Typically, refineries don’t calculate it themselves but get the companies selling them the packing to guarantee performance. Prior to that, they or the company contracted to do their construction, would come up with an estimate. There is no simple “grand unifying theory” for HETP.
For
the design of packed distillation columns it is simpler to treat the separation
as a staged process.
Use
the concept of the height of an equivalent equilibrium stage or HETP (Height
Equivalent to theoretical plate) to convert the number of ideal stages required to a height of packing.
HETP
is the height of packing that will give the same separation as an equilibrium
stage.
Eckert
(1975) has shown that in distillation the HETP for a given type and size of
packing is essentially constant, and independent
of the system physical properties;
providing good liquid distribution is maintained and the pressure drop is at
least above 17 mm water per meter of packing height.
Following
values for Pall rings can be used to make an approximate estimate of the bed
height required
Size
mm HETP, m
25 (1 in.) 0.4 - 0.5
38 (112in.) 0.6 - 0.75
50 (2 in.) 0.75 - 1.0
•
The HETP for saddle packings will be similar to that for Pall rings providing
the pressure drop is at least 29 mm per m.
•
The HETP for Raschig
rings will be higher than those for Pall rings or saddles, and the values given
above will only apply at an appreciably higher pressure drop, greater than 42
mm per m.
The
relationship between transfer units and the height of an equivalent theoretical
plate, HETP is given by:
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