batch reactors [1]. This process uses mechanical agitation
in conjunction with suspending agents to disperse the monomer in water
as a continuous phase. The initiator dissolves in the monomer droplets
and decompsed after adjusting the temperature, resulting in initiating
free radical polymerization in the droplets [2]. Typically, The reaction
takes place at a constant temperature. The kinetic aspects of the
polymerization reaction and generally final properties of produced
resin, strongly depend on the temperature in which the polymerization
reaction occurs. As the vcm suspension polymerization is carried out in a
batchwise reactors, the productivity of the process has been proven to
be low. Therefore, improving the productivity of the batch
polymerization process has been always considered as a challenging issue
[1-5]. The enhanced productivity in VCM suspension polymerization
process can be effectively achieved by reducing the batch time so that
predefined specifications of final product do not change . The
polymerization of vinyl chloride monomer (VCM) is carried out
isothermally in commercially producing plants. The cooling system of the
reactors must be designed in such a way that it is able to remove the
heat
developed at the highest reaction rate where exothermic peak occurs. The
typical trand of heat removal rate versus raection time in vinyl
chloride polymerization indicaes the full capacity of the cooling system
remains idle most of the time during the course of the polymerization.
As a consequent, the distribusion of heat load at
exothermic peak of reaction over the entire course of the polymerization
can lead to reduced reaction time and hence enhanced productivity. The
decomposition rate of the initiator and the polymerization temperature
are strategic factors that can be manipulated to control the reaction
rate at its maximum constant value. It is found that there are three
ways to install the full capacity of the cooling system for achieving an
almost constant rate of polymerization; temperature programming,
cocktail of initiators and fast initiator dosage system. In all the
strategies the polymerization reaction and correpounding heat removal
rates are kept constant at a desired value. Feldman et al.[2] and
Feldman and Macoveanu [3] suggested a temperature-programmed VCM
suspension polymerization with a constant rate to improve productivity.
They used a simple model developed by Abdel-Alim and Hamielec [6] to
develop a temperature trajectory for constant rate polymerization
reaction. They used a 0.5-L laboratory scale reactor to carry out
nonisothermal VCM polymerization. In their programmed temperature
policy,
the temperature was decreased from 64 to 40°C, followed by a
constant temperature at 40 °Cfor a period of time, and then an
increase to 68°C. This temperature profile led to a reduction in
time of reaction with slightly deteriorated volumetric properties
and better thermal stability of the product relative to the PVC
obtained isothermally. Longeway and Witenhafer [4] proposed
that programmed temperature of a series of the polymerization
reactors measuring 40m3 each (the first at 57 °C and the last at
36 °C, with an 80% final conversion) could decrease the total
reaction time by 28%. Tacidelli et al. [5] proposed a heat kick
policy in which the final temperature of the reaction increased
to reduce the reaction time. Recently, we have successfuly
implemented a variable temperature trajectory during the
course of the reaction to improve productivity by reducing the
polymerization time for a predefined K value [1]. Pinto et al. [3]
applied the technique of adding a mixture of different initiators
to reactor at the beginning of the raction and at a isothermal
condition.
Thereafter, we call these strategies the enhanced productivity
caused processes (EPCPs). In any event, the operational
condition variations which occur in all three strategies may
affect negatively or positively the quality of the final PVC product
in terms of morphological and molecular characteristics. The
aim of the present paper was to assess the influences which each
of EPCPs has on the final product compared to conventional PVC
regarded as what reported in the literature so far.
Result and Discussion
From the kinetic point of view, the course of VCM suspension
polymerization consists of three stages; when only the monomer
phase exists, when polymerization proceeds in both polymer
and monomer phases and once only the polymer-rich phase is
present. The evolution of conversion of vinyl chloride monomer
to polymer chains is accompanied with several physical
transitions during the polymerization in which polymer phase
is changed from the aggregation of the precipitated macroradicals
to nano-domains and then domain and finally to a threedimensional
network of primary particles which are surrounded
in a rigid skin. The process cycle of PVC production from
VCM polymerization is affected by the change of operational
conditions made by the abovementioned strategies which are
implemented to enhanced productivity.
The effect on productivity
Among works performed on the enhanced productivity of PVC
through temperature-trajectory [2], Darvish et al. [3] suggested
the most reduction of polymerization time (ca. 100min). While
the implementation of cocktail initiator policy could decrease
the total reaction time by 20%, Bijanmanesh et al.reported
that continuous dosing of a fast initiator during suspension
polymerization of vinyl chloride enhanced productivity by 40%;
the best approach in order to reduce the polymerization time of
a typical VCM suspension polymerization.
The effect on molecular characteristics
None of the performed studies considered the change in the
K value of the final product of PVC by implementations of EPCPs.
As the molecular weight of polymerized PVC is only slightly
affected by the initiation rate, the two modes of using a mixture
of different initiators and fast initiator continuous dosage
have no effect on molecular weight and subsequent K value
when compared to a those of conventional VCM suspension
polymerization.
The utilization of temperature trajectory for enhanced
productivity, however, is founded that have slightly influence
on molecular characteristics such as molecular weights and
molecular weight distribution[2].
The effect on grain morphology
While initiator cocktail shows no significant effect on the
mean particle size of the PVC resin, a non-isothermal trajectory
and continuous initiator dosage slightly broaden the particle size
distribution towards bigger mean particle size compared that
of a regular VCM suspension polymerization. It is believed that
continuous fast initiator dosage modes cause polymerization
initiated in the aqueous phase and ultrafine particles which is
subsequently adhered on the skin of the grains. In this case,
both temperature trajectory and initiator cocktail modes show
the same manner as a conventional approach. By comparing
the results obtained from micrographs related to grain shape
from different kinds of literature, we can see that the PVC grains
prepared by variable temperatures are more spherical with
smoother surfaces than those producedby two others.Theskin
porosity of the PVC grains prepared nonisothermal was greater
than that in the PVC grains produced other than. The most
effects on primary particle morphology belong to the mode of
fast initiator dosage. While initiator cocktail process causes
fused primary particles packed together structure, exactly like
to regular process, the grains produced in both temperature
trajectory and initiator dosage systems are composed of an
agglomeration of smaller primary particles promoting the
formation of porous PVC grains. In another word, the final size
of the primary particles formed by the cocktail initiator and the
conventional process is larger than that of PVC grains produced
other than, a temperature or initiator dosage trajectory results
in the formation of an early three-dimensional skeleton with
smaller primary particles and subsequently higher internal
porosity. Although PVC grains prepared by either trajectory
entertain higher porosity, the particle bulk density is also
comparable to the resin prepared regularly.
The effect on thermal stability
One of the interesting results is obtained for PVC synthesized
using suspension polymerization at variable polymerization
in temperatures or initiator dosage trajectory, in which the
Brabenderplastograph data indicate a lower fusion time and
higher stable time.The number of labile chlorines, internal double bonds, and dehydrochlorination rates in the PVC chains
synthesized with these two policies decreases in compared
with a regular process. As a result, the thermal stability can be
improved if either a variable temperature or continuous initiator
dosage is applied.
Conclusion
By the assessment of the final properties of PVC grains, it
can be concluded that the cocktail initiator mode leads to the
grains with the nearest properties to that of a conventional
polymerization process of VCM. The greatest productivity,
however, can be obtained by initiator dosage system. This is
while the purchased cost would spend more in the initiator
dosage strategy and in a lower extent, in the temperature
programming, because of special equipment needed to
satisfy two modes. Therefore it seems to achieve an enhanced
productivity accompanied with the equivalent properties of
a conventional VCM suspension polymerization, in terms of
molecular and morphological characteristics, the best approach
is using cocktail initiator system.
Acknowledgement
This research was partially supported by Yasuj univeersity
and amirkabir university of technology. We are thankful to
our colleagues prof. M. Nasr esfahany and prof. Bagheri who
provided expertise that greatly assisted the research, although
they may not agree with all of the interpretations provided in
this paper.
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