Membranes - Principles of Chemical Separations with Environmental Applications - page 245

Table 9.2 Existing and emerging applications for membrane vapor recovery.

Existing Emerging

Gasoline vapor recovery Recovery of feedstock from

oxidation reactor vents

Recovery of vinyl chloride Recovery of olefins from

monomer from PVC reactor vents reactor purge gas streams

Recovery of CFCs from process Recovery of gasoline vapor from

vent and transfer operations gas station storage tanks

Recovery of olefins from resin purge
bin off-gas in polyolefin production

In the spiral-wound mounting, a porous hollow tube is spirally wrapped with a porous
sheet for the feed flow, and a membrane sheet and a porous sheet for the product flow to
give a spiral sandwich-type wrapping. The spiral module is encased in a pressure vessel,
and the feed flow through the porous sheet is in an axial direction to the porous tube. As
the feed flow passes through the porous sheet, a portion of the flow passes through the
membrane into the porous sheet for the product. From there, the product flows spirally to
the porous center tube. The retentate stream is discharged from the downstream end of
the porous sheet for the feed flow.
Single membrane units can be evaluated based on their geometry and operation con-
ditions. Zolandz and Fleming [4] provide a good description for gas permeation systems
and models for design purposes. Seader [5] discusses the use of cascades (or staging) for
various series and/or parallel sets of membrane modules.

9.9 Membrane processes

The membrane processes described in this chapter are summarized in Table 9.2 [7], and
will be discussed in more detail later in this chapter.

9.9.1 Gas separations and vapor permeation

For the separation of gas mixtures (permanent gases and/or condensable vapors) where
the feed and permeate streams are both gas phase, the driving force across the membrane
is the partial pressure difference. The membrane is typically a dense film and the transport
mechanism is sorption-diffusion. The “dual-mode” transport model is typically used with
polymer materials that are below their glass transition temperature.
The ratio of the permeate to feed flowrate is called the cut. For a given feed flowrate,
the cut increases as the membrane area increases. The selectivity decreases as the cut
increases. So, there is a tradeoff between productivity and selectivity in the design of a
membrane unit for this application. This is illustrated in the Robeson log-log plot for