Conversion processes, reactivity contents

There are many different routes to organic chemicals from biomass because of its high polysaccharide content and reactivity. The practical value of the conversion processes selected for commercial use with biomass will depend strongly on the availabiUty and price of the same chemicals produced from petroleum and natural gas. 

Eastern coal conversion development may come to be favored because of market proximity, water availability, and coal sources which, because of their high sulfur content, are currently unuse-able and, hence, largely decoupled from other energy prices. Proximity reduces transport costs and allows an increased use of low and medium Btu syngas processes. Reactivity and swelling problems may be overcome by technology. 

Important biomass fuel properties for thermochemical conversion processes are reported as proximate and ultimate analyses. The proximate and ultimate analyses for selected biomass feedstocks are presented in Table 33.5. For comparison, the analyses from two selected coal samples are also presented. Biomass generally has a lower energy density than coal, oils, and natural gas it also has higher oxygen content. The higher volatiles and oxygen content of biomass translate into a higher reactivity compared to traditional fossil fuels. In terms of thermochemical conversions, this means that less severe process conditions (lower temperature and shorter residence time) are required for bio-... 

At present, this conceptual coal conversion process is limited in its application to western coals because of their noncaking character and lower sulfur content compared with eastern bituminous coals. It is of interest to note, however, that certain chemical pretreatment processes (10,11) have been shown to eliminate the caking tendency of the coal and reduce its sulfur content. Recent experiments at ORNL (12) have confirmed the suitability of several chemically treated coals to mild hydrocarbonization. In fact, there is evidence to suggest that some of these pretreated coals are actually more reactive under hydrocarbonization conditions than untreated coal, thereby enhancing the yield of oil or gas. 

Many factors influence the reactivity and digestibility of the cellulose fractions of lignocellulose materials. These factors include Hgnin and hemicellulose content, crystalhnity of cellulose, and the porosity of the biomass materials. Pretreatment of Hgnocellulosic materials prior to utiHzation is a necessary element in biomass-to-ethanol conversion processes. The objective of the pretreatment is to render biomass materials more accessible to either chemical or enzymatic hydrolysis for efficient product generation. The goals of the pretreatment are ... 

Borosilicate catalysts provide high approach to thermodynamic equilibrium of the xylenes, and offer high selectivity in the conversion of ethylbenzene (8.12.22.50 ). In addition, they have been shown to be less prone to the effects of thermal and steam treatments than corresponding aluminosilicate zeolite catalysts (51). The catalytic activity of borosilicate catalysts was demonstrated to be a function of the structural boron content of the molecular sieve (22.36,50). In addition, the by-product distribution obtained from a borosilicate catalyst in a xylene isomerization/ethylbenzene conversion process was found to be distinctive (50), with high transethylation reactivity relative to transmethylation. 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Prepolymer produced via the terephthalic acid (TPA) monomer route shows an increased reactivity in comparison with that produced by the dimethanol tereph-thalate (DMT) monomer process [49], This behavior is possibly caused by the enhanced CEG content, which is usually higher in products from the TPA process as a result of insufficient conversion of the acid monomer in the esterification reaction (Figure 5.23). The increased reactivity may be caused by an autocatalytic influence of the carboxylic groups which seems to be disturbed by an unbalanced content of OH groups in the case of degradation. 

All reactive stripping experiments showed that reducing the water content level (due to better stripping performance) increases the per-pass conversions, but has a negative effect on selectivity in the chosen model reaction system. Nonetheless, the water contents are the result of a balance between stripping efficiency and catalyst hold-up. As a consequence, the space-time yield was highest for katapak-S , whereas in DX -packings, the excellent separation efficiency optimized the use of catalyst, but decreased the selectivity. For industrial applications, the choice will always depend on the balance between mass transfer performance, the kinetics, the activity of the catalyst, and the process economics. 

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