Feed riser

Figure 10.2 Typical radial catalyst density profile in feed riser (From Ref. 11. 

Slug flow in feed riser Slug flow in rellux-drum inlet flow... 

Minimum Velocity for Vertical Feed Risers 15 to 20 Ft/S (At Bottom)... 

Feed Riser Residence Time 1.8 to 2.4 Seconds (Based on Outlet Moless)... 

The minimum velocities for risers are listed in Table 1. For a feed riser the lowest velocity is normally encountered at the bottom where the oil is first vaporized. In a vertical feed riser (e.g., Figure 6) it is important that the diameter of the riser at the feed injection point is sized to give at least 15 to 20 Ft/S (based on 100% vaporization of the feed). The riser diameter may be increased further up the riser as velocity picks up due to molar expansion. The bottom, narrow part of the riser is often referred to as the acceleration zone. Riser exit velocities are generally in the 60 Ft/S range. Riser velocities rarely exceed the limits of erosion resistant lining (90 Ft/S). 

Riser pressure drop and densities can be calculated by a number of different methods e.g., Jazayeri (1991). Typically feed risers have an average density of 5 Lb/FF 10 Lb/Ft" at the bottom, 1 Lb/FF at the top. Including entrance and exit (separator) effects, riser pressure drop is usually in the 4 psi range. For optimum conversion, riser residence times are designed to be about 1.8 to 2.4 seconds based on outlet moles (i.e., the volume of the riser divided by the volume of hydrocarbon and steam exiting the riser). Most state-of-art risers are 100+ Ft long with an L/D ratio of over 20. 

In regenerators, sets of cyclones are operated as pairs in series to give a combined collection efficiency of about 99.998% at an overall pressure drop of about 1 psi. In modem reactors with riser separators, one stage cyclones are used with about a 99.995% collection efficiency and about 0.5 to 0.7 psi pressure drop. The modified API (1985) method can also be used for rough cut cyclones, i.e., cyclones attached to the end of feed risers as riser separators. 

Cody GD, Bellows RJ, Goldfarb DJ, Wolf HA and Storch GV 2000 A novel non-intrusive probe of particle motion and gas generation in the feed injection zone of the feed riser of a fluidized hed catalytic cracking unit, Proc. ASNEIMECE 2000, Orlando, Florida, US, Nov 5-12... 

Distillation curves Composition (Feed, riser outlet. profile... 

Radial density gradients in FCC and other large-diameter pneumatic transfer risers reflect gas—soHd maldistributions and reduce product yields. Cold-flow units are used to measure the transverse catalyst profiles as functions of gas velocity, catalyst flux, and inlet design. Impacts of measured flow distributions have been evaluated using a simple four lump kinetic model and assuming dispersed catalyst clusters where all the reactions are assumed to occur coupled with a continuous gas phase. A 3 wt % conversion advantage is determined for injection feed around the riser circumference as compared with an axial injection design (28). 

Thus the ECCU always operates in complete heat balance at any desired hydrocarbon feed rate and reactor temperature this heat balance is achieved in units such as the one shown in Eigure 1 by varying the catalyst circulation rate. Catalyst flow is controlled by a sHde valve located in the catalyst transfer line from the regenerator to the reactor and in the catalyst return line from the reactor to the regenerator. In some older style units of the Exxon Model IV-type, where catalyst flow is controlled by pressure balance between the reactor and regenerator, the heat-balance control is more often achieved by changing the temperature of the hydrocarbon feed entering the riser. 

From die preheater, die feed enters die riser near die base where it eontaets die ineoming regenerated eatalyst. The ratio of eatalyst to oil is normally in the range of 4 1 to 9 1 by weight. 

The flowrate of die regenerated eatalyst to die riser is eommonly regulated dirough die use of eidier a slide or plug valve. The operation of a slide valve is similar to that of a variable orifiee. Slide valve operation is automatie. Its main funetion is to supply enough eatalyst to heat the feed and aehieve the desired reaetor temperature. 

For columns in which there is a substantial flash of the feed liquid, or in which the feed is a vapor of a different composition than the internal vapor, a collector plate can be installed above the feed point. The purpose of this plate is to provide mixing of the vapor phase in the gas risers so that a more uniform vapor composition enters the rectifying section of the column. 

Solution Since the distributor annular space w as capable of handling the vapor flow, the riser was simply blanked off. Adding a hat or relocating the feed pipe w ere possible alternate. solutions. 

Fluidized-bed catalytic cracking units (FCCUs) are the most common catalytic cracking units. In the fluidized-bed process, oil and oil vapor preheated to 500 to SOOT is contacted with hot catalyst at about 1,300°F either in the reactor itself or in the feed line (called the riser) to the reactor. The catalyst is in a fine, granular form which, when mixed with the vapor, has many of the properties of a fluid. The fluidized catalyst and the reacted hydrocarbon vapor separate mechanically in the reactor and any oil remaining on the catalyst is removed by steam stripping. 

In the DCC unit, the hydroearbon feed is dispersed with steam and eraeked using a hot solid eatalyst in a riser, and enters a fluidized bed reaetor. A known injeetion system is employed to aehieve the desired temperature and eatalyst-to-oil eontaeting. This maximizes the seleetive eatalytie reaetions. The vaporized oil and eatalyst flow up the riser to the reaetor where the reaetion eonditions ean be varied to eomplete the eraeking proeess. The eyelones that are loeated in the top of the reaetor effeet the separation of the eatalyst and the hydroearbon vapor produets. The steam and reaetion produets are diseharged from the reaetor vapor line and enter the main fraetionator where further proeessing ensure the separation of the stream into valuable produets. 

For distributors of any design, including the PAN, it is important to filter the feed or reflux liquid entering tlie distributor to reduce the possibilities of plugging of tlic orifice holes. Otherwise, the landom plugging will cause non-uniform distribution onto the packing below. It is important to avoid leakage around the risers because this can destroy the liquid distiibution pattern [131]. 

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