CC BY
88
ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
AZ3RBAYCAN KÎMYA J U RNA LI № 3 2018
33
UDC 66.012/665.652.2
CONTROL OF THE PROCESS OF STYRENE POLYMERIZATION IN NONSTATIONARY CONDITIONS
A.M.Aliyev, LLOsmanova, A.R.Safarov, A.M.Guseynova, I.V.Balayev
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
[email protected]. ru Received 16.02.2018
The mathematical model of the process of suspension polymerization of styrene has been developed under nonstationary conditions caused by a poisoning effect on the initiator of phenylacetylene entering to polymerization with styrene and leading to a decrease in productivity by polystyrene. To avoid this problem the nonstationarity function has been suggested using of which it together with the equation of the monomer consumption rate will permit correcting the change of initiator activity with time and keeping the productivity at the required level.
Keywords: styrene, polystyrene, suspension polymerization, phenylacetylene, control, nonstationarity.
Polystyrene (PS) is a thermoplastic material with high hardness and good dielectric properties, chemically resistant to alkalis and acids except for nitric and acetic acids. Polystyrene is insoluble in lower alcohols, aliphatic hydrocarbons, phenols, ethers. It dissolves in its own monomer, aromatic and chlorinated hydrocarbons, esters, acetone. Resistant to radioactive irradiation, but resistance to ultraviolet rays is not high. Polystyrene is easily molded and colored. Well processed by mechanical methods and easily glued. It has got low moisture absorption and high moisture and frost resistance. Physiologically harmless.
Various polymerization processes are used for production of polystyrene: block, in solution, suspension, emulsion, block-slurry which is carried out under various operating conditions: batch, continuous, semi-continuous, in continuous-stirred tank and tubular reactors. Recently for styrene suspension polymerization process they mainly used stirred tank reactors and only in batch performance.
In stirred tank reactors reliable mixing of reacting substances is ensured throughout the volume of the apparatus and periodicity provides the invariance of concentration and temperature fields in space.
Suspension polymerization takes place in drops of the monomer suspended in water. The diameter of the droplets and, consequently, the granules of the resulting monomer reaches 0.5-5 mm. Polymerization in a suspension is carried
out by simply dispersing styrene in water to which a stabilizer is added, for example, starch, gelatine, polyvinyl alcohol in an amount of 0.1-10% by weight of the aqueous phase.
The process is carried out at a temperature of ~90°C [1]. The amount of water in the slurry has almost no effect on the course of the polymerization process, but it is easier to remove the heat of reaction when the content of water is high. The polymerization temperature must be below the temperature of softening of the polymer not at least than 10°C. Suspension polymerization of styrene is carried out in the presence of soluble initiator in monomer - benzoyl peroxide and a stabilizer - polyvinyl alcohol. The main purpose of the stabilizer is to prevent sticking the polymer granules, the causes of which may be disturbances in the polymerization regime or the presence of a small amount of impurities present in the process. Sticking usually occurs when the degree of conversion of monomer in polymer is 30-70% and may end within a few seconds.
Mathematical model of the process
I. The kinetic model
The process of polymerization of styrene in suspension proceeds through a free-radical mechanism with sequential addition of monomer molecules to the growing macroradical and includes the stages described below.
1. Initiation - the process of formation of active centers on which the macromolecule grows. This stage of reaction is the decomposi-
34
CONTROL OF THE PROCESS OF STYRENE POLYMERIZATION
tion of the initiator and formation of a macro-radical with the first monomeric residue. It is described by the following kinetic scheme:
I-
R +M-
-» 2R
(1)
where I - initiator; R - primary radical; M -monomer; P*- macroradical with the first monomeric residue; k&, k\ - rate constants of decomposition of the initiator and the start of polymerization, respectively.
Hereinafter kt is not taken into account as the rate-limiting stage is the collapse of the initiator.
2. Stage of chain growth follows the initiation. During this stage molecules of the monomer are sequentially attached to the active sites, converting into the linked with each other monomer units of the polymer.
The equation of chain growth looks like
this:
P.+M-
kn
(2)
where P* - macroradical with n monomer resi-
*
dues; P„+1- macroradical with (//+1) monomer
residues; kp - rate constant of chain growth.
3. The growing active reaction center of the macromolecule may lose its activity. This process is called chain termination. The resulting product, unable to spontaneously continue its growth is called a "dead" polymer. The equation of the chain termination has the following form:
Pl+P.
> D.
(3)
where P* - macroradical with m monomer res-
m
idues; D/r m - polymer; kx - rate constant of chain termination by recombination.
The general stoichiometric scheme looks as follows:
//OHr CI I > Lc 11 CI I ;
(4)
The kinetic model of the process is a system of differential equations that describe the conversion of the monomer (xm) and initiator (xi), obtained on the basis of the kinetic scheme (1) (3):
<5)
dv|/d/=/,'d(l-A-|), (6)
where .v =
(i,-О ,
-, J-n
M-
M0micv:
Ks - кp
V К fV
k= 189-l#exPr^];
k& = 1.4*10 exp
kt0 = 6.52101-exp
14 , -125700
RT
-37170 RT
M0i„ic= 0.104 кг/моль; / = 0.6; 8 = -0.178; I, lo - initial and current concentrations of initia-tor, mol/м ; F(xm) - function that takes into account the correction to the rate constant of chain termination for the gel effect;/- factor of effectiveness of using the initiator; % - rate con-
3 11
stant of decomposition of initiator, м mol" s" ;
3 11
keff- effective reaction rate constant, м mol" s" ;
3 11
kp- rate constant of chain growth, mol" s" ; kt и kt0- the rate constant of the chain termination
3 11
and its pre-exponent constant, м mol""s" ; Mnic - mass of feed initiator, kg; .V/(,jnK. - molar mass of initiator, kg/mol; V° - initial volume of loaded styrene, м ; x\ - initiator conversion; xm - the degree of monomer conversion; s - coefficient of volume change during the polymerization reaction; pst, pps - density of styrene and polystyrene, kg/м3.
II. The equation of heat balance
Influence of the temperature regime on the polymerization processes is an important factor determining their quantitative characteristics and product quality [2]. In general, temperature changes affect such basic polymer properties as average molecular weight, molecular weight distribution (MWD). In applying apparatus with agitators there must be considered that increasing the agitator speeds with sig-
A.M.ALIYEV et al.
35
nificant viscosity reaction environ results in a significant increase in the power consumed by a stirrer and to the danger of local overheating of the reaction mass itself due to the friction layer of a viscous medium, which reduces the quality of the resulting product [2]. In addition, reaction media in polymerization processes are characterized by low values of the thermal conductivity coefficient, which also affects the efficiency of heat exchange.
In [1] the process of suspension polymerization of styrene (SPS) was investigated under conditions of automatically maintained constant temperature in the reactor and the speed of rotation of the stirrer.
The heat balance of the reactor is as follows:
q=<ihr-<ho^ (7)
where q - flow of the heat accumulated during the process, W:
q = p VCp^-, (8)
at
T - temperature of reaction medium, K; V -
"3
volume of reaction medium, m :
V = (9)
Vvmh(t) - volume of polymer-monomer particles (PMP):
v u = v +v
pmh st ps'
(10)
"3
Vst(t) - current volume of styrene, m :
Vst=V°{\-xm), 01)
Vs°t - initial volume of styrene, m3; Vps - current
"3
volume of polystyrene, m :
V =V u-V
ps pmh st ?
(12)
Vps=V:xm(e + \),
{ 'water - volume of water (assumed constant), m ; 8 - coefficient of volume change during the polymerization reaction.
As a result of the transformations the volume of the reaction medium (3) takes the form:
v = v +v +v
r r st r ps r water >
V = V°(l + sx ) + V
y y st V1 ^ tAm/T y water'
"3
p - density of reaction medium, kg/M :
(13)
P =
o V + o V + d V
r st st r ps PS r water v water
V
(14)
pst, Pps - the density of styrene and polystyrene, respectively, kg/M3; pwater - water density, kg/M3. =_9957_
Pwater 09g4 + 0 000483(7 - 273)'
The density of the reaction medium (7) takes the form:
K (pst (l ■- ■Xm )+ Pps*m (l + -))+ pwate/w
P =
Vs°t(l-8Xm) + ^
(16).
Cp - heat capacity of the reaction mixture,
Jkg'K1:
c _CpMst+CPpMps+Cp_Mwater p M +M +M
1V1 st ^lv± water
cPa , C,, , C„......... " heat capacity of styrene,
Pwater
polystyrene and water, Joule/(kg K); Mst, Mps, Mwater - mass of styrene, polystyrene and water, respectively, kg:
^st = ^stPst MPS = ^psPps "^^water ^water P water
i/h,- - heat flow from the chemical reaction of polymerization of styrene, W:
cbc
q* =MstAH-^ , (18)
at
Mgt ~ mass of styrene, kg; AH - enthalpy of the process (AH=716000 J/kg); xm - monomer conversion; i/|(is - the heat flux of losses to the environment, was determined experimentally, W.
The final form of the equation of the thermal balance of the reactor takes the form:
pVCpft<19>
Equations (5), (6), (19) constitute the mathematical model of the process proceeding under stationary conditions, which are observed when pure raw materials enter into the reactor. However, in the dehydrogenation of ethylben-zene, in addition to styrene, toluene and benzene, there are also byproducts containing a small amount of phenyl acetylene, which is very difficult to separate from the reaction mixture, so that into the polymerization process entering not pure styrene but with the addition of ~5
36
CONTROL OF THE PROCESS OF STYRENE POLYMERIZATION.
ppm of phenyl acetylene. Phenyl acetylene is rapidly and irreversibly adsorbed on the initiator, poisoning it and causing its decay.
Assuming that the initiator's activity decreases linearly with the amount of phenyl a-cetylene coming with the initial styrene, has been proposed the following dependence of the change in the activity of the initiator 8 with the reaction time t.
HO
= -k-Q-a-N (20)
at
where k is the Arrhenius dependence:
Eg
k — k0Q ^^ .
Integrating (20), we obtain:
e = eiaNstf, (21)
Here Nst - amount of styrene fed to the pol-ymerizer, kmol/hour; a - mole fraction of phe-nylacetylene in the feedstock, equal to -0.001; k, k() - constant and pre-exponent of the rate constant of initiator decay, mol"1; Ea - activation energy of the reaction of initiator decomposition under the action of phenyl acetylene, kJ/mol; R - gas constant, kJmor'k"1; T - process temperature, K.
Using combination of methods of nonlinear programming (Rosenbrock, McCormick, Powell) [3], we determined the values of the parameters of the kinetic model: A't,= 15,79 kJ/mol. A,, 70.71.
The main indicator of reactor productivity is the amount of monomer converted into the polymer during residence time in the reactor. By changing the flow rates of the reagents (monomer and initiator), can be carry out the control of the reactor. And as during the process
under the influence of phenylacetylene the rate of decomposition of the initiator is increased, so in order to maintain the polymerization rate and to obtain polystyrene of the same molecular mass and quality as under stationary conditions, it is necessary enter in the equation of the monomer consumption rate the nonstationarity function 0 that corrects the changes in activity initiator with reaction time:
<K d t
^effO "*»)<
loO-^)
0. (22)
If the performance of polystyrene is different from its stationary value, by changing the value of 0 by increasing the purity of the styrene fed to the reactor or the amount of initiator, we can adjust the process and obtain the desired yield of the product. Equations (6), (22), and (19) constitute the complete mathematical model of the process of styrene polymerization under nonstationary conditions, using of which will support the productivity of polystyrene at the required optimum level.
References
1. Safin MA. Razrabotka sistemy avtomatiche-s-kogo upravleniia reaktorom sinteza suspenzi-onnoi polimerizatcii stirola s uchetom kinetiki protcessa. Dis. ... kand. tekhn. nauk. M: RKHTU im. D.I. Mendeleeva, 2014. 124 s.
2. Egorova E.I., Koptenarvusov V.B. Osnovy tekh-nologii polistirolnykh plastikov. SPb: Hiniizdat. 2005. 277 s.
3. Sliakhtakhtinskii T.N.. Baklimanov M.F., Kelba-liev G.I. Metody optimizatcii protcessov lii-micheskoi teklinologii s programmami dlia I A M Baku: Elm. 1985. 260 s.
QEYRÍ-STASiONAR §ORAÍTÍNDO GED9N STÍROLUN POLÍMERL3§MeSÍ PROSESINÍN ÍDAR9 EDÍLM£>SÍ
А.М.ЭИуеу, LLOsmanova, A.R.Soforov, A.M.Hüseynova, LV.Balayev
Qeyri-stasiomr soraítdo gcdon slirolun polimcrlosmo prosesinin riyazi modeli yaradilib. Bu qeyri-stasionarligin asas sobobloiindon bin polimcrlosmo proscsino stirolla birgo daxil oían fenilasetilenin istirakini qeyd clmok olar. Fenilasetilen inisiatora /ohon crici tosir gostonr va polistirolun mohsuldathgini asagi salir. Bunun qarsismin alimnasi moqsodilo qeyri-stasionarliq funksiyasi loklif olunub. Bu funksiyam monomerin siirot tonliyino daxil ctmoklo inisiatoran aktivliyinin zamana gôro doyismosino tosii gôstorocok va mohsuldarhgi daimi saviyyada saxlanilmasina iinkan vcrocok.
Açar saz for: Stirol, polistirol, suspenziya polimerizasiyasi, fenilasetilen, qevri-stasionarhq, idaraetma.
A.M.ALIYEV et al.
37
УПРАВЛЕНИЕ ПРОЦЕССОМ ПОЛИМЕРИЗАЦИИ СТИРОЛА В НЕСТАЦИОНАРНЫХ УСЛОВИЯХ
А.М.Алиев, И.И.Османова, А.Р.Сафаров, А.М.Гусейнова, И.В.Балаев
Разработана математическая модель процесса суспензионной полимеризации стирола в нестационарных условиях, вызванных отравляющим действием на инициатор фенилацетилена, поступающего на полимеризацию вместе со стиролом и приводящим к снижению производительности по полистиролу. Для избегания этого предложена функция нестационарности, введение которой в уравнение скорости расходования мономера позволит скорректировать изменение активности инициатора со временем и сохранить производительность на требуемом уровне.
Ключевые слова: стирол, полистирол, суспензионная полимеризация, фенилацетилен, управление, нестационарность.
