Measurement of conversion

The factors that affect conversion were mentioned above, but the measure of conversion was not described. In a simple chemical reaction, conversion is easily determined by measuring the products formed or the disappearance of the starting material. A petroleum process, however, deals with a multitude of different compounds, many of which still cannot be identified, let alone measured. This makes the selection of a good measure of conversion difficult. 

Overall goal of this work was to maximize the amount of information obtained from chromatographic analysis of latex solutions. More specific aims were (1) measure amounts of unreacted monomers, as one measure of conversion, (2) determine amount of polymer, as a second measure of conversion, (3) estimate chemical composition of the polymer formed, and (4) look for evidence of grafting (in the AN/M polymerization) as evidenced by detection of insoluble polymer formation. 

Before discussing the results obtained from this study it is important to understand the method by which a measurement of conversion was obtained directly from the spectra acquired from the reaction being studied in situ. The reaction chosen for study was the esterification reaction of methanol and acetic acid to form methyl acetate and water ... 

Concurrent measurements of conversion and conductivity, and measurements of the DPD, throughout the reaction. 

Little quantitative work has appeared on the determination of the rate of conversion of the various purine analogues to their nucleotides with highly purified phosphoribosyltransferases from mammalian cells or any other source, but there would appear to be a rough correlation between the cytotoxicity of these analogues and their ability to serve as substrates (the Kj values for a number of purines and purine analogues have been determined [45a, 85a] but this value is not a measure of conversion to nucleotide). Table 2.1 lists a number of purine analogues and an estimate of their ability to serve as substrates for the phosphoribosyltransferases. 

In the first case, product purities are controlled indirectly by controlling front positions. In distillation columns the front positions are easily controlled with cheap, reliable and fast online temperature measurements on sensitive trays inside the column [27]. A similar procedure was recently proposed for moving-bed chromatographic processes with UV rather than temperature measurement [37]. However, the performance of such an approach is usually limited. Exact product specifications cannot be guaranteed because of this indirect approach. Furthermore, in combined reaction separation processes the relationship between the measured variable and the variable to be controlled is often non-unique, which may lead to severe operational problems as shown for reactive distillation processes [23], It was concluded that these problems could be overcome if in addition some direct or indirect measure of conversion is taken into account. 

The principal method of characterizing solar-cell performance is the measurement of conversion efficiency while the cell is exposed to 1 sun illumination (—100 mW cm-2). The conversion efficiency is determined by measuring the current - voltage characteristic (see Fig- 8), locating the maximum power point (Pm = JmVm), and also measuring the solar insolation... 

Measurement of conversions of various formulations at various EB doses can be used to rank the reactivity of the formulation. A particularly useful procedure has been to prepare a standard mixture of an acrylate resin with various reactive diluent monomers in order to compare the volatility and reactivity of new monomers. For these studies, a mixture of 40 weight % of a Bis-phenol A epoxy dlacrylate resin with 60% of the test liquid monomer has proved convenient. A viscosity measurement of the mixture also provides information on the relative viscosity reducing ability of the test monomer. Illustrative examples of these measurements are shown in Table I and Figure 1. Mote from these examples that a monofunctional monomer, Monomer B, can be used to provide the low volatility and high reactivity typical of the multifunctional monomers, while also serving to reduce the crosslinking. Many other available monofunctional monomers are found to be either more volatile or less reactive than Monomer B. 

Equivalent circuit analysis is well suited for analysis of EIS measurements of conversion coatings and is the primary method for interpreting EIS spectra from conversion coated metal surfaces. A widely accepted generalized equivalent circuit model for the EIS response of pitted conversion coatings is shown in Fig. 22a (66,67). Several related models discussed below are also shown. In the gener-... 

Two principal cases can be pointed out rheokinetics of production of linear or network polymers. In the first case, the viscosity variation of the reactive mass serves as a unified measure of conversion from the beginning to end of the process. In the second case, the reactive system becomes incapable of reversible deformations at a gelpoint, long before the curing is completed, i.e., the viscosity becomes indefinitely high but the process is continued. 

The theory also has relevance to the so-called seeded " emulsion polymerization reactioas- In these reactions, polymerization is initial in the presence of a seed latex under conditions such that new particles are unlikely to form. The loci for the compartmentalized free-radical polymerization that occurs are therefore provided principally by the particles of the initial seed latex. Such reactions are of interest for the preparation of latices whose particles have, for instance, a core-shell" structure. They are also of great interest for investigating the fondamentals of compartmentalized free-radical polymerization processes. In this latter connection it is important to note that, in principle, measurements of conversion as a function of time during nonsteady-state polymerizations in seeded systems offer the possibility of access to certain fundamental properties of reaction systems not otherwise available. As in the case of free-radical polymerization reactions that occur in homogeneous media, investigation of the reaction during the nonsteady state can provide information of a fundamental nature not available through measurements made on the same reaction system in the steady state. 

Aqueous emulsions of styrene, methyl methacrylate, methyl acrylate, and ethyl acrylate were polymerized with y-radiation from a Co source in the presence of sodium dodecyl sulfate or sodium laurate. The continuous measurement of conversion and reaction rate was carried out dilato-metrically. The acrylates polymerized fastest and the over-all polymerization rate increased as follows styrene < methyl methacrylate < ethyl acrylate methyl acrylate. The effects of radiation dose, temperature, and original monomer and emulsifier concentrations were studied with respect to the following factors properties of polymer dispersions, number and size of polymer particles, viscometrically determined molecular weights, monomer-water ratio, and kinetic constants. 

The aim of this investigation was therefore the continuous measurement of conversion and reaction rate in polymerizing emulsion systems of different mono-... 

Reaction monitoring is highly facilitated as routine experiments such as II NMR, 13C NMR, mass spectrometry, IR spectroscopy, elemental analysis can be performed. This is mainly due to the simplicity of the structure. Unlike polymeric supports where signals from the support often prevent accurate measurement of conversion or side product concentration, trimethylammonium salts have only a limited effect on NMR spectra with a singlet and a multiplet in the 3.0-3.2 ppm region. Additionally onium salt supported products can eventually be analyzed using HPLC and mass spectrometry. 

The course of polyesterification in an experimental crosslinking system of the type A—A -i- B—B + A/ is shown in Fig. 5.6. There occurs, at first, a general increase in viscosity and then a sudden big increase as the gel point is approached. Also the reaction, as measured by the conversion of the reactive group, proceeds very slowly near the gel point. These two factors make it difficult to determine the gel point from the measurement of conversion or viscosity. 

Note the simple relation between the conversion and the temperature variation in adiabatic situations the variation in temperature is a measure of conversion and vice versa. 

In the second part of this chapter, focus was on control of continuously operated RD processes. So far most control studies focus on processes that are operated close to chemical equilibrium. Emphasis was on the well-known esterification and etherification systems. The methods employed are similar to non-RD column control. It is worth noting that this is consistent with our conclusions on open-loop dynamics as drawn above. Additional problems may rise in indirect control schemes, where product compositions are inferred from temperature measurements. It was shown that these problems can be handled if in addition some direct or indirect measure of conversion is taken into account. 

---