Membrane Process design and operation

As the water flows along the length of a module the overall conversion is increased until there may come a point at which the element is running well below capacity. For example if, in Fig. 2.19, the conversion is 16% per element then the flow in stream 8 will be (1 - N 0.5 that of the feed. Under such circumstances, it is advantageous to combine streams from modules operating in parallel in order to maximise utilisation of the membrane elements, such that an array of modules is produced (Fig. 2.20). This is known as staging, and is very common in the RO treatment of thin to light brackish waters where the retentate solution osmotic pressure does not become excessive through its concentration along the length of the module.

It is not necessarily feasible or desirable for the retentate to be staged in order to maximise the overall recovery. For high-salinity feedwaters, typically seawater where osmotic pressures in the region of tens of bar prevail, further concentration of these waters would demand uneconomically large operating pressures. For these and other waters permeate staging is employed, and the retentate from the second stage is returned to thc feed of the first stage to produce a so-called twin pass system (Fig. 2.21). Twin pass systems also find use in highpurity water production.

Staging in electrodialysis differs from reverse osmosis in that further desalting of a solution that has already been desalinated by a maximum of 50% for a single passage through the stack, as constrained by the limiting condition imposed by depletion polarisation (Section 2.3.2), demands either increasing the volumetric flow, and/or reducing the current passed. Since the current passed is directly proportional to the total equivalent amount of ions transferred (Section 2.4.2), the simplest way of achieving additional desalination is by simply directing the product from the first passage through half the number of cell pairs in the second passage, thereby doubling the flow rate whilst leaving the current constant. This is known as hydraulic staging (Fig. 2.22). If further desalination is required, it may become expedient to reduce the current across the stack. This is referred to as electrical staging. It is normal to combine hydraulic with electrical staging to achieve reasonable desalination levels.



Specific energy demand Pressure-driven processes The energy consumption of any pressure-driven process operating continuously is given by product of the pressure and flow, thus the total energy consumption per unit mass of permeate product, ignoring pumping efficiency, is given by:

where p is the permeate density, Wel the supplementary plant electrical energy consumption (e.g. for instrumentation and aeration or other turbulence promotion methods) and M the mass flow of permeate. CAP represents the sum of the individual pressure changes, which can include the pressure drop across the membrane APm, as given by Equation (2.5), the concentrate-permeate osmotic pressure difference All*. from Equation (2.9), the hydraulic losses APlosses associated with forcing the retentate through the membrane channels and the pressure derived from the various contributions to membrane fouling. In the case of reverse osmosis, the total pressure for an individual membrane module is given by:

where Q, is the feed flow rate to the module and fouling is ignored. In Equation (2.24), the pressure losses per unit flow through the membrane channels on the retentate side are represented by dP/dQ. This factor can be estimated from the Hagen-Poiseulle equation, but is normally obtainable from the membrane supplier. An allowance can be made for fouling through an empirical correction factor. The rate of fouling in most reverse osmosis applications is normally low, since fouling is routinely suppressed by appropriate pretreatment based on chemical addition (Section 2.4.3). In the case of filtration, the osmotic pressure term (the last term in Equation (2.24)) does not apply but the effects of cake formation and fouling on operating pressure cannot be ignored. Large-scale dead-end filtration plant operates with a backflush cycle actuated either at fixed intervals or when the membrane permeability has decreased to some pre-identified level. Since the backflush invariably fails to entirely recover the original membrane permeability, a more rigorous cleaning cycle, usually involving aggressive chemicals, is initiated again either at fixed intervals or when once the permeability of the backflushed membrane has declined to some other level. The calculation of energy consumption per unit volume must therefore incorporate the effects of downtime, specifically the energy consumption of the backflush and duration of the backflush and cleaning cycles (Section 4.3.5). Since backflushing is normally at 3-4 times the forward-flow flux, the energy expenditure per unit time is commensurately higher.

Dead-end filtration plant may be operated at either constant pressure or at constant flux (Fig. 2 . 2 3 ) . In both cases trends in permeability decay over the backflush cycle, as reflected by flux decline at constant pressure operation (Fig. 2.23a) or pressure increase at constant flux operation (Fig. 2.23b), is exponential or pseudo-exponential, although the exact trend is dependent on the relative contributions of cake filtration, pore blocking and adsorption (Table 2.. For filtration of solids forming an incompressible cake, on the other hand, the trend is linear for constant flux operation. The trend in the residual membrane permeability, that is the permeability of the “permanently fouled� membrane as reflected in the trend in the minima for the backflush cycles in Fig. 2.22, also tends to be exponential or pseudo-exponential. Whether constant flux or constant pressure operation, the mean energy consumption relates to the mean membrane permeability over a cleaning cycle. Electrodialysis

In electrodialysis the total power consumption is the product of the voltage and the current. Since the voltage relates directly to the current by the resistance according to Ohm’s law, the specific energy demand is critically dependent upon the current and electrical resistance. The applied current i is directly proportional to the total equivalent quantity of counter-ions or co-ions extracted:

(2.25)

where Q is the flow through the stack, F is the Faraday constant, AC is the change in the diluate concentration in eq m-3 from inlet to outlet, N is the number of cell pairs (the number of desalinating or concentrating cells in the stack) and 6 is the current efficiency (normally 85-95%). The energy demand is then given by: E = i2R/OQ (2.26) where R is the overall electrical resistance in ohms, and relates to the cell pair resistance RCP: R = RcpN (2.27)

If scaling can be overcome and the path length can be extended by staging the principal constraint placed on the recovery 0 in Equation (2.26) is from electrodialysis reversal (see below). Because there is no osmotic pressure limitation in electrodialysis, and because the concentrate and diluate streams can be completely segregated, further efficiencies are obtained by recycling of the concentrate stream, which then decreases both the concentrate waste volume and the electrical resistance across the stack. For large-scale systems, recoveries in excess of 80% are normal. The cell pair electrical resistance is given by: (2.28)

where the subscripts conc and dil refer to the concentrate and diluate compartments and CEM and AEM refer to the cation and anion exchanging membranes. The concentrate resistance is normally no more than 20% of the diluate resistance and the resistance of the ion exchange membranes, although determined to some extent by the salt concentration, is normally low - between 2 and 10 f2 cm2 for most commercial ion exchange membranes in 0.5 eq mP3 NaCl (Strathmann, 1984). It therefore follows that the main contribution to the electrical resistance is from the diluate cell. The specific energy demand is increased, by 5-lo%, by electrodialysis reversal (EDR) operation. In this operational mode, which is very common in ED applications, the current is periodically reversed such that the concentrated stream becomes the diluate stream and vice versa. This adds to process complexity and reduces recovery but also virtually eliminates problems with scaling, since reversing the polarity of the electrodes produces a concomitant pH shift that suppresses the build-up of scale during the high-pH cathode electrode compartment.