How do refineries calculate Height of Equivalent Theoritical Plate (HETP) for distillation column ?

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: