Catalyst-to-oil ratio

Most FCC units use fired heaters for FCC feed final preheat. The feed preheater provides control over the catalyst-to-oil ratio, a key-variable in the process. In units where the air blower is constrained. 

The first step in E-cat testing is to bum the carbon off the sample. The sample is then placed in a MAT unit (Figure 3-13), the heart of which is a fixed bed reactor. A certain amount of a standard gas oil feedstock is injected into the hot bed of catalyst. The activity i.s reported as the conversion to 430°F (221°C) material. The feedstock s quality, reactor temperature, catalyst-to-oil ratio, and space velocity are four variables affecting MAT results. Each catalyst vendor uses slightly different operating variables to conduct micro activity testing, as indicated in Table 3-2. 

Increasing catalyst-to-oil ratio by decreasing the feed preheat temperature... 

Figure 6-11. Increased catalyst-to-oil ratio decreases gasoline sulfur [4],...

DO is the lowest priced product and the goal is to reduce its yield. The DO s yield largely depends on the quality of the feedstock and the conversion level. Naphthenic and aromatic feedstocks tend to yield more bottoms than paraffinic feedstocks. If the conversion is m the low- to mid-70 s, increasing catalyst-to-oil ratio or using a catalyst with an active matrix can reduce slurry yield. Raising conversion reduces bottoms yield. If the conversion is in the 80 s, little more can be done to reduce bottoms yields. 

Decrease in the feed preheat temperature and subsequent increase in the catalyst-to-oil ratio... 

The benzene content of FCC gasoline is typically in the range of 0.6 vol /i to 1.3 vol%. CAAA s Simple Model requires RFC to have a maximum of 1 vol% benzene. In California, the basic requirement is also 1 vol% however, if refiners are to comply with averaging provisions, the maximum is 0.8 vol%. Operationally, the benzene content of FCC gasoline can be reduced by reducing catalyst-oil contact time and catalyst-to-oil ratio. Lower reactor temperature, lower rates of hydrogen transfer, and an octane catalyst will also reduce benzene levels. 

Gas-oil cracking was carried out in a fixed bed tubular reactor at atmospheric pressure and 482 °C. Average yields of the different products -diesel, gasoline, gases (methane, ethane, ethylene, C, C ), and coke- were measured at different levels of conversion jy varying the catalyst to oil ratio in the range 0.025-0.40 g.g, but always at 60 sec on-stream. The operational procedure has been detailed elsewhere (6). 

Pilot plant tests were made in a cyclic fixed fluidized bed unit over a range of conditions. Catalyst-to-oil ratio was varied from 3 to 5 and WHSV was varied from 32 to 53, inversely. The reactor temperature was held at 975°F for the cracking and steam stripping cycles, and at 1200°F for the regeneration cycles. After regeneration, carbon on catalyst was effectively zero. 

The amount of SOx produced in the regenerator can be considerable. For example, consider a 50,000-barrels-per-day unit, with a catalyst inventory of 500 tons and a catalyst circulation rate of 50,000 tons a day. The unit operates at a catalyst to oil ratio of 6. The feedstock contains 2.0% sulfur, and 7% of the sulfur in the feedstock goes to coke. For this unit, the daily SOx emissions would be 23.3 tons, expressed as SO j or 11.7 tons, expressed as elemental sulfur. 

Performance Analysis. In order to determine the effect of hydrotreating on catalytic cracking performance, the above feedstocks were evaluated at a low severity cracking condition (catalyst-to-oil ratio of 6.0 and reactor temperature of 910° F) and a high severity cracking condition (catalyst-to-oil of 8.0 and reactor temperature of 1010° F). The results from the catalytic cracking of these feedstocks (shown in Tables I and II) are shown in Tables III through V. The results presented in these tables are... 

FIGURE 1.8 Effect of API gravity and Conradson carbon on catalyst to oil ratio and coke yield at 55% conversion. 

FCC catalyst, supplied by Grace Davison, at three different cat-to-oil ratios, 4,6, and 8. The feed was injected at a constant rate of 3 g/min for 30 seconds. The catalyst to oil ratio was adjusted by varying the amonnt of catalyst in the reactor. Two catalysts used for this evaluation were laboratory deactivated using the cyclic propylene steaming (CPS) method [6]. Properties of these catalysts after deactivation are listed in Table 12.3. 

Catalytic evaluation of the different pillared clays was performed using a microactivity test (MAT) and conditions described in detail elsewhere (5). The weight hourly space velocity (WHSV) was 14-15 the reactor temperature was 510 C. A catalyst-to-oil ratio of 3.5-3.8 was used. The chargestock s slurry oil (S.O., b.p. >354 C), light cycle oil (LCGO, 232 C < b.p. <354 C) and gasoline content were 62.7 vol%, 33.1 vol% and 4.2 vol% respectively. Conversions were on a vol% fresh feed (FF) basis and were defined as [VfVp/V ] x 100, where is the volume of feed... 

These catalysts were evaluated in an isothermal, bench-scale, riser pilot plant at 1000 F average riser temperature, 5 seconds oil residence time (ORT) and at least four different catalyst to oil ratios varying from 4 to 10. The feedstock used was a Nigerian gas oil with properties as shown in Table III. Gases were analyzed by GC. The liquids were analyzed by a GC to give the carbon number isomer breakdown. In addition there was enough sample to analyze for mini-micro Motor octane which has a reported reproducibility of 0.75 octane for a single determination. 

A vacuum gas-oil (characteristics given in Table II) was cracked at 755 K. The catalyst to oil ratio (g.SAPO/g.gas-oil) was varied between 0.20 and 0.84 g.g 1 in order to obtain different levels of conversion,... 

The experiments were carried out in a fixed-bed tubular reactor, heated by an electric furnace divided into three heating zones. Prior to each experiment the catalyst was stripped with N2 at 482°C (reaction temperature) for 20 minutes. Then the reactant (which characteristics are given in Table II) was charged at the top of the reactor by a constant-rate positive-displacement pump. The catalyst to oil ratio with respect to the zeolite content was varied between 0.09 and 0.23 g.g 1 by changing gasoil feed (4.45-1.78 g) while keeping the weight of zeolite constant (0.40 g). 

In the past, ORC experienced plugging from exactly this type of phenomenon. When processing residue-containing feedstocks, coke would build up just above the feed injection nozzle causing the flow to the riser reactor to become restricted. As a result, runs would have to be ended prematurely. The coke build-up tended to be the worst at low catalyst-to-oil ratios when catalyst flow rates were also low. 

This change in catalyst hopper operation virtually eliminated the formation of coke deposits in the feed mixing zone. Catalyst-to-oil ratios as low as 2.5 have been achieved even with very heavy feedstocks. 

Another aspect which needs to be considered is the effect of catalyst to oil ratio. In a commercial unit the effective meso pore activity will be a function of the meso pore activity of the catalyst and of the catalyst to oil ratio (CTO) ... 

DCC uses heavy VGO as feedstock and has the same features has the FCC but with the following differences special catalyst, high catalyst-to-oil ratio, higher steam injection rate, operating temperature, residence time, and lower operating pressure. 

If we assume that the poisoning effect will increase with the concentration of poisons on the catalyst, than the poisoning effect will be inversely proportional to the catalyst-to-oil ratio (CTO). Nitrogen poisoning of FCC catalyst [12] is often roughly correlated in this way. 

One option from UOP for olefin reduction is the revamp of an FCC unit to RxCat technology (10). In the RxCat process, Figure 4.6, a portion of coked catalyst is recycled to mix with regenerated catalyst at the bottom of the riser reactor. This feature allows the unit to run at a higher catalyst-to-oil ratio and a lower catalyst contact temperature. Moreover, ZSM-5 additive is more effective with RxCat because coked ZSM-5 retains more activity than coked Y zeolite. 

A more systematic study has been produced by Golden et alia with representative data given in Table 10.2. The table illustrates that higher propylene yield is a consequence of increasing severity in the FCC operation that is increasing temperature and the catalyst to oil ratio increases the propylene yield. There is a concomitant increase in the amount of coke deposited on the catalyst. 

Fujiyama et alid have proposed reconfiguring FCC operations to increase propylene yield. The group have demonstrated a down-flow reactor system operating at high catalyst to oil ratios (>15), high reaction temperature (> 550°C) and short residence time (< 0.5 sec) and obtained propylene yields over 15%. 

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