Riser termination

The reaetor-generator (Figure 4-45) is die heart of die FCC proeess. In a modern eat eraeker, virtually all of die reaetions oeeur in die riser over a short period 2-4 see before die eatalyst and die produets are separated in die reaetor. However, some diermal and nonseleetive eatalytie eraeking reaetions eontinue to oeeur in die reaetor housing. A number of refineries are modifying die riser termination deviees to eliminate diis problem. 

Riser termination. Good riser termination devices, such as closed cyclones, minimize the vapor and catalyst holdup time in the reactor vessel. This reduces unnecessary thermal cracking and nonselective catalytic re-cracking of the reactor product. The benefits are a reduction in dry gas and a subsequent improvement in conversion, gasoline octane, and flexibility for processing marginal feeds. 

Figure 9-7A. SWEC LD riser termination device (courtesy of Stone and Webster).

This chapter provided several cost recommendations that, once implemented, would provide cost-effective added value to the operation of the FCC. Examples of such items include tips on debottlenecking the air blower, wet gas compressor, and catalyst circulation. This chapter also discussed the latest technologies regarding the riser termination devices, as well as feed injection systems. Prior to implementing any new technologies, it is critical that the objectives and the limitations of the unit are clearly defined to ensure the expected benefits of the new technology are realized. 

RIPP separation sequence (removal of insolubles isolation purification polishing), 5 781-782 RISC enzyme, 27 620 Riser termination technologies, 22 685 Risk... 

Gledhill, D., and Pedersen, J. Operating Experience with the New Riser Termination Technology. Grace Davison FCC Technology Conference, Lisbon, Portugal, September 1-4, 1998, Paper 14. 

An improved riser terminator can lower delta coke, reduce dry gas up to 40%, lower regenerator temperature and allow more feed or higher conversion. Adding post-riser vapor quench can independently reduce dry gas, increase gasoline and reduce diolefins in the alkylation stream and the FCC gasoline. 

Dome steam shonld be added when any close-connected riser terminator is installed. This will rednce coke formation on the top of the reactor vessel and cyclones. The steam mnst be snperheated so that liquid droplets do not impinge on the cyclones and coke doesn t bnildnp on the steam ring. Typical dome steam rates are based on a snperficial velocity high enongh to sweep the hydrocarbons to the reactor catalyst separation system. 

Add dome steam with close connected riser terminators... 

Carryover of catalyst from the reactor disengager vessel to the main fractionator is always a concern and particularly susceptible during start-up operations. The modification of the riser termination as part of a short contact time riser revamp, depicted in Figure 7.3, has increased the concern for catalyst carryover. 

State of the art riser termination devices have significantly reduced the coke deposition in the reactor disengager vessel. These modifications have significantly reduced the hydrocarbon residence time and potential thermal cracking in the disengager. 

In the process, a residuum is desulfurized and the nonvolatile fraction from the hydrodesulfurizer is charged to the residuum fluid catalytic cracking unit. The reaction system is an external vertical riser terminating in a closed cyclone system. Dispersion steam in amounts higher than that used for gas oils is used to assist in the vaporization of any volatile constituents of heavy feedstocks. 

Silverman, M. A. New Stone Webster FCC Riser Terminator Commercial Performance, in Circulating Fluidized Bed Technology IV (Amos A. Avidan.ed.), pp. 577-582. Somerset, Pennsylvania (1993). 

Ramshom riser termination (S W), Direct coupled cyclones (Texaco), Post-riser vapor quench (Amoco)... 

Ultra Orthoflow Stacked configuration Vertical riser terminating in roughcut cyclones Internal baffled stripper... 

Shaw S W side-by-side configuration for maximum propylene production Straight vertical riser terminates into a reactor bubbling bed Low operating pressure unit Additional steam injection into riser for very low hydrocarbon partial pressure... 

In 1979, Exxon built its first Flexicracker which is a side-by-side configured FCC unit with an elevated reactor vessel. This unit contained a straight external vertical riser terminating in a proprietary separation system. 

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. 

Some of the first improvements in riser termination devices were in the form of inertial separators (Figure 20). The inertial or ballistic separator was a device attached to the end of the riser which deflected the catalyst in cracked product 180° in a downward direction toward the base of the reactor vessel. Gravity was used to draw the catalyst into the stripper and density differences to draw the hydrocarbons into the cyclones. 

Mobil Oil developed and commercialized a "closed cyclone system" in which a rough cut cyclone was directly to the riser termination which dramatically reduced the residence time of hydrocarbon vapors in the dilute phase of the reactor vessel thereby dramatically reducing "post-riser cracking". 

The next step is to estimate transition pressure losses, whieh inelude riser liftpot, riser termination, cyclone, spent catalyst distributor, and other flow transitions. By subtracting these transition pressure losses from the hydraulic pressure, the remainder would be the pressure drop available for slide valve control. While other parameters in pressure balance seldom change much at all, it is not uncommon to have the standpipe apparent density lower than the typical... 

After the spent catalyst/product vapor separation from a riser termination system, it is inevitable that some hydrocarbon will be entrained by the spent catalyst from the exit of the riser termination system. Part of the entrained hydrocarbon resides in the vapor space between catalyst particles, and the remainder is physically adsorbed on the surface and inside the pores of the catalyst. The primary objective... 

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