Backwash adsorption

Resias are seldom used oace and discarded. Whether the system is mn batchwise or ia columns, the resia must be periodically removed from service and regenerated. An exception is the use of a resia as a catalyst ia organic reactions. Each cycle consists of two principal steps, adsorption and regeneration, and one or more iatermediate steps, tinse and backwash. Eailure to use good practices results ia poor cycHc performance. 

As manufactured, most resias have a Gaussian-Hke distributioa of particle size. Very few are as small as 0.3 mm or as large as 1.0 mm. Most are betweea 0.5—0.8 mm. A backwash before usiag aew resia is common practice to assure uniform flow during the adsorption and regeneration steps. The backwash eliminates air pockets that may have formed while filling the column and sorts the beads such that the smaller sizes are at the top of the bed and... 

Adsorption efficiency can be optimized by using finer particle size products which will improve the diffusion rate to the surface of the activated carbon. However, there is a tradeoff in using finer particles with pressure drop and, hence energy use. Note that during start-up of an activated carbon filter bed, a bed expansion of 25 to 35 % is recommended in order to remove soluble matter and to stratify particles in order to ensure that the MTZ is maintained when future backwashing is performed. 

Activated carbon filters are employed primarily as RW contaminant removal systems for chlorine (by chemisorption) and various organics such as trihalomethanes (THMs), petroleum products, and pesticides (by adsorption). In addition, they act as physical filters and therefore incorporate sufficient freeboard in their designs to permit periodic backwashing. 

Activated carbon adsorption has very limited use in the removal of LNAPL. Adsorption is primarily effective for removal of low levels of soluble hydrocarbons. Groundwater applied to activated carbon adsorption units must be pretreated to prevent clogging and coating of the activated carbon with free oil. If the activated carbon adsorption units are not adequately protected, the units will have to be backwashed frequently and the activated carbon will have to be replaced with unacceptable frequency. 

In a typical fixed-bed carbon column, the column is similar to a pressure filter and has an inlet distributor, an underdrain system, and a surface wash. During the adsorption cycle, the influent flow enters through the inlet distributor at the top of the column, and the groundwater flows downward through the bed and exits through the underdrain system. The unit hydraulic flow rate is usually 2 to 5 gpm/ft2. When the head loss becomes excessive due to the accumulated suspended solids, the column is taken off-line and backwashed. 

Figure 16 Process flowsheet of a GAC system with regeneration. In this complete GAC adsorption and regeneration system, four GAC columns can be operated in parallel or in series. Spent carbon is transferred to a multiple-hearth furnace for thermal regeneration. Regenerated carbon is mixed with virgin makeup and pumped back to the GAC columns. The GAC columns are backwashed periodically. (From Ref. 27.)...

Figure 10 Carbon adsorption flow diagram. The carbon columns are operated in series backwash water is provided by a pump (from Ref. 11).

Model 4 system described contains steel pipes. Model 10 system described is backwashable. Systems include aU piping and manual valves to comprise a complete adsorption system, enabling all operations. Equipment costs include drawings, technical submittals, and provision of an operation and maintenance manual. Freight cost may need to be added for some models. 

In addition to advanced hltration, this chapter also discusses carbon adsorption. This is a unit operation that uses the active sites in powdered, granular, and hbrous activated carbon to remove impurities from water and wastewater. Carbon adsorption and hltration share some similar characteristics. For example, head loss calculations and backwashing calculations are the same. Carbon adsorption will be discussed as the last part of this chapter. 

The operation of carbon bed adsorption units is similar to that of filtration units. In fact, bed adsorption is partly filtration. Countercurrent flow operation is analogous to Alter backwashing, and co-current flow operation is analogous to normal downflow or gravity filtration. 

Data for liquid phase adsorption are typified by water treating for removal of small but harmful amounts of impurities. Some conditions are stated by Bemardin [Chem. Eng., (18 Oct. 1976)]. Water flow rates are 5-10 gpm/sqft. When suspended solids are present, the accumulation on the top of the bed is backwashed at 15-20 gpm/sqft for 10-20 min/day. The adsorbent usually is not regenerated in place but is removed and treated in a furnace. Accordingly, a continuous operation is desirable, and one is simulated by periodic removal of spent adsorbent from the bottom of the vessel with a design like that of Figure 15.18(b) and replenishing of fresh adsorbent at the top. The pulses of spent and fresh carbon are 2-10% of the total bed. Height to diameter ratio in such units is about 3. 

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.

Elegant slide valves are used to separate the adsorption, regeneration and resin backwash stages. The contactor operates in predetermined cycles and is an ideal process for feedstreams with no suspended solids. 

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. 

Permeate backwash is a possible means to remove cake deposit in UF. The choice of a more hy drophilic membrane can decrease adsorption significantly and changes in the solution chemistry can impact on the deposit structure and porosity. 

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