Temperature swing adsorption (TSA)

Principle of operation The TSA process normally operates in a cyclic batch mode where the adsorbent bed is saturated and regenerated alternatively. The regeneration is carried out by increasing the bed tem-perature usually by purging a hot inert gas at constant pressure. The principle of TSA operation is shown in Figure 5.3. It shows the effect of temperature on adsorbent equilibrium loading for a Type I isotherm at an adsorption pressure ofP_ads. As the temperature is increased from adsorption temperature ofT_ads to des-orption temperature ofT_des, the equilibrium loading is also reduced fromq_adstoq_des. After the adsorbent is regenerated, it must be cooled down ready for a new adsorption step. The main disadvantage of TSA is that the number of cycles obtainable in any given time is limited by the relatively slow heating and cooling pro-cess steps. For this reason, TSA is limited to the removal of small quantities of strongly adsorbed impurities.

Table 5.4 General considerations for selecting a regeneration method. Reprinted from [4] c1984, with permission from John Wiley & Sons, Inc

Method Advantages Disadvantages

TSA Good for strongly adsorbed species; small change in temperatures results in large change in adsorbent loading

Thermal aging of adsorbent

Desorbate may be recovered at high concentration

Heat loss mean inefficiency in energy usage

Unsuitable for rapid cycling so adsorbent cannot be used with maximum efficiency Gases and liquids In liquid systems high latent heat of

interstitial liquid must be added PSA Good where weakly adsorbed species is

required in high purity

Very low pressure may be required Mechanical energy is more expensive than

heat

Rapid cycling-efficient use of adsorbent Desorbate recovered at low purity Inert purge Operation at constant temperature and

pressure

Large purge volume required Displacement

desorption

Good for strongly held species Product separation and recovery needed (choice of desorbent is crucial) Avoids risk of cracking reactions during

regeneration

Avoids thermal aging of adsorbent

P_ads

Adsorbent loading (q)

Pressure (P)

T_ads

q_des q_ads

T_des

Figure 5.3 Operating principle of a TSA system

Two-bed TSA systems The simplest TSA system operates with two beds, one adsorbing and the other desorbing, in order to maintain continuous flow. The feed stream containing adsorbate is passed through the first bed at ambient temperature until the bed is saturated or breakthrough occurs. Then the bed is taken off-line and the feed is switched to the second bed. Simultaneously the first bed is regenerated by raising its temperature and purging with a hot inert gas normally in a flow direction opposite to that used for the adsorption step. The regenerated bed is then cooled to ambient temperature by using either cold feed or inert fluid. Although the desorption step can be accomplished in the absence of a purge by simply vaporizing the adsorbate at elevated temperature, re-adsorption of some solute vapor would occur upon cooling the bed.

When the adsorbate is valuable and easily condensed, the purge fluid might be a non-condensable gas.

When the adsorbate is valuable but not easily condensed, and is essentially insoluble in water, steam may be used. Condensation of the steam allows the desorbed adsorbate to be separated. When the adsorbate is not valuable, fuel and/or air can be used as the purge fluid, followed by disposal, for example by incineration.

The heating and desorption steps must provide sufficient energy to perform the following functions:

• to raise the adsorbent, its associated adsorbate and the containment vessel to the desorption temperature;

• to provide the heat of desorption;

• to raise the adsorbent and vessel temperature to final regeneration temperature (if greater than that for desorption).

The adsorbent bed cannot normally be heated and cooled quickly and hence the cycle time of a typical TSA process may range from several hours for a bulk separation to several days for purification. Long cycle times inevitably mean large bed lengths resulting in high adsorbent inventories.

During the period when Bed-1 is adsorbing, Bed-2 is being desorbed, which includes the time required for heating and cooling. The two-bed TSA process requires that the time taken for desorbing gases from one bed matches the time allowed for adsorption in the other bed. Otherwise product flow would be discontinuous. If a longer period is required for desorption, then due to the time constraint, only a fraction of the adsorbate can be removed during the desorption step of the cycle. Bed capacity is consequently not fully utilized.

Because of the long cycle times required for TSA processes this mode of operation is used almost exclusively for the removal of low concentrations of adsorbable gases from feed streams.

Three-bed TSA systems For a fixed-bed system, the amount of adsorbent required to remove the con-taminant from its inlet concentration to the desired level is termed the mass transfer zone (MTZ). As the MTZ progresses through the fixed beds, it reaches a point where the MTZ is longer than the remaining depth of the adsorbent in the vessel still capable of adsorption (not spent). At this point, the concentration of the contaminant begins to increase in the outlet of the adsorbent bed as the MTZ begins to exit the vessel; this is called breakthrough. Some processes may have a long length of unused bed (LUB, which is approximately one-half the mass transfer zone), which can result in huge adsorber size and inefficient usage of adsorbents. This problem can be overcome in a three-bed system where a guard bed is located between the primary adsorber bed and the bed undergoing regeneration [40]. Figure 5.4 is a schematic diagram of the operation.

In this operation, the feed first enters the adsorber bed in which adsorption occurs. When the concentra-tion of primary adsorber effluent reaches nearly the feed concentraconcentra-tion, the beds are switched. The guard bed becomes the primary adsorber, the regenerated bed becomes the guard bed, and the saturated bed goes for regeneration. This rotation is continued to keep the LUB section always in the guard bed and thus

Primary Adsorber C3

Primary Adsorber C2

Primary Adsorber C1 Regeneration Bed C2Regeneration Bed C1Regeneration Bed C3 Guard Bed C1Guard Bed C2 Guard Bed C3

Figure 5.4 Three-bed TSA system

the primary adsorber is completely loaded when regeneration begins. Thus a complete bed utilization and economic regeneration are achieved.

Minimum purge temperature Efficient desorption is achieved above the minimum purge temperature T_o. It is based on the equilibrium theory proposed by Basmadjian [41]. According to this theory,T_o is the temperature at which the slope of the adsorption isotherm at the origin is equal to the ratio of the heat capacities of the solid phase (adsorbent plus adsorbate) and the inert carrier gas (C_ps/C_pb). For a Langmuir isotherm

V = V_monK(T)P

1+K(T)P (5.19)

T_o is determined by

V_monK T_o

= C_ps

C_pb (5.20)

As the temperature is increased beyond T_o, energy cost increases without a significant gain in desorp-tion [3].

Common examples of TSA processes include solvent recovery with activated carbons, and drying of gases or liquids with type A zeolites, removal of water from VOCs with zeolites, gas sweetening, and so forth.