INNOVATIVE DESIGN OF HIGH ACTIVITY ZSM-5 ADDITIVES FOR MAXIMUM PROPYLENE PRODUCTION IN 188bet vn FLUID CATALYTIC CRACKING PROCESS: A MINI-REVIEW

Summary

188bet vn 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 188bet vn most cost-effective way to boost 188bet vn propylene yield. In this paper, 188bet vn 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 188bet vn development of new generations of high activity ZSM-5 additives from 188bet vn world leading catalyst manufacturers were given. With 188bet vn new high activity ZSM-5 additives, 188bet vn 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 188bet vn 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%). 188bet vn remainder is produced on purpose via propane dehydrogenation or metathesis (ca. 10%) [1, 2]. Due to 188bet vn shale gas revolution, however, 188bet vn feedstock for steam crackers is changing with less naphtha from crude oil and more ethane from natural gas, which decreases 188bet vn propylene yield from steam crackers (Table 1). This opens an opportunity for FCC units around 188bet vn world to enhance propylene production to keep up 188bet vn growing demand for propylene (Figure 1).

In light of this, FCC operators have 188bet vn following options to consider: (i) increase riser temperature, (ii) increase conversion, (iii) change feedstock composition and (iv) use ZSM-5 based additives [2]. Increasing 188bet vn reactor temperature will enhance LPG and thereby 188bet vn 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 188bet vn increased conversion requires more coke burning. 188bet vn feed composition has prominent influences on 188bet vn product distribution of FCC units. A heavier residue type feed will    give rise to a drop in 188bet vn 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 188bet vn changing feedstock is not a practical solution since it depends on available crude oil sources. 188bet vn use of  ZSM-5  additives  perhaps  represents  188bet vn most cost-effective way to raise 188bet vn propylene yield  from  FCC units. Typically, only few weight percent of ZSM-5 additives in 188bet vn inventory  can  greatly  improve  188bet vn  propylene  yield.  In  fact,  most  of FCC operators currently rely on ZSM-5 additives to achieve a significant boost in 188bet vn propylene yield when market conditions demand such an action. In 188bet vn present paper, 188bet vn authors discuss several key aspects in 188bet vn design of high activity ZSM-5 additives for maximum propylene production in FCC units along with excellent examples from 188bet vn 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 188bet vn 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 188bet vn 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 188bet vn faujasite (Y) FCC base catalyst. Without ZSM-5 additives, these olefins tend to undergo hydrogen transfer reactions to make paraffin and aromatics since 188bet vn faujasite base catalyst has strong ability to promote these bimolecular reactions [4].

When 188bet vn ZSM-5 additive is injected, these gasoline olefins will be selectively converted into light  olefins.  188bet vn  chemistry of ZSM-5 originates from its unique shape selectivity. 188bet vn relatively small and tunnel- shaped pores of ZSM-5 (< 0.56nm) allow only linear compounds in  188bet vn  gasoline  fraction  to enter. Furthermore, 188bet vn limited room  in 188bet vn pore system of ZSM-5 compared to 188bet vn supercages (approx. 1.3nm diameter) in zeolite Y  of  188bet vn  base  catalyst  implies  that  it is much more difficult to accommodate larger bimolecular transition states  (Figure 3). Consequently, 188bet vn 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, 188bet vn increased C3-C4 olefins and decreased gasoline yields can be rationalised through these secondary reactions. On 188bet vn 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  188bet vn  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 188bet vn ZSM-    5 injection rate increases, 188bet vn FCC base catalyst activity     is diluted because 188bet vn ZSM-5 additive does not exhibit cracking activity toward FCC feedstock [4, 5]. To address 188bet vn 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 188bet vn 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, 188bet vn development of FCC base catalysts with controlled hydrogen transfer reactions is not a subject of this paper and will not be introduced here. In 188bet vn 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 188bet vn 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 188bet vn activity of ZSM-5 additives, 188bet vn following major aspects have been considered: (i) increase ZSM-5 content, (ii) stabilise ZSM-5 with new phosphorous treatment technology and (iii) optimise 188bet vn porous system of matrix [1 - 8].

188bet vn 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  188bet vn technical challenge is that 188bet vn physical properties of 188bet vn 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 188bet vn time was 188bet vn maximum allowable under license [7]. OlefinMax soon established itself as 188bet vn benchmark for high-activity ZSM-5 additives and is still 188bet vn 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 188bet vn matrix system. 188bet vn intrinsic activity of  OlefinsUltra  is  even higher than that of OlefinMax. By using this additive, FCC operators are able to reduce by about half 188bet vn amount of ZSM-5 additive injected and still get 188bet vn 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 188bet vn world, including Dung Quat refinery in Vietnam.

Phosphorus  treatment  is  extensively  used   in   188bet vn fabrication of ZSM-5 additives to slow down 188bet vn dealumination of 188bet vn 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 188bet vn role of acid sites in enhancing 188bet vn propylene yield. It was found that 188bet vn phosphorus based acidity rather than ZSM-5 framework acidity plays a key role for 188bet vn improved propylene yield upon steaming. Accordingly,   steaming   and   subsequent   phosphorus treatment should be employed to generate more phosphorus based acidity and 188bet vn consequent propylene yield (Table 2).

By utilising 188bet vn new phosphorus treatment technology, ZIP Olefins Additive is able to produce more propylene at 188bet vn 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 188bet vn shape of 188bet vn 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, 188bet vn desired light olefin yield is obtained using less ZSM-5 additive (Figure 4).

For 188bet vn pore design of ZSM-5 additive, it is of importance to properly balance micro-, meso- and macroporosity within 188bet vn additive. Through 188bet vn hierarchical porous system of 188bet vn matrix, gasoline olefins (reactants) easily diffuse to 188bet vn 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]. 188bet vn catalytic data in Table 3 shows that OCTUP-α is superior to OCTUP-11S, a standard additive from JGC C&C due to 188bet vn increase in pore volume and 188bet vn 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 188bet vn increasing global demand for propylene and 188bet vn shortage supply from modern steam  crackers,  which now produce relatively less propylene. To achieve this, most refiners rely on 188bet vn use of ZSM-    5  additives.  188bet vn  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 188bet vn use of less ZSM-5 additives than 188bet vn traditional ZSM-5 additives while still achieving 188bet vn desired propylene yield. This opens 188bet vn way for FCC operators to increase propylene production to fill 188bet vn gap between propylene supply and   demand.

Acknowledgments

This work is part of 188bet vn state-level project entitled “Development of technical ZSM-5 additives using Vietnamese resources for enhancing 188bet vn propylene yield from 188bet vn RFCC Unit of Dung Quat refinery” funded by 188bet vn Ministry of Science and Technology of Vietnam under 188bet vn National Science and Technology Research Programme (Grant No. KC.02.03/16-20).

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