Reactor-regenerator unit

Description DCC is a fluidized process to selectively crack a wide variety of feedstocks into light olefins. Propylene yields over 24 wt% are achievable with paraffinic feeds. A traditional reactor/regenerator unit design uses a catalyst with physical properties similar to traditional FCC catalyst. The DCC unit may be operated in two operational modes maximum propylene (Type I) or maximum iso-olefins (Type II). Each operational mode utilizes unique catalyst as well as reaction conditions. DCC maximum propylene uses both riser and bed cracking at severe reactor conditions, while Type II utilizes only riser cracking like a modern FCC unit at milder conditions. 

Description DCC is a fluidized process to selectively crack a wide variety of feedstocks into light olefins. Propylene yields over 24 wt% are achievable with paraffinic feeds. A traditional reactor/regenerator unit design uses a catalyst with physical properties similar to tra-... 

The main drawback of kinetic models, based only on steady-state data, is associated with the fact, that start-up and transient regimes cannot be reliably modeled. Kinetic models for nonstationary conditions should be applied also for the processes in fluidized beds, reactions in riser (reactor) - regenerator units with catalyst circulation, as well as for various environmental applications of heterogeneous catalysis, when the composition of the treated gas changes continuously. 

In comparison, units that are designed with turbulent beds have a lower superficial velocity limit because of soflds entrainment and are unable to independently control the entrained soflds recycle. The soflds loading in the turbulent-bed regenerator configuration are equal to the reactor—regenerator circulation plus the entrained soflds via the cyclone diplegs. 

The process consists of a reactor section, continuous catalyst regeneration unit (CCR), and product recovery section. Stacked radial-flow reactors are used to minimize pressure drop and to facilitate catalyst recirculation to and from the CCR. The reactor feed consists solely of LPG plus the recycle of unconverted feed components no hydrogen is recycled. The liquid product contains about 92 wt% benzene, toluene, and xylenes (BTX) (Figure 6-7), with a balance of Cg aromatics and a low nonaromatic content. Therefore, the product could be used directly for the recovery of benzene by fractional distillation (without the extraction step needed in catalytic reforming). 

A clear understanding of the pressure balance is extremely imptiriant in squeezing the most out of a unit. Incremental capacity can come from increased catalyst circulation or from altering the differential pressure between the reactor-regenerator to free up the wet gas compressor or air blower loads. One must know how to manipulate the pressure balance to identify the true constraints of the unit. 

Using the drawing(s) of the reactor-regenerator, the unit engineer must be able to go through the pressure balance and determine whether it makes sense. He or she needs to calculate and estimate pressures, densities, pressure buildup in the standpipes, etc. The potential for improvements can be substantial. 

To maximize the unit s profit, one must operate the unit simultaneously against as many constraints as possible. Examples of these constraints are limits on the air blower, the wet gas compresst>r. reactor/regenerator temperatures, slide valve differentials, etc. The conventional regulatory controllers work only one loop at a time and they do not talk to one another. A skilled operator can push the unit against more than one constraint at a time, but the constraints change often. To operate closer to multiple constraints, a number of refiners have installed an advanced process control (APC) package either within their DCS or in a host computer. 

Catalyst circulation is like blood circulation to the human body. Without proper catalyst circulation, the unit is dead. Troubleshooting circulation problems requires a good understanding of the pressure balance around the reactor-regenerator circuit and the factors affecting catalyst fluidization. The fundamentals of fluidization and catalyst circulation are discussed in Chapter 5. 

The product alkenes are insoluble in the alcohol and phase separation takes place. After settling, the alcohol layer goes to a regeneration unit. The alkene layer is washed and ethene is recycled to the reactor. The products are distilled and the desired fractions are collected. 

Not listed in Table 6.4 of possible unit modifications are improved unit controls. These can be applied in many ways to both the reactor/regenerator and gas plant. Operating closer to limits increases revenue by forcing the operation to several limits rather than one or two. A good FCC model is needed if all the benefits are to be realized. 

Since CPP cracking effluent is molecularly similar to that of heavy distillate cracking it will be logical to construct integrated CPP-steam cracker units, or even to add a CPP reactor-regenerator as a revamp side-cracker expansion feature. Various plans are under review for such prospective projects. 

The fluidized reactor system is similar to that of a refineiy FCC unit and consists of riser reactor, regenerator vessel, air compression, catalyst handling, flue-gas handling and feed and effluent heat recovery. Using this reactor system with continuous catalyst regeneration allows higher operating temperatures than with fixed-bed reactors so that paraffins, as well as olefins, are converted. The conversion of paraffins allows substantial quantities of paraffins in the feedstream and recycle of unconverted feed without need to separate olefins and paraffins. 

Use oxygen enrichment in regeneration unit Integrate regeneration energy with air blower Eliminate fluid-bed reactors Fugitives... 

Early fluid-catalyst units employed a bank of many small-diameter cyclones in parallel, but this practice was superseded by the use of a smaller number of large-diameter cyclones (97). These are typically 3 to 5 ft. in diameter and 10 to 15 ft. high, although rough-cut separators with diameters up to 8 ft. have been reported (272). Two stages of cyclones in series are ordinarily used in the reactor. In units without Cottrells, two stages are usually employed also in the regenerator. Three... 

In commercial reactors, the acid phase is recirculated numerous times. A small amount of the used acid is removed and sent to the regeneration unit. A similar amount of feed acid is added to the recycle stream. At least with sulfuric acid, major reductions of acid consumption can be obtained when two and preferably more reactors are used in a refinery. Proper arrangements of the acid flows, method of adding different olefins, and adjusting operating conditions in the different reactors can often result in the reduction of acid composition from about 0.5-0.8 to 0.25-0.3 Ib/gal. 

In order to regenerate the activity of the catalyst completely, a small amount of fresh catalyst is added to the system from time to time. Figure 6.11 depicts typical reactor-regenerator sections of a catalytic cracking unit used in petroleum refineries. 

Finally, the two units (reactor, regenerator) are fluidized beds and it is well known how poorly understood the fluid mechanical characteristics of such units are. 

The SO additives are formed by an oxide function, usually Ce203, which can rapidly oxidize SO2 to SO3. This then reacts with a more basic oxide, such as MgO or MgO-A1203, also present in the additive, and forms the corresponding metal sulfate. All these processes occur in the regenerator unit. Then, when the additive is passed to the reactor, it reacts with H2 to either be regenerated, forming H2S and H2O, or to form a sulfide and H2O. The sulfide can then be hydrolyzed in the stripper to form H2S. The different reactions occurring are schematized in fig. 9. 

Description The DCC process selectively cracks a wide variety of feedstocks into light olefins, with a reactor/regenerator configuration similar to traditional fluid catalytic cracking (FCC) units (see figure). Innovations in catalyst development and process variable selection lead to synergistic benefits and enable the DCC process to produce significantly more olefins than an FCC that is operated for maximum olefins production. 

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