Continuous operations adsorption

A vehicle fuel vapor control system must be designed to meet both driving and refueling emission level requirements. Due to the nature of hydrocarbon adsorption, this emission control is a continuous operation. 

In these processes, a solid with a high surface area is used. Molecular sieves (zeolites) are widely used and are capable of adsorbing large amounts of gases. In practice, more than one adsorption bed is used for continuous operation. One bed is in use while the other is being regenerated. 

Both batch and continuous adsorption processes are used. In a batch process, the adsorbent bed is allowed to become saturated with adsorbed material and is subsequently regenerated in a cyclic manner. In a continuous process, usually the counter-current mode is adopted for adsorption and desorption, either in time form or in simulated mode. Continuous operation offers many advantages with respect to the efficiency of adsorbent utilization. Thus, for... 

Beta/montmorillonite composite was prepared under dynamic hydrothermal conditions. Firstly, montmorillonite calcined at 800 °C were added to a diluted solution of sodium hydroxide, potassium chloride and TEAOH in distilled water and the resulting mixture was vigorously stirred for 1 h secondly, silica sol was added into the above uniform mixture to allow at least 3 h stirring finally, the gel was moved into stainless steel autoclaves (1L) and heated at 413 K for 48 h. The samples were characterized by XRD, N2 adsorption-desorption, FT-IR and SEM-EDS. The catalytic assessment experiments were carried out in a flowing-type apparatus designed for continuous operation. 

It is not passive. The driving force, its continued operation, is based on the difference between the contaminant concentration at the velocity barrier (assumed to be equal to the ambient concentration) and the sorbent surface (the principle of diffusion). That difference is maintained by the continuous adsorption of the contaminant vapors by the sorbent. 

Perhaps the hypersorption process (7) of recent years may be thought new and it is new in applying the mechanical principle of continuous operation to charcoal adsorption, but such adsorption on a batch process was in use more than 25 years ago and became obsolete in competition with absorption. Now the continuous hypersorption method appears to be finding a real field of usefulness, especially in making very high recoveries of propane and in recovering substantial amounts of ethane. Recovery of ethane is beginning to be important, in connection with its use as a chemical raw material for the reactions mentioned previously in this paper. 

When continuous operation is necessary, at least two adsorbers are employed, one on adsorption and the other alternately on regeneration and cooling. In cases where breakthrough is especially harmful, three vessels are used, one being regenerated, the other two onstream with the more recently regenerated vessel downstream, as in Figure 15.16. 

The usual equipment for gas adsorption is a number of vessels containing fixed beds of the adsorbent, at least two vessels for achieving overall continuous operation. Figure 15.17 shows suitable vertical and horizontal vessels. The vertical ones are less likely to form channels and usually are favored. Bed depths as high as 45 ft are in use. Horizontal vessels are preferred when pressure drops... 

Industrial-scale adsorption processes can be classified as batch or continuous. In a batch process, die adsorbent bed is saturated and regenerated in a cyclic, operation. In a continuous process, a countercurrent staged contact between lire adsorbent and die feed and desorbent is established by cidier a true or a simulated recirculation of die adsorbent. The efficiency of an adsorption process is significantly higher in a eoiuinuous mode of operation than in a cyclic batch mode. For difficult separations, batch operation may require 25 times more adsorbent inventory and twice die desorbent circulation rate than does a continuous operation. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, wtiereas a continuous mode allows mure degrees of freedom with respect to these functions, and thus a better overall operation. 

Multistage Crosscurrent Adsorption The amount of adsorbent required for the separation of a given amount of solute can be decreased by employing multistage crosscurrent contact, which is usually operated in batch mode, although continuous operation is also possible. The required adsorbent is further decreased by increasing the number of stages. However, it is seldom economical to... 

Among hybrid separations not involving membranes, adsorptive distillation (87) offers interesting advantages over conventional methods. In this technique a selective adsorbent is added to a distillation mixture. This increases separation ability and may present an attractive option in the separation of azeotropes or close-boiling components. Adsorptive distillation can be used, for instance, for the removal of trace impurities in the manufacturing of fine chemicals (it may allow for switching some fine chemical processes from batchwise to continuous operation). 

The feasibility of combining chemical reaction and adsorption separation in a single unit has been discussed in this chapter. In particular, two units allowing continuous operation have been considered, namely annular reactive chromatography and simulated moving-bed reactors. 

The combination of a second unit in peroxidase reactors may be helpful in different situations, depending on the nature of the substrate to be treated. For instance, a two-stage reactor system for the continuous decolorization of direct dyes was used [44]. The first unit consisted on a fixed bed reactor connected to a second column of activated silica, which helped in the adsorption of toxic reactive species, therefore reducing the biotoxicity of the effluent. The same system was applied for the decolorization of textile effluents and was capable of decolorizing 40% effluent even after 2 months of continuous operation [45]. 

A cyclic adsorption process for citrus oil processing in supercritical carbon dioxide (SC-C02) was studied with silica gel adsorbent. Based on the adsorption equilibrium properties, where adsorbed amounts decreased with the increase in the solvent density and oxygenated compounds were selectively adsorbed on silica gel, a continuous cyclic operation between the adsorption step at 8.8 MPa and 313 K, and the desorption step at 19.4 MPa and 313 K was demonstrated Highly concentrated fraction of oxygenated com pounds was continuously obtained for the desorption and blowdown step. The proposed system showed the feasibility of the continuous operation for citrus oil processing. 

An adsorption-desorption-regeneration cycle using three fixed beds will be used. One bed will be regenerated and purged over an 8-h period. During this period, the other two beds will be on alternate 1-h adsorption and 1-h desorption cycles to permit a continuous operation. A single pass of the gas will be used. 

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. 

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