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NKB10704 MATERIAL AND ENERGY BALANCE JANUARY SEMESTER 2014 ASSIGNMENT 1 Answer ONE (1) of the following questions: Question 1: Sulfuric acid (H2SO4)...

NKB10704 MATERIAL AND ENERGY BALANCE JANUARY SEMESTER 2014 ASSIGNMENT 1 Answer ONE (1) of the following questions: Question 1: Sulfuric acid (H 2 SO 4 ) is made by burning sulfur (S) in air to make SO 2 , then further oxidizing SO 2 to SO 3 over a catalyst, using air as the source of O 2 , then finally cooling and absorbing the gases into water to produce H 2 SO 4 . We want to design a plant that produces 200 tons/day of concentrated H 2 SO 4 (98 wt% H 2 SO 4 and 2 wt% water), using water, sulfur, and air as raw materials. Develop a reaction pathway such that there is no net generation or consumption of SO 2 or SO 3 . Calculate the raw materials required as: tons sulfur/day, standard cubic feet of air/h, and tons water/day. Question 2: Natural gas usually contains some hydrogen sulfide (H 2 S), a very toxic and smelly gas. One way to remove H 2 S from sour natural gas streams is to pass the natural gas over a bed of zinc oxide, where the reaction ZnO + H 2 S ZnS + H 2 O takes place. Suppose you need to treat 1 million standard cubic feet of natural gas per day, and the gas contains 0.82 mol% H 2 S. Assume that water is not allowed in the natural gas product and you’d like to have to remove and discard the ZnS only once per day. Determine (a) how much ZnO is required, (b) how large is the vessel to hold this quantity, and (c) how much water must be removed (kg/day). You will need to look up some physical property data.
Question 3 Formaldehyde (HCHO) is produced by partial oxidation of methanol (CH 3 OH). Several side reactions also occur, producing formic acid (HCOOH), CO, CO 2 , and H 2 O. In one working process, air is mixed with fresh and recycled methanol and fed to a reactor. Your job is to evaluate how well this process is working. You take samples at several points in the process and find that (a) the reactor inlet stream is 35 mol% CH 3 OH, (b) the recycled methanol is pure, (c) the offgas contains 10.9 mol% H 2 , 6.0 mol% CO 2 , 0.3 mol% CO, 81.9 mol% N 2 , and 0.9 mol% O 2 , and (d) the liquid product stream contains 43 mol% HCHO, plus HCOOH and H 2 O. Write down a set of independent, stoichiometrically balanced chemical equations that completely describe all the reactions taking place in the reactor. Use a degree-of-freedom analysis to determine if you’ve got enough stream compositions analyzed to completely describe the process operation. Calculate the flow rate through the reactor, and the production rate for the two valuable products, formaldehyde and formic acid, per 100 moles methanol fed to the process. Finally, discuss how might you consider adjusting the process operation if formaldehyde is a more valuable product than formic acid. Question 4 Methanol (CH 3 OH) reacts to form formaldehyde (HCHO) either by decomposition to formaldehyde and hydrogen (H 2 ) or by oxidation to form formaldehyde and water (H 2 O): CH 3 OH HCHO + H 2 CH 3 OH + (1/2)O 2 HCHO + HO 2 A mixture containing 99 mol% methanol and 1 mol% of an inert contaminant is available as feed to a formaldehyde plant. 1000 kmol/h of this mixture, along with 200 kmol/h O 2 , are fed to a process as fresh feed. The fresh feed is mixed with a recycle stream and fed to a reactor. The fractional conversion of methanol achieved in the reactor is 25%. All of the oxygen is consumed in the reactor. The reactor output is sent to a separation unit, where formaldehyde, water, and hydrogen are removed, and methanol and the contaminant are recycled to the reactor feed. To control the contaminant level in the reactor, a purge stream is taken off the recycle stream. The maximum contaminant allowed in the recycle stream is
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