MODELLING OF THE PRODUCTION OF CHLORINATED POLYETHYLENE

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Virginia Gilbert
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1 MODLLIG OF TH PODUCTIO OF CHLOITD POLYTHYL Gabriella Kun 1, Ferenc Szeifert 1, Tibor Chován 1, ndrás Tóth 2, ttila Czeller 2 1 University of Veszprém, Department of Process ngineering 2 BorsodChem Ltd., Kazincbarcika ITODUCTIO The demand for process engineering software packages allowing the quick solution of special problems is getting higher and higher. This initiated the elaboration of so called problem-oriented packages. Usually these packages are suitable only for analysing specific processes or their parts. The main strength of these packages comes from the fact that the problem is well-defined and therefore they rely on parameters which can be determined very precisely. Their user interfaces are usually transparent and easy to use without extensive numerical background and they help significantly the work of operating personnel who do not necessarily have an engineering degree. Commercial simulation packages such as SP, CHMCD, BTCHCD, SYFLOW are feasible for the analysis of operating industrial systems, however it is often required to use in-house developed programs which are suited for special problems and purposes. In our department such problem-oriented simulator package has been developed for studying batch chemical reactor systems. This framework consists of three parts: BS Batch reactor simulator module describes the operation of a batch chemical reactor with a heating/cooling system and can be applied for determining operational conditions and analysing measured process data. KS eaction kinetic simulator module supports the development of reaction kinetic models. eaction systems with a number of reactions in several thermodynamic phases can be studied. KI eaction kinetic identification module supports the identification of reaction kinetic parameters from measured concentration profiles. Kinetic parameters such as rate coefficient, energy of activation, heat of reaction and partial order of component can be determined from experimental data. Chlorinated polyethylene (CP) is one of the oldest impact modifiers of PVC, but nowadays is used as cross-linked elastomer as well. Favourable market situation raises of the necessity of the extension of the capacity of the CP plant. In order to support the decision of BorsodChem Ltd. about how its CP plant should be extended, we have developed a problem-oriented software package for the modelling and analysis of CP production, where the objectives of the simulation studies are the following: understanding the most important processes of the technology detecting the most important processes influencing the product quality revealing the bottleneck units of the technology estimation of production capacity of the subsystems supporting the design an up-to-date control system In the paper we present the concept of the modelling of the CP production process and the development of the KS module of our framework for the analysis of the reactors and the washing-system.

2 MODLLIG OF TH POCSS Process description The polyethylene chlorination process (see Fig. 1) starts with making an aqueous suspension of the hydrophobic polyethylene-powder in the presence of wetting additives. This mixture is filled into an autoclave. fter reaching the requested initial temperature begins the feed of chlorine into the gas phase at an accurately controlled rate. The rate of the reaction is temperature-dependent. Hence, it would be advantageous to lead the process above the melting point of polyethylene. However, this cannot be done because of the agglomerization of the polyethylene. Therefore, the temperature is increased in two steps. The first reaction stage is carried out under the melting temperature, where only the amorphous part of the polyethylene reacts with the chlorine. The second stage of the reaction takes place upper the melting point of the polymer, hence the crystal phase of the raw material also reacts. fter feeding the calculated amount of chlorine, the reaction step is finished, the mixture is cooled down, and the CP is separated using centrifugal extractor. fter this procedure the CP is washed[1] to remove the by-products (HCl, residual chlorine, HOCl). The water content of CP is removed in a fluid bed dryer. DIOIZD WT DDITIVS HTIG-COOLIG UTLIZIG GT DYIG I P POWD Cl2 (L) P P T O Y P SUSP -SIO Cl2 (G) C T O CP DDITIVS W S H I G UIT CP D Y CP P C K G UIT PO- DUCT Fig. 1 The block diagram of the process. Model of CP reactor The software packages used in chemical engineering science are heavily relying on mathematical modelling. The starting point of the derivation of these models, including kinetic models which are used in this study, is a conception of the detailed reaction mechanism. Hence, the following tasks were taken into consideration: definition of the thermodynamical phases which influences the adequate description of the relevant processes definition of active chemical components involved in the processes for every thermodynamical phases definition of the chemical reactions in the phases by the reaction-rate equations and the reaction heat. In case of multi-phase processes, this requires the definition of formal chemical reactions between the phases (e.g. the component transport)

3 CP polymer is produced from high density polyethylene by a polymer-analogous substitutional chlorination. The technology is realized in aqueous suspension, therefore the reaction is a heterogeneous one [2]: C H + bc l C H C l + bh C l b b (1) where b indicates the degree of polymerization. The reaction mixture can be divided into three phases, such as gas (G), liquid and solid phase. The raw polyethylene is inherently a two-phase system of crystalline and amorphous parts [3]. s the solid phase contains liquid components, we had to distinguish two liquid phases. The first F1 indicates the bulb liquid phase, while the second F2 indicates the liquid phase closed as clusters in the solid phase. Component concentrations are different in each liquid phase. These considerations lead to the structure of the model of the reactor depicted on the Fig. 2. This scheme takes into account the heat transfer between the jacket of the reactor used for heating and cooling, the reactor and the wall. CTO CTO WLL ISID TH CTO ISID TH JCKT F1, LIQUID PHS GS PHS SOLID PHS H2O H2O POLYM PHS F2, LIQUID PHS HCl HCl MOPHOUS H2O HOCl Cl2 Cl2 CYSTLLI CP HCl HOCl H3O + Cl2 OH - H3O + Cl - OH - Cl - ClO - Fig. 2 Decomposition of the system ClO - Based on this decomposition, the following processes can be taken into consideration: Component-transfer (H 2 O, HCl, HOCl, Cl 2 )between the liquid (F1) and the gas phase (G) Component-transfer(H 2 O, HCl, HOCl, Cl 2, H 3 O +, OH -, Cl -, ClO - ) between the liquid phases (F1 and F2) Ionic reactions in the liquid phases (F1 and F2) 2H O H O OH (2) Cl + 2H O H O + Cl + HOCl HOCl + H 2O H3O + ClO (4) HCl + H O H O + Cl (5) 2 3 (3)

4 Solid phase chlorination reaction ( ( )) + ( 2 ) + * ( 2) ( ( )) + ( 2 ) + * ( 2) C H P bcl F C H b HCl F B C H P K bcl F C H b HCl F B (6) (7) The model based on these kinetic equations includes the volume, component and enthalpy balances for the reaction mixture: d( m* xj ) dt I= 1 dm ( t) = dt I = 1 F I () t (8) ()*, ( ) * ()* µ * () = F t x t + M m t r t dq I I J J IJ I I = 1 ( t) dt I = 1 () ( I * I () = m t H r t (9) ) (10) where m represents the mass(phase), F L the mass flowrate of the phase, x J the mass fraction of the component j, M J the molecular weight of the component j, µ J the stoichiometric coefficient, r I reaction rate, and Η the heat of the reaction given a priori and assumed to be constant in time. Similarly to other cases in our engineering practice, we were forced to work in an information rare environment, e.g. the parameters of the complete kinetic model could not be identified or found in the literature. However, by introducing tendency-models which contain only the decisive components and processes, we were able to reduce the dimensionality of the modelling problem to obtain practically usable model. The following reduction principles were followed: selection of the key components selection of the relevant processes consideration of the analytical constraints when selecting reactions and components application of incomplete reaction equations SIMULTIO STUDIS This section demonstrates some results of our simulation of the reaction and the washing units of the CP production process. Fig. 3 shows the concentrations of the main components in the solid phase. The huge difference between the concentration trajectories in the two phases shows that most important variable influencing the product quality is the temperature program that could be used to control the final propeties of the CP (the molecule structure is determined by the ratio of the concentrations of the amorphous and the crystal polymer). The increase of the concentration of the by-products in the F2 liquid phase reveals the necessity of the removal of the hydrochloric acid components (see Fig. 4). This task is performed in the washing unit. Figure 5 and 6 show how among of several possible solutions this unit is operated.

5 phase 1. T<Tmelt transient phase phase 2. T>Tmelt P() P(K) CP 0.5 Concentration time(min) Fig. 3 Concentrations of the main components in the solid phase phase 1. T<Tmelt transient phase phase 2. T>Tmelt Concentration of the component HCl HCl(F2) Cl2(F2) HCl(F1) Cl2(F1) Concentration of the component Cl time(min) Fig. 4 Concentration of the major components of the liquid phases in the reactor Concentracions in the liquid phase(f2) F21 F22 F23 F24 F25 F Concentracions in the liquid phase(f1) washing time(min) Fig. 5 Concentration of the component Cl 2 in the washing unit

6 Concentracions in the liquid phase(f2) F21 F22 F23 F24 F25 F Concentracions in the liquid phase(f1) washing time(min) Fig. 6 Concentration of the component HCl in the washing unit During the analysed washing process the solid phase taken from several batches of the reactor are collected and washed together. Hence, the whole washing feed-batch procedure can be decomposed into several, in this case five batches. s Fig. 5. and 6. show, these sub-batches can be considered almost independent, as the rate of extraction is almost identical. COCLUSIOS This paper presented the development of an in-house problem specific software for the analyis of the clorination of high density polyethylene. The developed software is based on a tendency model obtained by the reduction of the detailed kinetic model of the process. Simulation results were shown to illustrate the applicability of the software and the utilized modelling methodology. The results illustrate that the developed KS unit can be effectively used for the analysis of the alternatives of the intensification of CP processes, e.g. for the development of different washing strategies. CKOWLDGMT The authors would like to acknowledge the support of the Cooperative esearch Center (VIKKK). FCS [1] Kálmán Marossy:Structure-properties relationship of CP, Fatra 97 Zlin [2] ndrás Tóth-ttila Czeller: Development of the production of chlorinated polyethylene, extension of the applicability. OKK.3. Balatonfüred, [3] Kálmán Marossy: The multiphase system of CP, Műanyag és Gumi, / 3.

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