Stripper design, solvent

Generahzed prediction methods for fci and Hi do not apply when chemical reaction occurs in the liqmd phase, and therefore one must use ac tual operating data for the particular system in question. A discussion of the various factors to consider in designing gas absorbers and strippers when chemical reac tious are involved is presented by Astarita, Savage, and Bisio, Gas Treating with Chemical Solvents, Wuey (1983) and by Kohl and Ricseufeld, Gas Purification, 4th ed., Gulf (1985). 

Solvent losses constitute an expensive item which must be predicted when designing and evaluating commercial solvent extraction processes. A solvent make-up cost of 250,000 annually would not be unusual for a large lubricating oil solvent extraction plant of this loss, it is likely that only 40 to 60% would be found in the residual streams from raffinate and extract strippers. The balance would disappear as leakage, decomposition, and unexpected upset losses. 

Design of Absorber-Stripper Systems The solute-rich liquor leaving a gas absorber normally is distilled or stripped to regenerate the solvent for recirculation back to the absorber, as depicted in Fig. 14-3. It is apparent that the conditions selected for the absorption step... 

The number of independent variables required to define the operation of an absorber or stripper may also be determined by applying the description rule, stated in Section 5.2.1. The number of trays or the column height is set by construction and may, in the design phase, be used as design variables. Since, by definition, the feeds are introduced at the top and bottom of the column, the feed locations are not variable. The feed compositions and thermal conditions are set outside the column region and are therefore beyond the operator s control. The operator can, however, control the valves on the two feeds and the two products. One of these four valves, usually the bottoms product valve, cannot be controlled independently since it must be set at steady state such as to maintain the required liquid level in the bottom of the column. The overhead valve is usually used to control the column pressure. The two feed valves may be controlled independently one controls the main process stream rate and the other controls the solvent or stripping gas flow rate. 

The combination of effluent clean-up and solvent distillation should be considered in the design of a stream stripper. For water-miscible solvents that do not form water azeotropes, such as methanol and acetone, the conversion of the stripping column into a fractionating column presents few problems (Fig. 3.4). Similarly, the solvents that are sparingly water miscible can be passed through a decanter and the water phase returned to the stripper feed. 

Solvent or stripped liquid concentration > design boilup rate or steam stripping rate too low/feed concentration > expected/feed contamination for sour water stripper acid in feed may be chemically bonded with NH3 and prevent adequate stripping of NH3/[foaming] /leak in preheater exchanger/[column malfunction]. ... 

There are two key design optimization variables in this process the solvent-to-feed ratio and the pressure in the stripper. The amount of solvent is dictated by the specified purity of the hydrogen gas leaving the absorber. A purity of 90mol% hydrogen is assumed. The required solvent-to-feed molar ratio is 1.66 as shown in Figure 14.14. The CO2 is then stripped from the solvent with a design specification of only 0.16 mol% CO2 in the lean solvent recycled back to the absorber. 

Since the set-up is intended to perform research and development of solvents, it shall be flexible in its setting of parameters. A small solvent buffer tank is included as an absorber sump. Solvent refill or replacement will be possible in different ways, but the main one is via the stripper condenser vessel. It is intended that different solvents are to be tested at for example different gas/liquid ratio s, striper pressures/temperatures and gas compositions. However, it is important to note that the design takes into account amine solutions in water. After small modification, the pilot unit can also be tested with non-aqueous solvents. 

Although the rate of absorption of CO2 and H2S into a DEA solvent is less than into an MEA solvent, the DEA solution is somewhat easier to strip than an MEA solution. Usually the same Hol value is used to design DEA strippers as is used for the design of MEA strippers. The DEA solvent, however, can be regenerated to a lower acid gas concentration— about 0.07 to 0.08 mol CO2 per mol DEA—than is typical for an MEA solvent. Because of the low vapor pressure of DEA, a wash bed may not be required at the top of the stripper. 

Column heights for amine plant absorbers and strippers are usually established on the basis of experience with similar plants. Almost aU installations that utilize primary or secondary amines for essentially complete add gas removal are designed with about 20 trays (or a packed hei t equivalent to 20 trays) in the absorber. In bulk add gas rmnoval applications, experience has shown that if a 20-tray column is supplied widi sufSdent amine so dial the rich solvent leaving the absorber has an add gas loading that is 75 to 80% of the equilibrium value, then the amine on the upper 5 to 10 absorber trays is very close to equilibrium widi the... 

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