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distillation column design, Physics

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A mixture of methanol and water is to be separated in a new separation unit and you are required to provide a report detailing the design and costing of a distillation column with the specifications given below. The operating pressure is to be 1 atm and the feed temperature is 30° C.

Part A: Determining the Thermodynamic data

1. Compare

a) ideal VLE data obtained using Antoine''s equation with VLE data obtained using ChemCAD assuming

b) ideal mixture and

c) non-ideal mixture with

d) data available in reference literature, by using Excel to produce a single plot displaying all your results. The plot should be in placed in the main part of your report. All numerical values are to be tabulated clearly (and referred to from the main report) in an appendix in one table, i.e. x-values in the first column and then one column of y-values from each calculation method. (Separate hand-calculations, and tables for each method, are not required and should not be included.) Quote your literature sources clearly using proper referencing format (see lab. handbook).

2. Briefly discuss the findings with particular emphasis on the suitable of the thermodynamic model used in ChemCAD in predicting the thermodynamic data.

3. Make a clear recommendation of which method should be used and why.

Part B: Preliminary design

1. For the given specifications, find the distillate and bottom flow rates, as well as the light and heavy key splits into the distillate, for the column given the specifications above. (Make sure your mass balances over the column add up.)

2. Find an average relative volatility from the experimental VLE data obtained in Part A. (Make sure you do not emphasis some parts of the range more than others, i.e. by including relatively more points between 0 and 0.1 than in the rest of the range.)

3. Use the Fenske-Underwood-Gilliland (FUG) method assuming the constant average relative volatility to find the number of theoretical stages N required as well as the feed point location NF (Kirkbride''s method) to separate the mixture in a tray column assuming a total condenser and a kettle reboiler, using R/Rmin = 1.3. Quote the literature sources for your equations clearly (not ES lecture notes).

4. Using the short-cut column model (SHOR) in ChemCAD, find the number of theoretical stages N for the conditions given above assuming a) ideal mixture and b) non-ideal mixture. (Include a brief ChemCAD report in an appendix.) Present the three estimates (your FUG hand-calculations and the two sets of results obtained using ChemCAD, remembering how ChemCAD numbers its stages, as well as rounding rules) in a tabulated form for Nmin, N, NF, Rmin and R.

5. Compare the estimates (ideal vs non-ideal mixture and hand calculations vs using ChemCAD) and discuss the effect on the design (over/under design etc).

Part C: Detailed column design

1. Using the results from the short-cut calculations as a starting point, find the required number of stages in the column using the rigorous SCDS ChemCAD model, a kettle reboiler and a sub-cooled condenser (20° C sub-cooling). Your design should have a reasonable (you define and explain) trade-off between first

i) column height N vs heat input QR with a fixed relative NF and then

ii) feed location NF vs heat input QR for the N just found (optimising based on QR indirectly considers reflux ratio R so do not consider this separately). (When changing N, make sure you keep NF in its relative position, e.g. a third from the top.) Explain the reasoning behind your choices. (Present any supporting data as graphs in your report with corresponding tables in appendix.)

2. For your final design, report the temperature, pressure, total mass and total molar flow rates and mole fractions of the product streams (in a Stream Box) and submit your PFD in the main report. In addition, also report the number of stages, the feed location, the reflux ratio, the required heating and cooling duties (kW) in the report and comment on your findings (e.g. accuracy, chosen design following column optimisation, simple (i.e. SHOR) model vs. rigorous (i.e. SCDS) model but on the same basis etc.)

Part D: Column sizing and costing

1. Determine the size (overall height and diameter) and then the total purchased capital cost of the column section using the data provided below for a) sieve trays and b) random packing using ChemCAD. (Include a brief ChemCAD report in appendix for each of the two final designs.)

2. Calculate by hand the total column heights based on either the tray spacing and tray efficiency or the HETP. Tabulate the height with the column diameter and the purchased capital cost (in £) for each column alternative for easy comparison.

3. Present the relevant (you decide) technical and cost data (in £) for each alternative (but not in the form of ChemCAD printouts) and compare and discuss these.

4. Make a clear recommendation for which column type should be chosen and why.

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