ZSM-5 additive maximises propylene
Multi-stage phosphorus treatment improves ZSM-5 zeolite stabilisation and creates more propylene producing acid sites.
CARL KEELEY, VASILEIOS KOMVOKIS, MODESTO MIRANDA, DAVID HARRIS and JASON SMITH
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Globally, there is strong demand for light olefins, especially propylene. Propylene is an important feedstock for the petrochemicals industry in producing polypropylene, acrylonitrile, propylene glycols, cumene, butyraldehydes, acrolein and other valuable products. Primarily produced by steam crackers, the fluid catalytic cracking (FCC) unit is the second largest propylene producer. Due to the shale gas revolution in the US, propylene yields from steam crackers have declined, opening an opportunity for FCC units around the world to increase propylene production. To meet this opportunity, FCC operators rely on ZSM-5 based additives to change the yield distribution and quality of FCC liquefied petroleum gas (LPG) and gasoline.1-4 Since demand for propylene can vary significantly, ZSM-5 usage rates are different for different FCC units.4 With the growing demand to increase propylene from FCC units, BASF has invested in the understanding of ZSM-5 chemistry to increase its effectiveness.
ZSM-5 is used to fine-tune the production of gasoline and light olefin products.1-4 Its usage rates in FCC units vary across the industry according to specific unit goals, which depend on objectives, capacity and market. FCC units targeting gasoline octane enhancement have low usage requirements (for example, 1-4 wt% ZSM-5 additive loading), while those targeting a maximum light olefins yield have relatively high usage requirements (10-20 wt% ZSM-5 additive loading).
The ZSM-5 crystal is a shape selective zeolite used in FCC units to convert light gasoline range olefins (C6-C9) to LPG (C3 and C4 olefins).1 Without ZSM-5 the light gasoline olefins are converted to low octane paraffins via a hydrogen transfer mechanism. Since rare earth promotes hydrogen transfer, low rare earth FCC catalysts are preferred for maximum light olefin production. Consequently, the use of ZSM-5 increases both light olefin production and gasoline octane. As can be imagined, there are different pathways in which C6-C9 olefins may crack. For example, a C9 molecule may crack to a C6 molecule and a C3 molecule, or a C5 molecule and a C4 molecule. An accepted rule of thumb states additives that only use ZSM-5 zeolite should have an LPG product selectivity of approximately 50-60 wt% C3=, 20-40 wt% C4=, and 10-20 wt% iso-butane. Small amounts of propane and normal butane may also be generated. Small changes in LPG product selectivity have also been observed for ZSM-5 with different feeds.
Feed quality and FCC catalyst properties play a key role in the amount of LPG generated from the ZSM-5 additive. These effects are related to the hydrogen transfer tendency of the feed and the catalyst. The higher the hydrogen transfer, the lower the olefinicity of the C6-C9 hydrocarbons; therefore, lower amounts of LPG olefins are generated by ZSM-5.2-4
As the amount of ZSM-5 applied to a unit increases, a refiner will experience diminishing returns in terms of additional LPG, mostly due to lower quantities of readily crackable C6-C9 olefins. The larger olefins in this range are more easily cracked, leaving behind C6 and small amounts of C7. These represent targets to improve the total LPG olefins yield (see Figure 1).
It is important to note the impact of ZSM-5 on activity dilution of FCC catalyst. As ZSM-5 additive is inert in terms of conversion, higher levels of ZSM-5 can dilute the activity of the FCC catalyst. By fine-tuning the additive injection rate, the refinery can optimise light olefin production and gasoline octane. However, as the ZSM-5 injection rate increases, FCC catalyst activity is diluted. The rule of thumb is every 5 wt% of ZSM-5 additive reduces FCC catalyst activity by 1-2 wt%. Therefore, when targeting a maximum light olefins yield (that is, using 10-20 wt% ZSM-5 loading), it is more profitable to integrate most of the ZSM-5 functionality within the FCC catalyst formulation to avoid activity dilution, and have a small amount of separate additive addition for fine-tuning yields. When done properly, the ZSM-5 loading can be increased significantly without any detrimental effect on the catalyst physical and catalytic functions.2
Development of a ZSM-5 additive
To address the need to use less ZSM-5 to maximise propylene yield, BASF has developed a novel ZSM-5 additive called ZIP Olefins Additive. Many approaches were explored from different zeolite properties (for instance, silica to alumina ratio, crystal size, and so on), to exploring zeolite modifications (such as phosphorus alternatives), to synthesis process improvements. An interesting improvement came in the form of a novel multi-stage phosphorus treatment.
Phosphorus helps to slow down the dealumination of the ZSM-5 framework. This stabilisation means more active acid sites and thus higher activity.1 In some additives, phosphorus also acts as a binder. Depending on the composition of the rest of the additive, phosphorus’s effect on binding and attrition resistance may be more or less important.
Figure 2 shows the effect of ZSM-5 additive, with and without phosphorus, on propylene yield. For these experimental conditions, the base propylene yield is ~6 wt% without any ZSM-5 additive. Addition of ZSM-5 additive without phosphorus increases the propylene yield to nearly 10 wt%, and treating the additive with phosphorus further increased the propylene yield to about 14 wt%.
Table 1 shows the impact of steam and phosphorus on the performance of BASF’s Maximum Olefins Additive (MOA) combined with a low hydrogen transfer FCC catalyst. MOA is an optimised, ZSM-5 based additive produced using conventional manufacturing techniques. Based on these results, un-steamed ZSM-5 additive is less active for propylene generation, with or without phosphorus. The introduction of steam increases propylene yield, and the combination of steam and phosphorus gives the highest selectivity to propylene.
Based on these experiments, active sites necessary for propylene production involve phosphorus and are enhanced by steaming.
BASF developed a new methodology using Fourier transform infrared (FT-infrared) spectroscopy with pyridine adsorption to characterise the nature of the phosphorus inside ZSM-5.7 This new technique allows us to separate and quantify phosphorus based acid sites from zeolite framework and non-framework acid sites. A study using this technique suggested maldistribution of phosphorus within ZSM-5 additives produced via conventional manufacturing techniques. Figure 3 shows the effect of steam deactivation on framework ZSM-5 acid sites and propylene yield. As the steam severity increases, the acidity decreases; however, the propylene yield increases. Thus, ZSM-5 zeolite framework acidity does not generate more propylene as steam severity increases, so propylene yield must be due to another acid site.
Figure 4 shows propylene yield increases as the phosphorus based acidity increases with more severe steam deactivation. Phosphorus based acid sites therefore play a key role in propylene formation after steam deactivation.
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