CH-102 Solution Energy Balance

1.

Ethanol is produced commercially by the hydration of ethylene: C2H4 (g) + H2O (v)  C2H5OH (v); Some of the product is converted to diethyl ether in the undesired side reaction: 2 C2H5OH (v)  (C2H5)2O (v) + H2O (v) The combined feed to the reactor contains 53.7 mole% C2H4, 36.7% H2O, and the balance N2 which enters the reactor at 310oC. The reactor operates isothermally at 310oC. An ethylene conversion of 5% is achieved, and the yield of methanol (moles ethanol produced/mole ethylene consumed) is 0.90. a) Assuming 1 mol of feed to the reactor as basis, calculate the molar amount of each species in reactor output stream. [5] b) Calculate the reactor heating or cooling requirement in kJ/mol feed. Also, write down the assumptions involved, if any. Use heat of formation method. [11] c) Briefly state what would have been different in your calculations if you take molecular species at [2] 25oC as reference in part (b). d) Note that the reactor is designed to yield low conversion of ethylene. If same conversion is to be maintained, what processing step (or steps) would probably follow the reactor in a commercial implementation of this process? [2] Data for diethyl Ether:

 H   272.8 kJ/mol for the liquid,  f

 H v   26.05 kJ/mol (assume independent of T) c p (kJ/mol. C)  0.8945  40.33 105 T ( C)  2.244  107 T 2 2. A natural gas containing 95 mole% methane and the balance ethane is burned with 20.0% excess air. The stack gas which contains no unburned hydrocarbons or CO, leaves the furnace at 900oC and 1.2 atm and passes through a well insulated pre-heater (heat exchanger). The air on its way to the furnace also passes through pre-heater, entering it at 20oC and leaving it at 245oC. a. Taking as a basis 100mol/s of natural gas fed to the furnace, calculate the required molar flow rate of air (with individual flow of oxygen and nitrogen) fed to the furnace and the molar flow rate and composition of the stack gas. [8] b. Calculate the rate of heat transfer Q in the pre-heater, and the temperature at which the stack gas leaves the pre-heater. c. What would Q be if the actual feed rate of natural gas were 350 SCMH?

[9.5] [2.5]

3. A proposed method for producing methanol is to react CO2 with H2: CO2 + 3H2 →CH3OH + H2O. The flowchart for the process is shown below. Assume that gross feed enters the reactor in the stoichiometric quantities needed for the reaction. Also 0.5% N2 flows in with the fresh feed and single pass conversion in reactor is 57%. The concentration of N2 in the gross feed into the reactor is 2% and 1 kmol/h CO2 is present in the feed to the reactor. a. Draw a completely labeled flowchart for the process and determine the sequence in which material balance problem must be solved for the given flowchart. [4+6] b. Calculate all the unknown compositions and flow-rates. [7] c. The heat removed from the entire system is 70570 kJ/h. Determine the temperature of methanol and water stream emerging out of the separation unit. Use heat of formation method. [13]

CH3OH (liq), T oC Product gas  o

Fresh Feed, 25 C  Gross Feed  Gas  CO2, H2, N2 (g) 

Reactor 

Separator

H2O (liq), T oC

o 90 C Recycle gas 

Purge gas, 100 oC

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