Lead membrane modules

Shims Shims are used to prevent modules from moving back and forth during pressurization and depressurization. Such movement could wear on the internal O-ring seals. Shims are plastic spacer rings similar to washers. They are typically 0.20-inches thick, and can be purchased from the manufacturer of the pressure vessel or fashioned from polyvinylchloride (PVC) pipe. Shims fashioned from PVC pipe must be cut parallel and free of burrs to work correctly. They are installed between the face of the lead membrane module and the adapter hub (see Figure 6.16) after all the... 

Figure 6.16 Placement of shims between the lead membrane module and the adapter hub. Courtesy of Dow Water and Process Solutions.

Pumps should be started slowly to prevent water hammer (a surge resulting from a sudden change in liquid velocity). Water hammer can cause cracks in the outer shell of the membrane modules as well as compaction of the membrane itself (compaction results in lower flux through the membrane at constant pressure). Also, water hammer causes the membrane modules to move in the vessel, which can cause wear to the O-rings used on standard interconnectors and lead to leaks of feed water into the permeate (see Chapter 4.3.3). An increase in pressure of no more than 10 psi per second is recommended.3 Some motors may be equipped with a "soft start" that regulates the speed with which they start up. Other considerations to minimize water hammer include ... 

The energy lost from pressurized feed water is absorbed by the membrane module materials, which can cause the materials to shift within the module when the degree of fouling or scaling is severe. This can lead to telescoping of the membrane leaves, resulting in... 

A number of factors can lead to high pressure drop, including membrane scaling, colloidal fouling, and microbial fouling. These three factors all involve deposition of material onto the surface of the membrane as well as onto components of the membrane module, such as the feed channel spacer. This causes a disruption in the flow pattern through the membrane module, which, in turn, leads to frictional pressure losses or an increase in pressure drop. 

High pressure drop causes disruptions to the system hydraulics. Because of the high pressure drop, the lead membranes tend to operate at very high fluxes while the lag membranes operate at low flux. This increases the rate of membrane fouling for both the lead and lag membranes. Lead membranes foul faster because more water is forced to the membrane module faster and the rate of contaminant accumulation in the boundary layer on the membrane surface increases. The lag membranes, on the other hand, experience low flows since most of the water is removed through... 

Shims are installed at the feed end of the module/pressure vessel assembly (refer to figures 4.18 and 4.19). Because pressure vessels are constructed with slight variations in length (known as "freeboard"), membrane modules can slide during pressurization and depressurization. Shims are installed between the face of the lead module and the adapter hub to prevent this motion. Membrane modules should be pushed completely against the thrust ring prior to installation of the shims. Shims are washer-like plastic rings that may be purchased from the pressure vessel manufacturer or fashioned out of PVC (must be free of burrs and be cut parallel to work properly). 

Membrane modules can be operated in the dead-end or cross-flow modes (see Figure 18.8). Dead-end ultrafiltration is used mostly for laboratory-scale applications and industrial ultrafiltration processes are usually carried out in the cross-flow mode. The main advantage of cross-flow ultrafiltration is the lower extent of concentration polarization. The cross-flow mode also allows recirculation of the retentate stream to the feed tank followed by its mixing with fresh feed that leads to several operational advantages. 

Because polymeric membranes are operated at near-ambient temperatures, matching the coefficient of thermal expansion (CTE) of materials used to construct the membrane module is not so critical. However, ceramic and metal membranes that will be operated at several hundred degrees may experience unacceptable strain, leading to failure, due to mismatched CTE. A simple example is a stainless steel... 

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