In this investigation, we demonstrate our methodology in developing a comprehensive computer simulation model for the low-density polyethylene process in a tubular reactor using Polymers Plus. We use the perturbed-chain statistical associating fluid theory to describe the thermodynamic properties of the system. A comparison with literature data shows that the selected equation of state does a very good job in describing the physical properties and phase equilibria of the system. A detailed reactor model was proposed on the basis of transport literature that provides insight into the various resistances to heat transfer that arise during polymerization, and a comprehensive free-radical kinetic model was developed that describes the various individual mechanisms of the polymerization of ethylene and the properties of the polymer product. Results from the proposed simulation model were used in comparison with plant measurements from an Equistar Chemicals plant, in both correlative and predictive modes, for several polymer grades. In all cases considered, very good agreement was observed between simulation results and plant data on reactor temperature profiles, polymer properties, and production rates.

Fouling in a low-density polyethylene (LDPE) tubular polymerization reactor is caused by the polyethylene/ethylene mixture forming two phases inside the reactor. Some of the polymer-rich phase is deposited on the reactor's inside wall, which considerably reduces heat-transfer rates. At a given reactor pressure, the reactor inside wall temperature is the critical parameter in determining when fouling occurs and this is controlled by the coolant stream temperatures. In this work, plant data and a heat-transfer model were used to determine the fouling thickness in a LDPE industrial reactor and the speed at which the foulant material is deposited.

In this work, we investigated various approaches for the modeling of the high-and low-pressure separator units downstream from a low-density polyethylene tubular reactor using the Polymers Plus software package. First, we examined the performance of thermodynamic equilibrium by using the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Experimental data taken from the open literature were used to obtain the model parameters.Comparison with data from an Equistar plant showed that the PC-SAFT simulations agreed very well with the low-pressure separator residual-ethylene solubility measurements. There were, however, significant discrepancies between the model and the plant data for the highpressure separator, indicating that the high-pressure separator is not operating at equilibrium conditions. A further investigation was performed where a physical mechanism based on a bubble formation model was evaluated and a mathematical correlation using dimensionless numbers developed. The resulting model yielded high-pressure separator predictions that agreed adequately with plant data.

In a high-temperature high-pressure high-density polyethylene (HDPE) solution polymerization process it is imperative that the solution remains a super-critical single-phase throughout the reaction and adsorption steps to prevent severe operational problems. This work describes how an on-line phase equilibrium predictor was developed to alert plant operators of processing conditions that can lead to two-phasing, crossing the polymer solubility boundary from a single liquid phase to two liquid phases during steady-state operation of the plant. The dimensionless form of the Benedict-Web-Rubbin equation of state was used to develop the predictor. A comparison of the model predictions to plant observations for different sets of processing conditions shows that the average model prediction error for two-phasing pressures were in the range from 3.45% to -4.37% for cyclohexane and from -3.09% to 5.28% for hexane.

Fouling in a low-density polyethylene (LDPE) tubular polymerization reactor is caused by the polyethylene/ethylene mixture forming two phases inside the reactor. Some of the polymer-rich phase is deposited on the reactor's inside wall, which considerably reduces heat-transfer rates. Phase equilibria calculations show a high degree of sensitivity of the single-phase/two-phase process fluid boundary to temperature. Almost all of the process stream is single phase and the fluid mixture is only two phase in the boundary layer close to the reactor wall where the temperature is low enough to cause phase separation. At a given reactor pressure, the reactor inside wall temperature is the critical parameter in determining when fouling occurs, and this is controlled by the coolant stream temperatures.