Recycle, Bypass and Purge - Process Calculations - Chemical Engineering Previous Year Questions

1

2007 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2007

A simplified flowsheet is shown in the figure for production of ethanol from ethylene. The conversion of ethylene in the reactor is 30 % and the scrubber following the reactor completely separates ethylene ( as top stream ) and ethanol and water as bottoms. The last (distillation) column gives an ethanol-water azeotrope (90 mol % ethanol) as the final product and water as waste. The recycle to purge ratio is 34.
The reaction is : C₂H₄(g) + H₂O(g) → C₂H₅OH (g)
For an azeotrope product rate of 500 mols/hr, the recycle gas flowrate in mols/hr is

A

30

B

420

C

1020

D

1500

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2

2008 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2008

Carbon black is produced by decomposition of methane:
\( CH_{4(g)} \rightarrow C_{(s)} + 2H_{2(g)} \)
The single pass conversion of methane is 60%. If fresh feed is pure methane and 25% of the methane exiting the reactor is recycled, then the molar ratio of fresh feed stream to recycle stream is

A

0.9

B

9

C

10

D

90

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3

2009 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2009

Pure water (stream W) is to be obtained from a feed containing 5 wt % salt using a desalination unit as shown below.

If the overall recovery of pure water (through stream W) is 0.75 kg/kg feed, then the recycle ratio (R/F) is

A

0.25

B

0.5

C

0.75

D

1.0

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4

2011 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2011

Ammonia is synthesised at 200 bar and 773 K by the reaction \( N_2 + 3H_2 \rightleftharpoons 2NH_3 \). The yield of ammonia is 0.45 mol/mol of ammonia. Flow sheet for the process (along with available compositions) is shown below. The single pass conversion for \( H_2 \) in the reactor is 20 %. The amount of \( H_2 \) lost in the purge as a PERCENTAGE of \( H_2 \) in fresh feed is

A

10

B

20

C

45

D

55

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5

2012 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2012

The reaction \(A_{(l)} + B_{(g)} \rightarrow C_{(l)} + D_{(g)}\) is carried out in a reactor followed by a separator as shown below. Notation: Molar flow rate of fresh B is \(F_{FB}\), Molar flow rate of A is \(F_A\), Molar flow rate of recycle gas is \(F_{RG}\), Mole fraction of B in recycle gas is \(Y_{RB}\), Molar flow rate of purge gas is \(F_{PG}\), Molar flow rate of C is \(F_C\). Here, \(F_{FB} = 2\) mol/s, \(F_A = 1\) mol/s, \(F_B/F_A = 5\) and A is completely converted. If \(Y_{RB} = 0.3\), the ratio of recycle gas to purge gas (\(F_{RG}/F_{PG}\)) is

A

2

B

5

C

7

D

10

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6

2012 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2012

If the ratio of recycle gas to purge gas (\(F_{RG}/F_{PG}\)) is 4 then \(Y_{RB}\) is

A

3/8

B

2/5

C

1/2

D

3/4

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7

2013 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2013

A fraction f of the feed is bypassed and mixed with the permeate to obtain treated water having a fluoride concentration of 1 mg/L. Here also the flow rate of the reject stream is 60% of the flow rate entering the reverse osmosis unit (after the bypass). The value of f , up to 2 digits after the decimal point, is

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8

2014 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2014

Two elemental gases (A and B) are reacting to form a liquid (C) in a steady state process as per the reaction A + B → C. The single-pass conversion of the reaction is only 20% and hence recycle is used. The product is separated completely in pure form. The fresh feed has 49 mol% of A and B each along with 2 mol% impurities. The maximum allowable impurities in the recycle stream is 20 mol%. The amount of purge stream (in moles) per 100 moles of the fresh feed is __________

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9

2020 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2020

A feed stream containing pure species L flows into a reactor, where L is partly converted to M as shown in the figure.

The mass flow rate of the recycle stream is 20% of that of the product stream. The overall conversion of L (based on mass units) in the process is 30%. Assuming steady state operation, the one-pass conversion of L (based on mass units) through the reactor is

A

34.2%

B

30%

C

26.3%

D

23.8%

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10

2024 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2024

Consider the process in the figure for manufacturing \( B \). The feed to the process is 90 mol% \( A \) and a close-boiling inert component \( I \). At a particular steady-state:
• \( B \) product rate is 100 kmol h⁻¹
• Single-pass conversion of \( A \) in the reactor is 50%
• Recycle-to-purge stream flow ratio is 10
The flow rate of \( A \) in the purge stream in kmol h⁻¹, rounded off to 1 decimal place, is ________

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11

2026 · Chemical Engineering · Process Calculations · Recycle, Bypass and Purge

Chemical Engineering (CH) 2026

Cyclohexane (C₆H₁₂) is produced from benzene (C₆H₆) and hydrogen (H₂) according to the following reaction.
C₆H₆ + 3H₂ → C₆H₁₂
Consider the process flow diagram shown in the figure.
In this process, the overall and single-pass conversions are 90% and 25%, respectively. The recycle stream contains 25 mol % C₆H₆ and 75 mol % H₂. If the fresh feed contains 25% excess H₂, at steady state, the ratio of the molar flow rate of recycle stream to that of the fresh feed is ______ (rounded off to one decimal place).

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