Acid temperature control input

Input acid temperature is controlled by indirect water cooling in an acid cooler, Figs. 9.5 and 24.6. 

Meanwhile attempts to find an air oxidation route directly from p-xylene to terephthalic acid (TA) continued to founder on the relatively high resistance to oxidation of the /Moluic acid which was first formed. This hurdle was overcome by the discovery of bromide-controlled air oxidation in 1955 by the Mid-Century Corporation [42, 43] and ICI, with the same patent application date. The Mid-Century process was bought and developed by Standard Oil of Indiana (Amoco), with some input from ICI. The process adopted used acetic acid as solvent, oxygen as oxidant, a temperature of about 200 °C, and a combination of cobalt, manganese and bromide ions as catalyst. Amoco also incorporated a purification of the TA by recrystallisation, with simultaneous catalytic hydrogenation of impurities, from water at about 250 °C [44], This process allowed development of a route to polyester from purified terephthalic acid (PTA) by direct esterification, which has since become more widely used than the process using DMT. 

A first control scheme proposed in [90] is shown in Fig. 10.26. In this scheme, product purities of methyl acetate (MeAC) and water (HjO) are inferred from temperatures on trays 3 and 12, respectively, and the feed rates of methanol (MeOH) and acetic acid (AcH) are used as manipulated variables. For this configuration, three different temperature profiles exist with identical temperature values at the sensor locations but different feed rates and completely different product compositions. The solid line in Fig. 10.26 represents the desired temperature profile with high conversion. This situation corresponds to input multiplicity as introduced at the beginning of section 10.2 on multiplicity and oscillations. Here, the same set of output variables (temperatures) is produced by (three) different sets of input variables (feed rates). Because the steady state values of the output variables are fixed by the given setpoint of the controllers, this input multiplicity will lead to steady state multiplicity of the closed loop system as illustrated in Fig. 10.27. 

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