19.3.3 Technical questions
1. Why is oxalic acid added in the acrylonitrile purification column ?
Ans: One of the byproducts of the ammonoxidation of propylene are cyanohydrins. These organic compounds readily dissociate to form volatile compounds. These volatile compounds are severely polluting compounds. Therefore, to avoid this, oxalic acid is added to the purification column in order to form complex compounds with these cyanohydrins and these compounds eventually enter the heavy end products.
2. A careful analysis of the process flowsheet shown indicates that while absorption is favoured at lower temperatures and higher pressures, exactly opposite conditions exist for the reactor outlet stream (at about 1 atm pressure and 400 – 500 oC). What additional process modifications are suggested ?
Ans: Cooling the vapour product stream from 400oC to about 50°C in a series of heat exchangers. Since vapour is involved, extended area exchangers will be beneficial. Heat integration with the reboilers of any of the distillation columns is also beneficial.
Pressurizing the vapour pressure to higher pressure and allowing it to enter the scrubber at the same temperature. This is beneficial but compressor costs will be enormous.
Therefore, in the light of the process costs, cooling the vapour stream is beneficial than compression to favour good absorption.
3. In certain processes for acrylonitrile production, cyanohydrins removal is desired. If so, what process modifications are suggested ?
Ans: Cyanohydrins are in the bottom product obtained in the product splitter. Therefore, the bottom product can be sent to a reactor where cyanohydrins can be converted to acrolein and these acroleins can be separated and sent back to the ammonoxidation reactor (fluidized beds). In that case, oxalic acids are not used and the heavy ends will not also get produced significantly and therefore process topology will be somewhat different from what is being shown here. For further details upon how the process flowsheet changes please refer to Chemical Engineering Design (Book) by Sinnott where in Appendix these modifications have been explained thoroughly in the process description.
4. Is a partial condenser required in the acetonitrile azeotropic column ?
Ans: No, the reason is that the bottom product from the product splitter consists of heavy ends, acetonitrile and water and does not consist of lighter ends and HCN. Therefore, a partial condenser is not required and a total condenser producing two separate liquid phase streams that separate upon gravity is required.
5. A feed stock heater is not shown in the process. However, reactor operating conditions are indicate high temperature operation. How is the feed stock heated ?
Ans: The heat for achieving the feed to desired temperature is provided by superheated steam that is mixed along with the feedstock. Typical feed molar composition is propylene 7, ammonia 8, steam 20 and air 65. Additional heat for the reaction is obtained from the highly exothermic reaction in the fluidized bed catalytic reactor.
6. What are the advantages of the fluidized catalytic reactor when compared to a packed bed reactor ?
Ans: It is well known that the heat and mass transfer coefficients of gases are predominantly lower than those of the liquids. Therefore, fludization principle effectively enhances bulk phase mass and heat transfer coefficients of the gas solid catalytic reaction. In other words, due to fluidization, less contact time that is required in the process, higher conversions can be achieved.
References:
1. Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008
2. Kirk R. E., Othmer D. F., Encyclopedia of Chemical Technology, John Wiley and Sons, 1999-2012