Regenerator Stripper

The second CO2 removal is conducted using the same solvent employed in the first step. This allows a common regeneration stripper to be used for the two absorbers. The gases leaving the second absorption step stiU contain some 0.25—0.4% CO and 0.01—0.1% CO2 and so must be methanated as discussed earlier. The CO, CO2, and possibly small amounts of CH, N2, and Ar can also be removed by pressure-swing adsorption if desired. 

The salt process employs scrubbing of the gas with cuprous ammonium salt solution. Carbon monoxide forms a complex with the solution at high pressure and low temperature in the absorption column. The absorbed pure carbon monoxide is released from the solution at low pressure and high temperature in the regenerator/stripper section. Any carbon monoxide that is liberated in the stripper section is removed by subsequently washing the gas with caustic. Sulfur has to be removed in the pretreatment stage to prevent the formation of solid sulfide. 

In addition, the complex problem of unit design (feed nozzles, regenerators, strippers, etc.) and operation has to be addressed properly (15) in order to further extend the limits (higher Ni, V, Fe contaminants as well as more refractory feedstocks) of feeds processable by FCC (16). 

If a waste sulfuric acid regeneration plant is not available, eg, as part of a joint acrylate—methacrylate manufacturing complex, the preferred catalyst for esterification is a sulfonic acid type ion-exchange resin. In this case the residue from the ester reactor bleed stripper can be disposed of by combustion to recover energy value as steam. 

Fig. 7. UOP linear detergent alkylate process Rx = reactor S = settler HR = HF regenerator HS = HF stripper BC = benzene column ...

Release of and regeneration of the metal oxide adsorbent in the stripper. 

More often than not the rate at which residual absorbed gas can be driven from the liqmd in a stripping tower is limited by the rate of a chemical reaction, in which case the liquid-phase residence time (and hence, the tower liquid holdup) becomes the most important design factor. Thus, many stripper-regenerators are designed on the basis of liquid holdup rather than on the basis of mass transfer rate. 

FIG. 14-1 Gas absorber using a solvent regenerated by stripping, a) Absorber, (h) Stripper. 

Not all hydroearbon vapors ean be displaeed from the eatalyst pores in the stripper. A fraetion of them are earried with the spent eatalyst into the regenerator. These vapors have a higher hydrogen-to-earbon ratio than the eoke on the eatalyst. The drawbaeks of allowing these hydrogen-rieh hydroearbons to enter the regenerator are ... 

The flow of the spent catalyst to the regenerator is typically controlled by the use of a valve that slides back and forth. This slide valve (Figure 4-50) is used to control the catalyst level in the stripper. The catalyst level in tlie stripper provides the pressure head that allows tlie catalyst to flow into tlie regenerator. The exposed surface of tlie slide valve is usually lined with a suitable refractory to withstand erosion. 

This segment foeuses on the breaker trip event and how both the train speed and the differential pressure between the regenerator and reaetor stripper ean be eontrolled during this event. Based on the breaker status, aetion is immediately initiated on the expander. Due to the improved reliability in train operation with these patented eontrol teehniques, the PRT ean be better utilized without saerifieing plant reliability. This leads to more effieient FCCU operation and the pay-baek period for tlie improved eontrol system ean be extremely short. 

Breaker opening eauses the expander inlet valve to elose. This, however, would disturb the differential pressure between the regenerator and the reaetor stripper. To keep this pressure eonstant, the bypass valve needs to be opened to keep pressure, P, upstream of the inlet and bypass valves eonstant. Again, this needs to be done earefully. Opening the bypass valve too mueh ean eause the pressure to drop to sueh a level that eatalyst enters the expander. This must be prevented under all eireumstanees. 

Step 7 Calculate the differential in Ae One PRT eontrol objeetive is to maintain the differential pressure between the regenerator and the reaetor stripper. At the time of the breaker opening, it is assumed that the reaetor stripper pressure will not vary. Therefore, to keep the differential pressure eonstant, the regenerator pressure needs to also remain eonstant. Eor the expander, this means that must remain eonstant. To keep P eonstant, the mass flow before and after the breaker opening must remain eonstant (Equations 7-7 and 7-8). This implies that whatever mass flow is redueed on the inlet valve must be rerouted over the bypass valve. 

The capturing agent be physically compatible with the FCC catalyst and easily regenerated in the riser and stripper. 

As discussed in Chapter 1, a portion of the feed is converted to coke in the reactor. This coke is carried into the regenerator with the spent catalyst. The combustion of the coke produces H2O, CO, CO, SO2, and traces of NOx. To determine coke yield, the amount of dry air to the regenerator and the analysis of flue gas are needed. It is essential to have an accurate analysis of the flue gas. The hydrogen content of coke relates to the amount of hydrocarbon vapors carried over with the spent catalyst into the regenerator, and is an indication of the rcactor-stripper performance. Example 5-1 shows a step-by-step cal culation of the coke yield. 

A heat balance can be performed around the reactor, around the stripper-regenerator, and as an overall heat balance around the reactor-regenerator. The stripper-regenerator heat balance can be used to calculate the catalyst circulation rate and the catalysi-to-oil ratio. 

If a reliable spent catalyst temperature is not available, the stripper is included in the heat balance envelope (II) as shown in Figure 5-4. The combustion of coke in the regenerator satisfies the following heat requirements ... 

Using the operating data from the case study. Example 5-5 shows heat balance calculations around the stripper-regenerator. The results are used to determine the catalyst circulation rate and the delta coke. Delta coke is the difference between coke on the spent catalyst and coke on the regenerated catalyst. 

The coke calculation showed the hydrogen content to be 9.9 wtVt. As discussed in Chapter 1, every effort should be made to minimize the hydrogen content of the coke entering the regenerator. The hydrogen content of a well-stripped catalyst is in the range of 5 wt% to 6 wt%. A 9.9 wt% hydrogen in coke indicates either poor stripper operation and/or erroneous flue gas analysis. 

The spent catalyst slide valve is located at the base of the standpipe. It controls the stripper bed level and regulates the flow of spent catalyst into the regenerator. As with the regenerated catalyst slide valve, the catalyst level in the stripper generates pressure as long as it is fluidized. The pressure differential across the slide valve will be at the expense of consuming a pressure differential in the range of 3 psi to 6 psi (20 kp to 40 kp). 

The reactor or stripper catalyst level controller is controlled with a level controller that regulates the movement of the spent catalyst slide valve. The regenerator level is manually controlled to maintain catalyst inventory. 

Normally, the reactor temperature and the stripper level controllers regulate he movement of the regenerated and spent catalyst slide valves, le algorithm of these controllers can drive the valves either fully Of [ or fully closed if the controller set-point is unobtainable. It is ext nely important that a positive and stable pressure differential be mail ined across both the regenerated and spent catalyst slide valves. r safety, a low differential pressure controller overrides the tempera re/level controllers should these valves open too much. The shutdov is usually set at 2 psi (14 Kp). 

Catalyst circulation coke is a hydrogen-rich coke from the reactor-stripper. Efficiency of catalyst stripping and catalyst pore size distribution affect the amount of hydrocarbons carried over into the regenerator. 

A properly designed stripper minimizes the quantity of entrained and adsorbed hydrocarbons that are carried over to the regenerator with the spent catalyst. This goal should be accomplished by the use of... 

Catalyst flux is defined as catalyst circulation rate divided by the full cross-sectional area of the stripper. For efficient stripping, it is desirable to minimize the catalyst flux to reduce the carryover of hydrogen-rich hydrocarbons into the regenerator. 

It is important to note that, depending on the stripper pressure and temperature, a certain fraction of stripping steam is carried with the spent catalyst into the regenerator. Example 7-1 shows how to determine this amount. 

Calculate the amount of entrained stripping steam into the regenerator from a reactor-stripper with the following conditions ... 

A gradual loss of the catalyst level in the reactor stripper and/or in the regenerator... 

Verify the catalyst bed levels in the stripper and regenerator vessels. 

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