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1 Process Notes Acrylic acid (AA) is nsod as a precursor for a wide variety of chemicals in the polymers and textile industries. There are several chemical pathways to produce AA, but the most common one is via the partial oxidation of propylene. The usual mechanism for producing A A utilizes a two-step proceas in. which propylene is first oxidized to acrolein and then further oxidized to AA. Each reaction step usually takes place over a separate catalyst and at different operating conditions. The reaction stoichiometry is given below: C3H4+O2C3H4O+21O2C3H4O+H2O(Actolein)C3H4O2(AcrylicAeid) Several side reactions may occur, most resulting in the oxidation of reactants and products. Some typienl side reactions are given below: C3H4O+21O2C3H4OC3H63CO2+2H2O+23O2C2H4O2+CO2+29O23CO2+3H2O Therefore, the typical process set-up consists of a two-reactor system with each reactor containing a separate catalyst and operating at conditions 50 as to minimize the prodinction of AA. The first reactor typically operates at a higher temperature than the second. 2 Process Description The process shown in Fig. 1 produces 50.000 metric tons per year of 99.98 by mole A. product. The: number of operating hours is taken to be 8000/yt, and the process is somewhat simplified since there is only one reactor. It is assumed that both reactions take place on a single catalyat to yield AA and byproducts. It is imperative to cool the products of reaction quickly to avoid further asidation reactions, and this is achieved by rapidly quenching the reactor effluent with a cool recycle. Stream 8 , of dihute aqueous AA in T-301. Additional recovery of AA and acetic acid (a by-product) is achived in the absorber, T-302. The stream leaving the absorption sections is a dilute aqucous acid, Stream 9. This is sent to a liquid-liquid extractor, T-303, to remove preferentially the acid fraction from the water prior to purification. There are many possible solvents which can be used as the organic phase in the separation; high solubility for AA and low solubility for water are desirable. Some examples include ethyl acrylate, ethyl acetate, xylene, diisobutyl ketone, methyl isobutyl ketone, and disopropyl ether (DIPE) which is used here. The organic phase from T-303 is sent to a solvent recovery columm. T-304. where the disopropyl ether (and some water) is recovered overhead and returumd to the extractor. The bottom stream from this column, Stream 14, contains virtually all the AA and acetic acid in Stream 3 Reaction Kinetics and Reactor Configuration The reactions taking ptace are kinetically controlled nt the conditions sud in the proges ic- oqutlibrium lies far to the rigat. The reaction kinetica for the catalygt test in the proces ate gheb Figure 1: Acrylic Acid Process Flow Diagram. below: C3H6+23O2C3H4O2+H2OC3H6+25O2C2H4O2+CO2+H2OC3H6+29O23CO2+3H2OReaction1Reaction2Reaction3 The reactor configuration used for this process is a fluidized bed, and it is assumed that the bed of catalyst behaves as a well-mixed tank, that is, it is isothermal at the temperature of the reaction (310C). The gas flow is assumed to be plug flow through the bed, with a 10% of the gas bypassing the catalyst. This latter assumption is made in order to simulate the gas chamneling which occurs in real fluid bed reactors. 4 Safety Considerations As with any reaction involving the partial oxidation of a fuel-like feed material (propylene), consid.crable attention must be paid to the composition of hydrocarbons and oxygen in the feed streum. Operation outside the explosive limits is recommended for this reaction. However. the conditions used in this process lie within the explosive limits. The addition of a large amount of steam and the use of a fluidized bed reactor allow safe operation within the explosive limits. The second safety concers is that associated with the highly exothermic polymerization of AA which occurs in two ways. First, if this material is stored withont appropriate additives, then free radical initiation of the polymerization can occur - this potentially disastrous situation is discussed by Kurland and Bryant (1987). Sccond, AA dimerizes when in high concentrations at temperatures greater than 90, thus much of the separation sequence must be operated under high vacuum in order to keep the bottom temperatures in the columns below this temperature. 5 Vapor-Liquid-Liquid-Equilibrium The nse of a liquid-liquid extractor requires the use of a thermodynamic package for pliysicil property data) which reflects the fact that two phases are formed and that significent partioning of the AA and acetic acid occurs with the majority going to the organic phase (in this case DIPB). Distribution coefficients for the organic acids in water and DIPE as well as mutual solubility data for water DPE are desirable. The process given in Fig. 1 is simulated using UNIFAC. The kinetics presented above are fictitious but should give reasonable preliminary estimates of reactor size