Highly Selective Olefin Skeletal Isomerization Process - ACS

Chapter 22

Highly Selective Olefin Skeletal Isomerization Process J . Barin Wise and Don Powers

Downloaded by CORNELL UNIV on May 20, 2017 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch022

Lyondell Petrochemical Company, 8280 Sheldon Road, P.O. Box 777, Channelview, T X 77530-0777

The 1990 Clean Air Act has increased the demand for blendable ethers while at the same time reducing vapor pressure limits. The California Air Resources Board (CARB) has recently required further olefin reduction as well. Iso-olefins are required for the production of several of these ethers. One attractive route for production of additional ethers is the isomerization of normal olefins to iso-olefins using new high selectivity catalysts. In the case of C olefins, the isomerization and subsequent etherification of these components provides additional oxygenates while reducing vapor pressure and olefin content. Previous development work has illustrated the difficulty in achieving high selectivities with reasonable cycle lengths. The new Lyondell Process meets these objectives while maintaining relatively low capital costs. 5

Recent implementation of the 1990 United States Clean A i r A c t has created an increased demand for oxygenates for use i n gasoline blending. M T B E , currently the primary oxygenate used i n gasoline blending, is produced using isobutylene as a primary component. Isobutylene is also required for production of E T B E . In addition, future regulations restricting the level of olefins i n gasoline are expected since olefins have been shown to promote the formation of ozone i n the atmosphere (7). Figure 1 shows a distribution of olefins i n gasoline based on carbon number. Olefins enter the atmosphere primarily through the evaporation of gasoline. The olefin evaporation rate increases as the carbon number decreases. Figure 2 shows the average vapor pressure of olefins versus carbon number. The relative potential for the formation of ozone by various olefins can be represented by the product of the reactivity, vapor pressure and distribution in gasoline. Figure 3 shows the relative potential of olefin formation by carbon number.

0097-6156/94/0552-0273$08.00/0 © 1994 American Chemical Society Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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ENVIRONMENTAL CATALYSIS

Figure 1: Distribution of Gasoline Olefins (Reproduced with permission from reference 1. Copyright 1993 K . Rock)

Figure 2: Vapor Pressure of Gasoline Olefins (Reproduced with permission from reference 1. Copyright 1993 K . Rock)

Figure 3: Relative Ozone Potential of Gasoline Olefins (Reproduced permission from reference 1. Copyright 1993 K . Rock)

Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

with

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Highly Selective Olefin Skeletal Isomerization Process

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In order to meet the increased demand for oxygenates while reducing olefin content and vapor pressure i n gasoline, the petrochemical and refining industries w i l l not only require additional quantities of iso-olefins such as isobutylene and isoamylene to form ethers such as M T B E , E T B E and T A M E but w i l l also require an alternate disposition for current olefins in gasoline. Lyondell Licensing, Inc. a wholly owned subsidiary of Lyondell Petrochemical Company, is working to increase the supply of iso-olefins while reducing the level of olefins i n gasoline with a unique Olefin Skeletal Isomerization Process (2).


Background Three years ago, Lyondell Petrochemical Company identified a need for technology to convert normal butylènes, produced during operation of its steam cracker, to isobutylene for the production of M T B E . A t the time, no technology was commercially available for the isomerization of normal butènes to isobutylene. The only proven technology available was to isomerize η-butane to i-butane followed by a dehydrogenation step to produce isobutylene. Capital requirements for these technologies are high with significant licensing fees. A review of literature and patent information suggested that difficulties with rapid catalyst deactivation and substantial by-product formation made most options commercially unattractive (5-6). Lyondell concluded that i n order for this process to be commercially feasible, a low cost fixed bed catalytic process was required. Such a system was expected to require less capital investment and be easier to operate than a moving bed system. In addition, the injection of halogen or acid was to be avoided due to safety, environmental and metallurgical concerns. In order to meet these criteria, Lyondell set out to develop a process which would exhibit long cycle times between regenerations while providing near equilibrium yields of isobutylene with high selectivity. Isobutylene and isoamylene yields are limited to equilibrium concentrations in the product stream. For C4 olefin isomerization, 42 wt.% isobutylene (based on butènes i n the feed) represents the equilibrium concentration at optimal reactor temperature, pressure and space velocity. Equilibrium yields of isoamylenes, which consist of 2-methyl-2-butene and 2-methyl-l-butene, at reaction temperature, pressure and space velocity total 73 wt.% based on pentenes i n the feed. Once a successful catalytic process was developed for butene isomerization, pentene isomerization would be even easier since the activation energy for skeletal isomerization of C5*s is lower than that of C4S. Catalyst Screening and Commercial Testing A small Bench Scale Reactor was used to screen catalysts and evaluate process conditions. The size of this unit was ideally suited for testing catalyst preparations produced at the laboratory scale. This reactor operates 24 hours per day on a 7-day per week schedule. These studies led to early initial success and many subsequent improvements i n catalyst performance. Figure 4 shows this unit, which occupies three hoods within the laboratory. This unit continues to operate to evaluate catalyst improvements.


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Figure 4: Bench Scale Reactor (under vent hoods)

Figure 5: Process Development Unit



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After initial success with the bench scale reactor, Lyondell began work i n a larger existing pilot plant called the "Process Development Unit" (Figure 5). Catalyst samples for this unit were prepared at a larger semi-works scale i n order to obtain information on scale up of the catalyst manufacturing process. This unit was then used to test the effect of multiple catalyst regenerations on catalyst performance. The larger unit also provided an opportunity for intermediate scale up before going on to larger scale commercial equipment. In order to fully demonstrate the operability of this process, a commercial demonstration was performed using existing equipment at the Channelview plant. The process was operated for a three month period using raffinate Π feed at an average rate of 3,000 B P S D . During this time, commercial M T B E product was produced using an existing M T B E unit. A t the conclusion of this three month test, the demonstration unit was converted back into its original service due to product inventory requirements. Figure 6 shows a photograph of the commercial reactors used for this demonstration. When available, this unit w i l l be used to test further improved catalyst formulations. The catalyst that was used to test n-butene isomerization to isobutylene was also used to isomerize n-pentenes to isoamylene at the laboratory scale. The initial results from this testing were extremely positive. Results and Discussion The goal of the catalyst development program was to develop a catalyst that would convert normal olefins to iso-olefins at near thermodynamic equilibrium concentrations with minimal by-product formation. The operating conditions are low pressure, typically just above atmospheric, with temperatures between 720 and 810 ° F . The catalyst of choice is a zeolite containing extrudate exhibiting long cycle lengths between regenerations. This catalyst converts η-olefins to iso-olefins at yields slightly below equilibrium with high selectivity to the iso-olefin being produced. The catalyst does not require the addition of halogens, acids or steam to maintain activity. Conversion and selectivity reflect the feed (FD) and effluent (EFF) concentrations of butene-1 ( B l ) , cis and trans Butene-2 (B2) and isobutylene (IB1). Conversion is defined as follows: (wt.% B l + wt.% B 2 ) F D - (wt.% B l + wt.% B 2 ) E F F χ 100 (wt.% B 1 + w t . % B 2 ) F D selectivity is calculated as: (wt.% I B 1 ) E F F - (wt.% I B 1 ) F D Selectivity, wt.% = χ 100 (wt.% B l + wt.% B 2 ) F D - ( w t . % B l + wt.% B 2 ) E F F and yield is calculated as: (wt.% IB 1)EFF - (wt.% IB 1)FD Y i e l d , wt.% = χ 100 (wt.% B 1 + w t . % B 2 ) F D Conversion, wt.% =




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Figure 6: Commercial Demonstration Unit


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Figure 7 graphically illustrates the improvement i n catalyst performance achieved as a result of our catalyst development program. This figure shows the isobutylene yield versus run time characteristic of this process. The time scale represents a single catalyst cycle. Isobutylene yields have been normalized for butene content i n the feed. Commercial raffinate-Π feed with 72% butene content was used to feed the reactor. A component breakdown of commercial raffinate-Π is presented i n Table I.


Table I. Raffinate-II Composition Isobutane Isobutylene 1-Butene n-Butane t-2-Butene c-2-Butene C +

4.86 1.69 41.87 23.31 17.20 10.95 0.12

wt.% wt.% wt.% wt.% wt.% wt.% wt.%

5

Catalyst improvements i n the development program have focused on reducing the catalyst coking rate to improve cycle time while maintaining conversion and selectivity. The improvement illustrated i n Figure 7 is a result of modifications made i n the catalyst preparation process. Figure 8 shows the high selectivity to isobutylene that is typically achieved using our catalyst. W i t h each catalyst improvement, the data clearly shows a significant increase i n run time at equilibrium isobutylene yield. Another way to represent this data is to look at the average yield and selectivity over a given catalyst cycle time. Figure 9 shows the average conversion, selectivity and yield for this catalyst over a 290 hour (12 day) cycle. Over this cycle time, the average conversion of normal butènes is 44% with a selectivity to isobutylene of greater than 90%. Figure 10 shows the same information to an average cycle time of 480 hours (20 days). In this case, the average selectivity to isobutylene increases to 92 wt.%. The increase i n average selectivity partially compensates for the decrease i n conversion exhibited by the longer cycle. The resulting yield of isobutylene decreases by only 2.7 wt.%. The primary by-products formed are gasoline boiling range material and small amounts of propylene. Through experimentation, we have also seen evidence of saturates conversion i n butene isomerization early on i n the cycle. This conversion of saturates is demonstrated i n figure 11. Figure number 12 shows the results from a recent laboratory scale test of our improved second generation catalyst using T A M E raffinate as the feedstock. Feedstock characteristics for T A M E unit raffinate feeding an isomerization unit are shown i n Table Π. The average conversion, average selectivity and yield of isoamylene are shown for a cycle time of 206 hours. The selectivity to isoamylene starts out at greater than 90 wt% and approaches 100 wt.% as the cycle progresses. This increase i n average selectivity partially compensates for the decrease i n



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B e n c h Scale Reactor - Generation Results I B 1= Y i e l d v s . R u n T i m e

300

100 200 R u n T i m e , hrs Figure 7: Isobutylene Y i e l d vs. R u n Time

Bench Scale Reactor - Generation Results Selectivity vs. Run Time

100

m m Mγ

V

Sri

M

—^— I st Generation

ta •

φ

2nd Generation

—I— Tmproved 2nd Generation

l 50

100

ι l ι l ι I 150

200

250

300

Run Time, hrs Figure 8: Selectivity vs. R u n Time


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Process

Improved Second Generation 290 Hour Cycle T i m e 44 wt.% Conversion 90 w t % Selectivity


Lights Propylene Isobutylene C+

Highly Selective Olefin Skeletal Isomerization Process - ACS

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