Recovery conversion, overall

Paskall (25) has recently reviewed the various modifications to the Claus process that result in optimum sulfur recovery efficiency. Overall plant conversion efficiencies in the range of 97% were considered to be the upper limit at the beginning of the 1970 s (26). While this is a very respectable conversion efficiency for an industrial process the unrecovered 3% in a 2,000 tonne/d sulfur plant represents 60 tonnes/d of sulfur lost, mainly to atmosphere as 120 tonnes/d of SO2. Modifications to the four stage Claus converter train however, can raise overall conversions to over 98.5% thus halving the sulfur loss to the plant tail gas. This either reduces environmental impact or the load on tail gas desulfurization units that will be discussed later. 

Using a Pd-MR can give a rather high acetic acid conversion, with 30-35% hydrogen recovery. The overall reaction system can be optimized by tuning the right amount of Ru-based catalyst and Ni-based catalyst [59]. 

The i j -configuration of the 6,7-double bond in pre-vitamin D is critical to its subsequent thermal rearrangement to the active vitamin. A photochemical isomerization of pre-vitamin D to yield the inactive trans-isoTnen occurs under conditions of synthesis, and is especially detrimental if there is a significant short wavelength component, eg, 254 nm, to the radiation continuum used to effect the synthesis. This side reaction reduces overall yield of the process and limits conversion yields to ca 60% (71). Photochemical reconversion of the inactive side product, tachysterol, to pre-vitamin D allows recovery of the product which would otherwise be lost, and improves economics of the overall process (70). 

Ammonia Synthesis and Recovery. The purified synthesis gas consists of hydrogen and nitrogen in about 3 1 molar ratio, having residual inerts (CH Ar, sometimes He). The fresh make-up gas is mixed with the loop recycle and compressed to synthesis pressures. AH modern synthesis loops recycle the unreacted gases because of equiUbrium limitations to attain high overall conversions. The loop configurations differ in terms of the pressure used and the point at which ammonia is recovered. 

Dehydrogenation of t-amylenes over a dehydrogenation catalyst produces isoprene. The overall conversion and recovery of t-amylenes is approximately 70%. 

It is the formation of this material which makes the reaction have a low atom economy and, owing to the cost of disposal (usually by conversion to calcium phosphate and disposal as hazardous waste), has limited its commercial usefulness to high value products. Several methods have been developed to recycle (Ph)3PO into (Ph)3P but these have proved more complex than might be expected. Typically the oxide is converted to the chloride which is reduced by heating with aluminium. Overall this recovery is expensive and also produces significant amounts of waste. 

Sulfreen American Lurgl Claus tall gas. Sulfur compounds 1-3Z Raises overall sulfur recovery to 99%. 2000-3000 ppmv sulfur In treated gas. No COS/CS2 conversion... 

Beavon Hark 11 Parsons H2S content of <5Z or Claus tail gas Up to 99% overall sulfur recovery. Some COS and CS2 conversion... 

Beavon Mark 1 Parsons Claus tall gas 99.8% overall sulfur recovery, <300 ppmv sulfur compounds in treated gas, generally <10 ppmv H2S. Some COS/CS2 conversion... 

Assumptions Synthesis of phenylpyruvic acid Batch synthesis process for precursors overal yield of 95+% of theoretical to pheny Ihydantoin overall yield of 90+% of theoretical from phenylhydantoin to phenylpyruvic acid recovery and recycle of acetic acid no byproduct crec taken for acetic acid formed from acetic anhydride addition. Conversion of phenylpyruvic acid and aspartic acid. Bioreactor productivity of-18 g PHE/L/h (four columns in parallel) 98% overall conversion no byproduct credit taken for pymvic acid (recovery cost assumed to be of by revenue from sale) 80% recovery of L-PHE downstream of bioreactor. 

These sub-dew point processes can increase the overall Claus plant sulfur recovery to up to 99%, as limited by equilibrium conversion and sulfur vapor pressure losses. Elf Aquitaine s Sulfreen process, Amoco s cold bed adsorption (CBA) process, and the Mineral and Chemical Resource Company (MCRC) process licensed by Delta Hudson are all variations on the cold bed sub-dew point process. 

Inhibitors are introduced al specific points in the process to prevent polymerization Sulfuric acid serves as catalyst in a combined hydrolysis-esterilieaiion of methacrylamide sulfate to a mixture of methyl methacrylate and methacrylic acid. Conversion of methacrylamide sulfate to methyl methacrylate can be carried out using a variety of procedures for die recovery of crude methyl methacrylate and fur separation of methanol and methacrylic acid for recycling. A schematic of the overall process is given in Figure I. The overall yield based on acetone cyanohydrin is approximately 90D Most of Ihe world supply of MMA is still produced by this process. 

In addition to the identification of the appropriate biosynthetic enzyme, the feasibility of all biotransformation processes depends heavily on other criteria, such as the availability of inexpensive starting materials, the reaction yield, and the complexity of product recovery, hi the case of transaminase processes, the reversible nature of the reaction (Scheme 3.1) and the presence of a keto acid by-product is a concern that limits the overall yield and purity of product and has led to efforts to increase the conversion beyond the typical 50% yield of product.99 100 Additionally, there are cost considerations in the large-scale preparation of keto acid substrates such as 2-ketobutyrate, which are not commodity chemicals. 

Racemic l-dimethylaminopropan-2-ol (18) was acylated with propanoyl chloride, and the reaction product was analyzed by gas chromatography (GC). The resultant (R)- and (5)-esters 19 (R1 = COEt) were resolved by GC using an a-cyclodextrin column. Reaction of the racemic alcohol 18 was then carried out in the presence of Novozyme 435 and vinyl propanoate, and the reaction was followed by GC. After 4 hours the reaction was approximately 2% complete and the ee of the propanoate ester 19 (R1 = COEt) was 95.9%. After 88 hours the conversion had reached 50% and the ee was still 95.6%. The remarkable specificity of Novozyme 435 for the (/O-amino alcohol 1 A )-18 was evident because even after 3.5 days reaction time, only a very small amount of the (5)-ester was detected. The reaction could be scaled up thus, 1 kg of the racemic amino alcohol 18 was treated with vinyl propanoate (0.5 equivalents) and Novozyme 435 (3% by weight). After 3 days, both optically active products were isolated by distillation at reduced pressure. The (5)-amino alcohol (.V)-18 was recovered in 45% yield, which compared favorably with a yield of 32% for resolution on a small scale. The (R)-propanoate 19 (R1 = COEt) distilled as a colorless oil in 36% yield — slightly higher than that obtained from the small-scale resolution. The overall recovery was 81% from the scaled up reaction. 

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