Olefin Paraffin Alkylation Evolution of a Green Technology

Anne M. Gaffney and Philip J. Angevine

12.1

Introduction

Today's gasoline has many refining sources: fluid catalytic cracking (FCC) naphtha, straight-run naphtha, coker and visbreaker naphtha, reformate, isomerate and alkylate. High compression engines are pushing the market to increased amounts of high-octane gasoline, and environmental factors are constraining gasoline composition in many other ways. Table 12.1 lists some of the key specifications of gasoline - now and in the near future:

• In addition to SOx emissions, sulfur causes indirect, inhalable particulates. Since sulfur poisons catalytic converters, lower sulfur dramatically reduces CO, NOx and unburned hydrocarbon emissions.

• Benzene is a known carcinogen and is carefully regulated. Although other aromatics are less problematic, their acceptable levels are also being lowered. The presence of benzene/aromatics may increase particulate formation and/or the formation of polycyclic aromatic compounds.

• Olefins are photochemically reactive and are a leading cause of smog/ozone formation.

• Although not shown, the evaporative emissions ofhydrocarbons (i.e., volatility) are another cause of smog formation and are also regulated via Reid vapor pressure (RVP) specifications.

• The US EPA toxicity equation identifies heavy gasoline as a factor in overall toxicity; therefore, T90 - the temperature at which 90vol.% of the gasoline has been evaporated - must be controlled.

With all of these regulatory restrictions, refiners' gasoline blending options are becoming more limited.

Gasoline's two main blending components are FCC and naphtha reformer gasoline. Both contain large concentrations of aromatic compounds and, in the

Table 12.1 Some key gasoline specifications.

USA Phase II RFG (2004)

California Carb (2005)

EU

RON, min

91

MON, min

81

(RON + MON)/2

87/89/91°

87/89/91°

S (ppm, max)

80

30

50 ! 10 (2009)

Benzene (vol.% max)

1.0

1.1

1.0

Aromatics (vol.% max)

35

35

Olefins (vol.% max)

10

18

aSet by industry for regular, mid-grade, and premium.

aSet by industry for regular, mid-grade, and premium.

case of FCC gasoline, significant amounts of olefins. Fortunately, alkylate represents the nearly ideal gasoline component. It contains no aromatics, no olefins, essentially no sulfur or nitrogen; it also has a high research octane number (RON) and motor octane number (MON), and low vapor pressure. However, olefin/paraffin alkylation historically has been carried out with mineral acids, such as HF or H2SO4, both of which have many negative qualities.

This chapter discusses alkylation and its evolution into a modern refining process. We review the basic chemistry of alkylation, assess the properties and other merits of HF versus H2SO4, identify key drivers in the process and discuss the evolution of one particular process - the AlkyClean solid acid catalyst alkylation process.

12.2

Liquid Acid Catalysts

Olefin/isoparaffin alkylation dates back to the 1930s, and the need for high-octane aviation gasoline in World War II was the major impetus for the technology's development. While various catalysts were tried, by the 1940s HF and H2SO4 had become the catalysts of choice. The highly active HF enabled refiners to broaden the feedstocks to include C3= and C5=, thereby boosting the overall alkylate capacity (currently about 30 billion gallons, or 80 million tons, per year on a worldwide basis).

Table 12.2 summarizes the key properties ofHFand H2SO4. The critical properties for alkylation are acidity and isobutane availability. The catalyst's acidity generally determines the olefin protonation rate. Isobutane availability determines the carbo-cation formation. H2SO4 is a stronger acid than HF. Isobutane is more readily available in HF as it has higher solubility. In addition, isobutane mass transfer from hydrocarbon to acid phase is more expedient in HF versus H2SO4.

With HF, water should be completely removed. In the case of H2SO4, water and, to a lesser extent, hydrocarbons (acid soluble oils) lower the acid strength. As excess acidity can cause unwanted by-products there is an optimal acid concentration for each set of conditions and feed mix. Since HF has higher isobutane solubility, it provides higher alkylate quality with less isomerization and oligomerization.

HF

H2SO4

Molecular weight

20

98

Boiling point (° C)

19.4

290

Freezing point (° C)

—83

10

98% acid

3

Specific gravity

0.99

1.84

Viscosity (cP)

0.26 (0 °C)

33 (15 °C)

Surface tension (dynecm-1)

8.1 (27 °C)

55 (20°C)

Specific heat (Btulb °F—^

0.83 (—1 ° C)

0.33 (20°C)

Hammett acidity (—H0) at 25 ° C

10.0

11.1

98% acid at 25 ° C

8.9

9.4

Dielectric constant

84 (0 °C)

114 (20°C)

Liquid solubility (wt%)

i-C4H10 in 100% acid at 27 °C

2.7

i-C4H10 in 99.5% acid at 13 °C

0.10

HF in i-C4H10 at 27 °C

0.44

HF in C3H8 at 27 °C

0.90

With H2SO4, acid strength needs to be optimized. An acid strength below 90% may suddenly become too low, and polymerization reactions will begin to dominate, leading to a so-called "acid runaway." As such, H2SO4 may oxidize the polymers and form large quantities of SO2. This is a highly undesired situation.

The minimum operating temperature of H2SO4 is constrained by the freeze point, viscosity and other properties, which in turn limits alkylate quality. High surface tension and lower hydrocarbon solubility require very strong mixing within the H2SO4 system to obtain reasonable yields and product quality.

Both acids are spent (diluted) during the process and require regeneration. HF is volatile and can be recovered by distillation inside a refinery alkylation unit. Net consumption of HF is low. While the operating dilution range for H2SO4 is higher than HF, spent H2SO4 is normally regenerated outside the refinery. Dilute acid leaves the refinery and higher concentration acid is brought in. As a practical matter, refiners consider acid use to be the quantity ofacid that must be brought in through the gate. By this measure, in H2SO4 alkylation, acid consumption is high (75-150 kg of acid per ton of alkylate produced). HF consumption is low (less than 0.4 kg per ton alkylate). Although HF is more expensive than regenerated H2SO4, the overall acid cost for HF alkylation is normally far lower than H2SO4 alkylation.

Spent sulfuric acid is often recovered offsite in a specially designed regeneration plant by incineration and conversion of the sulfur oxides formed into sulfuric acid. However, large quantities of fresh and spent acid must be transported to the offsite regeneration plant. (The total worldwide consumption of existing H2SO4 alkylation units is about 10-20 billion lb per year or 4-8 million tons per year.) In some cases, refiners have built a captive H2SO4 regeneration unit, which is normally far smaller

Table 12.3 Typical products of liquid acid alkylation [5].

Component

H2SO4 alkylate (vol.%)a

HF alkylate (vol.%)a

Propane

0.05

Isobutane

0.04

0.13

n-Butane

0.9

4.9

Isopentane

8.8

5.1

n-Pentane

0.23

0.01

2,2-Dimethylbutane

2,3-Dimethylbutane

5.4

2.4

2-Methylpentane

1.3

0.9

3-Methylpentane

0.64

0.4

n-Hexane

2,2-Dimethylpentane

0.25

0.2

2,4-Dimethylpentane

3.6

2.0

2,2,3-Trimethylbutane

0.01

3,3-Dimethylpentane

0.01

2,3-Dimethylpentane

2.2

1.3

2-Methylhexane

0.22

0.24

3-Methylhexane

0.14

0.12

3-Ethylpentane

0.01

0.01

2,2,4-Trimethylpentane

24.2

38.0

n-Heptane

2,2,3,3-Tetramethylbutane

2,2-Dimethylhexane

0.04

2,4-Dimethylhexane

2.9

4.2

2,5-Dimethylhexane

4.9

3.6

2,2,3-Trimethylpentane

1.5

1.4

3,3-Dimethylpentane

2,3,4-Trimethylpentane

13.2

9.6

2,3-Dimethylhexane

3.4

4.9

4-Methylheptane

2-Methylheptane

0.08

0.09

2,3,3-Trimethylpentane

11.47

8.14

3,4-Dimethylhexane

0.26

0.59

3-Methylheptane

0.23

0.19

2,2,5-Trimethylhexane

7.20

3.20

Other C9+

6.82

8.38

Total

100.00

100.00

aBased on total finished alkylate.

aBased on total finished alkylate.

than the size judged economical by commercial acid suppliers. The economics of this approach vary with location and circumstances.

Table 12.3 summarizes the typical products obtained in H2SO4 and HF processes. One key observation is that trimethylpentanes/dimethylhexanes (TMP/DMH) formation is far from thermodynamic equilibrium, a desirable factor given the octane spread - TMPs have RON numbers of 100 and higher, DMHs about 50-60. The various products formed are present for all feeds, catalysts and process conditions, only in different proportions.

Olefin feed

C3—

i-C4—

2-C4—

1-C4-

Feed (vol.%)

Olefin

15.9

15.8

16.7

14.9

Isoparaffin

77.0c

80.5c

80.3c

78.9c

n-Paraffins, C3-C5

7.1

3.7

3.0

6.2

Reactor hydrocarbon (vol.%)

Product6

30.2

25.0

29.8

29.6

Isoparaffin (feed type)

62.9c

70.3c

65.3c

64.5c

n-Paraffins, C3-C5

6.9

4.7

4.9

5.9

Product6 (vol.%)

Isobutane

Isopentane

3.8

10.0

4.2

4.7

2,3-Dimethylbutane + methylpentane

4.2

5.2

4.6

4.4

2,4-Dimethylpentane

20.8

3.9

2.4

2.6

2,3-Dimethylpentane

50.4

2.6

1.4

1.5

2,2,4-Trimethylpentane

4.4

28.7

30.6

30.5

Dimethylhexane

1.7

9.5

9.0

11.0

2,2,3 + 2,2,4-Trimethylpentane

3.7

23.1

41.6

39.1

2,2,5-Trimethylhexane

0.9

4.9

1.9

1.8

Other C9s

0.4

1.7

0.5

0.7

C10

5.3

2.5

0.7

0.6

Cii

3.7

2.1

0.7

0.7

Cl2 +

0.8

5.9

2.6

2.6

c c a0.22 olefin (liquid-hourly) space velocity.

bC4 and heavier product exclusive of n-paraffins and feed-type isoparaffin. cIsobutane.

a0.22 olefin (liquid-hourly) space velocity.

bC4 and heavier product exclusive of n-paraffins and feed-type isoparaffin. cIsobutane.

Table 12.4 shows the effect of different olefin feeds on the alkylate quality with a sulfuric acid system. All of the butenes give better alkylate yields and product quality than propylene, and 2-butene is the preferred C4= feed. For an HF unit, 1-C4= gives a lower RON product because of its higher DMH yield. As such, most HF units have an isomerization unit upstream of the alkylation unit to isomerize the 1-C4= to 2-C4=.

Reaction Mechanism

The generally accepted alkylation reaction mechanism has four desirable key steps and four undesirable secondary reactions. The four desirable steps are:

1. Initiation (or olefin protonation): In this step, a t-butyl cation is formed from isobutene. A sec-butyl cation is formed from 1-C4= or 2-C4=. The sec-butyl cation can form a t-butyl cation by methyl shift, or it can undergo hydride transfer from isobutane, forming n-C4 and a t-butyl cation.

2. Alkylation (or t-butyl cation/olefin condensation): here, the various TMP or DMH carbocations are formed.

3. Isomerization: The C8 carbocations formed in step 2 may isomerize via hydride transfer or methyl shift to form various TMP cations. DMHs are thermodynami-cally favored; thus, residence time preferably should be short (high temperature reduces the required residence time).

4. Termination via hydride transfer: The carbocations react with isobutane to form the various octane products, along with a t-butyl cation to continue the reaction sequence.

The following unwanted secondary reactions generally result in reduced yield and quality loss:

1. Oligomerization: After the primary reaction forms a C8 + carbocation, a second olefin reacts to form a higher molecular weight hydrocarbon (e.g., C12) and another t-butyl cation. Further reactions can result in even larger products (e.g., C16s, etc.).

2. Disproportionation: The typical reaction here is a bimolecular reaction of two equivalent alkylate molecules (e.g., C8s form a C7 and C9 product). Of the major products, 2,3,4-TMP is most reactive for disproportionation.

3. Cracking: the larger isoalkyl cations can undergo b-scission to form smaller olefins and isoalkyl cations.

4. Self-alkylation: This reaction occurs readily with HF and with most zeolites, albeit to a reduced extent. Using 2-C4= as a typical reagent, the butene can be protonated to a sec-butyl cation, which undergoes hydride transfer from an isobutane molecule to form a low-valued n-C4. Self-alkylation reaction rates increase with molecular weight and alkene branching. With higher alkenes (e.g., C5 +), H2SO4 will also become active.

The two major reactants are isobutane and butene. While a wide range of hydrocarbons is formed, the predominant product is the C8s, and the preferred products are TMPs versus DMHs. The TMPs have excellent RON and MON - both are at and above 100. As such, TMPs represent the highest octane, nonaromatic gasoline component. DMHs have lower octane values, with RONs of 50-60.

A good process and catalyst system will drive alkylation to minimize light and heavy by-products as well as to maximize TMPs versus DMHs. There are several interactive reactions in the proposed mechanism. First, initiation of the alkylation cycle takes place by reaction of olefins, leading to the formation of i-C4 + species on the acid sites. This adsorbed i-C4 + further reacts with a C4= to form an adsorbed i-C8 + carbocation. The i-C8 + reacts with i-C4 via hydrogen transfer, and the i-C8 product is formed along with another i-C4 + to feed back into the cycle, and the cycle repeats.

As shown in Figure 12.1, there are many more competing reactions. For example, the C8 + can further react with another olefin (e.g., C4=) to form a heavier cation, in this case i-C12 +. This i-C12 + can leave the loop via hydrogen transfer, react again

I transfer '

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