Jan-2009

FCC reactor design: part II

In part I, reaction kinetics, cracking chemistry, pressure balance, catalyst and vapour residence times, and fluidisation were considered. In part II, feed injection technology and riser termination devices to minimise dilute-phase cracking are reviewed

Warren Letzsch, Chris Santner and Steve Tragesser
Shaw Energy & Chemicals Group

Article Summary

The feed injection system is the most important aspect of any fluid cracking process, since this is where the feedstock is brought into contact with the hot catalyst. In addition to vapourising the feed so catalytic cracking can proceed, the entire feed injection system must perform other essential functions, including quenching the hot catalyst coming from the regenerator as quickly as possible and contacting all of the feed with the catalyst.

Since vapourisation of the feed is essential to the process, small oil droplets must be produced to ensure rapid heat transfer. This is illustrated by the following analysis. If oil droplets and catalyst particles are assumed to be spheres of diameter Do and Dc respectively, the weight of the catalyst and oil in the riser is:
                       π
wt of catalyst   Nc • 6  Dc3  • ρc        (1)

                          π
wt of oil          No • 6  Do3  • ρo        (2)

where Nc, No are the number of catalyst and oil particles and ρc, ρo are the densities of the catalyst and oil particles. The catalyst-to-oil ratio is simply (1) divided by (2) or:

CAT/OIL = Nc * Dc3 * ρc        (3)
          No * Do3 * ρo

Substituting for the densities (56 and 90 lb/ft3 for the oil and catalyst respectively), assuming a catalyst-to-oil ratio of 5.0 and a catalyst particle size of 70 microns into Equation 3, a single oil droplet of 280 microns must contact 199 catalyst particles, while a 70 micron oil droplet (the same size as the catalyst particles) would contact about three particles. It is highly unlikely a single droplet can contact about 200 catalyst particles. As a consequence of these hydrodynamics, the larger droplets will not be able to directly quench all of the catalyst particles. This results in a higher average reactor temperature, a higher delta coke and more dry gas in the case of large oil droplets.

A large amount of heat transfer must take place between the catalyst and oil. The smaller droplets have four times the external surface area in the previously mentioned case. The higher heat transfer rate and the smaller diameter drops greatly increase the rate of vapourisation of the oil. Table 1 by Mauleon shows the value of a small oil droplet size for intimate catalyst/oil contacting and rapid vapourisation.

Buchanan’s analysis gave similar results. He also found that heat transfer between the catalyst and gas was almost instantaneous, that heat transfer by radiation is much slower than for convection and that the droplet temperature will be equal to the wet-bulb temperature based on the surface composition. The number of large droplets is important, since a few of these may comprise a relatively large percentage of the feed on a weight basis.

All of the feed nozzles used in fluid catalytic cracking can be classified as either of two fluid nozzles: oil and steam or pressure nozzles. Pressure nozzles atomise by applying pressure to a liquid and then forcing it through an orifice. Today, all of the commercially available feed nozzles offered by FCC licensors are of the two fluid nozzle type. Nukiyama and Tanasawa found the following correlation for two fluid nozzles:

Where:
Dsm    = Sauter mean diameter, microns
σ    = surface tension liquid, dynes/cm
ρ,, ρg    = liquid and gas density, gm/cm3
μ,    = viscosity liquid, poise
U    = relative velocity of gas and liquid
L/G    = liquid/gas mass ratio.

This equation has two terms that contain both the fluid properties and nozzle parameters. The second term suggests the droplets will be smaller if more gas is used relative to the liquid rate, while the first term relates to how the two streams are brought into contact with each other. Higher relative velocities create more shear and smaller droplet sizes. When resid is processed, the droplet sizes tend to increase due to increases in the surface tension and viscosity of the oil. The modified Mogele equation shows this relationship:

Dresid   = dσresid n 0.5dμresidn0.2 dρvgon0.3
Dgasoil     σgasoil      μvgo      ρresid    (5)

Typical numbers for gas oil and resid suggest resid would produce droplets about twice the size of gas oil feeds. Correlations of each of these fluid properties vs temperature show each increases significantly as the feed temperature is reduced from 700–350°F. The changes are largest below 450°F.

Other nozzle features that play a role in feed/catalyst contact are the spray angle, evenness of the spray (uniform flow rate) and penetration into the centre of the riser. These features control the overall distribution of the feed across the riser cross-section in the feed injection zone. If everything is done properly, the temperature in the riser should drop quickly and catalytic cracking will be maximised, and thermal reactions will be suppressed. This is illustrated in Figure 1.

The improvement in yields can be quite significant, with payouts of three months or less for revamps to existing systems. As an example of this, Table 2 gives the before and after yields for one revamp. The reduction in dry gas and delta coke frees the unit of gas and coke limits, and the increase in gasoline of 6.2 vol% is quite valuable. Research octane number may drop by 0.5–1.0 due to the lower average reactor temperatures, even though the riser outlet temperature is unchanged.