Page 293 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.4 Fractionator 295

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condenser. Overhead condensers are typically designed to have 0.15e0.25 kg/cm pressure drop and
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the vapour line with w15 m/s vapour velocity culminating to 0.1e0.15 kg/cm pressure drop in
superatmospheric columns. Vapour line pressure drop is kept much lower in vacuum columns, e.g., in a
vacuum column with 80 mm Hg(abs.) column top pressure, the condenser may operate at around
70e75 mm Hg(abs.).
The liquid exit temperature attainable in the condenser depends on the coolant temperature. In
Indian condition, cooling water from cooling tower is available at around 33e35 C and is returned

back to the tower at 45e47 C. Accordingly, in a condenser cooled by cooling water, the temperature

of the exiting liquid shall be 53e55 C for a realistic minimum approach of around 20 C. The reflux

drum receives liquid from the condenser that is at its bubble point temperature. Therefore, the
bubble pressure of the liquid corresponding to the condenser exit temperature is the reflux drum
pressure. Adding the condenser pressure drop to the reflux drum pressure sets the condenser inlet
pressure. Column top pressure is arrived at by further adding to it the pressure drop in the overhead
vapour line.
However, in case of heat recovery from the overhead vapour, exchangers are installed in the
overhead vapour line and in such case, the pressure difference between the column top and reflux drum
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can be up to 0.8 kg/cm . Also in case of large columns, there can be more than one set of overhead
vapour line and condensers working in parallel. This is often seen in large vacuum distillation columns
where the pressure drop limits are lower as the column operating pressure is low and the vapour
volumetric flow rate is high due to the pressure being low. The overhead vapour line is sized for this
maximum allowable pressure drop.
Vapour flows through the packed beds/trays from the bottom to the top at a lower pressure. Once
the column (top) pressure is fixed, the feed zone and the bottom pressures are
estimated from the actual number of trays and/or the packed bed depth that are
arrived at through approximate calculations.
Pressure profile
The column pressure profile is required for detailed fractionation calcula-
tions. Thereafter, the column internals are designed (Chapter 14) ensuring that
the pressure profile in the column is close to that considered at this stage.
Table 11.3 shows the typical height equivalent to a theoretical plate (HETP) values for packing of
various nominal sizes. A first estimate of the required depth (h bed in m) of
packing for typical packed tower distillation systems can be arrived at by
multiplying HETP and the number of theoretical stages/plates. Depending on
Packed column
the gaseliquid traffic, ðDP=h bed Þ typically varies between 5 and 125 mm
water column per m of packing depth and this information can judiciously be
used to estimate the pressure drop across the active packed bed. However,
there are other components of pressure drop due to the presence of an extra top layer, redistributor(s),
etc., that may be added to improve the estimation. Detailed estimation and calculations related to
design of packed section is covered in Chapter 14.