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16CGA002 stagewise processes Assignment

Loughborough University
STAGEWISE PROCESSES 
16CGA002

Q1 Solute (A) should be extracted from the aqueous solution by the organic solvent (S). The organic solvent (S) and H₂O are insoluble in each other. The feed aqueous solution consists of 20 kg of solute (A) and 100 kg of H₂O. 100 kg of the organic solvent (S) is available for the extraction process. The equilibrium distribution for the solute (A) in organic solvent (S) and H₂O is given by:

Y = 2.5X

where Y = kg A/kg S and X = kg A/kg pure water

When answering each question, sketch the process, define all variables and label all inlet and outlet streams. 

• If a single equilibrium stage is used, how much solute (A) will be extracted? Use an analytical approach to obtain the solution. [3 marks]

• Consider a three cross-current stage system. If 20 kg of solvent is used on the first stage, 30 kg of solvent on the second stage and 50 kg on the third stage, calculate how much solute (A) will be extracted on each stage. Determine the percentage removal (extraction) of solute that can be achieved by the system and state whether it satisfies a 90% extraction criterion. How can the separation be improved? Use an analytical approach to obtain the solution.[10 marks]
• How many equilibrium stages connected in counter-current mode are required to achieve 96% removal of solute using 80 kg of solvent? Show stage-by-stage calculations (either graphically or analytically) and write down the compositions of the extract and raffinate streams leaving each stage. Sketch the system, label all the streams and define all variables. Calculate how much total solute will be extracted and how much is left in the raffinate? [12 marks]

Q2 Bromine (Br₂) is to be extracted from water (H₂O) by the solvent carbon tetrachloride

(CCl₄). CCl₄ and H₂O are insoluble in each other. The feed solution consists of 25 kg of Br₂ and 100 kg of H₂O. 150 kg of CCl₄ is available for the extraction process. The equilibrium distribution for Br₂ between CCl₄ and H₂O is given in the table below.

Equilibrium data for bromine distribution in water and CCl₄.

where X ≡ kg Br₂/kg H₂O; Y ≡ kg Br₂/kg CCl₄

When answering each question, sketch the process, define all variables and label all inlet and outlet streams. 

• If a single equilibrium stage is used, how much bromine will be extracted? Use a graphical approach to obtain the solution. [3 marks]

• Consider three cross-current stages employing 50 kg of CCl₄ in each stage. Calculate how much total Br₂ will be extracted. Use a graphical approach to obtain the solution.
• It is desired to absorb 90% of acetone from a gas (acetone/air) in a counter- current stage tower. The total inlet gas flow to the tower is 30.3 kg h^-1

(containing 0.3 kg h^-1 of acetone) and the total inlet pure water flow to be used to absorb acetone is 90 kg h^-1. The process is to operate isothermally at 300 K and absolute pressure of 101.3 kPa. Consider the system to be dilute. The equilibrium distribution for the acetone (A) in the gas-liquid is given by:

Y_A = 2.53X_A

where X = kg A/kg H₂O; Y = kg A/kg air.

Determine the number of theoretical stages required for this separation and write down the compositions of the gas and liquid streams leaving each stage.

Use a graphical approach to obtain the solution. [12 marks]

Q3 A counter-current gas absorption tower with 4 equilibrium stages is used to absorb Br₂ from air using pure water. The entering water flow rate is 70 kmol h^-1 and the tower operates at an absolute pressure of 7 bar. The outlet gas flow rate is 5 kmol h^-1 and the outlet gas contains 0.01 mole fraction of Br₂. For this problem, you may assume that water and air are mutually insoluble. 

Hint: You will need to work on a solute-free basis.

• Sketch the absorption tower, labelling all streams and defining all variables.
• Use Henry’s law to find an equation linking the mole ratio of bromine in the gas phase to the mole ratio of bromine in the water phase.

• Calculate the inlet gas Br₂ mole fraction. Use an analytical approach to obtain the solution.

• Determine the concentration of bromine (Br₂) in the water stream leaving the tower in kmol h^-1.

Relevant Data

The Henry’s Law constant for Br₂/H₂O is 46.6 bar per mole fraction Br₂.

Q4 A counter-current gas absorption tower is used to absorb hydrogen sulphide from air using pure mono-isopropyl amine (MIPA). The tower operates at 20°C and a total pressure of 10 bar. The inlet gas flow rate is 20 mol h^-1 and the inlet gas contains 15% mol of hydrogen sulphide. The required outlet gas molar composition is 0.03 or less. For this problem, you may assume that MIPA and air are mutually insoluble. Hint: You will need to work on a solute-free basis. 

• Use Henry’s law to find an equation linking the mole ratio of hydrogen sulphide in the gas phase to the mole ratio of hydrogen sulphide in the solvent phase. How many equilibrium stages are needed for the separation if the entering MIPA flow rate is 20 mol h^-1? Use an analytical approach to obtain the solution.

• Determine the minimum MIPA flow rate to achieve the required separation (i.e. operating under pinch conditions).

Relevant Data

The Henry’s Law constant for H₂S/MIPA is 12.2 bar per mole fraction of H₂S.