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1000 kmol/h of biogas consisting of 65 mol % CH4, 25 mol% CO2 and 10 mol% N2 is fed to a
membrane purification process in order to reduce CO2 content. The outlets from the membrane
process are the retentate and the permeate stream. The permeate contains 50 mol% CO2, 20 mol%
CH4 and the rest is N2. The permeate stream is 25% of the retentate stream.

(a) Draw and label the flowchart. (5 Marks)
(b) Calculate the composition of the retentate and unknown flow rates. (15 Marks)

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Related questions

Gas boiler uses methane (CH4) as its fuel source. As a newly appointed engineer, you are assigned by your superior to determine whether the current boiler operation is running at the optimum conditions. You are given the average daily Orsat gas analysis of the stack gas as shown in Table Q3.

Table Q3: Composition of Orsat stack gas.

Component Mol (%)

CH4 0.55

O2 12.00

N2 82.40

CO2 5.05

Given the element molecular weight of C=12, H=1, O=16 and N=14. Assume the air stream fed to the boiler only consists of 21 mol% O2 and 79 mol% N2. In order to perform the analysis, you need to solve few fundamental question below.

a. Write the main stoichiometric combustion reaction.

b. Draw the input-and output flowchart of the process.

c. By using element balances and taking 100 kmol/s of Orsat flue gas as the basis of calculation, determine the following:

i. The mass flow rate of natural gas into the boiler.

ii. The percentage of excess air used in the combustion

iii. The actual fractional conversion of methane

iv. The actual water produced.

v. Composition of stack gas in wet basis.

Methanol is a potential clean fuel for gasoline engines and its synthesis reactions occur in gas phase, where the products are subsequently liquefied to split the un-reacted reagent gases - carbon dioxide (CO2), carbon monoxide (CO) and hydrogen (H2). In addition to these metha- nol synthesis reactions, reverse water-gas shift reaction occurs in the process, where CO2 and H2 react to form CO and water vapour. The liquid stream leaving the separator consists of methanol and water with the separation of un-reacted reagent as exhaust gases.

A methanol synthesis plant is designed to produce 50,000 kg/hr of 99.8% methanol, using the feed gas with the composition of 5 mol% methane (CH4), 25 mol% CO, 5 mol% CO2 and 65 mol% H2. The product liquid stream leaving the separation process has the methanol purity of 99.8 wt%. The single pass conversions of CO and CO2 are assumed to be 45% and 30%, re- spectively. A graduate engineer has devised a simulation model of the reactions of the metha- nol synthesis plant and based on the kinetic equations of those reactions, following conver- sions of CO and CO2 were predicted.

Table 1.1 Reactions and simulated conversions

Reaction Conversion (%)

CO reaction 45%

CO2 reaction 30%

Reverse water-gas shift reaction 0%

(i) Write down the three equilibrium reactions involved in the process.

(ii) Draw a flowsheet to represent the methanol production process - clearly label each process streams and also indicate all known and unknown flowrates and compositions

of the streams.

(iii) Using the provided conversion of CO and CO2, calculate the amount of feed required

to produce methanol and also determine the molar composition of each stream.

(iv) Is this process efficient? If not give a reason why and suggest what can be done to

make it more efficient.

Anequimolarliquidmixtureofbenzeneandtolueneisseparatedintotwoproductstreamsbydistillation. A process flowchart and a somewhat oversimplified description of what happens in the process follow:
Feed
100 mol 0.500 mol B/mol 0.500 mol T/mol
n1(mol) n1(mol) CONDENSER
DISTILLATION COLUMN
Boilup yB[mol B(v)/mol]
Vapor—97% mole benzene
Reflux
Overhead product
Bottoms product
REBOILER xB[mol B(l)/mol]

Inside the column a liquid stream flows downward and a vapor stream rises. At each point in the column some of the liquid vaporizes and some of the vapor condenses. The vapor leaving the top of the column, which contains 97 mole% benzene, is completely condensed and split into two equal fractions: one is taken off as the overhead product stream, and the other (the reflux) is recycled to the top of the column. The overhead product stream contains 89.2% of the benzene fed to the column. The liquid leaving the bottom of the column is fed to a partial reboiler in which 45% of it is vaporized. The vapor generated in the reboiler (the boilup) is recycled to become the rising vapor stream in the column, and the residual reboiler liquid is taken off as the bottom product stream. The compositions of the streams leaving the reboiler are governed by the relation
yB/1 ␣ yB 2:25 xB/1␣xB
where yB and xB are the mole fractions of benzene in the vapor and liquid streams, respectively. (a) Take a basis of 100 mol fed to the column. Draw and completely label a flowchart, and for each of four systems (overall process, column, condenser, and reboiler), do the degree-of-freedom analysis and identify a system with which the process analysis might appropriately begin (one with zero
degrees of freedom). (b) Write in order the equations you would solve to determine all unknown variables on the flowchart,
circling the variable for which you would solve in each equation. Do not do the calculations in this part. (c) Calculatethemolaramountsoftheoverheadandbottomsproducts,themolefractionofbenzenein the bottoms product, and the percentage recovery of toluene in the bottoms product (100 × moles
toluene in bottoms/mole toluene in feed).

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