Pulsed beds

Pulsed beds of ac tivated carbon are used in water and wastewater treatment systems. The adsorber tank is usually a vertical cylindrical pressure vessel, with fluid distributors at top and bottom, similar to the arrangement of an ion exchanger. The column is filled with granular carbon. Fluid flow is upward, and carbon is intermittently dis-... 

Focusing discussions on carbon adsorption processes, in a pulsed-bed adsorber, the carbon moves countercurrent to the liquid. The effeet is of a number of staeked, fixed-bed eolumns operating in series. Spent earbon is removed from the bottom of... 

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. 

In the pulse bed shown in Figure 32, the liquid enters the bottom cone and leaves through the top cone. The flow of liquid is stopped periodically, spent carbon is withdrawn (pulsed) from the bottom, and virgin or reactivated carbon is added into the top of the adsorber. In a fixed-bed adsorber (illustrated in Figure 33), the liquid... 

It may also be replaced with new carbon and disposal of the exhausted carbon Most adsorbers are pressure vessels constructed in carbon steel, stainless steel or plastic. Large systems for drinking water are often eonstructed in concrete. In some cases, a moving or pulsed bed adsorber is employed to optimixe the use of the granular activated carbon. 

According to the vendor, ISEP is less expensive than traditional fixed or pulse-bed treatment technologies (D22289J, p. 2). The vendor estimates that a 1-million-gallon-per-day ISEP system for treating nitrate would cost 137.44 per million gallons of water treated (D20023N, p. 6). Additional vendor-supplied cost information is summarized in Table 1. 

Figure 15.25. Liquid phase adsorption processes for water treated with activated carbon and petroleum treated with clay adsorbents, (a) A two-stage slurry tank and filter process, (b) Continuous pulsed bed operation, individual pulses 2-10% of bed volume as needed.

If the liquor is not to be enzyme-converted, it is pumped to mud centrifuges and rotary drum filters which remove the suspended fats and insoluble impurities from the filtrate. Amino acids and peptides which may react with carbohydrates are also removed. Then the filtrate is passed through pulsed beds of activated carbon for clarification and bleaching. The temperature in the carbon column is maintained at 150-170°F (69-77°C) with a typical contact time of 90-120 minutes for optimum removal of impurities. Usually these columns contain packed granular carbon, although powdered carbon may also be used. 

The treatment of liquids with higli flow rates and/or high concentrations of contaminants requires a frequent replacement of carbon adsorbent. In such cases pulsed (Fig. 5.) or moving bed systems are installed. In this technological solution the purified water enters the filtration column from the bottom and flows upward. At the same time, GAC is dosed downward from the top and it is removed continuously or in pulses at the bottom of the filtration column. Thus, the filter with moving or pulsed bed can be maintained with no downtimes. 

Pulsed Beds. A moving or pulsed-bed system may be used in which some carbon is removed at intervals from the bottom of the column and replaced at the top by fresh adsorbent. The rate at which the adsorbent is replenished should be balanced by the rate at which the adsorbent is used in practice, the mass transfer zone should be held at a constant position within this bed. 

Pulsed beds are normally operated with the columns completely full of adsorbent so there is no free board to allow bed expansion during operation. This prevents mixing, which would disturb the sorption zone and reduce the adsorption efficiency. Usually, the adsorbent is maintained in plug flow so that a sharp adsorption zone will be retained. The pulsed bed... 

FIGURE 15 J3 Schematic diagram of pulsed-bed sorption unit. 

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