The specific energy consumption of crossflow filtration system is widely variable depending on how well the internal pressure drop is controlled. The pressure drop in tubular membrane is largely unavoidable and mostly given based on liquid velocity and the diameter/length of the membrane. But the pressure drops in headers, turns, connectors, etc. have rooms to be minimized by using single tube membranes, reducing the number of connections among tubes, maintaining a constant ID in the entire liquid channels, making turns smooth, etc.
Fig. 1 shows a typical tubular membrane system, where feed water is distributed to multi-tubes from the inlet header and the retentate (or concentrate) are merged again in the outlet headers. Due to the changes in channel diameters, where velocity head converts to pressure head back and forth and flow pattern changes sharply, pressure loss in the headers is significant. Therefore the total length of membrane channel hardly exceeds 10-15 m while TMP in inlet is controlled at a reasonable level (3-7 bar). In other words, the high operating pressure of tubular membrane is required not for obtaining a desired flux, but for keeping the flow move at a desired velocity at long channels.
The number of connections can be largely reduced by using longer tubes. The pressure drop in headers can be also prevented by using single tubes to from a water channel with a uniform diameter from the entrance to the exit. In this case, feed water splits right after feed pump, flows through multiple flow channels, and merges again right before the pressure control valve. For instance some of Koch ABCOR® tubular modules consist of single tubes with an ID of 2.54 cm (or 1 inch) and a length of 3 meters. All the tubes are connected either straight couplings or U-tubes when flow return is required. Most importantly channel diameter is kept constant at 2.54 cm from the entrance to the exit seamlessly. If larger surface area is required, more channels are added in parallel. As a result, the channels share only the entrance and exit in the entire streamline. Due to the constant channel size throughout the channels, longitudinal pressure drop can be kept remarkably lower than that with multi-tube modules.
Fig. 1.Membrane module with multiple tubular membranes (or multi-tube module)
© Seong Hoon Yoon