Fluidized backwash

Stratification of the particles making up the bed, caused by the fluidization (fines on top), is not desirable. The soflds holding capacity of the bed is best utilized if the filtration flow encounters progressively finer sand particles. This is achieved in upflow filters where the fluidization due to backwash produces the correct stratification in the bed. Unfortunately, the filtration flow and the backwash take place in the same direction the disadvantage is that the washwater goes to the clean side of the filter. 

GAC may be used in fixed or moving beds and in downflow or upflow mode. Eixed beds are operated in downflow mode and as such, provide some amount of soflds filtration however, influent soflds concentration must be kept low (less than 5 mg/L suspended soflds) to prevent rapid plugging of the bed. Entered soflds are periodically removed by backwashing. Upflow beds are more tolerant of soflds 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. 

Carbon should be prewetted prior to being placed in the test columns. Backwashing the carbon at low rates (2.5 m/hr) does not remove the air. Rates that would expand the bed 50 percent or 15-30 m/hr, are required. The liquid used for prewetting can either be water, if it is compatible with the liquid to be treated, or a batch of the liquid to be treated which has been purified previously. There are three types of carbon systems (1) fixed beds, (2) pulse beds, and (3) fluidized beds, and these can be used singly, in parallel, or in combination. The majority of systems are either fixed or pulse beds. The two basic types of adsorbers which can be designed to operate under pressure or at atmospheric pressure are the moving or pulse bed and the fixed bed. Either can be operated as packed or expanded beds. 

Provide space to fluidize the resin during backwash... 

The column is backwashed to remove suspended solids collected by the bed during the service cycle and to eliminate chaimels that may have formed during this cycle. The back- wash flow fluidizes the bed, releases trapped particles, and reorients the resin particles according to size. 

Granular solid filters (see Fig. 16-4) are cleaned by backwashing with water. The backwashing is usually preceded by an air scouring to assure better cleaning.30 The backflow water rate should be fast enough to fluidize the bed. 

Columns are designed to have a larger internal volume than the quantity of resin they will contain. The extra space is to provide the necessary volume for a fluidized bed during backwash. Most units are designed for the space above the resin bed (free-board) to be between 50 and 100% of the packed resin bed. Small columns are, on occasion, designed for one-use applications. Since backwashing is of no importance, there is a tendency to fill the unit with as much resin as possible. That practice can be hazardous, especially if the resin swells as a result of oxidative attack or through conversion from one ionic form to another. 

Determine the backwash rate at which the bed in Problem 7.22 will just begin to fluidize, assuming the bed is expanded to 150%. 

Figure 8.8 Asahi moving packed bed. A, adsorption section B, elution section C, fluidized resin backwash D, resin collection hoppers with screen top and non-return valve outlet for resin E, transfer and backwash water overflow F, feed G, barren effluent H, eluant I, eluate product J, backwash supply K, resin flow [1]. Copyright 1987 John WUey Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

An important requirement for the successful operation of a mixed bed is the careful separation of the strong base anion resin from the strong acid cation resin by backwash fluidization. This is followed by contact of each type of resin with its respective regenerant in a manner which minimizes the... 

Fixed bed batch size on cycle time load, backwash/clean, regenerate, swing on-stream. Load typical flow rate 2 to 3.5 BV/h with the load time based on the ratio of the adsorption isotherm to the feed concentration of the target species (usual range varies with the application 18 to 100 min, while for water treatment 70 to 1(X) days). Backwash with velocity to fluidize the bed (see liquid fluidization, Section 16.11.7.5) velocity 0.8 BV/h. Time such that <5% feedrate used in backwash. Usually, carbon is removed and regenerated about four times per annum. Try to match the loading cycle to the regeneration cycle. 

Stirred tank crystallizers, see Section 16.11.4.6 and reactors. Sections 16.11.6.23 through 16.11.6.25. Liquid fluidized bed reactors. Section 16.11.6.26 hquid adsorption, Section 16.11.4.12 ion exchange, Section 16.11.4.13 backwash fixed-bed operations such as deep-bed filters. Section 16.11.5.13 liquid adsorbers. Section 16.11.4.12 and ion exchangers. Section 16.11.4.13. 

Stirred tank paddles power input suspend solids, 0.2 to 1.6 kW/m UD = 0.7 to 1.05/1. Baffle, four 90° baffle width = 0.08 x tank diameter off-the-wall distance = 0.015 x tank diameter. Minimum level of liquid = 0.15 x tank diameter for impeller tank diameter 0.28 1 and minimum level = 0.25 x tank diameter for impeller tank diameter = 0.4 1. Use a foot bearing plus a single, main axial hydrofoil impeller diameter = 0.28 x tank diameter located 0.2 x tank diameter from the bottom plus a pitched blade impeller diameter = 0.19 x tank diameter located 0.5 x tank diameter from the bottom. Liquid fluidized bed in general, particle diameter 0.5 to 5 mm with density and diameter of the particle dependent on the application. The superficial liquid velocity to fluidize the bed depends on both the diameter and the density difference between the liquid and the particle. Usually, the operation is particulate fluidization. Particle diameter 0.2 to 1 mm reactors superficial liquid velocity 2 to 200 mm/s. Fluidized adsorption bed expands 20 to 30% superficial liquid velocity for usual carbon adsorbent = 8 to 14 mm/s. Fluidized ion exchange bed expands 50 to 200% superficial liquid velocity for usual ion exchange resin = 40 mm/s. Backwash operations fixed-bed adsorption superficial liquid velocity = 8 to 14 mm/s fixed-bed ion exchange superficial backwash velocity = 3 mm/s. 

The upper surface of the resin bed acts as an efficient Alter for any fine particles present in the feed during normal service. It is therefore necessary to backwash or fluidize the reain with dear water lo remove tlieas solids to waste at the end of a service cycle. The flow distributor at the base and a collecting manifold at the top of the vessel ase used in this operation. 

Fluidized beds of resin have always been used for backwashing in ion-exchange practice. The use of fluidized beds as a means of con reeling fluids with particles has been applied since ihe 1940s and 1950. ... 

Example 7.3. A bed of ion-exchange beads 8 ft deep is to be backwashed with water to remove dirt. The particles have a density 1,24 g/cm and an average size of 1.1 mm. What is the minimum fluidization velocity using water at 20°C, and what velocity is required to expand the bed by 25 percent The beads are assumed to be spherical... 

Mixed-media filters use more than one type of fill. Since density and particle shape both influence fluidization during backwashing as well as the rates of settling, there is a mixture of particle sizes at any level in a bed. This gives downflow filtration some of the character of depth filtration. 

The Associated Octel Company have on the other hand patented a method involving cementation [33]. The effluent at pH 7.5-9 is passed down a column containing clean particles of zinc. Extraction efficiencies of 80-90% are maintained over long periods, 120-160 days. Occasional backwashing, sufficient to fluidize the zinc, is necessary to maintain the effectiveness of the zinc. Clean zinc particles are essential to the process and can be obtained by pre-washing with a... 

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