Pulsed bed adsorbers

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... 

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. 

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-... 

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 The flow of liquid is stopped periodically, spent carbon is withdrawn (pulsed) from the bottom, and virgin or reactivated carbon is added into the top of the adsorber.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... 

The adsorbate concentration in the Nth stage along the charcoal bed can be found by solving the series of N differential equations. These solutions represent the concentration profile in the Nth stage. For a unit pulse of adsorbate at time t = 0, the solution reduces to... 

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.

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... 

The chromatographic experiment, based on introducing a pulse of adsorbable tracer of concentration Co and duration time r into the entrance of the bed, is illustrated in Fig. 6.1. For quantitative analysis of the effluent peak Ce(r), there are several alternative techniques to determine model parameters by comparing the mathematical solution of the fundamental equations and the experimental results. These are 1)... 

A stochastic model could be applied satisfactorily in the simulation of the behavior of the fixed-bed adsorber and the prediction of the breakthrough curves for single-solute adsorption. The parameters such as the number of compartment and backflow ratio were estimated from a least-squares fit of the compartmental model to the observed data obtained from the pulse response test. The intensity mi2, which is expected to be a function of the flow pattern at the interphase between liquid and solid, was determined by fitting the initial portion of the breakthrough curve. An equation was introduced for improving the estimation of the parameter miv... 

In such a case,pulse techniques can be conveniently used to examine the reactivity of the adsorbed species as follows. After the catalyst had been reduced by a pure stream at 160°C for 2 hrs, the catalyst bed was flushed by He and then the sream changed to an 2 (12.8%) -He stream. O2 and 0 gases were separa-... 

Selectivity is a relative term and is defined in the Molex process as the adsorbent s preference for desired component (in this case, normal paraffins) over the undesired feed components (cyclic paraffins, iso-paraffins, aromatics) while employing a particular desorbent. One can easily determine an adsorbent and desorbent combination selectivity using a pulse test screening apparatus. This apparatus consists of a known volume of adsorbent placed in a fixed bed. A stream of desorbent is then passed over the bed to fill the pore and interstitial volume of the bed. A known quantity of feed is introduced to the feed at the top of the adsorbent bed and passed across the column as a pulse of feed. This pulse of feed is then pushed through the adsorbent bed using a known desorbent flow rate. Effluent from the column is monitored for the various feed components and the concentrations of each component noted (with respect to time) as they elude from the... 

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