Regenerator pressure balance

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. 

Model IV Regenerator and reactor at approximately equal elevation and pressure. Catalyst circulates through U-bends, controlled by pressure balance and variable dense-phase riser. 

Pressure balance deals with the hydraulics of catalyst circulation in the reactor/regenerator circuit. The pressure balance starts with the static pressures and differential pressures that are measured. The various pressure increases and decreases in the circuit are then calculated. The object is to ... 

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. 

The pressure balance survey indicates that neither the spent nor the regenerated catalyst standpipe is generating optimum pressure head. This is evidenced by the low catalyst densities of 20 Ib/ft (320 kg/m ) and 25.4 Ib/ft (407 kg/m ), respectively. As indicated in Chapter 8, several factors can cause low pressure, including under or over ... 

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 pressure balance should be examined to determine the normal pressure readings in the reactor, regenerator, air system, flue gas system, and main fractionator and overhead system. These need to be followed on a time basis and plotted against variables such as feed rate, wet gas rate, and dry gas rate to see if and where problems may occur. Adjustments may be possible if the spent or regenerated catalyst slide valve delta P is at a minimum to provide more operating room. 

Generally, FCC units operate in a heat-balanced mode whereby the heat generated by the burning of coke is equal to the heat needed for the vaporization of the feed plus the heat of cracking. Also, the pressure balance of an FCC unit is very important to ensure proper catalyst circulation and to prevent contact between the hydrocarbons (reactor) and air (regenerator). Overall, this makes the optimal operation of a unit a very interesting challenge. 

Since FCC catalyst is kept above minimum fluidization conditions everywhere in this catalyst circulation loop, the fluidized catalyst is free to flow from one place to another. Thus the catalyst circulation is driven by the overall pressure balance of the unit, and the circulation rate is regulated by the two slide valves, i.e., the stripper slide valve and the regenerator slide valve. A minimum pressure drop across each slide valve is set in the control system to guard against flow reversal, which is a very serious safety issue. For instance, a reverse flow of hydrocarbon vapor from the reactor to the oxygen-rich regenerator can lead to a sudden increase in combustion reaction and regenerator temperature. In the extreme case, a catastrophic explosion could occur. The overall pressure balance of the unit determines the pressure drops available for the slide valve control and hence the maximum catalyst circulation rate. 

Overall comparison between amine and carbonate at elevated pressures shows that the amine usually removes carbon dioxide to a lower concentration at a lower capital cost but requires more maintenance and heat. The impact of the higher heat requirement depends on the individual situation. In many appHcations, heat used for regeneration is from low temperature process gas, suitable only for boiler feed water heating or low pressure steam generation, and it may not be usefiil in the overall plant heat balance. 

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). 

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. 

Regeneration gas requirements are readily obtained once the adsorber is sized and the pressure vessel has been designed. The requirements for gas, a flow rate and duration for heating arise from a heat balance. 

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