Process and catalysis factors to maximise propylene output
Catalyst selection and process design enable a new RFCC unit to maximise propylene production from a range of feeds.
Takuya Amano, Jack Wilcox and Carel Pouwels, Albemarle Corporation
Takakazu Matsuura, Taiyo Oil Co
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The fluid catalytic cracking (FCC) unit has been the primary source when it comes to meeting the growing market demand for propylene. This has led to a significant challenge for licensors to develop new process technology and for refiners to optimise cracking severity. Achieving maximum propylene yield and conversion from a wide range of feed qualities also presents considerable challenges to catalyst designers. The impact of feed composition and process variables on the yields and heat balance is significant and therefore requires a good understanding of the chemistry to design the right FCC catalyst for individual FCC units.
This article describes the most important elements of chemistry and catalyst design considerations for maximising propylene make from the FCC unit.
Maximising propylene in the FCC unit
In the refining industry, several cracking processes are applied to cleave carbon-carbon bonds, including non-catalytic thermal coking and visbreaking, as well as catalytic hydrocracking and, most importantly, FCC. Coking, visbreaking and hydrocracking will not be discussed further, as there is essentially no production of propylene from these processes. While FCC is predominantly a catalytic process, the reaction temperature is high enough to initiate some thermal cracking.
Thermal cracking reactions proceed through a non-selective free radical chain mechanism that involves three primary steps: initiation, which involves the formation of the highly reactive, electrically neutral free radical ion; propagation, which is essentially the cracking step plus the generation of another radical; and termination, as the radicals are eventually reduced to methane, olefinic C2 and C2 fractions, butadiene and small amounts of isobutene and isobutylene. While the thermal cracking reactions occurring in the FCC reactor do produce some olefinic C2, indicating some hydrogen transfer, the contribution to the overall propylene yield is relatively small. The degree of branching is minimal, indicating very little isomerisation.
The predominant cracking mechanism utilised in the FCC process is driven by the use of a highly selective acid-catalysed system. Catalytic cracking involves the formation of carbo-cation intermediates as follows: a carbonium ion is initially formed either by abstraction of a hydride ion from a paraffin or from the acid site provided by the catalyst. The highly selective cracking reaction, referred to as β-scission, plus the formation of another carbonium ion, then occurs. Catalytic cracking produces highly olefinic liquid products and essentially no C2 and lighter products. In addition, secondary reactions, such as hydrogen transfer and isomerisation occur, which impacts LPG and gasoline olefinicity, as well as the degree of paraffin branching. Figure 1 illustrates the differences in selectivity by comparing the products derived from thermal and catalytic cracking of n-hexadecane at 500°C.1
A leap in FCC technology occurred when zeolites were incorporated into the catalyst formulation, significantly upgrading catalytic cracking. This change produced a much more intrinsically active catalyst, which resulted in not only increased unit conversion but also significantly better product selectivities, as well as reduced delta coke and dry gas production. However, at equivalent conversion, the gasoline research octane decreased, reflecting the reduced olefinicity. The reduced gasoline, as well as LPG olefinicity, is a direct result of the hydrogen transfer reactions. These reactions dictate the hydrogen redistribution through the entire product slate. Hydrogen transfer is reflected by increased iso/normal paraffins, reduced gasoline and LPG olefinicity, reduced over-cracking to LPG, and more hydrogen-deficient heavy products as hydrogen is shifted to lighter products (see Table 1).
Maximising propylene yield from the FCC unit is typically accomplished by combining a low rare earth catalyst system with severe reaction conditions. Significant quantities of incremental propylene may be generated by augmenting the catalyst inventory with the shape-selective zeolite ZSM-5. (Albemarle’s AFX catalyst and technology, with its unique zeolite system discussed later, maximises propylene without augmenting the catalyst inventory with ZSM-5 additive.) This zeolite utilises pore openings smaller than Y-zeolite, driving different catalytic reactions. The smaller pores restrict the access of branched and cyclic hydrocarbons to the active sites and only allow straight-chain and mono methyl paraffins and olefins to enter. These molecules are essentially centre-cracked, generating predominantly propylene and small amounts of butylene and ethylene. The primary reactants are in the C6 and C7 range, but other gasoline molecules with a carbon atom number between 5 and 10 can be converted by ZSM-5 to some extent. Figure 2 shows the effect of ZSM-5 on product selectivity shifts, giving clear evidence of the increase in the production of C3, C4 and C2 olefins, with a corresponding reduction in C6 and C7 components.
FCC gasoline can be typically classified into five types of hydrocarbons: paraffins (P), iso-paraffins (iP), olefins (O), naphthenes (N) and aromatics (A). Based on these hydrocarbon types, conversion of the most easily cracked C6 and C7 atoms by ZSM-5 is shown in Figure 3. An olefin is cracked by ZSM-5 and produces two smaller olefins. As such, in order to maximise the light olefin yield, the cracking mechanism generating the C6 and C7 olefins produced in the riser during the primary cracking with the FCC catalyst must be carefully managed.
The numbers on the bars in Figure 3 show a reduction in the specific hydrocarbon type. The decrease in light isoparaffins is the largest. This is attributed to the reduced formation of light isoparaffins in the presence of ZSM-5 and other reactions, including cracking to light olefins. The light olefins, however, contribute the most to propylene make, as olefins are more reactive and crack to form two smaller olefin molecules. In general, gasoline-range olefins are the primary reactants for propylene. The molecules that can be readily cracked by ZSM-5 are in the gasoline boiling range and often referred to as gasoline precursors. Although iso-paraffins are good gasoline precursors, the small pores of ZSM-5 restrict access primarily to straight-chain paraffins or those with a methyl branch. Larger branched or multiple-branched molecules are consequently not desirable. Since Y-zeolite catalyses isomerisation reactions, the catalyst must be formulated to minimise these reactions.
As Figure 3 shows, there is a small increase in the gasoline benzene and methyl-benzene with the use of ZSM-5.
FCC manufacturers have a variety of tools available for designing the optimal catalyst system to maximise the propylene yield from the FCC unit. The impact of these various parameters on catalyst performance in general and on light olefin selectivity in particular follows (see Table 2).
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