Page 49 - Adsorbents fundamentals and applications

34 SORBENT SELECTION: CRITERIA

The CD step was first used by Union Carbide in the lsoSiv process in a Texas
PSA plant in 1961 (Avery and Lee, 1962; Keller, 1983; Cassidy and Holmes,
1984). The process separated 1,000 barrels per day of “natural gasoline” feed
into n-paraffins and branched/cyclic hydrocarbons. The feed contained 54.4%
n-paraffins and 45.6% branched/cyclic hydrocarbons (Symoniak, 1980). The n-
paraffin (strong adsorptive) product purity was 95–98%, whereas the purity of
the “isomer” was 98–99% (Symoniak, 1980). Additional and larger lsoSiv plants
have been built since 1961. The CD step was mentioned, as part of more complex
PSA processes, in several patents that were all filed later than its first commercial
practice in 1961 (Kiyonaga, 1965; Wagner, 1969).

Pressure Equalization. The term “pressure equalization” (PE) refers to the
action by which the pressures in two interconnected beds are equalized. The main
purpose of the PE step is to conserve the mechanical energy that is contained in
the gas of the high-pressure bed. With the PE steps, the pressure in a regenerated
bed is increased in a sequence of steps by the gases admitted from other beds,
which are in various stages of depressurization. The energy reduction, as well as
other improvements resulting from the PE steps (to be discussed shortly in this
section), made large-scale PSA separations economically feasible.
The idea of pressure equalization was first suggested in a patent granted to
Marsh et al. in 1964 (Marsh et al., 1964). The process described in the patent
required an empty tank in addition to the two beds used in the Skarstrom cycle.
The tank was used to store a portion of the compressed gas from a saturated
bed, and the gas was used to later purge the same bed. The primary objective
was to recover the components contained in the compressed gas. The pressure
equalization step, as it is currently used in commercial processes, was disclosed in
the patents to Berlin (1966) and Wagner (1969). Four- to five-bed arrangements
were given by Wagner. No empty tanks are required in this process.
Besides energy conservation and increased product recovery, the flow of the
strong adsorptive product is “smoothed” by the pressure equalization steps. The
four-bed process has been subsequently developed into the Polybed process,
consisting of 9 to 10 beds, which is successfully used for large-scale production
of high-purity hydrogen.
Pressure equalization is accomplished by connecting the ends of two beds.
It is natural to use the CD step of one bed to re-pressurize another bed that
has been purged. The problem with the EQ step is that the concentration of the
unwanted component in the effluent from the CD step is increasing with time. The
“cleanest” portion enters the bed that is to be re-pressurized first. Consequently
the concentration profile in the re-pressurized bed is in the wrong direction, i.e.,
with the feed end the cleanest and the discharge end the dirtiest. This prevents the
production of a clean product during the ensuing cycle. To alleviate this problem,
multibeds are used to accommodate more than one PE step (Yang, 1987). In the
multibed PSA (or “Polybed” PSA of Union Carbide), typically three PE steps are
used. The effluent from the CD step is divided into three portions, with the first
portion fed to the bed that is in its third stage of repressurization (Fuderer and