Page 284 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

286 Chapter 11 Distillation

parameter to be varied to minimise the annualised cost of the system. Increasing the reflux ratio
monotonically increases the operating cost while the annualised fixed cost passes through a minimum.
The reflux ratio corresponding to minimum total annualised cost (fixed þ operating) is the optimum
reflux ratio and the corresponding process details of the equipment, e.g., number of trays, height and
diameter of column, etc., is the optimum design. As ballpark estimates, optimum reflux ratio is 1.2 to
1.35 times minimum reflux ratio (R m ). For refrigerated systems, R opt ¼ 1.1 to 1.2 times R m can be
used. Use of shortcut design procedures, approximate empirical correlations, etc., are often used in lieu
of detailed design at this stage as long as the accuracy of these are better than the accuracy of cost
estimation. Once the optimum is fixed, the detailed design of the system for the optimum case is
generated using rigorous process design calculations. If the annualised fixed cost recalculated for the
detailed design is within 5%e10% of the initial estimate, the design may be taken as the optimum
design.

11.3 Detailed design

Deliverable from conceptual design is the gross details of the optimum process option. This includes
features/facilities for safe start-up, normal operation, shut down and any special requirement during
plant maintenance like flushing or cleaning with flushing oil, etc. Instrumentation and control scheme
and the plant hydraulics are also covered.
The optimum design option is detailed based on input design specifications and applicable design
codes.
Process design
Process design starts with defining the battery limit. The battery limit is set depending on the scope
of the design problem posed. Stream conditions at the battery limit are firmed up, the process func-
tionality targets and constraints are identified and documented. Examples of such a target and con-
straints can be fractionating a feed to three streams of specified quality and availability of steam and
cooling water as hot and cold utility at specified pressure and temperature, respectively.
The deliverable in this phase, for a distillation plant, is the ‘process package document’ that in-
cludes a brief process description, start-up and shutdown procedures, emergency handling notes,
potential/approved vendors for the equipment and any other specific information. It includes BFD/
PFD, P&ID, general arrangement drawing for the column and auxiliary equipment and summarises the
salient features of the distillation system. The column configuration, dimensions and other details are
schematically represented at this stage. The scope of design may however remain limited only to the
design of the column and its auxiliaries, i.e., condenser and reboiler, the associated piping, pumps,
instrumentation scheme and safety devices.
Process design generates the design operating parameters and gross specification of the equipment
including the tower internals. This forms the basis of generating the mechanical details.
Mechanical design
Detailing the mechanical design of the equipment, auxiliaries and the hydraulic circuits is carried
out by following the applicable code of practice as already discussed in Chapter 1. The output from this
step is the set of fabrication drawings for the equipment. In case of a complete distillation facility, it