Page 292 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

294 Chapter 11 Distillation

Table 11.2 Process design output e fractionator.
Process equipment parameters
Column
· Composition, pressure and temperature profile
· Flow rates, pressure, temperature, phase (V, L, V þ L) of feed, reflux, distillate and bottom product; vapour boil up
rate and noncondensable vapour stream rates
Trays
· Number of theoretical stages and efficiency, number of actual trays, feed tray location
· Type of tray, tray diameter and tray layout
· % flooding and pressure drop across tray
Packed sections
· NTU or number of theoretical stages/plates
· HTU or HETP
· Packed section (column) diameter, depth, % flooding at design condition and pressure drop
· Packing material and specifications
· Packing support and top cover arrangement
· Distributor/redistributor
Reboiler and condenser
· Type, pressure, temperature, heat duty
Cold and hot utility
· Flow rates, inflow and outflow conditions
Dimension-related parameters
Column
· Height, diameter, support type and height
· Nozzle sizes, elevation and orientation e all services (process fluid, sampling, instrumentation, utilities, vent,
drain, safety)
· Location of trays, packing support, packed section
· Design pressure drop for overhead line, vapour return from reboiler, etc.
· Location (elevation and orientation) of instrument tapping on column shell for measurement of temperature,
pressure, level
· Size and location of manholes

Column pressure
Conventionally the term operating pressure of a column refers to the pressure on the top tray. This
2
pressure needs to be kept steady since even a small pressure fluctuation of 0.01 kg/cm across a tray in a
2
1.5 m diameter column will generate force in the order of 0:01 ðp =4Þ ð1:5 100Þ z 177 kg, which
is sufficient to dislodge trays. Trays are designed to be loaded from top and usually fail due to pressure
surges below the tray. Pressure surges in packed columns are notorious for disarraying the packed bed.
General heuristic considerations relating to column pressure have already been presented under
conceptual design (Section 11.2).
During design, the column top pressure is estimated by esti-
Estimating column pressure mating the reflux drum pressure and then adding a small margin to
account for the pressure drop in the overhead vapour line and