Back flushing

Dislodging of Cake by Reverse Flow. Intermittent back-flushing of the filter medium can also be used to control cake growth, leading to filtration through thin cakes in short cycles. Conventional vacuum or pressure filters can be modified to counter the effects of the forces during the back-flush (23,24). 

Many filters in chemical process units are either changed very rarely or are back-flushed automatically so there is hardly any exposure. Some filters, however, require frequent manual changing or cleaning and significant exposure may occur unless operators foUow the proper procedure. The filter container should be drained of any toxic material and then flushed and purged as needed so that when it is opened there is only minimal exposure. Zero exposure is difficult to achieve in situations where a disposable paper filter cartridge may retain and slowly release a material that cannot be removed by multiple flushes and purges. 

Cleaning is frequently aided mechanically. Foam balls scour the center of tubes, and hoUow-filter systems can be back-flushed. HoUow fibers and membranes attached to rigid supports can be back-pressured, thereby eliminating the pressure drop that holds redispersed films on the membrane surface. 

Figure 5.1 Schematic diagrams of a typical enricliment system (a) forward-flush position (b) back-flush position.

Figure 5.2 Schematic representation of the final column-switching system (a) foi ward-flush position (b) back-flush position (further details are given in the text). Reprinted from Journal of Chromatography, A 828, A. K. Sakhi et al. Quantitative determination of endogenous retinoids in mouse embiyos by high-performance liquid cliromatography with on-line solid-phase exti action, column switcliing and electi ochemical detection , pp. 451 -460, copyright 1998, with permission from Elsevier Science.

The packed column section contains a stripper pre-column (column 1), which separates the Cg+ fraction by back-flushing all compounds above -pentane in one peak. HjS, CO2, C2, O2, N2 and Cj are trapped in columns 3 and 4, while C3-C5 hydrocarbons elute from column 2 to the TCD. The remaining components are... 

Figure 14.15 Typical SFC chromatogram of total olefins in gasoline 1, saturates 2, aromatics 3, olefins t(L), time of loading sample on to columns and eluting saturates t(AR), time of eluting aromatics t(C), time of eluting remaining saturates from olefin trap t(BF), time of eluting olefins by back-flush.

Figure 14.17 Schematic diagram of the on-line coupled LC-GC system VI, valve foi switcliing the LC column outlet to the GC injector V2, valve for switching the LC column to back-flush mode V3, LC injection valve RI, refractive index monitor detector UV, ulti avio-let monitor detector FID, flame-ionization detector.

D. R. Deans, An improved teclinique for back-flushing gas cliromatographic columns , J. Chromatogr. 18 477-481 (1965). 

This type of exchanger usually provides relatively high heat transfer coefficients and does allow good cleaning by mechanically separating the plates, if back-flushing does not provide the needed cleanup. An excellent discussion on the performance and capabilities is presented by Carlson. To obtain a proper design for a specific application, it is necessary to contact the several manufacturers to obtain their recommendations, because the surfece area of these units is proprietary to the manufacturer. 

A baghouse system consists of the following pneumatic-conveyor system, filter media, a back-flush cleaning system, and a fan or blower to provide airflow. 

Fig. 16.13. Time dependence of membrane permeation rate during cross-flow filtration (a) low cross-flow velocity, (b) increased cross-flow velocity, (c) back flushing at the bottom of each saw-tooth .

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]. 

Due to a resisting minor sand production in most of the wells and some problems with precipitation of iron, a well maintenance programme has been developed. In this program the wells are back flushed twice a year and in addition to that, some wells are redeveloped with air lifting once a year. 

Simple and comprehensive 2D HPLC was reported in a reversed-phase mode using monolithic silica columns for the 2nd-D separation (Tanaka et al., 2004). Every fraction from the lst-D column, 15cm long (4.6 mm i.d.), packed with fluoroalkylsilyl-bonded (FR) silica particles (5 pm), was subjected to the separation in the 2nd-D using one or two octadecylsilylated (Cig) monolithic silica columns (4.6 mm i.d., 3 cm). Monolithic silica columns in the 2nd-D were eluted at a flow rate of up to lOmL/min with separation time of 30 s that provides fractionation every 15-30s for the lst-D, which is operated near the optimum flow rate of 0.4-0.8 mL/min. The 2D-HPLC systems were assembled, as shown in Fig. 7.6, so that the sample loops of the 2nd-D injectors were back flushed to minimize band broadening. 

FIGURE 15.6 Schematic of constant flux maintenance manifold, which can be used with SBCR utilization of active back-flushing of membrane surface with permeate solution. 

An active flux maintenance procedure was initiated at this point (about 330 h TOS), beginning with a 2 s back-flush of clean permeate through the filter membrane. This active flux maintenance cycle was continued every 30 min for just over 24 h. The flux initially recovered to 0.90 lpm/m2 (32.0 GPD/ft2), but declined again within 24 h to a baseline value of 0.76 lpm/m2 (26.7 GPD/ft2) without clean permeate back-flush. The flux maintenance method was then returned to passive (no back-flush with clean permeate) mode, only increasing the flux off-time to 60 s every 30 min. Thereafter, the flux steadily declined over the next 120 h TOS from 0.77 to 0.58 lpm/m2 (27.3 to 20.4 GPD/ft2). At 480 h TOS, a 1 h flux off-cycle was attempted, resulting in an increase of the flux back to 0.82 lpm/m2 (29.1 GPD/ ft2), a 42.6% increase. When the flux off-cycle was returned to the 60 s off-cycle for the next 48 h, it was found that the permeate flux decreased to 0.62 lpm/m2 (21.9 GPD/ft2). Applying another 1 h flux off-cycle returned the flux to 0.721pm/... 

Menges, R.A., Bertrand, G.L., Armstrong, D.W. (1990) Direct measurement of octanol-water partition coefficients using centrifugal partition chromatography with a back-flushing technique. J. Liq. Chromatogr. 13, 3061-3077. 

Bag filters are composed of woven material that intercepts small particles on the filter surface by impingement and electrostatic attraction. The efficiency of collection increases as the depth of the filter cake increases, making these filters highly efficient for small diameter, even sub-micron, particle size. Filters are periodically shaken or back-flushed to remove particulate accumulation. These filters are usually constructed of woven materials suitable for operation at low temperatures up to about 350°C. 

Other applications of the model 8700 system include fore-flushing and back-flushing of the pre-column, either separately or in combination with heart cutting, all carried out with complete automation by the standard instrument software. 

Candle filters. These consist of multiple high-temperature ceramic or sintered-metal candles fitted within a plenum and a nitrogen supply for online periodic back flushing. Captured particulate matter is discharged into a steel container through a rotary air lock. The particulates are processed in the MPT to ensure 5X decontamination. 

In the second type of filtration, depth or deep-bed filtration, the particles penetrate into the pores of the filter medium, where impacts between the particles and the surface of the medium are largely responsible for their removal and retention. This configuration is commonly used for the removal of fine particles from very dilute suspensions, where the recovery of the particles is not of primary importance. Typical examples here include air and water filtration. The filter bed gradually becomes clogged with particles, and its resistance to flow eventually reaches an unacceptably high level. For continued operation, it is therefore necessary to remove the accumulated solids, and it is important that this can be readily achieved. For this reason, the filter commonly consists of a bed of particulate solids, such as sand, which can be cleaned by back-flushing, often accompanied by... 

Some applications also involved direct injection techniques in which BZDs are preferentially absorbed onto a precolumn and are back-flushed onto the analytical column using column switching techniques (Lauber et al., 1994 Iwase et al., 1994). 

An injector valve operates in two modes— the fixed-loop mode or the partial-loop mode. In the fixed-loop mode, a sample is overfilled into the loop at 2-4 times the loop volume and the entire loop content is injected. In the partial-loop fill mode, a variable sample aliquot, measured precisely by a syringe at <50% of the loop volume, is injected. Note that the sample slug is introduced into the end of the sample loop and is back flushed onto the column to minimize band dispersion by the sample loop (Figure 9). Due to the emphasis on productivity, manual injectors are seldom used in the pharmaceutical laboratory except for preparative applications. 

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