Membrane Process Operation
The key elements of any membrane process relate to the influence of the
following parameters on the overall permeate flux are:
· The membrane permeability (k)
· The operational driving force per unit membrane area (Trans Membrane Pressure, TMP)
· The fouling and subsequent cleaning of the membrane surface.
- 1. Flux, Pressure, Permeability
-
The total permeate flow from a membrane system is given by following equation:
Qp = Fw*A
where, Qp is the permeate stream flowrate [kg·s-1], Fw is the water
flux rate [kg·m-2·s-1] and A is the membrane area [m2].
The permeability (k) [m·s-2·bar-1] of a membrane is given by the next equation:
K = Fw / PTMP
The trans-membrane pressure (TMP) is given by the following expression:
PTMP = (Pf+ Pc) / 2 - Pp
where, PTMP is the Trans-membrane Pressure [kPa], Pf the inlet pressure of feed stream [kPa];
Pc the pressure of concentrate stream [kPa]; Pp the pressure if permeate stream [kPa].
The rejection (r) could be defined as the amount of particles that have been removed from the feed-water.
r=(Cf - Cp) / Cf * 100
The corresponding mass balance equations are:
Qf = Qp + Qc
Qf * Cf = Qp * Cp + Qc * Cc
In order to control the operation of a membrane process, two modes, with respect to the flux and to the TMP
(Trans Membrane Pressure), can be used. These modes are: (1) constant TMP and (2) constant flux.
The operation modes will be affected when the rejected materials and particles in the retentate tend to accumulate in the membrane.
At a given TMP, the flux of water through the membrane will decrease and at a given flux, the TMP will increase, reducing the permeability (k).
This phenomenon is known as fouling, and it is the main limitation to membrane process operation.
- 2. Dead-End and Cross-Flow Operation Modes
-
Two operation modes for membranes can be used. These modes are:
2-1 Dead-End Filtration, where all the feed applied to the membrane passes through it,
obtaining a permeate. Since there is no concentrate stream, all the particles are retained in the membrane.
Raw feed-water is sometimes used to flush the accumulated material from the membrane surface.
2-2 Cross-Flow Filtration, where the feed water is pumped with a cross flow tangential
to the membrane and a concentrate and permeate streams are obtained. This model implies that for a flow
of feed-water across the membrane, only a fraction is converted to permeate product. This parameter
is termed ˇ°conversionˇ± or ˇ°recoveryˇ± (S). [S = Qpermeate / Qfeed = 1 - Qconcentrate / Qfeed]
The recovery will be reduced if the permeate is further used for maintaining processes operation, usually for membrane cleaning.
-
Filtration leads to an increase of the resistance against the flow. In the case of dead-end filtration process,
the resistance increases according to the thickness of the cake formed on the membrane.
As a consequence, the permeability (k) and the flux rapidly decrease, proportionally
to the solids concentration and, thus, requiring a periodic cleaning. For cross-flow filtration processes,
the deposition of material will continue until the forces of the binding cake to the membrane
will be balanced by the forces of the fluid. At this point, cross-flow filtration will reach a steady-state condition,
and thus, the flux will remain constant with time. Therefore, this configuration will demand less periodic cleaning.