Membrane and Desalination Technologies - TCE Moodle Website

304 J. Paul Chen et al.
a Feed b
Membrane
module
Permeate
As cross-flow mode is preferred over a dead-end one, the proper choice of the module is
the next crucial step. For a given module design and feed solution, the cross-flow velocity is
the main parameter that determines mass transfer in the module. Four cross-flow operations
shown in Fig. 7.17 are used:
1. Co-current
2. Counter-current
3. Cross-flow with perfect permeate mixing
4. Perfect mixing
Feed
Membrane module
Permeate
Dead-end operation Cross-flow operation
Fig. 7.16. Dead-end and cross-flow operation.
Retentate
In co- and counter-current operations, the feed and permeate streams flow co-currently or
counter-currently along the membrane. In the cross-flow mode, it is assumed that mixing
occurs so rapidly on the permeate side that the composition distributes equally. Among these
modes, counter-current flow gives the best results and the perfect mixing gives the worst
result.
In practice, systems usually operate in the cross-flow mode with perfect mixing on the
permeate side. In general, two basic flow schemes can be used in the system design (1) a
single-pass system and (2) a recycling system, as shown in Fig. 7.18.
In the single-pass system, the feed stream passes through the system only once, and there is
no recycling. In a recycling system, a recirculation pump is used to recycle the retentate
stream. For small-scale applications, a batch system can be used, as shown in Fig. 7.19.
Usually, a single-stage operation does not give the desired product quality, and a second
stage is required. A combination of stages is called a cascade. In a cascade operation, a large
number of units are used, and the permeate of the first stage is the feed for the second stage
and so on. An example of a two-stage operation process is shown in Fig. 7.20. When more
stages are required, the optimization of the process becomes very complex and difficult.
6.3.2. Nanofiltration and Reverse Osmosis
A typical NF or RO unit includes raw water pumps, pre-treatment, membrane modules,
disinfection units, storage, and distribution elements (3). The principal design considerations
for NF and RO units are: