Downflow filter

Deoiled water leaves the unit via a drawoff pan leading to the outlet piping. Large drops of coalesced oil rise past the water draw-off pan, into the separation section where it accumulates, and is later drawn off Deep bed downflow separator. Another approach to oil removal using lose media is the downflow filter Oily water enters near the top of the vessel and flows downward through the coalescing media which consists of three... 

The next division to be made is between upflow and downflow pressure filtration. Downflow filters operate with shorter beds (about 50% of typical upflow depth). They can achieve high unit throughputs when the solids concentration is low and are most frequently chosen in such cases. Their mechanism is essentially surface filtration by the fine particles at the top of the bed. 

The throughput of an upflow filter is limited by the danger of fluidization. This will dismpt the bed and allow a major increase of solids in the effluent liquid. On the other hand, upward flow allows depth filtration, in which particles can be removed from the brine anywhere in the bed. Upflow filters therefore have solids capacities two or three times as great as downflow filters, resulting in longer on-stream times. They also are more stable in the face of a clarifier upset. Some filters can tolerate up to 500 ppm solids in the feed brine while maintaining a 4-hr cycle [109]. 

Figure 1.9 A downflow filter in potable water treatment showing the typical operating conditions... 

Most deep bed filters are gravity fed but there are also some pressure types in use. The most common arrangement is the downflow filter, with backwash in the upward direction in order to fluidize the bed. The stratification of the particles that make up the bed, caused by the fluidization (fines on top), is not desirable, however. The solids holding capacity of the bed is best utihzed if the filtration flow encounters progressively finer sand particles. This is achieved in the upflow filters, where the fluidization due to backwash produces the correct stratification in the bed. Both the filtration flow and the backwash take place in the same direction and the disadvantage is that the wash goes to the clean side of the filter. 

Along with flocculation upstream from dissolved air flotation or through downflow filters. 

Conventional downflow sand filters are effective for liquid-solid separation at flow rates up to about 15m /h.m of filter area, although higher rate downflow filters are available. With proper selection of filter media, gelatinous as well as granular suspended matter can be filtered out, without a rapid differential pressure build-up. 

Typical filter media for the downflow filter consist of selected silica sands, and coal or anthracite, which are tough inert solids, available in a range of particle sizes. One solution to the problem of matching the pore sizes in the bed is to use... 

Fig. 5. Example of a deep bed filter, ie, a downflow gravity filter.

Anaerobic Filter. The anaerobic filter is similar to a trickling filter ia that a biofilm is generated on media. The bed is fully submerged and can be operated either upflow or downflow. For very high strength wastewaters, a recycle can be employed. 

The well-known aerobic downflow process is a trickled bed filter. Attached growth is used in the biological treatment of wastewater. Air passes through the bed while the liquid is forced to down by gravity. Figure 13.2 shows the liquid gas system for the mass transfer... 

The rating of this filter is 20-40 gpnv sq rt essentially twice mat ot tire downflow multimedia filter because of the dual flow. Backwash water enters at the bottom and expands the bed in the same fashion as the graded sand bed and multimedia filters In some cases, raw water may be used, it would be prudent to follow backwashing with a short rinse before returning to service. 

This is analogous to deep bed downflow filtration except the flow is in the opposite direction During the filtering step, oily water enters a lower chamber in the bottom of the vessel. The water is then spread uniformly across the bed through a large number of (low distribution noz zles in a bottom support plate. 

The trickle bed reactors that operate in the downflow configuration and have a number of operational problems, including poor distribution of liquid and pulsing operation at high liquid and gas loading. Scaleup of these liquid-gas-solid reactors is much more difficult than a gas-solid or gas-liquid reactor. Nevertheless, the downflow system is convenient when the bed is filled with small catalyst particles. And, because the catalyst particles are small, these reactors are quite effective as filters of the incoming feed. Any suspended fine solids, such as fine clays from production operations, accumulate at the front end of the bed. Eventually, this will lead to high pressure differentials between the inlet and outlet end of the reactor. 

Fresh, filtered feedstock is heated together with hydrogen and recycle gas and charged to the downflow reactor from which the liquid product goes to fractionation after flashing to produce the various product streams. Each process type is basically similar to its predecessor (Figure 9-11) but will differ in the number of reactors. For example, modifications necessary to convert the Type II to the Type III process consist of the addition of a reactor and related equipment, while the Type III process can be modified to a Type IV process by the addition of a... 

The process centers on a fixed-bed downflow reactor that allows catalyst replacement without causing any interruption in the operation of the unit (Figure 9-28). Feedstock is introduced to the process via a filter (backwash, automatic) after which hydrogen and recycle gas are added to the feedstock stream which is then heated to reactor temperature by means of feed-effluent heat exchangers whereupon the feed stream passes down through the reactor in trickle flow. Sulfur removal is excellent (Table 9-18), and substantial reductions in the vanadium content and asphaltenes content are also noted. In addition, a marked increase occurs in the API gravity, and the viscosity is reduced considerably. 

The separation of suspensions is the selective removal of suspended solids, say, by the ordinary processes of filtration. Application can also made to the separation of colloidal suspensions of minute or microscopic solid particles, and even of emulsions, the suspension of minute immiscible liquid droplets within another liquid phase. A distinguishing feature of ordinary filtration is usually that the discharged liquid phase does not form a continuum on the downflow or reject side of the membrane, or filter, and more or less exists at atmospheric pressure. If otherwise, if a contiunuum is formed, the process is more that of reverse osmosis, also called hyperfiltration. In common use, notably for the upgrading or desalination of salt water or brackish water, reverse osmosis is a subject for special consideration. 

GAC may be used in fixed or moving beds and in downflow or upflow mode. Fixed beds are operated in downflow mode and as such, provide some amount of solids filtration however, influent solids concentration must be kept low (less than 5 mg/L suspended soHds) to prevent rapid plugging of the bed. Filtered solids are periodically removed by backwashing. Upflow beds are more tolerant of soHds because they are fluidized and expanded by the wastewater entering at the bottom. In moving beds, the flow is countercurrent and makeup, fresh carbon is added continuously at the top of the unit while an equal amount of spent carbon is removed from the bottom. 

In the operation of a downflow-type saturator, water is introduced at the top of the saturator tank (there is an air gap to avoid the possibility of a cross-connection) and the level is regulated with a float-operated controller valve on the water supply line. The water then trickles down through the bed of sodium fluoride solids the solution is clarified in the sand and gravel filter bed and ends up as a clear, saturated solution at the bottom of the tank where it is withdrawn by the feeder. The fluoride-water solution pump withdraws the solution and delivers it into the water system at a desired dosage rate. Operator attention is required only to ensure that an adequate quantity of sodium fluoride solids is kept in the saturator and that the saturator is kept in a reasonably clean condition. 

Low-pressure Processes. Three processes for the polymerization of ethylene have recently been developed. The commercial process of the Phillips Petroleum Company for the polymerization of ethylene is carried out at relatively low pressures (100-500 psi) in either fixed-bed or slurry-type operations. The catalyst consists of 2-3 weight per cent chromium as oxide on silica alumina, and the reaction temperature varies from 90— 180°C. In fixed-bed operation, purified ethylene and hydrocarbon solvent streams are passed downflow, liquid phase over the catalsrst bpd. Solvent and polymer are collected, and the solvent is flashed overhead. Unreacted gases are removed from the solvent, taken overhead, and metered the solvent is recycled to the reactor. The solvent and polymer in the first receiver are cooled to room temperature to precipitate the polymer, which is then filtered and dried in a vacuum oven. In the slurry-type operation (indicated in Fig. 15-33 by a proposed flow diagram), solvent and a small... 

Deep bed cycle load 8 h backwash 15 min 93 % availability. Cake formation rate 1 X 10 mm/s solids flux 0.02-0.5 g/s m of filter area filtrate flux 1.3-16.7 L/s m of filter area with fluid loading decreasing as solids feed concentration increasing the usual value is 2.7 L/s m. Area per unit < 25 m maximum depth 4.5 m. Gravity upflow or downflow with single or multiple media pressure up-flow or downflow. Use pressure units for small to medium fluid capacities where high terminal headloss is expected. Backwash at 6 L/s m. ... 

Rapid filter. A coarser grain size, and sometimes a narrower size distribution than slow filters are used ofl en pressurised flow is employed. A variety of rapid filters exist, for example, downflow, upflow, mixed-media and continuous. These will be discussed in greater detail in the following sections. 

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