Molecular weight distribution product design

One major emphasis in this book is the focus of reactor design on the control of temperature, simply because temperature plays such a dominant role in reactor operation. However, in many reactors the control of other variables is the ultimate objective or determines the economic viability of the process. Some examples of these other properties include reactant or product compositions, particle size, viscosity, and molecular weight distribution. These issues are discussed and studied in subsequent chapters. 

The focus of this study is the chemistry of the partially pyrolyzed and oxidized liquid fuel which survives as pyrolysis and oxidative pyrolysis proceed. The experiments have been designed so that these products are not completely destroyed and can be recovered for analysis. The analyses include measurements of basic nitrogen, average molecular weight, molecular weight distribution and unsubstituted aromatics by gas chromatography, and Hi NMR studies to determine average molecular structure parameters. 

The rational design of a reaction system to produce a polymer with desired molecular parameters is more feasible today by virtue of mathematical tools which permit prediction of product distribution. New analytical tools such as gel permeation chromatography are being used to check theoretical predictions and to help define molecular parameters as they affect product properties. There is a laudable trend away from arbitrary rate constants, but systems other than styrene need to be treated in depth. A critical review of available rate constants would be useful. Theory might be applied more broadly if it were more generally recognized that molecular weight distributions as well as rates can be calculated from combinations of constants based on the pseudo-steady-st te assumption. These are more easily determined than the individual constants in chain reactions. 

Available reaction-transport models describe the second regime (reactant transport), which only requires material balances for CO and H2. Recently, we reported preliminary results on a transport-reaction model of hydrocarbon synthesis selectivity that describes intraparticle (diffusion) and interparticle (convection) transport processes (4, 5). The model clearly demonstrates how diffusive and convective restrictions dramatically affect the rate of primary and secondary reactions during Fischer-Tropsch synthesis. Here, we use an extended version of this model to illustrate its use in the design of catalyst pellets for the synthesis of various desired products and for the tailoring of product functionality and molecular weight distribution. 

Copolymer properties are known to be a function of the molecular weight distribution (MWD), the copolymer composition distribution (CCD) and in some cases the sequence length distribution (SLD). The optimal design, operation and control of reactors to produce high quality copolymers with efficient production rates requires ... 

You are asked to design a semibatch reactor to be used in the production of specialized polymers (ethylene glycol-ethylene oxide co-polymers). The semi-batch operation is used to improve the molecular-weight distribution. Reactant B (EG) and a fixed amount of homogeneous catalyst are charged initially into the reactor (the proportion is 6.75 moles of catalyst per 1000 moles of Reactant B). Reactant A (EO) is injected at a constant rate during the operation. The polymerization reactions are represented by the following liquid-phase chemical reactions ... 

In current industrial practice, coordinated anionic catalysts differ considerably from the original ones, developments by Ziegler, Natta, and others. Using the same basic chemistry, new compounds were developed over the years that yield large quantities of polyolefins from small amounts of catalysts. In addition, catalysts can now be designed to yield products that are either wide or narrow in molecular weight distribution, as needed. The new catalysts for ethylene polymerization can be divided into three groups ... 

The acrylamide-based polymers used in the study were obtained commercially in powder form. The homopolymers designated as PS-2806, SPP-34 and ALD-18127-7 were supplied by Polysciences, Scientific Polymer Products and Aldrich respectively. Supplied also by Polysciences were our two poly(acrylamide-co-acrylic acid) samples of high and low carboxyl content, PS-2220 and PS-4652. Table I illustrates the molecular weight distribution of these samples as determined by aqueous GPC/Laser light scattering. The acrylic acid contents of PS-2220 and PS-4652 copolymers measured by 90 MHz C NMR were approximately 63% and 15% respectively. Both the GPC and the NMR analyses indicated PS-2220 contained appreciable amounts of impurities. 

The objective of a rational reactor design scheme is the development of a polymer reactor configuration optimal in some sense. The measures of product polymer properties typically available to the reactor engineer are the molecular weight distribution, expressed in terms of the distribution function itself or in terms of the moments of this distribution, and the composition and sequence distributions in the case of copolymers. 

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