A two-bed field molecular sieve system is used to dry a gas
(molar mass of 22) to a dew point of...

Question

A two-bed field molecular sieve system is used to dry a gas
(molar mass of 22) to a dew point of 40°F and 500 psia (4°C and 34 bara). Gas
to the unit is water saturated at 110°F (43°C) and 300 psia (21 bara) and the
flow rate is 80 MMscfd (89 × 103 Nm3/h). The loading averages 10% (by weight)
of water on each adsorbent bed, the superficial velocity in the adsorbent
vessel is 40 ft/min (0.20m/s). The dehydrator uses an adsorption cycle time of
8 h, and heats and cools in 4 h each. a. What mass of adsorbent is needed to
remove the water? b. Assuming ideal gas behavior, what are the dimensions of
the adsorbent bed? For the adsorbent system, assume the vessel wall thickness
is 2″ (50mm) and uses steel with a density of 500 lbm/ft3 (6.4kg/m3 ).
The heat capacity of the steel is 0.12 Btu/lbm-°F (0.5 kJ/kg-K). Ignore the
vessel heads but include 5 ft (1.5m) of extra shell for bed supports and
screens. Ignore heat losses to the surroundings and assume the heat of
adsorption is equal to the heat of vaporization of water (see Appendix B1 ). c.
What is the required regeneration duty to heat the bed to 450°F (230°C)? Assume
the regeneration gas heater has an efficiency of 75%. d. Compute the daily
energy cost for both heater fuel and compression (see Chapter 9 for
compression). The adsorbent system has a pressure drop of 15 psi (1 bar).
Utility costs are $0.04 per kWh for the compressor motor and $5.00 per MMBtu
($4.75/GJ) for fuel gas

Dr Shweta · Accepted Answer

Figure 1. Water phase in equilibrium with either ice or gas hydrate, depending upon the gas composition and pressure.
Table 1.

A two-bed field molecular sieve system is used to dry a gas (molar mass of 22) to a dew point of 40°F and 500 psia (4°C and 34 bara). Gas to the unit is water saturated at 110°F (43°C) and 300 psia...

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