Temperatures, industrial Temperature control, acid

In industrial production of acid-modified starches, a 40% slurry of normal com starch or waxy maize starch is acidified with hydrochloric or sulfuric acid at 25—55°C. Reaction time is controlled by measuring loss of viscosity and may vary from 6 to 24 hs. For product reproducibiUty, it is necessary to strictly control the type of starch, its concentration, the type of acid and its concentration, the temperature, and time of reaction. Viscosity is plotted versus time, and when the desired amount of thinning is attained the mixture is neutralized with soda ash or dilute sodium hydroxide. The acid-modified starch is then filtered and dried. If the starch is washed with a nonaqueous solvent (89), gelling time is reduced, but such drying is seldom used. Acid treatment may be used in conjunction with preparation of starch ethers (90), cationic starches, or cross-linked starches. Acid treatment of 34 different rice starches has been reported (91), as well as acidic hydrolysis of wheat and com starches followed by hydroxypropylation for the purpose of preparing thin-hoiling and nongelling adhesives (92). 

In the electroplating industry, the use of titanium as hooks " and as heating and cooling coils for temperature control of certain acidic liquors has improved the control of plating baths Perhaps the most significant... 

The oldest way to produce caramel is by heating sucrose in an open pan, a process named caramelization. Food applications require improvement in caramel properties such as tinctorial power, stability, and compatibility with food. Caramels are produced in industry by controlled heating of a rich carbohydrate source in the presence of certain reactants. Carbohydrate sources must be rich in glucose because caramelization occurs only through the monosaccharide. Several carbohydrate sources can be used glucose, sucrose, com, wheat, and tapioca hydrolysates. The carbohydrate is added to a reaction vessel at 50°C and then heated to temperatures higher than 100°C. Different reactants such as acids, alkalis, salts, ammonium salts, and sulfites can be added, depending on the type of caramel to be obtained (Table 5.2.2). 

Typical industrial process for the synthesis of phenyl boronic acid from phenylmag-nesium bromide and boronic acid trimethoxy ester requires strict temperature control (—25 to —55 °C) to minimize the formation of side products. Recently, Hessel and coworkers reported that a micromixer (width 40 pm and depth 300 pm)/ tubular reactor system gave the phenyl boronic acid at high yield (>80%) even at higher temperatures (22 or 50 °C) with minimum amounts of the side products (Scheme 4.48) [66]. They also achieved a pilot-scale production by employing a caterpillar minimixer (width range 600-1700 pm and depth range 1200-2400 pm). 

Temperature control is therefore essential in order to prevent polynitration and to avoid selfgenerating runaway reactions that can have catastrophic consequences. With due care, nitrations are perfectly safe and are commonly carried out on a very large scale in industry. Indeed, mild nitrations carried out in 80% acetic acid, for example, are very safe. However, literature methods should be carefully followed. New nitrations should be carried out initially on a very small scale and with extreme care. Nitrations are rarely carried out above 120-140 °C reactions at higher temperatures would be considered hazardous. 

At the present time, all industrial plants operate in the vapor phase, although certain developments have been conducted to achieve the oxychlorinatkm of ethylene in the liquid phase, particularly by Kellogg. In this case, transformation takes place around 170 to 185 C at between 15 and 2.10 Pa absolute, in the presence of a catalyst system based on promoted cuprous and cupric chlorides, with more efficient temperature control by vaporization of part of the reaction medium and better performance, but with acid... 

Not mentioned in the table are the catalysts methanesulphonic acid, xylenesulphonic acid and certain modified zeolites ail of which have found some application. Other alkenes include the pentenes and isomers of the compounds in the table such as non-1-ene. Of importance in industrial reaction conditions are procedures to avoid alkene/alkene side reactions and promote the desired alkene/phenol reaction, with temperature control to avoid dealkylation of the product. 

Transesterification is a key transformation in organic synthesis, considering its industrial as well as academic importance. The well-known transesterification catalysts, like Ti(0 Pr)4, BuSn(OH)3 or Al(OR)3 commonly involve heating ester substrates with the catalyst in an alcohol solvent at reflux conditions and require elevated reaction temperatures and acidic conditions. Compounds 1-7 were reacted with phenyl acetate and methanol at 50 °C under neutral conditions, as shown in Scheme 17.1. This reaction quantitatively formed methyl acetate within 0.9-20 days whilst a control reaction performed in absence of the complexes gave only a low conversion of the ester to the product in an identical reaction time. 

It is advisable to include some carboxy functionality also as the cure reaction between acrylic and melamine resins is acid catalysed. In the more sophisticated industrial markets for acrylics, stoving temperatures are well controlled and adequate cure cycles are achievable. However, in the competitive general metal finishing market, low temperatures and short times are common, so whilst it is possible to externally catalyse using PTSA (para toluene sulphonic acid), this polar material remains in the film after curing to act as a pathway for water. Blistering on exposure to humidity is a common problem in acrylic-melamine systems which have been catalysed by sulphonic acids. 

---