Spent catalyst regeneration, riser

In addition to the remarkable ingenuity observed in successive unit designs, substantial creativity was brought to the market in many areas such as feed distribution, riser termination, spent catalyst regeneration, etc. 

In earlier Model II and Model III FCC units, spent catalyst was transported into the regenerator using 50% to 100% of combu.stion air. This spent cat riser was designed for a minimum air velocity of 30 ft/sec (9.1 m/sec). 

Higher catalyst circulation usually requires opening the regenerated and spent catalyst slide (or plug) valves. Higher circulation increases the pressure drop in the riser and in the reactor cyclones, lowering the differential pressure across the slide valves. This causes the valves to open further, until the unit finds a new balance. 

Other more capital-intensive modifications include installing a dedicated air blower for the spent catalyst riser. The spent catalyst riser often requires a higher back-pressure than the main air blower to deliver the catalyst into the regenerator. Therefore, less total combustion air would be available if one common blower is used to transfer spent catalyst and provide combustion air to the air distributors. 

The regenerator review will include spent catalyst distribution, air distribution, and cyclones. If the test run with heavy feed indicates a temperature limitation, catalyst coolers, partial combustion, or riser quench should be considered. 

In the process (Figure 8-12), the feedstock is vaporized upon contacting hot regenerated catalyst at the base of the riser and lifts the catalyst into the reactor vessel separation chamber where rapid disengagement of the hydrocarbon vapors from the catalyst is accomplished by both a special solids separator and cyclones. The bulk of the cracking reactions takes place at the moment of contact and continues as the catalyst and hydrocarbons travel up the riser. The reaction products, along with a minute amount of entrained catalyst, then flow to the fractionation column. The stripped spent catalyst, deactivated with coke, flows into the Number 1 regenerator. 

The catalyst/oil disengaging system is designed to separate the catalyst from the reaction products and then rapidly remove the reaction products from the reactor vessel. Spent catalyst from the reaction zone is first steam stripped, to remove adsorbed hydrocarbon, and then routed to the regenerator. In the regenerator all of the carbonaceous deposits are removed from the catalyst by combustion, restoring the catalyst to an active state with a very low carbon content. The catalyst is then returned to the bottom of the reactor riser at a controlled rate to achieve the desired conversion and selectivity to the primary products. 

The junction of the spent-catalyst standpipe with the riser carrying catalyst and air to the regenerator and the junction of the regenerated-catalyst standpipe with the reactor riser are potential sources of erosion... 

The spent catalyst mixes with air and clean catalyst at the base of the regenerator. Here the coke deposited during cracking is burned off to reactivate the catalyst and provide heat for the endothermic cracking reactions. The recirculating loop of clean catalyst provides added heat for initiation of the carbon bum. The catalyst and air flow up the regenerator riser and separate at a T-shaped head. The flue gas is further cleaned of catalyst in cyclones at the top of the regenerator. 

The latest model is known as Exxon Flexicracker (Figure 22) (56). In this model, the U-tubes are replaced by a standpipe that is followed by upwardly sloped laterals, referred to as J-bends. This model takes advantage of riser cracking and spent catalyst in the J-tube is controlled by a slide valve. Reactor temperature is controlled by pressure differential between the regenerator and reactor. 

UOP Side-by-side configuration for delta coke limited operations Spent catalyst recycle line from reactor stripper to mixing vessel at base of riser with slide valve control Regenerated catalyst and spent catalyst mixed prior to acceleration and contacting with fresh feed ... 

As feedstock vaporizes and cracks during its sec-onds-long journey through the riser pipe, feedstock vapors are separated from the catalyst at cyclones at the top of the reactor and fed into a fractionator. There, different fractions condense and are extracted as separate products. The catalyst becomes inactive as coke builds on its surface. Spent catalyst is collected and fed into the regenerator, where coke deposits are burned off. The regenerated catalyst is recycled into the reactor at temperatures of 650 to 815 degrees Celsius. 

In this case, it is the fluid medium, the fuel oil preheated to its evaporation point, that undergoes the value-adding conversion. Its long hydrocarbon chains are quickly cracked when encountering the catalyst particles in the fluid mix. The fluid is sent to a reactor. Cyclones at the top of the reactor separate the gaseous hydrocarbon fluid from the spent catalyst and transport it into a distillation unit. The solid catalyst particles are collected at the bottom of the reactor and sent to a regenerator, from where they are fed back into the catalytic riser in a continuous process. 

Gas oil feed is introduced at the bottom of the riser, where it meets hot catalyst from the regenerator. The cracking reaction starts immediately as the mixture of catalyst and gas oil passes through the riser into the reactor. During this encounter, the catalyst is deactivated because of the carbonaceous material deposited on its surface. The product vapor is transferred to the fractionation section and the spent catalyst flows down through the reactor standpipe into the regenerator, where the carbonaceous deposits are burned off. 

As the regenerator temperatures drop, the regenerated catalyst flowing back to the reactor through the regen slide valve cools. This in turn drops the riser outlet temperature, unless the catalyst circulation rate is increased. The cooler riser results in less conversion and hence less coke make. This effect further reduces the coke on the spent catalyst and lowers the regenerator temperature. 

Control of the FCC is achieved by the two slide valves. The regenerated catalyst standpipe slide valve controls the riser reactor outlet temperature by controlling the flow of hot regenerated catalyst supplied to the riser. The spent catalyst standpipe slide valve controls stripper level by adjusting the efflux of catalyst from the stripper. 

Risers are used primarily in FCCs to effect the conversion of the feed oil. They are also used in some older FCC designs (e.g., ESSO Model IV, Kellogg Model III) to air lift spent catalyst up into the regenerator. 

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