INNOVATIVE DESIGN OF HIGH ACTIVITY ZSM-5 ADDITIVES FOR MAXIMUM PROPYLENE PRODUCTION IN link 188bet cho dien thoai FLUID CATALYTIC CRACKING PROCESS: A MINI-REVIEW
Summary
link 188bet cho dien thoai strong globaldemandforpropylenealongwiththeshortagesupplyfrommodernsteamcrackers provides a significant incentive for refiners to maximise propylene production from FCC units. To respond to this need, most FCC operators currently apply high activity ZSM-5 additives as link 188bet cho dien thoai most cost-effective way to boost link 188bet cho dien thoai propylene yield. In this paper, link 188bet cho dien thoai authors discussed several key aspects for innovative design of high activity ZSM-5 additives, i.e. increased contents of ZSM-5 crystals, stabilisation of ZSM-5 framework by phosphorus treatment and optimisation of porous matrix. To illustrate these points, prominent examples in link 188bet cho dien thoai development of new generations of high activity ZSM-5 additives from link 188bet cho dien thoai world leading catalyst manufacturers were given. With link 188bet cho dien thoai new high activity ZSM-5 additives, link 188bet cho dien thoai desired propylene yield could be obtained with less ZSM-5 usage requirements. This translated in reduced dilution impacts and additive costs; thus enabling FCC operators to produce more propylene to meet link 188bet cho dien thoai growing propylene demand.
Key words: ZSM-5 additives, FCC, propylene, light olefins, shale gas.
1. Introduction
Propylene is an important raw material for producing many valuable products including polypropylene, acrylonitrile, propylene glycols, cumene, butyraldehyde and acrolein. Major sources of propylene supply are from steam crackers (ca. 60%) and fluid catalytic cracking (FCC) units (ca. 30%). link 188bet cho dien thoai remainder is produced on purpose via propane dehydrogenation or metathesis (ca. 10%) [1, 2]. Due to link 188bet cho dien thoai shale gas revolution, however, link 188bet cho dien thoai feedstock for steam crackers is changing with less naphtha from crude oil and more ethane from natural gas, which decreases link 188bet cho dien thoai propylene yield from steam crackers (Table 1). This opens an opportunity for FCC units around link 188bet cho dien thoai world to enhance propylene production to keep up link 188bet cho dien thoai growing demand for propylene (Figure 1).
In light of this, FCC operators have link 188bet cho dien thoai following options to consider: (i) increase riser temperature, (ii) increase conversion, (iii) change feedstock composition and (iv) use ZSM-5 based additives [2]. Increasing link 188bet cho dien thoai reactor temperature will enhance LPG and thereby link 188bet cho dien thoai propylene yield. But it also produces more undesired products, i.e. dry gas and coke. More propylene can be obtained by increasing conversion. For every 1wt% gain in conversion, about 0.3wt% additional yield in propylene can be obtained. However, this option is limited because link 188bet cho dien thoai increased conversion requires more coke burning. link 188bet cho dien thoai feed composition has prominent influences on link 188bet cho dien thoai product distribution of FCC units. A heavier residue type feed will give rise to a drop in link 188bet cho dien thoai propylene yield as it has a high concentration of naphthenic molecules which have a strong tendency to act as hydrogen donors and to saturate olefin precursors. But link 188bet cho dien thoai changing feedstock is not a practical solution since it depends on available crude oil sources. link 188bet cho dien thoai use of ZSM-5 additives perhaps represents link 188bet cho dien thoai most cost-effective way to raise link 188bet cho dien thoai propylene yield from FCC units. Typically, only few weight percent of ZSM-5 additives in link 188bet cho dien thoai inventory can greatly improve link 188bet cho dien thoai propylene yield. In fact, most of FCC operators currently rely on ZSM-5 additives to achieve a significant boost in link 188bet cho dien thoai propylene yield when market conditions demand such an action. In link 188bet cho dien thoai present paper, link 188bet cho dien thoai authors discuss several key aspects in link 188bet cho dien thoai design of high activity ZSM-5 additives for maximum propylene production in FCC units along with excellent examples from link 188bet cho dien thoai world leading FCC catalyst manufacturers including Grace Davison, BASF and JGC C&C.
2. Chemistry of propylene production by ZSM-5 additives
Addition of ZSM-5 additives to FCC units resulted in an increase in link 188bet cho dien thoai gasoline octane rating, a decrease in gasoline and an increase in C3-C4 gas products, particularly propylene [3 - 6]. These results can be explained by link 188bet cho dien thoai fact that ZSM-5 is shape selective zeolite which mainly cracks gasoline range olefins (typically C6-C9 hydrocarbons) into C3-C4 olefins. A simplified diagram of main reaction pathways for formation of light olefins upon addition of ZSM-5 additives is shown in Figure 2. Gasoline-range olefins are initially formed by conversion of FCC feedstocks (residues, gas oil, etc.) over link 188bet cho dien thoai faujasite (Y) FCC base catalyst. Without ZSM-5 additives, these olefins tend to undergo hydrogen transfer reactions to make paraffin and aromatics since link 188bet cho dien thoai faujasite base catalyst has strong ability to promote these bimolecular reactions [4].
When link 188bet cho dien thoai ZSM-5 additive is injected, these gasoline olefins will be selectively converted into light olefins. link 188bet cho dien thoai chemistry of ZSM-5 originates from its unique shape selectivity. link 188bet cho dien thoai relatively small and tunnel- shaped pores of ZSM-5 (< 0.56nm) allow only linear compounds in link 188bet cho dien thoai gasoline fraction to enter. Furthermore, link 188bet cho dien thoai limited room in link 188bet cho dien thoai pore system of ZSM-5 compared to link 188bet cho dien thoai supercages (approx. 1.3nm diameter) in zeolite Y of link 188bet cho dien thoai base catalyst implies that it is much more difficult to accommodate larger bimolecular transition states (Figure 3). Consequently, link 188bet cho dien thoai secondary cracking reactions of gasoline olefins over ZSM-5 produce predominant C3-C4 olefins [3 - 6]. An accepted rule of thumb states that ZSM-5 additives should yield an LPG fraction with 50 - 60wt% propylene, 20 - 40wt% butene and 10 - 20wt% iso-butane. Hence, link 188bet cho dien thoai increased C3-C4 olefins and decreased gasoline yields can be rationalised through these secondary reactions. On link 188bet cho dien thoai other hand, it should be noted that gasoline paraffins have low octane ratings and thus their reduced concentration in gasoline induces an improved gasoline octane number.
To maximise propylene production, in principle, gasoline olefins generated by link 188bet cho dien thoai base catalyst should be effectively converted over ZSM-5 additives to light olefins. This could be realised by using high ZSM-5 additive loadings, i.e. 15 - 30wt%. However, as link 188bet cho dien thoai ZSM- 5 injection rate increases, link 188bet cho dien thoai FCC base catalyst activity is diluted because link 188bet cho dien thoai ZSM-5 additive does not exhibit cracking activity toward FCC feedstock [4, 5]. To address link 188bet cho dien thoai need to use less ZSM-5 additives for maximum propylene production, catalyst manufacturers have focused on innovative design of ZSM-5 additives with high activity which allows FCC operators to lower ZSM- 5 additive loadings and still achieve link 188bet cho dien thoai desired yield of propylene. Furthermore, to make ZSM-5 additives even more effective, a base catalyst with controlled hydrogen transfer reactions should be used since these secondary reactions consume olefin precursors to form undesired products (Figure 2). However, link 188bet cho dien thoai development of FCC base catalysts with controlled hydrogen transfer reactions is not a subject of this paper and will not be introduced here. In link 188bet cho dien thoai next section, several key points, i.e., using high content of ZSM-5, stabilisation of ZSM-5 by phosphorous treatment and optimisation of porous matrix for link 188bet cho dien thoai design of high activity ZSM-5 additives for maximum propylene yield will be discussed in detail.
3. Design of high activity ZSM-5 additives for maxi- mum propylene production
In order to improve link 188bet cho dien thoai activity of ZSM-5 additives, link 188bet cho dien thoai following major aspects have been considered: (i) increase ZSM-5 content, (ii) stabilise ZSM-5 with new phosphorous treatment technology and (iii) optimise link 188bet cho dien thoai porous system of matrix [1 - 8].
link 188bet cho dien thoai formulation of ZSM-5 additives with high content ZSM-5 crystals has been proven as an efficient approach to obtain high activity ZSM-5 additives. But link 188bet cho dien thoai technical challenge is that link 188bet cho dien thoai physical properties of link 188bet cho dien thoai additive (apparent bulk density, attrition resistance) deteriorate at high ZSM-5 contents [5]. Nevertheless, catalyst manufacturers have made it possible to keep these properties by using suitable catalyst preparation technology. Grace Davison first developed successfully a high activity ZSM-5 additive with a high ZSM-5 content namely “OlefinMax”. This additive contained ca. 25wt% ZSM-5 crystals which at link 188bet cho dien thoai time was link 188bet cho dien thoai maximum allowable under license [7]. OlefinMax soon established itself as link 188bet cho dien thoai benchmark for high-activity ZSM-5 additives and is still link 188bet cho dien thoai most widely used ZSM-5 additive, accounting for nearly half of all ZSM-5 uses. Recently, Grace Davison has introduced a new high activity ZSM- 5 additive called “OlefinsUltra” by optimising link 188bet cho dien thoai matrix system. link 188bet cho dien thoai intrinsic activity of OlefinsUltra is even higher than that of OlefinMax. By using this additive, FCC operators are able to reduce by about half link 188bet cho dien thoai amount of ZSM-5 additive injected and still get link 188bet cho dien thoai desired yield of propylene. This means that they can lower fresh catalyst consumption by more than 10%. Up to date, OlefinsUltra has been used successfully in more than 17 refineries around link 188bet cho dien thoai world, including Dung Quat refinery in Vietnam.
Phosphorus treatment is extensively used in link 188bet cho dien thoai fabrication of ZSM-5 additives to slow down link 188bet cho dien thoai dealumination of link 188bet cho dien thoai ZSM-5 framework [6 - 8]. This stabilisation retains more active acid sites and thus higher activity for treated ZSM-5 additives. In this connection, BASF has recently developed a new generation of high activity ZSM-5 additives, called “ZIP Olefins Additive”, based on multi-stage phosphorus treatment technology [8]. For this purpose, they have made intensive efforts on understanding link 188bet cho dien thoai role of acid sites in enhancing link 188bet cho dien thoai propylene yield. It was found that link 188bet cho dien thoai phosphorus based acidity rather than ZSM-5 framework acidity plays a key role for link 188bet cho dien thoai improved propylene yield upon steaming. Accordingly, steaming and subsequent phosphorus treatment should be employed to generate more phosphorus based acidity and link 188bet cho dien thoai consequent propylene yield (Table 2).
By utilising link 188bet cho dien thoai new phosphorus treatment technology, ZIP Olefins Additive is able to produce more propylene at link 188bet cho dien thoai same ZSM-5 loading and crystal content as MOA (Maximum olefin additive, a standard additive product of BASF). Since product selectivity for ZSM-5 additives is primarily linked to link 188bet cho dien thoai shape of link 188bet cho dien thoai zeolite itself, there is no change in selectivity for ZIP Olefins Additive with respect to other ZSM-5 additives. However, because of ZIP’s higher activity, link 188bet cho dien thoai desired light olefin yield is obtained using less ZSM-5 additive (Figure 4).
For link 188bet cho dien thoai pore design of ZSM-5 additive, it is of importance to properly balance micro-, meso- and macroporosity within link 188bet cho dien thoai additive. Through link 188bet cho dien thoai hierarchical porous system of link 188bet cho dien thoai matrix, gasoline olefins (reactants) easily diffuse to link 188bet cho dien thoai active site of ZSM-5 and thereby give an increased performance. Based on this concept and employing high content ZSM-5 formulation technology, JGC C&C has developed an advanced ZSM-5 additive named “OCTUP-α” [5]. link 188bet cho dien thoai catalytic data in Table 3 shows that OCTUP-α is superior to OCTUP-11S, a standard additive from JGC C&C due to link 188bet cho dien thoai increase in pore volume and link 188bet cho dien thoai increase in acidity from a greater amount of ZSM-5 crystals (Figure 5).
4. Conclusions
Maximisation of propylene production has become a focus of most refineries because of link 188bet cho dien thoai increasing global demand for propylene and link 188bet cho dien thoai shortage supply from modern steam crackers, which now produce relatively less propylene. To achieve this, most refiners rely on link 188bet cho dien thoai use of ZSM- 5 additives. link 188bet cho dien thoai paper discusses three key points for innovative design of high activity ZSM-5 additives, including formulation with high ZSM-5 contents, stabilisation of ZSM-5 crystals with new phosphorus treatment technology and optimisation of porous matrix. By utilising these techniques, catalyst manufacturers have successfully developed new generations of high activity ZSM-5 additives which allow link 188bet cho dien thoai use of less ZSM-5 additives than link 188bet cho dien thoai traditional ZSM-5 additives while still achieving link 188bet cho dien thoai desired propylene yield. This opens link 188bet cho dien thoai way for FCC operators to increase propylene production to fill link 188bet cho dien thoai gap between propylene supply and demand.
Acknowledgments
This work is part of link 188bet cho dien thoai state-level project entitled “Development of technical ZSM-5 additives using Vietnamese resources for enhancing link 188bet cho dien thoai propylene yield from link 188bet cho dien thoai RFCC Unit of Dung Quat refinery” funded by link 188bet cho dien thoai Ministry of Science and Technology of Vietnam under link 188bet cho dien thoai National Science and Technology Research Programme (Grant No. KC.02.03/16-20).
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