Page 156 - Gas Purification 5E

144 Gas Purification

In equation 2-41, TREF is any suitably chosen reference temperature, and the specific heats
are averages for the temperatures between TW and the relevant process temperature.
Next, an enthalpy balance around the condenser is used to determine the composition and
temperature of the overhead stream just upstream of the condenser. The amount of water in
this stream is given by the following equation:

The subscript “SO refers to the stripper overhead, i.e., the stream leaving the regenerator
column. See Figure 2-95. TSo and several terms in this equation, which are functions of Tso,
are not known. Neglecting relatively small terms, equation 2-42 can be simplified to

{
WSO,H~O QRC + WA,H~O)~A (2-43)
=
}@SO
With an initial estimate of hso, a first approximation of Wso,H20 can be calculated by use
of equation 2-43. Steam tables can then be used to determine Tso and a more accurate value
of Ws0,H20 can be calculated by repeated direct substitution into equation 2-42. When deter-
mining Tso, about 2 psi should be allowed for the pressure drop through the condenser and
associated piping to the reflux drum.
When these calculations are completed, equipment sizing data can be generated from the
results. For example, the regenerator diameter is determined by conventional column design
correlations (see Chapter 1) based on the lean amine flowrate and the vapor flow generated
by the reboiler duty.

Commercial Plant Operating Data

The availability of operating data on a full size plant, operating under similar conditions,
is extremely helpful in the design of a new installation. Although many papers present infor-
mation on commercial plants, most do not provide complete and consistent sets of operating
data. This is particularly true for proprietary processes. However, an appreciable amount of
useful plant data is available for the commonly used amines. Obviously, only a small portion
of such material can be included in this text, and the reader is referred to the original publica-
tions for more complete information.

Design and operating data from two large MEA plants are summarized in Table 2-21.
Case A is a plant designed primarily to move H2S from 200 psig natural gas and produce a
sweet gas containing less than X grain per 100 scf. Although no data are given on C02
removal, it is expected that essentially all of the COz in the feed gas would also be absorbed
in a plant of this type. Case B is a plant intended to recover C02 from flue gas at atmospher-
ic pressure. Two 233 high-packed beds are used in a single tall absorber to provide efficient
C02 removal with less pressure drop than an equivalent tray column. Additional operating
data on an MEA plant using tray and packed towers are given in Table 2-15 in the previous
section of this chapter entitled “Tray Versus Packed Columns.”
Operating data from a large test unit used to absorb C02 from ammonia synthesis gas are
given in Table 2-22 (Butwell et al., 1979). The test runs were made to obtain data for the