Computer model for tubular high‐pressure polyethylene reactors

Abstract

A computer model was developed for tubular high-pressure polyethylene reactors. Plug flow and absence of axial mixing were assumed. Emphasis was placed on realistic modeling of the reaction kinetics and the variation of physical properties along the reaction coordinate. A good simulation of axial temperature profiles, conversion, molecular weights, molecular weight distribution, and transport properties along the reaction coordinate is believed to have been achieved. The model can be extended readily to cases where radial diffusion is significant. SCOPEA computer model was developed for tubular highpressure polyethylene reactors. Simplifications (plug flow, absence of axial mixing) were made which are shown to be justifiable in high throughput reactors. However, the steady state free radical approximation was not made, and variations in the density and viscosity of the fluid along the reaction coordinate were taken into account explicitly. Emphasis was placed on realistic kinetics including long chain branching, use of the best rate constants available, and realistic simulation of all physical properties along the reaction coordinate. Temperature profiles, monomer and initiator conversion, the number average molecular weight (sn), and the molecular weight distribution (MWD) were calculated for a typical set of operating conditions as they might exist in a simple, high throughput reactor. The response of the model to changes in operating variables and kinetic constants was then examined. The model can be extended readily to simulate reactors with multiple initiator or monomer injection and to reactors in which laminar conditions prevail in the high conversion zone. CONCLUSIONS AND SIGNIFICANCEThis model permits a calculation, for any point along the reaction coordinate, of transport properties, the first several moments of the radical and polymer size distributions, and of long chain branching. Considerable insight is therefore gained into the kinetics of this most important vinyl polymerization, as carried out under commercial conditions, and into the physical properties of the product. The reaction is found to be rapid, and the concentration of the active intermediates (radicals) is larger by about two orders of magnitude than that encountered in the much more familiar isothermal case, where the steady state approximation for the free radicals is normally made.Because the reaction is exothermic and the rate of initiation has a large positive temperature coefficient, the polymerization reaction accelerates rapidly after a certain rate of initiation has been achieved. The temperature profile is therefore S-shaped between the reactor inlet and the point at which the maximum temperature is reached, with the appearance of a point of inflection at which the reaction may be said to take off. Beyond that point, little heat is exchanged with the environment, and only limited control can be exerted over the course of the reaction. To a first approximation, therefore, conversion and polymer properties are determined by...

From Molecular to Plant‐Scale Modeling of Polymerization Processes: A Digital High‐Pressure Low‐Density Polyethylene Production Paradigm

“…The method of moments [23][24][25][26] is based on the statistical representation of the average molecular properties (i.e., number and weight average molecular weights) of the polymer chains in terms of the leading moments of the number chain length distribution (NCLD). The method of double moments [10,27,28] can provide additional information on the number and weight average copolymer composition as well as on the number and weight average degree of long-chain branching.…”

Emerging Issues in the Mechanisms of High Pressure Free Radical Ethylene Polymerization: A Review

“…Over the last few decades, a lengthy list of academicians and industrialists incessantly attempted to establish a unifed tangible understanding of ethylene polymerisation in high-pressure autoclave reactors [21,[58][59][60] and tubular reactors [28,61] mainly because the technical propert ies of polymer products are mainly determined by the conditions of the polymerizat ion reaction. Kiparissides et al [49] reported that low p ressure ionic ethylene poly merization processes have been developed for the production of h igh density polyethylene and mediu m density polyethylene.The process requires low pressure(8-80 at m) and temperatures less than 150 o C using a t ransition metal catalyst of the Ziegler-Natta or Ph illip type.…”

Modeling of imperfect mixing and its effects on polymer properties

“…Figure 5 shows the flow sheet and reaction conditions of an imperfectly mixed LDPE autoclave. The kinetic parameters used for the simulation are obtained from the available literature (Chen et al, 1976;Goto et al, 1981) and are listed in Table 4. The three reaction zones are assumed to be adiabatic CSTRs under constant pressure, typical of the real industrial autoclaves.…”