Back-pulse filtration

Back-pulse Filtration using GORE-TEX Membrane Filter Cloths... 

The soluble ions of iron and aluminium are usually reduced to a minimum by adjusting the electrolyte pH. For the removal of solid iron hydroxide and aluminium hydroxide Bayer decided to use a new pre-coat-free brine purification technology -back-pulse filtration using GORE-TEX membrane filter cloths. 

Each filter has demonstrated the capacity to filter the full brine flow of 195 m3 h 1. The pressure drop through the filter medium is measured and monitored continuously. Typically, it is nearly constant over a 2-h filtration at 195 m3 h 1. Back-pulse cleaning restores the initial pressure drop from cycle to cycle, with only a slow increase over time. After 12 months running time, the initial pressure drop at the beginning of the filter cycle had increased by 0.6 bar. The filter membranes were chemically cleaned with 5 % hydrochloric acid. After a cleaning time of 2 h the filter was started again and the pressure drop was less than 0.1 bar greater than that of new filter socks. 

The operation cycle is a stepwise process. The essential steps are filling the vessel, filtration (service), back-pulse (cleaning), cake settling, and sludge discharge. In practice, the steps will vary in length from application to application, but all working operation sequences include these basic steps, which will be addressed in order. 

Fig. 22.3 Back-pulse filtration principles (a) filling step ...

The filtrate that is back-pulsed from the dome first expands the filter socks thereby cracking the cake. The movement of the socks and the subsequent liquid flow through them serves to dislodge the filter cake solids from the GORE-TEX membrane surface. Because of the expanded PTFE material of the membrane, the cake releases from... 

The final principles of back-pulse filter technology are the nature and properties of the GORE-TEX membrane. The membrane is composed of expanded polytetra-fluoroethylene, or e-PTFE. The membrane traces its roots to the invention of e-PTFE by Robert W. Gore in 1969. Since that time, e-PTFE has found application in many areas including medical devices, electronics, fabrics and fuel cells to name a few. In the filtration area, e-PTFE is used in the form of a membrane to capture and remove particles from both gaseous and liquid streams. 

The success of this and the other operations derives from the correct application of back-pulse filtration principles, in particular the back-pulse step itself in which the filter cloths are cleaned and made ready for further filtration. The properties of the GORE-TEX membrane, composed of expanded polytetrafluoroethylene, give rise to the desirable results of these brine filtrations, in particular ... 

The most commonly adopted filtering media is a high-density ceramic characterized by an asymmetric structure, a support material covered with a thin membrane layer containing very small pores. The membrane collects the fine particles and by making it thin the differential pressure of the filter element is kept low. The ideal solution is to have a very thin layer without defects that just covers the support material, so that purely surface filtration takes place. Experimental evidence at industrial scale shows that this ideal solution can be achieved in practice, penetration of particles into the support structure of the filter element being prevented, and that the element can be effectively regenerated by the clean gas back-pulsing procedure described by Cocco et al. [56]. It has also been confirmed at... 

M. J. Raimer, Back-Pulse Filtration using Gore-Tex Membrane Filter Cloths. In J. Moorhouse (ed.). Modem Chlor-Alkali Technology, vol. 8, Blackwell Science, Oxford (2001), p. 272. 

Most of hydrodynamic methods have focused on increasing the particle back transport from the membrane-liquid interface by increasing the shear rate and the flow instability in the boundary layer. These techniques include secondary flows, spacers and inserts, pulsed flow, high shear rate devices, vibrations, and two-phase flow. The physical methods that are currently been tested to enhance filtration performance of membranes include the application of electric fields and ultrasound. 

Pilot-scale particle collection efficiency has been found to be similar to the cold and dry experiments over 300 hours filtration time, Figure 12 presents a relatively constant pressure drop over time for the hot and dry experiments conducted on the pilot-scale filter under a gas flowrate of 20 Nm /h (80 Nm /hr/m ), with a particle load of 3000 mg/Nm and using Ottawa sand as the filtering media. Deep holes on Figure 12 are air pulses to back-flush the solids plugging the exits. 

Tuhtdar Presses As the name implies, this press is composed of a candle filter inside a cyhndrical hydrauhc casing (Fig. 18-192). The filter cloth is wrapped around the filter candle, and a diaphragm is attached to the inner side of the outer casing. During the filtration step, the space in between two cylinders is filled with slurry, and pressure filtration is conducted. At the end of the filtration step, the diaphragm is inflated to squeeze the cake around the filter candle. At the end of expression, the bottom of the hydrauhc casing tube is opened and the filter assembly is lowered. Air is then introduced to pulse the cake off the candle. Alter the cake is discharged, the inner filter candle moves back, and the bottom is closed for the next filtration cycle. 

---