年產50萬噸苯乙烯工藝設計(已附翻譯)-資料下載頁

　　

【正文】 eacts instantly with the monomer and changes to active sites before any propagation occurs. (ii) The propagation rate is independent of chain length. (iii) There are no termination or chaintransfer reactions during the polymerization. (iv) As chain length, n, increases, the polymer concentration, Pn decreases so that nPn and n2Pn can be neglected for sufficiently large n. (v) Polymerization proceeds isothermally.The reaction mechanism of an ideal anionic polymerization isinitiation （3）propagation （4）Since the initiation reaction is very fast, only the propagation reaction is of concern. The ponent balances for active chains in a batch reactor are（5） ，for 1（6）The associated initial conditions are（7）（8）（9）Subject to assumption (iv), this infinite set of ordinary differential equations is readily solved, for example, by the method of moments. Kim and Nauman give the solutions for a batch reactor and various idealized flow reactors.The actual reaction mechanism is more plex than that of eqs 3 and 4. The organolithium pounds exist in associated states, and the polymerization mechanism depends on the concentrations of the monomer, initiator, and solvent. There have been many suggestions for the reaction mechanism, but the following mechanism is generally accepted for the system of styrene_nbutyllithiumtoluene.initiation （10） （11）propagation （12）（13）Some chain transfer to solvent occurs in anionic polymerization. A metalation reaction can be detected especially if toluene is used as a solvent. Chain transfer is significant at high temperatures or for extended durations of polymerization. Normally, the chaintransfer reaction rate is approximately 5 orders of magnitude lower than the main polymerization rate. The initiation and propagation rates have been experimentally verified to be （14）（15）As in the ideal reaction system, the initiation reaction is sufficiently fast to neglect. The equilibrium reactions, eqs 10 and 13, are also fast. Even though associated active chains are involved in polymerization, the reaction mechanism of active centers can be approximated as bimolecular with the monomer. That is, the reaction rates of active centers in a batch reactor are（16） （17）The rate constant in eqs 16 and 17 is related to the true propagation rate constant by（18）With this adjustment, the results for batch and ideal flow reactors are unchanged. The relation between xn and xw in a real system is the same as that in an ideal system. The only effect of a fast, equilibrium association is to retard the polymerization rate so that more time is required to achieve the same final results. Slow approaches to equilibrium, however, broaden the molecularweight distributions significantly.Since anionic polymerization is very sensitive to contaminants, published reaction rates are often quite different. Bandermann collected the published data for the anionic polymerization of styrene and determined an averaged propagation rate as （19）The initiator is very active toward contaminates which cannot be removed pletely. It is therefore mon practice to introduce an initiator efficiency, 216。, which reflects the presence of contaminates which act instantly to lower I0. Thus,（20）A priori prediction of _ remains difficult. It is normally treated as an adjustable parameter used to correct observed conversions or observed molecular weights. Other adjustable parameters have been introduced to reflect gradual rather than instant deactivation of the growing polymer chains. Priddy27 treated gradual deactivation as a firstorder process with the rate constant （21）where the activation temperature of 4040 K was obtained by regression of Priddy’s data. The preexponential factor is fit to experimental results.Process Model for a Tubular ReactorWe use what has bee the standard transport model for a tubular polymerization reactor in laminar ,28,29 Axial diffusion and conduction are ignored. Thermal conductivity, heat capacity, and the heat of polymerization are treated as constants. Molecular diffusivity varies with polymer concentration and temperature but is independent of chain length. The fluid is Newtonian, but its viscosity is a strong function of position and temperature. The density is a weak function of position and temperature. The radial velocity ponent is ignored when solving the equations of motion but is recaptured using the continuity equation and is included in the convectivediffusion equations for heat and mass. Some of these assumptions and particularly that of constant molecular diffusivity are further considered in what follows.The overall mass balance and the zponent equation of motion give the axial and radial velocities as（22）（23）The energy balance and its associated boundary conditions are（24） at z=0（25） at r=0（26） at r=R（27）The mass balance for the monomer and its associated boundary conditions are （28） at z=0（29） at r=0,R（30）Assuming the molecular diffusivity is independent of chain