Highlights 2019
tion pressure ∆pF according to equation (1) is negative. However, the filtration effectively occurs only in the lower part of the module (see fig. 10). Therefore, the effective average filtration pressure is higher.
Fig. 10: Schematic diagram of the +Flux process. Source: Jens Potreck, Pentair
Fig. 11: Progression of the transmembrane pressure over time in the +Flux process and a conventional filtration process. Source: Jens Potreck, Pentair
flow of filtrate to the feed side near the module outlet (see fig. 10). This backflow reduces the average filtrate flow rate for the entire module. With progressing filtration time, the height of the covering layer increases, too. To realise a constant filtrate flow rate, the pressure at the module inlet is adjusted (increased) accordingly, and the backflow vanishes. The backflow can be prevented entirely by lowering the fluid level on the filtrate side of the upright modules by applying a pressurised gas buffer at the beginning of the filtration process (see fig. 10). With progressing filtration time and increasing pressure at the module inlet, the fluid level on the filtrate side is adjusted (increased).
40
The average filtrate flow rate for the entire module is increased by the greater effective membrane area. The working principle of the +Flux process has been confirmed in laboratory tests using modules with a length of lM = 750 mm (see fig. 11). At the same controlled, constant filtrate flow rate, the average filtration pressure ∆pF according to equation (1) increases more slowly, before the maximum pressure at the module inlet is reached. Then the filtration process is interrupted and back-flushing is applied to the module. Thus, the +Flux process enables longer filtration times and back-flushing intervals. At the beginning of the filtration process the average filtra-
(1) The +Flux process enables longer filtration times and back-flushing intervals. When larger modules with a length of lM = 1500 mm are used, the increase of the filtration time is even greater. The process is already employed by several breweries of different sizes around the world since 2015. Daniel Janowitz from the STEP Consulting GmbH has reported on the recovery of magnesium oxide (MgO) from the concentrate of seawater desalination plants. The waste water (concentrate) of seawater desalination plants pollutes the environment. On the other hand, the contained salts constitute valuable reusable materials, e.g. magnesium as a material commonly employed in lightweight construction. The following recovery process is suggested for magnesium oxide: concentration of the saline solution, precipitation with sodium hydroxide (NaOH), filtration and washing of the precipitated magnesium hydroxide (Mg(OH)2), sintering to produce magnesium oxide (MgO). Early experiments of concentration by nanofiltration have shown the precipitation of a considerable amount of gypsum together with the magnesium hydroxide. The gypsum constitutes a serious impurity of the magnesium hydroxide. As an alternative, concentration in evaporation ponds has been suggested: In this process variant, the gypsum precipitates in the concentration stage already. Thus, in the subsequent precipitation stage, only small amounts of gypsum are produced together with the magnesium hydroxide and a higher purity is achieved. The operating costs are caused mainly by the sodium hydroxide employed for precipitation and the energy required for sintering. A feasibility study of the process has been conducted with actual sea water concentrate from the sea water desalination plant Shuwaikh (Kuwait). Reference literature: [1] Bell, C.M.: Comparison of polyelectrolyte coated PVDF membranes in thermopervaporation with porous hydrophobic membranes in membrane distillation using plate-and-frame modules Chemical Engineering and Processing: Process Intensification. 2016, 104, pp. 58–65.
F & S International Edition No. 20/2020