Increasing FCC propylene yield

A test run with ZSM-5 additive demonstrated to a refinery that it could significantly raise production of propylene in the FCC.

TOM VENTHAM, Johnson Matthey

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Article Summary

A new 22000 bpd FCC unit was constructed by Slovnaft in 1999. Slovnaft is Slovakia’s only modern, fully integrated refinery. The FCC processes mainly hydrotreated VGO feed with typical unit objectives of LPG and gasoline production. High value LPG components are sought for downstream processes such 
as polypropylene, ETBE and 
alkylation. The FCC limitation is typically found in the gas concentration section (LPG Merox, C3 splitter).

FCC propylene study
In 2014 Slovnaft investigated modifications to increase propylene production. The typical propylene yield from the Bratislava refinery FCC unit is approximately 6 wt% on a fresh feed basis. Industry experience has shown standard FCC units with VGO feed are typically capable of producing up to 10-12 wt% propylene by implementing relatively simple techniques that maximise gasoline overcracking to LPG and retain olefinicity.

Slovnaft performed a significant amount of research in this area and identified several options that could increase propylene output from the FCC unit. The company decided to take a stepwise approach by performing tests to determine the capabilities and sensitivities of methods to define the optimum strategy to be implemented. This research concluded that the most effective way for Slovnaft to increase FCC propylene yield would be the use of a high activity ZSM-5 additive.

Propylene market demands
Slovnaft was pursuing methods to elevate propylene production because of the worldwide market trend of increased propylene demand and value. The drive for propylene is mainly to satisfy petrochemical industry demands, especially growth in polypropylene demand (see Figures 1 and 2).1,2 Based on 2011 data, approximately 68% of refinery produced propylene is supplied to the chemicals industry.3 Of the chemical industry’s total propylene requirement of 109 million t/y, 35% is produced by refinery processes (of which the FCC process itself accounts for 97%), 57% is produced by steam crackers and 8% by ‘on-purpose’ propylene processes (see Figure 3).3 Propylene demand is expected to continue to increase by almost 5% a year, with new propylene production capacity not expected to keep up with this demand growth, resulting in the appearance of a ‘propylene gap’.3

Supply-side effects are also impacting this situation. In China, demand is outstripping investment in traditional sources of propylene production, and in North America, the shift to shale gas based ethane feedstocks for steam crackers has reduced propylene generation from this traditional source of the commodity. This trend is expected to be observed in Europe too as imports of American shale gas to European steam crackers will add to the propylene imbalance.4 Although various propylene on-demand projects will look to fill this propylene gap, there will be a lag before this extra supply is realised. In addition, many of these projects will be sensitive to commodity and crude oil prices as well as political and environmental pressures that are apparent in the regions where many of these projects are located, such as China. The FCC unit remains a flexible and cost-effective method for increasing propylene production to fill the demand gap.

ZSM-5 test run plans
The additive chosen by Slovnaft for this test run was Intercat Super Z Excel supplied by Johnson Matthey. Designed for high propylene FCC operations, Super Z Excel has high activity and stability, resulting 
in the efficient generation of propylene.

In cooperation with Intercat, Slovnaft developed an estimate to anticipate the yield and octane improvements that would be observed when adding Super Z Excel (see Table 1).

The main yield shift expected was an increase in the propylene yield followed by a boost in butylenes yield. The yield of isobutane was also expected to increase. Although ZSM-5 only produces olefins, because isobutene is the most readily saturated LPG molecule via hydrogen transfer, isobutane yield also increases 
with ZSM-5 due to the secondary effect of converting some of 
the produced isobutylene via hydrogen transfer on the main catalyst.

It was predicted that, following the increase in LPG, the net yield reduction would be from gasoline. Although the estimate shows a small reduction in LCO yield this is due to the reported low gasoline/LCO cutpoint at Slovnaft being below the standard 221°C cutpoint (gasoline 90% boiling point being 170°C). An unchanged bottoms yield was predicted as ZSM-5 only cracks molecules found in lighter intermediate product ranges. There is also no change expected in the coke yield, as ZSM-5 is heat balance neutral, so will not typically alter the coke requirements of the system.

 As part of this estimate an addition plan was developed (see Table 2) for loading the additive over the course of the test run. This started with a base load phase during the first seven days to quickly boost the additive concentration to the working level. Following the base load, a maintenance dose regime was planned to preserve the correct level of ZSM-5 activity for the remainder of the test run period.

Due to unrelated disturbances in upstream units during the initial stages of the test run Slovnaft were not able to precisely follow the planned loading protocol. However, sufficient amounts of Super Z Excel were added to attain the concentrations needed to observe yield responses.

ZSM-5 is a fast acting additive, meaning yield changes can typically be seen after the first week of addition. The octane improvement with ZSM-5 requires the full concentration of the additive to be present at steady state to determine the entire benefit.

LPG yield effects
When Super Z Excel additions to the FCC unit began, the propylene yield responded almost immediately. Figure 4 shows the change in propylene yield as ZSM-5 was introduced.
Slovnaft estimated that the typical FCC propylene yield without ZSM-5 was averaging 6 wt% prior to the start of the test run. As ZSM-5 was added in base load, with over 2.5 tonnes of additive added in the first four days of the test run, the propylene response was major, with propylene yield exceeding 9 wt% in the first week. This represents a 50% increase.

A constant ZSM-5 addition rate followed the base load, leading to a gradual increase in the inventory concentration. The addition rate was increased again towards the end of the test run, leading to a second propylene yield peak above 9 wt%. When the test run ended, the ZSM-5 concentration decayed along with the propylene yield, with the propylene half-life of Super Z Excel measured to be in the range of 23 days. The propylene yield remained above the base line for six weeks, falling to only 7 wt% at the end of the data set compared to the base line of 6 wt%. This shows that residual ZSM-5 activity was still present six weeks after the final addition. Based on the period when ZSM-5 concentration exceeded 5 wt% in the inventory to the final day of additions, the average propylene yield reached 8.9 wt%, or a 47% increase on the base line.

It was also observed that the total LPG yield (see Figure 5) increased when ZSM-5 was introduced to the FCC unit and continued to climb for the rest of the test run as ZSM-5 concentration increased. By the end of the test run, the total LPG yield increased 7 wt% compared to the pre-test run base line. It was expected that the total LPG increase would be made up of 55-60% propylene and 45-40% butylene with some saturation of produced isobutylene via hydrogen transfer on the main catalyst.

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