Acid temperature control construction materials

High-temperature/low-pressure inorganic digestions are an area of application that has benefited from recent advances in vessel and sensor design. The inert properties of Teflon and its resistance to acid attack make it the material of choice for microwave pressure-vessel construction. Improved commercial systems offer additional safety precautions and improved facilities for pressure and/or temperature control. Also, the distribution of microwave radiation inside the oven cavity is fairly homogeneous. Low-pressure systems allow decomposition temperatures of about 180 °C. However, for many matrices, such temperatures are not sufficient to guarantee the complete ashing of thermoresistant sample components. 

Carbon steels and stainless steels are the most common construction materials for equipment used in the manufacture, storage, and transportation of sulfuric acid. Anodic protection was successfully used to control the corrosion of the various equipments made of these materials in this acid over a wide range of temperatures. While the corrosion rates of carbon steels in sulfuric acid are mainly functions of temperature, velocity, and acid concentration and purity, particularly the iron content, these rates are greatly affected by minor alloying elements in steel, particularly copper. The corrosion rates of steel in 77 to 100% concentrations are in the range of 20 to 40-mils per year (mpy) at 24 °C [14]. 

In general, acetic acid production via acetaldehyde oxidation takes place continuously in a bubble column at 50-80 °C with pressures of 1-10 bar. The construction material of choice for the reactor is austenitic Cr-Ni-steel. The acetic acid product serves as process solvent and the concentration of acetaldehyde is kept at 3%. It is necessary to keep the temperature over 50 °C to obtain a sufficient peroxide decomposition and oxidation rate. To remove the heat of the exothermic reaction, the reaction mixture is circulated through an external heat exchanger. Accurate temperature control is important to decrease oxidative degradation of acetic acid to formic acid, CO2, and water. The reaction mixture is separated by several distillation units. The process yields are typically in the range of 90-97% and the purity of acetic acid is higher than 99%. Typical by-products are CO2, formic acid, methyl acetate, methanol, methyl formate, and formaldehyde. 

Conversely, synthesis is a constructive process. Reactants that are not particularly stable to strong acids and bases may be required and in much larger amounts than those used for digestion. They may be expensive and highly reactive, even on the laboratory-scale. The ability to define and control conditions, including temperature, time, sample stirring, addition or withdrawal of materials, and post-reaction cooling, is almost always vital for satisfactory outcomes and for reproducibility. 

High silicon austenitic stainless steels, such as SARAMET 23, SARAMET 35, ZeCor, and SX , were developed to handle hot, concentrated sulfuric acid. Acid towers are more commonly being constructed of these materials. Careful control of acid concentrations and temperatures flowing through the towers are critical in ensuring slow corrosion of these materials. 

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