Physical absorption process

Hybrid Processes. A number of processes have been developed which use both chemical and physical absorption solvents to offer high purity treat gas and low energy solvent regeneration. The operation of these processes is usually similar to that of the individual chemical or physical absorption processes. The solvent composition is typically customized to meet the requirements of individual appHcations. 

There are two main schemes proposed for sequestration of carbon dioxide. The first (referred to as a chemical absorption process), suitable for use at low pressures and temperatures, is usually adopted where the CO2 is to be removed from exhaust flue gases. The second (usually referred to as a physical absorption process), for use at higher pressures, is recommended for separation of the CO2 in syngas obtained from conversion of fuel. 

Fig. 8,2, The physical absorption process (after Chiesa and Consonni [."I)).

Figure 5. Simplified scheme of an integrated gasification combined cycle (IGCC) coupled with a pre-combustion C02 capture and storage unit using a physical absorption process [5].

In the physical absorption process, the CO2 is absorbed in a solvent according to Henry s Law and then regenerated using heat, pressure reduction or both heat and pressure reduction. Typical solvents are Selexol (dimethylether of polyethylene glycol) and Rectisol (cold methanol) which are applied at high pressure. At lower pressures, the chemical absorption processes are more economical. The Selexol physical solvent process is frequently specified for coal gasification applications199. 

The C02 is removed in either a chemical, a hybrid, or a physical absorption process. Residual C02 contents are usually in the range of 50 to 1000 ppmv, depending on the type and design of the removal unit. The physical absorption processes may be designed for zero heat consumption. But for comparison with the chemical processes, the mechanical energy requirements have to be considered.53... 

Such situations would exist, for instance, for NH3 absorption into an acid solution, for S02 absorption into an alkali solution, for vaporization of water into air, and for H2S absorption from a dilute-gas stream into a strong alkali solution, provided there is a large excess of reagent in solution to consume all the dissolved gas. This is known as the gas-phase mass-transfer limited condition, when both the liquid-phase resistance and the back pressure of the gas equal zero. Even when the reaction is sufficiently reversible to allow a small back pressure, the absorption may be gas-phase-controlled, and the values olhG and HG that would apply to a physical-absorption process will govern the rate. 

Chemical absorption processes Physical absorption processes ... 

Hybrid solvents attempt to combine the high gas purity offered by chemical absorption and the flash regeneration along with lower energy requirements of physical absorption. Processes such as Sulfinol are a mixture of sulfolane, DIPA or MDEA, and water. If the requirement is the total removal of the acid gases, then DIPA is used. If only H2S is to be removed, then MDEA can be used. Another process is the Flexisorb PS which can treat gases to less than 50 ppm C02 and 4 ppm H2S. 

In addition to the processes described above, there are a number of other physical absorption processes which will not be discussed here in detail either because they can be reasonably used for the purification of methanol syngas from coal with economically justifiable expenditure only in exceptional cases, or because they have not yet been tested on an industrial scale. These processes include... 

The removal of acid gases from gas streams can be generally classified into two categories (1) chemical absorption processes and (2) physical absorption processes. There are several such processes that fit into these categories (Tables 23.3 and 23.4) the features of the individual process may vary (Table 23.4 van den Berg and de Jong, 1980 Bodle and Heubler, 1981). 

Physical absorption processes suffer from the fact that they frequently encounter difficulty in reaching the low concentrations of hydrogen sulfide required in the sweetened gas stream. However, there are processes that, with proper attention and care to regeneration cycles, can meet this specification. 

Whilst in physical absorption processes the partial pressure Pj of the absorbed component i increases approximately proportional to the mole fraction x,- in the solution, during chemical absorption when x,-is small p,- is initially very low (good initial loading capacity of the solvent is the basis of the case Vp > 1). With increasing jf,-, Pi shows a sharp increase. 

For the calculation of gas solubilities for physical absorption processes both approaches discussed in Section 5.1 (Eqs. (5.9) and (5.10)) for VLE calculations can be applied. 

Physical absorption processes are temperature and pressure dependent, with absorption occurring at high pressures and low temperatures. Such processes are typically used for carbon dioxide separation when partial pressures of carbon dioxide are high. For low CO2 partial pressure a significant amount of compression is needed. Thus, a significant energy penalty will be incurred making the process uneconomical. 

Absorption of Impurities by Reactive Solvents, 1330 Physical Absorption Processes, 1332 Condensation, 1332... 

Conventional high and low temperature shift conversion carbon dioxide removal preferably by a physical absorption process methanation. 

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