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Jul-2008

Effect of hydrocarbon partial pressure on FCC propylene production

Results of cracking experiments are discussed, where the hydrocarbon partial pressure was varied by altering the total reactor pressure, feed rate and lift gas

Ruizhong Hu, Gordon Weatherbee, Hongobo Ma, Terry Roberie and Wu-Cheng, Grace Davison Refining Technologies
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Article Summary
Many refiners have continually revamped and debottlenecked their FCC units to increase feed throughput and improve profitability. Most FCCUs are running at a significantly higher feed rate than their original design. In order to maintain catalyst and vapour velocity in the riser, and cyclones at a higher throughput, the unit pressure and consequently the hydrocarbon (HC) partial pressure need to be increased. Current laboratory methods for evaluating FCC catalysts and additives cannot match the HC partial pressures in commercial FCCUs. One reason for this is that the available laboratory testing equipment, such as ACE and MAT, typically operates at atmospheric pressure. The Davison Circulating Riser (DCR), a pilot plant-scale testing unit, is regularly operated under total pressure similar to commercial FCCUs.1 However, due to the small diameter of the DCR riser, a relatively large amount of nitrogen is needed to lift the catalyst, thus decreasing the HC partial pressure. Studies documenting the effect of HC partial pressure on FCC yields are scarce.

It is generally expected that a rise in HC partial pressure will increase the rate of all bimolecular reactions, including hydrogen transfer, relative to cracking, which is unimolecular. An increase in the rate of hydrogen transfer will result in a reduction of olefins in both gasoline and LPG, and an increase in gasoline-range aromatics and paraffins. The change in the rate of hydrogen transfer could also affect the gasoline sulphur concentration as well as the effectiveness of gasoline sulphur-reduction catalysts and additives. Moreover, the effectiveness of ZSM-5 additives, which are used to produce light olefins, especially propylene, could be affected by the HC partial pressure. As ZSM-5 works by cracking gasoline-range olefin molecules, changing the rate of hydrogen transfer could have a profound impact on the propylene yield.

The results of a series of cracking experiments conducted in a DCR are presented, where the HC partial pressure was varied by changing the total reactor pressure, feed rate and amount of lift gas. The effect of changing the HC partial pressure on HC yields, especially that of light olefin, and gasoline sulphur will be further discussed.

Experimental
Figure 1 shows a schematic diagram of the standard DCR setup. The range of operating conditions in the DCR is shown in Table 1. Operation of the DCR has been described previously.1 Similar to commercial FCCUs, the DCR is operated in adiabatic mode. In typical DCR operation, the regenerator temperature, riser outlet temperature and feed rate are set. The catalyst circulation rate and thus the catalyst-to-oil ratio are varied by changing the feed preheat temperat.ure. During DCR operation, a metering pump precisely controls the feed rate as feed is pumped from the load cell through a preheater. Nitrogen and steam, injected through a separate preheater/vapouriser, are used as a feed dispersant. Catalyst and product pass from the riser to the stripper overhead disengager. Products exit the disengager through a refrigerated stabiliser column to a control valve, which maintains unit pressure at the desired level. A section of the stripper-regenerator spent catalyst transfer line consists of a shell and tube heat exchanger. The rate of heat transfer across this exchanger provides a precise and reliable method to calculate the catalyst circulation rate. The stabiliser column, also called the debutaniser column, is operated to separate C4 minus (C4-) from the liquid product, which is condensed and collected. The collected liquid is analysed by gas chromatography (GC) simulated distillation ((SIMDIS) to provide gasoline (ibp: 430°F), LCO (430–700°F) and 700°F+ bottoms fractions. The gaseous products are metered and batch collected for subsequent analysis by GC.

Two methods of changing the HC partial pressure were investigated. The first method involved keeping the total pressure, feed rate and steam injection rate constant while reducing the nitrogen lift gas. The second method involved keeping the nitrogen lift gas and steam injection rate constant while increasing the total pressure and feed rate. The latter case is similar to some commercial FCCU revamps, where the total pressure of a FCCU is increased to accommodate the higher feed and catalyst circulation rate.

Table 2 shows the three DCR operating conditions. Condition 3 is a commonly used DCR operating condition, while Conditions 1 and 2 are modifications to raise the HC partial pressure closer to the value in commercial FCC operations. As cracking is a molecular weight-reduction process, the HC mole fraction and, therefore, the partial pressure increase along the riser. The molar expansion (moles of product/moles of feed) in a typical FCCU is between 4 and 5. For the purpose of engineering calculations, it is common to approximate the HC mole fraction as equal to one-third of the mole fraction at the inlet and two-thirds of the mole fraction at the outlet of the riser. The total moles of the HC products are calculated by using GC analyses of the light gases and gasoline paraffins, isoparaffins, olefins, naphthalenes and aromatics (PIONA), and assuming average molecular weight values of 220 and 350 for LCO and bottoms respectively.

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