V

Multiple alkylation or of C

Cracking

Cracking

Figure 12.1 Conceptual mechanism for olefin/paraffin alkylation [10].

to form higher molecular weight hydrocarbons or break into smaller products such as an olefin and a sorbed carbocation. The olefin rapidly reacts with the acid sites to form another carbenium ion.

Since hydrogen transfer is essential for the desired product formation, a high isobutane-to-olefin (I/O) ratio is required in the feed. A high hydride transfer rate lowers the formation of non-C8 (e.g., C5-C7 and C9 + ) products, impedes the isomerization of kinetically favored TMPs to thermodynamically favored DMHs and limits oligomerization to heavies. A high feed I/O ratio is typically 8-15 for liquid alkylation processes. Owing to internal mixing and relative solubility of olefins and i-C4, the local I/O ratio can be as high as 1000. The high recycle of isobutane is a key cost factor in the process.

Operating Variables

Several operating variables in both liquid catalyst alkylation processes impact product yields and quality as well as overall costs:

1. Acid strength and composition: In H2SO4 processes, the optimal acid concentration is about 95-97%. At low levels (e.g., below 90%) catalyst activity is significantly diminished. At high levels (e.g., above 99%) isobutane reacts with SO3. Acid level is dictated by consumption and fresh catalyst make-up rates. Hence, acid concentration affects kinetics, alkylate yield and quality, and catalyst life. While HF plants are similar, a primary difference is that HF needs to be water-free because any water will rapidly deactivate the HF catalyst and can lead to severe corrosion problems.

0 20 40 60

Isobutane/Olefin Volume Ratio Figure 12.2 Alkylate octane number versus I/O ratio for an HF unit [7].

2. Isobutane-to-olefin (I/O) ratio: Since butene-butene reactions can form unwanted dimers and polymers, the I/O ratio is critical for driving yields and alkylate quality. It also dictates acid consumption. Since isobutane is essential for the hydride transfer step, a high I/O ratio drives desired alkylation and hydrogen transfer versus oligomerization reactions. At low I/O, high MW hydrocarbons are formed and catalyst consumption increases. Figure 12.2 [7] shows the alkylate octane number versus I/O ratio for an HF unit. At I/O ratios below 10, the octane and yield (not shown) decrease significantly. Consequently, the standard I/O ratio for HF is 10-15. H2SO4 alkylation is more complex. RON will also drop, but H2SO4 is less demanding. The external I/O ratio can be 5-8. A high isobutane recycle is required to maintain a very high internal I/O ratio because the olefins adsorb readily into the acid phase and the low olefin concentration is required to mitigate oligomerization.

3. Reaction temperature: Figure 12.3 [8] shows a typical octane versus temperature response curve for an HF unit with C4 feeds. One factor in this effect is the product

RON-Clear 97 -

RON-Clear 97 -

15 20 25 30 35 40 45 Temperature, °C

Figure 12.3 Effect of reactor temperature on research octane number (RON) of alkylate produced in an HF alkylation unit [8].

shift in TMP/DMH, which favors DMH at higher temperatures. H2SO4 units typically operate at 0 to10 °C, constrained at the lower end by viscosity and hydrocarbon solubility and at the upper end by unwanted oxidation and subsequent acid consumption.

4. Feedstock effects:

a) Olefins: Table 12.5 [9] shows that the specific olefins used can have a major impact on products and each catalyst system has a different response to each olefin. For sulfuric acid, the preferred reactant is 2-butene, but due to the rapid isomerization of 1-butene to 2-butene, each gives a somewhat similar product. Isobutylene oligomerizes more readily, causing a lower C8 yield and resulting in higher acid consumption. C3= (not shown) gives a significantly lower product RON (more C7 compounds). The HF system is far less impacted by olefin variation. The major effect is the 1-butene/2-butene difference: 1-butene does not isomerize easily to 2-butene and the RON debit is 3-4 numbers for 1-C4= versus 2-C4=. Consequently, most HF units have an isomerization unit upstream of the alkylation unit for isomerization of 1-C4= feed and selective hydrogenation of butadiene. C3= again produces a lower RON product and up to 15% propane by hydrogen transfer reactions, which are typically enhanced by the use of HF. The hydrogen transfer reactions also explain the somewhat higher RON that is observed in the case of C3= and HF. By hydrogen transfer from i-C4, the i-C4 + ions formed will continue to react according to the general mechanism to C8 compounds.

b) Feed impurities: As water is a major issue with HF units because of corrosion, feed pre-drying is critical. Diolefins, such as those found in coker or visbreaker naphtha, lower alkylate quality and increase acid consumption. They can be sharply reduced by a selective hydrogenation pretreatment step. Sulfur compounds (e.g., mercaptans) can increase acid consumption and lower alkylate quality and yields. The mercaptan concentration is often more pronounced for C5= feeds.

Table 12.5 Isobutane alkylation with different butene isomers (H2SO4 catalyst) [9].

Alkylate Olefinic components i-C4= 1-C4= 2-C4=

Composition (vol.%)

C8 59 90 77

Composition of C8 product (vol.%)

2.3.4-TMP 15 20 18 2,3,3-TMP 19 25 24 DMHs 15 10 11

5. The effect of space velocity is interrelated with reactor geometry and the effect of olefin concentration. As long as alkylation and hydrogen transfer reaction rates are in balance (effect of T, I/O, etc.), yield and selectivity show little dependence on olefin space velocity (OSV). In general, a very high space velocity will increase acid consumption and formation of acid-soluble oils because of the higher probability of multiple olefins reactions.

Advantages Versus Disadvantages

Both liquid acid processes have major safety issues. H2SO4 is very corrosive, but leaks are generally localized because of its lower volatility. Hence, there is reduced catastrophic potential other than that caused by fire from hydrocarbon-air mixtures. HF is more volatile, and even a pinhole leak can cause aerosol formation that can drift for miles. The resulting potential for fatalities from HF inhalation and burns is high, and many communities require extensive mitigation steps to be retrofitted into existing HF units (US$40-50 million). In some areas, new HF units are being banned altogether.

Owing to various compensating costs, the capital associated with HF and H2SO4 units are comparable. The operating costs of H2SO4 units are higher due to catalyst consumption and the refrigeration costs of the lower temperature process. However, safety mitigation costs are significantly higher for HF. H2SO4 has more flexibility within C4 olefins than HF, but HF is more flexible for handling C3 and C5 olefins.

12.3

Zeolite Catalysts

Several types of solid catalysts have been explored for use in alkylation. Several groups have attempted to use "hybrids" (i.e., liquids that have been adsorbed or immobilized on solid supports), such as BF3/zeolites/aluminas, triflic acid on a carrier, H2SO4 on SiO2, and so on. Since most have shown limited success, we restrict our discussion to the most promising true solid acid materials - zeolite catalysts.

The above-mentioned safety issues summarize the key environmental drivers for solid catalysts. Hybrids have had limited success and ionic liquids are undesirable since they can leach toxic compounds, and so on. Compared with AlCl3 and phosphoric acid catalysts, zeolites are significantly safer and, in many instances, more selective.

The earliest zeolite catalysts used in alkylation date back to the 1960s with rare earth-stabilized X and Y - "REX" and "REY" [11,12]. Historically, the three major catalyst attributes - activity, selectivity and aging stability - could not be achieved. Quite often, stability was the key limitation.

An early, detailed study of the product mix is shown in Figure 12.4 [13], where C5-C7 "lights," C8 alkylate and C9-C12 "heavies" are plotted against time on

60 80 ' 200 Time on stream (min) Figure12.4 Product distribution (carbon number) versus time on stream (2-C4=/isobutane with CeY zeolite) [13].

stream for a cerium-exchanged Y ("CeY") zeolite. For the first 25-minute period, conventional alkylation occurs. Then the hydride transfer activity declines and oligomerization begins to be more important. Unfortunately, the DMH composition increases with time, which is consistent with oligomerization/cracking/re-alkylation (Table 12.6) [13]. As hydride transfer decreases, the product quality shifts downward, consistent with the demise of strong acid sites.

Zeolite Factors Impacting Alkylation Performance

1. Zeolites show strong hydrocarbon sorption, which increases reactant concentration in the zeolite. This effect is more pronounced at low temperature.

2. There is a high concentration of acid sites in the zeolite cage, which drives consecutive bimolecular reactions.

3. The preferential sorption of olefins versus paraffins in zeolites favors oligomerization over alkylation.

4. Diffusional effect in zeolites impact the allowed transitional states of certain TMP isomers (2,3,4- and 2,3,3-TMP have lower steric hindrance), thereby influencing the product distribution. This effect is more pronounced for medium-pore zeolites. Overall product similarity between zeolites and mineral acids suggests that similar or equivalent mechanisms predominate for each.

5. Zeolites have numerous total acid sites, which is more important than the strength of each acid site.

Table 12.6 C5-C7 and C8 product distribution during initial alkylation stage [13].

Time on stream, min

Time on stream, min

Table 12.6 C5-C7 and C8 product distribution during initial alkylation stage [13].

1

5

15

30

"5-C7 hydrocarbons (wt%)

2-Me-butane

51.8

44.3

37.2

35.7

Other C5 or C7

0.2

0.3

0.5

1.7

2-Me-pentane

4.0

5.2

5.8

5.2

3-Me-pentane

6.1

6.6

6.7

8.1

2,3-diMe-butane

17.2

17.9

17.4

16.8

2-Me-hexane

1.2

1.8

2.5

1.9

3-Me-hexane

1.4

2.3

3.5

3.5

3-Et-pentane

0.2

0.3

0.4

0.7

2,3-diMe-pentane

6.0

9.1

12.6

13.4

2,4-diMe-pentane

10.7

11.2

12.4

11.1

2,2,3-triMe-butane

1.2

1.0

1.0

1.4

100.0

100.0

100.0

100.

"8 Hydrocarbons (mol.%)

2-Me-heptane

0.2

0.3

0.3

0.2

3-Me-heptane + 3-ET-hexane

0.4

0.7

0.8

0.7

4-Me-heptane

0.1

0.1

0.2

0.1

2,3-diMe-hexane

4.8

8.4

13.1

10.8

2,4-diMe-hexane

6.2

6.4

7.4

4.5

2,5-diMe-hexane

3.0

3.1

3.8

2.1

3,4-diMe-hexane

6.9

11.3

13.5

22.4

2-Me-, 3-Et-pentane

0.6

1.0

1.3

1.7

2,2,3-triMe-pentane

4.4

3.6

3.2

2.6

2,2,4-triMe-pentane

22.3

18.3

17.7

11.3

2,3,3-triMe-pentane

27.9

24.9

20.7

18.1

2,3,4-triMe-pentane

23.2

21.9

18.0

17.8

Octenes

7.7

Zeolites should have the following general properties of a good alkylation catalyst:

• sufficient acidity to form and stabilize carbocation intermediates;

• good hydrogen transfer capability to desorb C8 carbocations as well as to generate t-butyl cations from sec-butyl cations and isobutanes;

• sufficiently large pores to enable trimethyl alkanes to exit;

• low concentration of Lewis acid sites, which promote polymerization.

Impact of Reaction Conditions for Zeolites

A high I/O ratio drives hydride transfer versus polymerization. This process variable is a major factor in selectivity and overall operating cost.

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