Backflushing

Some more drastic measures than increasing crossflow velocity need to be taken to help reduce the flux decline over an economically acceptable period of operating time. One such technique frequently adopted in many porous inorganic membrane systems today is 

From Eq. (5-3) it is clear that, when the membrane pores are completely blocked (i.e. 0=1)  

In practice, air pressure or pumps can be used for generating backpulses. Air pressure may be preferred since pumps sometimes generate particles which can possibly plug membrane pores on the permeate side [Norton Co., 1984]. The backflushing system 

Although most of the discussions on backflushing so far use the crossflow configuration as an example, backflushing also can be and has been employed in through-flow membrane separation applications. 

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Backflushing techniques, well-known to analysts, are also extremely valuable tools. 

This type of analysis requires several chromatographic columns and detectors. Hydrocarbons are measured with the aid of a flame ionization detector FID, while the other gases are analyzed using a katharometer. A large number of combinations of columns is possible considering the commutations between columns and, potentially, backflushing of the carrier gas. As an example, the hydrocarbons can be separated by a column packed with silicone or alumina while O2, N2 and CO will require a molecular sieve column. H2S is a special case because this gas is fixed irreversibly on a number of chromatographic supports. Its separation can be achieved on certain kinds of supports such as Porapak which are styrene-divinylbenzene copolymers. This type of phase is also used to analyze CO2 and water. 

Cleanable Granular-Bed Filters The principal objective in the development of cleanable granular-bed filters is to produce a device that can operate at temperatures above the range that can be tolerated with fabric filters. In some of the devices, the granules are circulated continuously through the unit, then are cleaned of the collected dust and returned to the filter bed. In others, the granular bed remains in place but is periodically taken out of service and cleaned by some means, such as backflushing with air. 

Cake-discharge methods are the chief distinguishing feature among the various designs. That of the Industrial Filter Pump Hydro-Shoe, for example, removes cake from the tubes by filtrate backflushing assisted by the shocking action of a compressed-gas pocket formed... 

Air Backflush A configuration unique to microfiltration feeds the process stream on the shell side of a capillaiw module with the permeate exiting the tube side. The device is rim as an intermittent deadend filter. Eveiw few minutes, the permeate side is pressurized with air. First displacing the liquid permeate, a blast of air pushed back-... 

The sampling system consists of a condensate trap, flow-control system, and sample tank (Fig. 25-38). The analytical system consists of two major subsystems an oxidation system for the recovery and conditioning of the condensate-trap contents and an NMO analyzer. The NMO analyzer is a gas chromatograph with backflush capabihty for NMO analysis and is equipped with an oxidation catalyst, a reduction catalyst, and an FID. The system for the recovery and conditioning of the organics captured in the condensate trap consists of a heat source, an oxidation catalyst, a nondispersive infrared (NDIR) analyzer, and an intermediate collec tion vessel. 

Following the backflush of the primary column and separation of the analytes on the second column, the system can then be returned to its original prefractionation position, ready for the next sample injection. 

Figure 3.7 shows some early examples of this type of analysis (39), illustrating the GC determination of the stereoisomeric composition of lactones in (a) a fruit drink (where the ratio is racemic, and the lactone is added artificially) and (b) a yoghurt, where the non-racemic ratio indicates no adulteration. Technically, this separation was enabled on a short 10 m slightly polar primary column coupled to a chiral selective cyclodextrin secondary column. Both columns were independently temperature controlled and the transfer cut performed by using a Deans switch, with a backflush of the primary column following the heart-cut. 

Separation in column 1 (C-1) removes early-eluting interference compounds, and so considerably increases the selectivity. The fraction of interest separated in C-1 is then transferred to column 2 (C-2) where the analytes of the fraction are separated. These transfers can be carried out either in forward mode or backflush mode. The forward mode is preferred because the backflush mode has two disadvantages for polar to moderately polar analytes. For most polar compounds, it leads to additional band broadening, while for more retained analytes there is a decrease in the separation obtained earlier in the process (31). 

The characteristics of the sorbent in the precolumn may lead to problems when coupling the two systems. Therefore, when the analytes are more retained in the precolumn than in the analytical column, peak broadening may appear, even when the analytes are eluted in the backflush mode (40). This has been solved with a special design in which the analytes retained in the precolumn are eluted with only the organic solvent of the mobile phase and the corresponding mobile phase is subsequently formed (40, 41). 

A five-column configuration of Such an analyser system is depicted in Figure 14.6. The first event in the process is the analysis of Hj by injection of the contents of sample loop 2 (SL2) onto column 5 (a packed molecular sieve column). Hydrogen is separated from the other compounds and detected by TCD 2, where nitrogen is used as a carrier gas. The next event is the injection of the contents of sample loop 1 (SLl), which is in series with SL2, onto column 1. After the separation of compounds up to and including C5, and backflushing the contents of column 1, all compounds above C5 (Q+) are detected by TCDl. The fraction up to and including C5 is directed to column 2, where air, CO, COj, Cj, and 2= (ethene) are separated from... 

Backflushing is another way of cleaning heavily fouled membranes. During back flushing a slight overpressure is applied to the permeate side of the membrane forcing solution from the permeate side to the feed side of the membrane. The flow of solution lifts deposited materials from the surface. Typical back flushing pressures are 5-15 psi [48]. 

Kim FIG, Park Ch, Yang J et al (2007) Optimization of backflushing conditions for ceramic ultraflltration membrane of disperse dye solutions. Desalination 202 150-155... 

One interesting concept mentioned in the literature to reduce cycle time is that of backflushing.60 In this method, the column flow is reversed just after the product of interest has eluted. After a delay time, the next injection is made at the opposite end of the column from the first. This is designed to minimize the time spent eluting strongly retained contaminants. 

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