Page 283 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.2 Conceptual design 285

• Batch distillation is usually best for separation of volatile product or volatile impurities.
• If purity is not a major concern for the lighter or the heavier stream, flash distillation is a cheap
alternative.
• Optimum based on economic calculations must be evaluated for all alternative configurations to
choose the best option. For this, all competing conceptual designs are compared based on some
chosen economic criterion. Selection and computation of the economic optimisation function is
based on available cost data. Shortcut design procedures are normally used to quickly estimate the
size and cost of different configuration options. The optimum for each competing configuration
option needs to be calculated separately and the best among those are selected for overall
comparison.
• Practical considerations like ease of operation, proven/reliable design, existence of a similar plant
in the same complex, ease of maintenance due to simplicity of design or common spare
equipment, etc., may override the computed economic optimum consideration in choosing plant
configuration.
• Separation by distillation at lower pressures is easier. The number of stages found by
McCabeeThiele construction for the same change of composition in the stripping or in the
enriching section is lower at lower pressure as the binary equilibrium (x-y diagram) curve shifts
further away from the 45 degree (x ¼ y) line with reduction in pressure. Reboiler and the
condenser temperatures are also lower at lower pressure. Hence, the lowest practical operating
pressure is used for design. On the other hand, deciding on column pressure lower than necessary
increases the volumetric vapour flow rate that may lead to a larger diameter (more expensive)
column.
• A practical distillation column will always have some leakages through flanges, fittings, valve
glands, etc. Thus, a vacuum column will suck in ambient air and pressurised column will bleed
vapour to the atmosphere, however small the quantity may be. In case of hydrocarbon columns,
accumulation of air inside results in potential explosion hazard. Even when distillation is feasible
at atmospheric pressure, columns handling hydrocarbons are designed and operated slightly
above atmospheric pressure. This practical positive gauge pressure may vary from few cm of
2
water column in small systems to 0.1e0.4 kg/cm (g).
• Vacuum columns are preferred mostly when the components have high boiling point and/or are
heat sensitive, i.e., they may crack, decompose, polymerise or get denatured. Operation under
vacuum allows the column to operate at lower temperature.
• The temperature of the bottom product may be limiting for heat-sensitive materials.
An example of optimisation can be the design of a continuous distillation facility to split a binary
component feed stream to an overhead and a bottom product. The design
needs to be done for a particular feed flow rate in order to meet the compo-
sition requirements of the product streams. Optimum design is obtained by
Optimum design
minimising the total annualised cost for the system. Annual operating cost for
the system includes the cost of cooling in the overhead condenser, reboiler
energy input and the energy for pumping the streams. The total annualised cost is arrived at by adding
to this term, the annual financial cost of the fixed capital investment required for the erection, fabri-
cation and commissioning of the system. The reflux ratio for the column is taken as the independent