CHEMICAL ENGINEERING ASSESSMENT BRIEF 2025-26

Core Information

CENG0010 SEPARATION PROCESSES I

Project

A mixture of methanol and propanol is to be separated in a tray distillation column. You are required to provide a report detailing the preliminary design of a distillation column that can meet the specifications given below.

Please note that emphasis will be placed on the presentation of your report with a max page limit 10 pages. Any Aspen printouts included are to be placed in appendix at the end of the report and not in the main report. The final Aspen input file must also be submitted in a separate submission link on Moodle.

Specifications:

Part A: Distillation [50 marks]

Q1. Hand-calculations (Equations with worked-out solution and explanation)

a) Use the Fenske-Underwood-Gilliland (FUG) method and the constant average relative volatility from data given in Table 1 to find the number of theoretical stages, N, as well as the feed point location, NF, using Kirkbride’s method, which is required to separate the mixture in a tray column assuming a total condenser and  a  kettle  reboiler,  using  R/Rmin    =  1.4  and  operating  pressure  of  1.5  atm.  Briefly  comment  on  the assumptions employed and the accuracy of your calculations for the design of this column.

(Calculation hint: Note that FUG equations for binary systems can be found in reference textbooks, e.g. Coulson & Richardson, Vol 2 [1]. Quote the literature sources foryour equations (not lecture notes). [20]

Q2. Computer aided design using AspenPlus

b) Using Aspen and shortcut calculations (DSTWU Model), find the number of theoretical stages N for the given specifications. Comment on the column pressure and inlet temperature and how it affects the performance of separation.

Present  the  column   design  estimates   in  the  main   report,  i.e.  your   FUG  hand-calculations  and  the corresponding set of results obtained using Aspen, in a tabulated form. for Nmin, N, NF, Rmin and R.

Compare the estimates and briefly discuss the reasons for any differences on the design between hand calculations and simulation. [10]

c) Repeat your simulation with a more rigorous column model (RadFrac) with the estimates of N, NF and R found from the shortcut  model  (DSTWU),  using  the experimentally found tray  efficiency from your distillation experiments. Comment on how the design results change and why. If your design specification is not met, provide a suitable alternative design that meets them.

Compare the predicted separation performance between the two Aspen methods (not the hand calculations) and again briefly discuss the differences on the design results. [20]

Table 1 Isobaric VLE data for the binary system methanol – propanol from experiments (P=1 atm)

Part B: Absorption [25 marks]

A gas stream consists of 95 mol% N2  and 5 mol% CO2. The CO2  is to be removed in an absorption column operated at 10 bar and 5 oC using pure water as the absorbent, and 95% of the CO2  is to be absorbed. Because of      internal       cooling       coils,       the       operation       can       be       assumed       to       be       isothermal. The Henry’s coefficient of CO2  in water at the operating conditions is 800 bar.

a) Determine the equilibrium coefficient K in the equation below based on the data given

y* = K x

where y* and x are   mole   fractions   of   CO2     in   vapour   and   liquid   at   equilibrium,   respectively.

In the following, assume that mole ratio can be used instead of mole fractions, i.e. K remains the same in Y* = K X

where Y* and X are mole ratios of CO2  in vapour and liquid at equilibrium, respectively.

Find the required solute free solvent flow rate L, when L = 1.2 Lmin where Lmin is the minimum solute free solvent flow rate, as well as the outlet concentration of CO2  in water (as mole fraction). [10]

b) Above it was assumed that the equilibrium conditions were linear, i.e. K remained the same for mole ratios as for mole fractions. Figure B1 gives the equilibrium diagram for CO2  in water for linear conditions and for actual conditions that shows that this assumption is incorrect. Discuss briefly what impact this assumption is likely to have had on your results. [5]

c)  In the laboratory experiment, you investigated the pressure drop and flow rate characteristics of packed columns by varying air and water loading. How did the pressure drop (ΔP) change with increasing air flow rate (Q) during the experiment? How did you identify the loading and flooding points in the packed columns? How do the loading and flooding points affect the performance of packed columns? [10]

Figure B1. Equilibrium data for CO2 in water at 10 bar.

Part C: Liquid-liquid extraction [25 marks]

A feed of 10 kg min-1  of a 1 wt% mixture of acetic acid in water is to be extracted with pure benzene at         1 atm and 25 oC in a counter-current extraction column. The maximum amount permissible in the outlet water is 0.1 wt% acetic acid. A schematic of the column is given in Fig. C1, the equilibrium information for extraction in Figure C2 and relevant data are given in Table 2 and Table 3.

a)  Set  up  the  material  balance  for  acetic  acid  over  the  column.  List  all  your  assumptions  clearly. A solvent feed stream of 400 kg min-1  has been proposed. What will the concentration of acetic acid be in the outlet solvent stream if this quantity of solvent is used? [7]

b)  With the solvent flow rate of 400 kg min-1 , a further suggestion is to operate the separation in an existing extraction column consisting of 4 theoretical separation stages. Would you recommend the use of this column and this solvent flow rate? Explain your answer briefly. [7]

c)   It  has been suggested to operate the separation at a different temperature.  Do you agree with this suggestion, and if yes, what temperature would you recommend? Explain your answer briefly. [4]

d)   In the laboratory experiment you extracted ethanol from an oil mixture that had 10% ethanol (by weight) using water as solvent. What challenges were associated with using oil in this experiment? Are there any sustainability, ethical or safety considerations in selecting these materials? [7]

Table 2. Densities at 1 atm and 25 0C.

Table 3. Distribution coefficients for immiscible extraction.

Figure C1. Schematic of the extraction column.

Figure C2. Equilibrium line for acetic acid (A) in water (B) with benzene (C) as solvent.

References:

1.   Coulson, J.M. and J.F. Richardson, Chemical Engineering, Butterworth Heinemann, Vol. 2, 5th  ed., 2002.