Carbon bed

When the bed is saturated, regeneration of the adsorbent is necessary. Carbon beds are typically regenerated with steam, hot air, or a combination of vacuum and hot gas. 

Gaseous Effluents. Twenty percent of the carbon disulfide used in xanthation is converted into hydrogen sulfide (or equivalents) by the regeneration reactions. Ninety to 95% of this hydrogen sulfide is recoverable by scmbbers that yield sodium hydrogen sulfide for the tanning or pulp industries, or for conversion back to sulfur. Up to 60% of the carbon disulfide is recyclable by condensation from rich streams, but costly carbon-bed... 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

Some natural gases have also been found to contain mercury, which is a reformer catalyst poison when present in sufftciendy large amounts. Activated carbon beds impregnated with sulfur have been found to be effective in removing this metal. 

When the adsorption capacity of a carbon unit is exceeded, there is breakthrough of the contaminant in the treated stream. Eixed beds may be operated in series to allow continuous treatment while spent or exhausted units are replaced with fresh carbon. In series operation, there are two or more units. The majority of the contaminant is removed by the first unit in the series with the downstream units acting as polishing units. When breakthrough occurs in the first or primary unit, it is replaced with fresh carbon and becomes a polishing unit while the next unit in the series takes over and becomes the primary treatment unit. An example of this round-robin operation of a three-carbon bed is shown in Eigure 2. 

Process and environmental air is compressed and passed through activated beds to reduce air emission levels to <5 ppm. Process wastewater is air stripped to remove CCl. The solvent containing air is also passed through the activated carbon beds. The total air flow through the beds averages about 3965 mVmin (140,000 SCFM). 

Liquid-liquid extraction is used primarily when distillation is imprac-tic or too costly to use. It may be more practical than distillation when the relative volatility for two components falls between 1.0 and 1.2. Likewise, liquid-liquid extraction may be more economical than distillation or steam-stripping a dissolved impurity from wastewater when the relative volatility or the solute to water is less than 4. In one case discussed by Robbins [Chem. Eng. Prog., 76 (10), 58 (1980)], liquid-liquid extraction was economically more attractive than carbon-bed or resin-bed adsorption as a pretreatment process for wastewater detoxification before biotreatment. 

Premature shutdown of fans/venti-lation system immediately following shutdown of heat input (prior to sufficient cooling) resulting in hot spots and flammable pockets (dryers, carbon beds, and thermal oxidizers). Possibility of subsequent ignition resulting in fire or explosion. 

The measured pressure drops were slightly greater than literature data would indicate for packed carbon beds. However, they are certainly not prohibitive and a successful outcome of the Westinghouse trial of the SOFC guard bed is anticipated. 

The resulting adsorption behavior in an unsteady-state fixed bed adsorber is illustrated in Fig. 7 [32], As the gas stream enters the carbon bed, which is initially free of adsorbate, the adsorbate is rapidly adsorbed, and the gas is essentially free of adsorbate as it continues through the carbon bed. As the adsorbent at the inlet... 

During the process of adsorbing fuel vapors onto a bed of activated carbon, the higher molecular weight components of the vapor mixture tend to build up within the smaller pores in the carbon. While purging (desorption) of the carbon bed is capable of removing a large portion of the adsorbed fuel vapor, these heavier components are not completely removed, and this residual vapor is left in the activated carbon, and is commonly referred to as the "heel" of the carbon bed. 

Once the heel has been established in the carbon bed, the adsorption of the fuel vapor is characterized by the adsorption of the dominant light hydrocarbons composing the majority of the hydrocarbon stream. Thus it is common in the study of evaporative emission adsorption to assume that the fuel vapor behaves as if it were a single light aliphatic hydrocarbon component. The predominant light hydrocarbon found in evaporative emission streams is n-butane [20,33]. Representative isotherms for the adsorption of n-butane on activated carbon pellets, at two different temperatures, are shown in Fig. 8. The pressure range covered in the Fig. 8, zero to 101 kPa, is representative of the partial pressures encountered in vehicle fuel vapor systems, which operate in the ambient pressure range. 

In this example, the one liter canister is designed as a cylinder with a length-to-diameter (L/D) ratio of five. The vapor feed stream to the canister is a 50/50 mixture of n-butane and air, and the inlet flow rate is set at 40 grams per hour of n-butane. The curves in the Fig.9 show that break through occurs shortly after the 100 minute point in the load. Up to break-through, the activated carbon bed has adsorbed about 65 grams of HC. 

Hydrocarbon vapor migration within the carbon canister is a significant factoi during the real time diurnal test procedure. The phenomenon occurs after the canister has been partially charged with fuel vapors. Initially the hydrocarbons will reside primarily in the activated carbon that is closest to the fuel vapor source. Over time, the hydrocarbons will diffuse to areas in the carbon bed with lower HC concentration. Premature break through caused by vapor migration for twc different canisters is shown in Fig. 17. The canister with the L/D ratio of 5.0 shows substantially lower bleed emissions than the canister with an L/D ratio of 3.0. 

Table 6. Other parameters/operating conditions affecting canister performance and design Recirculating Fuel System Non-Recirculating Fuel System Single vs. Multiple Carbon Beds Gasoline vs. Alcohol-based Fuels Liquid Fuel Ingestion into Carbon Bed Water Ingestion into Carbon Bed Dispensed Fuel Temperature...

Adsorption for gas purification comes under the category of dynamic adsorption. Where a high separation efficiency is required, the adsorption would be stopped when the breakthrough point is reached. The relationship between adsorbate concentration in the gas stream and the solid may be determined experimentally and plotted in the form of isotherms. These are usually determined under static equilibrium conditions but dynamic adsorption conditions operating in gas purification bear little relationship to these results. Isotherms indicate the affinity of the adsorbent for the adsorbate but do not relate the contact time or the amount of adsorbent required to reduce the adsorbate from one concentration to another. Factors which influence the service time of an adsorbent bed include the grain size of the adsorbent depth of adsorbent bed gas velocity temperature of gas and adsorbent pressure of the gas stream concentration of the adsorbates concentration of other gas constituents which may be adsorbed at the same time moisture content of the gas and adsorbent concentration of substances which may polymerize or react with the adsorbent adsorptive capacity of the adsorbent for the adsorbate over the concentration range applicable over the filter or carbon bed efficiency of adsorbate removal required. 

The adsorbers are usually built of steel, and may be lagged or left unlagged the horizontal type is shown in Figure 28. The vapor-laden air is fed by the blower into one adsorber which contains a bed of 6- to 8-mesh activated carbon granules 12 to 30 inches thick. The air velocity through the bed is 40 to 90 feet per minute. The carbon particles retain the vapor only the denuded air reaches the exit, and then the exhaust line. The adsorption is allowed to continue until the carbon is saturated, when the vapor-laden air is diverted to the second adsorber, while the first adsorber receives low-pressure steam fed in below the carbon bed. The vapor is reformed and carried out by the steam. The two are condensed and if the solvent is not miscible with water, it may be decanted continuously while the water is run off similarly. After a period which may be approximately 30 or 60 minutes, all the vapor has been removed, the adsorbing power of the charcoal has been restored, and the adsorber is ready to function again, while adsorber No. 2 is steamed in turn. 

Even at 1,500 F, equilibrium eonstants for the first two reactions are high enough (about 10) to expect reaction to go essentially to completion except for kinetic-rate limitations. The reaction zone might be expected to be sized by volume of rabbled carbon bed, considering that the carbon gasification reactions that occur in it are governed by kinetics and are reaction-rate limited. Actually, it is sized by hearth area. The area exposed to the gases controls mass transfer of reactants from the gas phase to the carbon and heat transfer to support the endothermic reactions. 

It is also preferred where suspended solids create a high pressure drop, or dissolved gases create bubbles in the carbon bed. For a downflow or percolation system, an influent line should be installed at the top of the column, with an effluent at the bottom. To prevent the column from draining during operation, the effluent line from the last column should extend from the bottom of the column to above the top of the column. This will keep the column filled with liquid at all times during operation and prevent siphoning from occurring. 

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