Lean-rich exchanger

In the amine regenerator, the rich amine solution is heated to reverse the acid-base reaction that takes place in the contactor. The heat is supplied by a steam reboiler. The hot, lean amine is pumped from the bottom of the regenerator and exchanges heat with the rich amine in the lean-rich exchanger and a cooler before returning to the contactor. 

The sweetened gas goes overhead and is sent to the fuel gas system. The rich amine exits the bottom of the scrubber and is heated in the lean—rich exchanger. It then enters the amine regenerator, where a steam reboiler is used to heat the amine to 225—250 °F. At these temperatures, the salts are thermally dissodated to regenerate the amine. The add gas, composed primarily of H2S, is sent to a sulfur recovery unit that generates elemental sulfur. 

In determining the lean/rich exchanger duty, Qm, it is first necessary to determine the regenerator bottoms temperature, Tg. This temperature is fixed by the lean amine composition, the lean amine type, and the regenerator pressure profile. As previously noted, a pressure drop of about 5 psi from the reflux drum to the stripper bottoms should be allowed. Available aqueous amine solution vapor pressure data are then used to determine the stripper bottoms temperature, Tg. See Figures 2-56,2-57,2-58,2-59,2-60, and 2-61 for vapor pres-sure/temperature data for MEA, DEA, DGA, lEA, DIPA, and MDEA, respectively. 

Figures also generally apply to heal exchanger tubing. However, Manning and Thompson (1991) recommend a maximum tubeside velocity of 2 to 4 ji/sec for lean/rich exchangers where rich amine is on the tubeside. 

Recommended velocity applies only to rich DEA flowing through the tubeside of a lean/rich exchanger. Dailey (1970) and Smith and Younger (1972) rqport Aat severe corrosion occurred in the tubes of a lean/rich exchanger when the tubeside velocity exceeded 5.25 ft/sec. 

The hot lean amine proceeds to the rich/lean amine exchanger and then to additional coolers to lower its temperature to no less than 10°F above the inlet gas temperature. This prevents hydrocarbons from con-... 

Rich/lean amine exchangers are usually shell-and-tube exchangers with the corrosive rich amine flowing through the tubes. The purpose of these exchangers is to reduce the reboiler duty by recovering some of the sensible heat from the lean amine. 

Fig. 2.13. Split flow amine sulfur ronoval process, (a) Absorber, (b) regenerator (c) lean/rich solution heat exchanger (d) cooler, (e) reboiler, (f) reclaimer, (g) condenser...

Qlr = Lean/rich heat exchanger duty. exchanger... 

The lean oil from the lean-oil fractionator passes through several heat exchangers and then through a refrigerator where the temperature is lowered to —37° C. Part of the lean oil is used as a reflux to the lower section of the rich-oil deethanizer. Most of the lean oil is presaturated ia the top section of the deethanizer, is cooled again to —37° C, and is returned to the top of the absorber, thus completing the oil cycle. 

The third key section of the process deals with ethylene oxide purification. In this section of the process, a variety of column sequences have been practiced. The scheme shown in Figure 2 is typical. The ethylene oxide-rich water streams from both the main and purge absorbers are combined, and after heat exchange are fed to the top section of a desorber where the absorbate is steam stripped. The lean water from the lower section of the desorber is virtually free of oxide, and is recirculated to the main and purge absorbers. The concentrated ethylene oxide vapor overhead is fed to the ensuing stripper for further purification. If the desorber is operated under vacuum, a compressor is required. 

Rich glycol is leaking into lean glycol in the glycol heat exchanger or in the glycol pump... 

Check the glycol to glycol heat exchanger in the accumulator for leakage of wet, rich glycol into the dry, lean glycol. 

Throughout this bocdt, several mass-exchange operations will be considered simultaneously. It is therefore necessary to use a unified terminology such that y is always the composition in die rich phase and x is the composition in the lean phase. The reader is cautioned here that tiiis terminology may be different ftom other literature, in which y is used for gas-phase composition and x is used for liquid-phase composition. 

As has been previously mentioned, the minimum TAC can be identified by iteratively varying e. Since the inlet and outlet compositions of the rich stream as well as the inlet composition of the MSA are fixed, one can vary e at the rich end of the exchanger (and consequently the outlet composition of the lean stream) to minimize the TAC of the system. In order to demonstrate this opdmization procedure, let us first select a value of e at the rich end of the exchanger equal to 1.5 X 10 and evaluate the system size and cost for this value. 

As can be seen from Fig, 3.7, the pinch decomposes the synthesis problem into two regions a rich end and a lean end. The rich end comprises all streams or parts of streams richer than the pinch composition. Similarly, the lean end includes all the streams or parts of streams leaner than the pinch composition. Above the pinch, exchange between the rich and the lean process streams takes place. External MSAs are not required. Using an external MSA above the pinch will incur a penalty of eliminating an equivalent amount of process lean streams from service. On the other hand, below the pinch, both the process and the external lean streams should be used. Furthermore, Fig. 3.7 indicates that if any mass is transferred across the pinch, the composite lean stream will move upward and, consequently, external MSAs in excess of the minimum requirement will be used. Therefore, to minimize the cost of external MSAs, mass should not be transferred across the pinch. It is worth pointing out that these observations are valid only for the class of MEN problems covered in this chapter. When the assumptions employed in this chapter are relaxed, more general conclusions can be made. For instance, it will be shown later that the pinch analysis can still be undertaken even when there are no process MSAs in the plant. The pinch characteristics will be generalized in Chapters Five and Six. 

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