Page 321 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.6 Flash distillation 323

to fix a temperature, compute the corresponding (V/F) splits by varying the drum pressure
between the bubble and dew pressure of feed and tabulating the splits that meet the specification
requirements. The design operating condition is chosen from the tabulated data, considering the
balance between the yield and the feasibility of adopting the corresponding design pressure and
temperature.
• Choice between the horizontal and the vertical drum e Horizontal drums provide a longer travel
path and also a larger cross section for vapour travel that reduces vapour velocity. This helps
separation of the phases. Vertical drums on the other hand occupy less floor space. Length to
diameter ratio of vertical drums is usually between 3 and 5. While sizing a vertical drum, if the
length/diameter estimate exceeds 5, a horizontal drum is designed.
• Demister pads e These are provided when the chance of liquid carryover is to be minimised.
Typical demister pads are 80e100 mm thick wire gauge pads and offer below 25 mm water
column pressure drop. These are bought out items. In horizontal drums the demister is provided
only for a section of the drum in the area below the vapour outlet nozzle. This is schematically
shown in Fig. 11.14 and 11.15.
• One may refer to Chapter 17 for sizing the features of the flash drum.

11.6.3 Design steps
The design steps consist of
(i) Deciding the design operating conditions e the drum temperature and pressure required to meet
the composition specification assuming equilibrium conditions (T V ¼ T L & P V ¼ P L ). This step
generates the flow rate and composition of the vapour and liquid streams leaving the drum,
preheat temperature requirement, heat load through preheat and/or through heating arrangement
integral to the drum.
(ii) Firming up the general arrangement of the process. This includes arrangement for preheating
the feed and/or heating arrangement for the drum, arrangements for controlling the feed rate,
drum temperature, pressure and liquid level. This generates the P&ID of the system.
(iii) Sizing of the drum is based on applicable design equations and industry practices to ensure safe
and trouble-free operation of the system. This includes decision on the drum configuration
(orientation) e vertical or horizontal; requirement of mist eliminator/demister pad to minimise
liquid entrainment in the vapour stream; sizing of feed inlet and vapour and liquid outlet
nozzles; location and sizing of nozzles for drain, vent, safety valve fixing, utility connections;
location and sizing of nozzles for instrumentation e temperature, pressure and level
measurements.
(iv) Detailed mechanical design of the drum and the associated system.
Sizing of the flash drum
Maximum allowable vapour velocity on the liquid surface is

1=2
L V L (11.35)
u V;max ¼ SF fðr r Þ=r g
3
where r and r are the density of liquid and vapour (kg/m ) and m L ; m V are the corresponding flow
L
V
rates (kg/sec).