The physical absorption process

The CO2 rich solvent is drained from the bottom of the tower, and led first to a hydraulic turbo-expander and then to four flash drums connected in series, where CO2 is de-absorbed as the pressure is lowered. Lean solvent is pumped back to the top of the absorber tower 

X - HEAT EXCHANGER RB - REBOILER 1C - INTERCOOLED COMPRESOR AC- AFTER COOLER 

CONDENSATE TO I STEAM FROM TURBINE CYCLE TURBINE CYCLE 

A - ABSORBER FD - FLASH DRUMS C - COMPRESSORS 1C - INTERCOOLED COMPRESSORS P- PUMP T - HYDRAULIC TURBINE D - DEHYDRATOR AC - AFTER COOLERS X- THROTTLE G GENERATOR 

Manfrida [4] argues that the heat demand and the substantial power loss associated with presssure-swing physical absorption makes it less attractive than chemical absorption, even for high pressure sequestration. The expansion work in the fonner is difficult to recover as. several expanders are needed. 

---

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

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... 

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. 

Reactive absorption represents a process in which a selective solution of gaseous species by a liquid solvent phase is combined with chemical reactions. As compared to purely physical absorption, reactive absorption does not necessarily require elevated pressure and high solubility of absorbed components because of the chemical reaction, the equilibrium state can be shifted favorably, resulting in enhanced solution capacity [2]. Most of the reactive absorption processes involve reactions in the liquid phase only, but in some of them both liquid and gas reactions occur [3, 4]. 

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. 

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. 

Pretreatments of the cellulose fiber surface or physical modifications can clean the fiber surface, chemically modify the surface, stop the moisture absorption process, and increase the surface roughness. - ... 

Native cellulose are commonly modified by physical, chemical, enzymic, or genetic means in order to obtain specific functional properties, and to improve some of the inherent properties that limit their utility in certain application. Physical/surface modification of cellulose are performed in order to clean the fiber surface, chemically modify the surface, stop the moisture absorption process, and increase the surface roughness. " Among the various pretreatment techniques, silylation, mercerization, peroxide, benzoylation, graft copolymerization, and bacterial cellulose treatment are the best methods for surface modification of natural fibers. 

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. 

---