Standpipes pressure balance

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 ... 

Figure 5-10. Pressure balance survey with calculated standpipe densities,...

If the unit pressure balance indicates that either the pressure gain in the standpipes is inadequate or the delta P across the slide valves is erratic, standpipe aeration and instrumentation should be examined. Redesigning the aeration systems or replacing the standpipes can gain valuable pressure drop. Proper instrumentation can include independent aeration flow to each tap, flow indicators/controllers on each, and differential pressure indicators between the taps. 

To achieve a pressure balance, Reh did not require a standpipe of considerable height. Providing a slide valve for his operating temperature, 1,100°C, might be difficult, and he elected a device with no moving parts, the J-leg, for balancing the pressure drop across his fast bed. 

Pressure Balance - Operability. Overall operation, as in catalytic cracking, depends on use of standpipes to generate the pressure differentials necessary to cause shale and balls to flow into the process vessels. Excess pressures are taken out by slide valve control which also dampens the transfer of pressure surges between vessels. 

As an example of the operation of a standpipe, we will consider how an overflow standpipe operating in fluidized bed flow reacts to a change in gas flow rate. Figure 8.17(a) shows the pressure profile over such a system. The pressure balance equation over this system is ... 

Consider now the case of an underflow standpipe operating in packed bed flow (Figure 8.18), the pressure balance across the system is given by ... 

If the gas flow from the lower bed increases, the pressure drop across the upper bed distributor increases to Apd(new)- The pressure balance then calls for an increase in standpipe pressure drop. Since in this case the standpipe length is... 

Figure 8.18 Pressure balance during operation of an underflow standpipe effect of increasing gas flow through fluid beds... 

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... 

Another type of instability can occur when an upflow riser is directly coupled with a downcomer that returns entrained particles to the bottom of the riser. A pressure balance between the riser and the downcomer is required to maintain steady operation. If the gas velocity is decreased at a given solids circulation rate, a critical state may be reached at which steady operation at a given solids flux is impossible instability occurs because solids cannot be fed to the riser at the prescribed rate. Such an instability, referred to as standpipe-induced (Bi et al., 1993), occurs at a lower critical velocity for a higher solids holdup in the riser. The point of instability can be predicted based on an analysis of the pressure balance in the riser-downcomer loop (Bi and Zhu, 1993). To circumvent stand-pipe-induced instability, the solids inventory in the standpipe needs to be sufficiently high or, alternatively, the riser needs to be uncoupled from the downcomer, e.g., by employing screw feeders. 

A nonmechanical device operating in the valve mode is always located at the bottom of an underflow standpipe operating in moving packed bed flow. The standpipe is usually fed by a hopper, which can either be fluidized or nonfluidized. Knowlton and Hirsan (1978) and Knowlton et al. (1978) have shown that the operation of a nonmechanical valve is dependent upon the pressure balance and the geometry of the system. 

For a fluidized standpipe, the drag force of particles balances the pressure head as a result of the weight of solids. If the Richardson and Zaki form of equation (Eq. (8.55)) is proposed for the drag force, derive an expression for the leakage flow of gas in this standpipe. Discuss the effect of particle size on the leakage flow assuming all other conditions are maintained constant. 

This pressure drop must be balanced by the pressure drop generated in the overflow standpipe. If the stand-... 

If the gas flow rate through the column is increased, APd will increase. If APy remains constant, then APsp must also increase to balance the pressure drop loop. This is shown as Case II in the pressure diagram of Fig. lOB. Unlike the overflow standpipe case, the solids level in the standpipe cannot rise to increase the pressure drop in the standpipe. However, the AP/Lg in the standpipe must increase in order to balance the pressure drop aroimd the loop. This occurs in a packed bed standpipe because of an increase in in the standpipe. This can be visualized with the aid of Fig. IOC. 

Study on application of closing valve in the return leg, observation of an identifiable particle, using a device to record the force imparted by returning solids form the cyclone, measuring the pressure drop across the constriction in the return loop, and estimating solids mass flow from the heat balance on a calorimetric section in the standpipe... 

---