Question

Problem 4.20: A 100 kmol/h stream that is 97 mol% carbon tetrachloride (CCl₄) and 3% carbon disulfide (CS₂) is to be recovered from the bottom of a distillation column. The feed to the column is 16 mol% CS₂ and 84% CCl₄, and 2% of the CCl₄ entering the column is contained in the overhead stream leaving the top of the column.

(a.) Draw and label a flowchart of the process and do the degree-of-freedom analysis.

(b.) Calculate the mass and mole fractions of CCl₄ in the overhead stream, and determine the molar flow rates of CCl₄ and CS₂ in the overhead and feed streams.

(c) Suppose the overhead stream is analyzed and the mole fraction of CS₂ is found to be significantly lower than the value calculated in Part (b). List as many reasons as you can for the discrepancy, including possible violations of assumptions made in Part (b).

Answer 1

(c) This can be possible due (i) to difference between vapor and liquid dynamics inside the column which contribute to changes in composition of vapor (ii) those operating near critical pressure of the light key produce an immediate change in the vapor product (iii) feed tray location and number of stages inside the column. (iv) hydraulic response of the tray