Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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Olefin metatheses are equilibrium reactions among the two-reactant and

two-product olefin molecules. If chemists design the reaction so that one

product is ethylene, for example, they can shift the equilibrium by

removing it from the reaction medium.

Because of the statistical nature

of the metathesis reaction, the equilibrium is essentially a function of the

ratio of the reactants and the temperature. For an equimolar mixture of

ethylene and 2-butene at 350°C, the maximum conversion to propylene

is 63%. Higher conversions require recycling unreacted butenes after

fractionation.

This reaction was first used to produce 2-butene and eth-

ylene from propylene (Chapter 8). The reverse reaction is used to prepare

polymer-grade propylene form 2-butene and ethylene:

CH=CHCH

+ CH

=CH

2CH

CH=CH

The metathetic reaction occurs in the gas phase at relatively high tem-

peratures (150°–350°C) with molybdenum or tungsten supported cata-

lysts or at low temperature (≈50°C) with rhenium-based catalyst in either

liquid or gas-phase. The liquid-phase process gives a better conversion.

Equilibrium conversion in the range of 55–65% could be realized,

depending on the reaction temperature.

In this process, which has been jointly developed by Institute Francais

du Petrole and Chinese Petroleum Corp., the C

feed is mainly composed

of 2-butene (1-butene does not favor this reaction but reacts differently

with olefins, producing metathetic by-products). The reaction between 1-

butene and 2-butene, for example, produces 2-pentene and propylene.

The amount of 2-pentene depends on the ratio of 1-butene in the feed-

stock. 3-Hexene is also a by-product from the reaction of two butene

molecules (ethylene is also formed during this reaction). The properties

of the feed to metathesis are shown in Table 9-1.

Table 9-2 illustrates

the results from the metatheses reaction at two different conversions. The

main by-product was 2-pentene. Olefins in the range of C

–C

and higher

were present, but to a much lower extent than C

Figure 9-3 shows a simplified flow diagram for the olefin metathesis.

Olefins and Diolefins-Based Chemicals 247

Table 9-1

Properties of feed to the metathesis process

Composition Wt%

n-Butane 2.8

Butene- 1 7.2

Butene-2 90.0

Chapter 9 1/22/01 11:07 AM Page 247

Table 9-2

Results of metathesis of 2-butene at two conversion levels

Item Case 1 Case 2

Ethylene feed, kg/h 8.1 8.1

Total C

feed, kg/h 14.3 13.4

recycle, kg/h 4.4 9.6

Butene-2 conversion

% per pass 62.3 59.6

% overall 87.8 94.6

Propylene product

% selectivity 93.8 96.6

% yield from butene-2 82.4 91.3

248 Chemistry of Petrochemical Processes

Figure 9-3. A flow diagram showing the metathesis process for producing poly-

mer grade propylene from ethylene and 2-butene.

OLIGOMERIZATION OF BUTENES

2-Butenes (after separation of l-butene) can be oligomerized in the

liquid phase on a heterogeneous catalyst system to yield mainly C

and

olefins.

The reaction is exothermic, and requires a multitubular car-

bon steel reactor. The exothermic heat is absorbed by water circulating

around the reactor shell. Either a single- or a two-stage system is used.

The process can be made to produce either more linear or more branched

oligomers. Linear oligomers are used to produce nonyl alcohols for plas-

ticizers, alkyl phenols for surfactants, and tridecyl alcohols for detergent

Chapter 9 1/22/01 11:07 AM Page 248

intermediates. Branched oligomers are valuable gasoline components.

Figure 9-4 shows the Octol oligomerization process.

A typical analysis

of A-type oligomers (branched) is shown in Table 9-3.

CHEMICALS FROM ISOBUTYLENE

Isobutylene (CH

=C(CH

)

) is a reactive C

olefin. Until recently,

almost all isobutylene was obtained as a by-product with other C

hydro-

carbons from different cracking processes. It was mainly used to produce

alkylates for the gasoline pool. A small portion was used to produce

chemicals such as isoprene and diisobutylene. However, increasing

demand for oxygenates from isobutylene has called for other sources.

n-Butane is currently used as a precursor for isobutylene. The first step

is to isomerize n-butane to isobutane, then dehydrogenate it to isobuty-

lene. This serves the dual purpose of using excess n-butane (that must be

removed from gasolines due to new rules governing gasoline vapor pres-

sure) and producing the desired isobutylene. Currently, the major use of

iosbutylene is to produce methyl-ter-butyl ether.

The following section reviews the chemistry of isobutylene and its

important chemicals.

Olefins and Diolefins-Based Chemicals 249

Figure 9-4. The Octol Oligomerization process for producing C

’s and C

’s and

’s olefins from n-butenes:

(1) multitubular reactor, (2) debutanizer column,

(3) fractionation tower.

Chapter 9 1/22/01 11:07 AM Page 249

OXIDATION OF ISOBUTYLENE

(Methacrolein and Methacrylic Acid)

Much like the oxidation of propylene, which produces acrolein and

acrylic acid, the direct oxidation of isobutylene produces methacrolein

and methacrylic acid. The catalyzed oxidation reaction occurs in two

steps due to the different oxidation characteristics of isobutylene (an

olefin) and methacrolein (an unsaturated aldehyde). In the first step,

isobutylene is oxidized to methacrolein over a molybdenum oxide-based

catalyst in a temperature range of 350–400°C. Pressures are a little

above atmospheric:

250 Chemistry of Petrochemical Processes

Table 9-3

Typical analysis of branched oligomers (Type A)

Densily (20°C), kg/l 0.755

Flash point °C –4

Ignition temperature, °C 240

Pour point °C below –40

Hydrocarbon no. distribution % by mass

0.7

1.0

66.2

2.0

3.0

1.2

16.6

to C

0.5

7.8

16+

1.0

RON MON

Gasoline hase stock

(unleaded, low in olefins) 97.0 85.7

+5% oligomers 97.0 85.3

+ 10% oligomers 96.8 85.0

In the second step, methacrolein is oxidized to methacrylic acid at a

relatively lower temperature range of 250–350°C. A molybdenum-

supported compound with specific promoters catalyzes the oxidation.

Chapter 9 1/22/01 11:07 AM Page 250

Methacrylic acid is esterified with methanol to produce methyl

methacrylate monomer.

Methacrylic acid and methacrylates are also produced by the hydrocya-

nation of acetone followed by hydrolysis and esterification (Chapter 8).

Ammoxidation of isobutylene to produce methacrylonitrile is a simi-

lar reaction to ammoxidation of propylene to acrylonitrile. However, the

yield is low.

EPOXIDATION OF ISOBUTYLENE

(Isobutylene Oxide Production)

Isobutylene oxide is produced in a way similar to propylene oxide and

butylene oxide by a chlorohydrination route followed by reaction with

Ca(OH)

. Direct catalytic liquid-phase oxidation using stoichiometric

amounts of thallium acetate catalyst in aqueous acetic acid solution has

been reported. An isobutylene oxide yield of 82% could be obtained.

Direct non-catalytic liquid-phase oxidation of isobutylene to isobuty-

lene oxide gave low yield (28.7%) plus a variety of oxidation products

such as acetone, ter-butyl alcohol, and isobutylene glycol:

Olefins and Diolefins-Based Chemicals 251

Hydrolysis of isobutylene oxide in the presence of an acid produces

isobutylene glycol:

Isobutylene glycol may also be produced by a direct catalyzed liquid

phase oxidation of isobutylene with oxygen in presence of water. The

catalyst is similar to the Wacker-catalyst system used for the oxidation

Chapter 9 1/22/01 11:07 AM Page 251

of ethylene to acetaldehyde. Instead of PdCl

/CuCl

used with ethylene,

a TlCl

/CuCl

catalyst is employed

252 Chemistry of Petrochemical Processes

Liquid-phase oxidation of isobutylene glycol produces othydroxyisobu-

tyric acid. The reaction conditions are 70–80°C at pH 2–7 in presence of

a catalyst (5% pt/C)

Dehydration of the acid produces 95% yield of methacrylic acid:

ADDITION OF ALCOHOLS TO ISOBUTYLENE

(Methyl- and Ethyl-Ter-Butyl Ether)

The reaction between isobutylene and methyl and ethyl alcohols is an

addition reaction catalyzed by a heterogeneous sulfonated polystyrene

resin. When methanol is used a 98% yield of methyl-ter-butyl ether

MTBE is obtained:

The reaction conditions have been noted in Chapter 5.

Ethyl-ter-butyl ether (ETBE) is also produced by the reaction of

ethanol and isobutylene under similar conditions with a heteroge-

neous acidic ion-exchange resin catalyst (similar to that with MTBE):

Chapter 9 1/22/01 11:07 AM Page 252

MTBE and ETBE constitute a group of oxygenates that are currently in

high demand for gasoline octane-number boosters. Both MTBE and

ETBE have a similar research octane number of 118, but the latter ether

has a motor octane number of 102 versus 100 for MTBE.

However, the

oxygen content of MTBE is 18.2% compared to 15.7% for ETBE. The

lower oxygen content of ETBE is related to the air/fuel ratio, which may

not require a change in the automobile carburetors. A comparison

between the two ethers regarding phase separation, antiknock behavior,

and fuel economy has been reviewed by Iborra et al.

HYDRATION OF ISOBUTYLENE

(Ter-Butyl Alcohol [(CH

)

COH])

The acid-catalyzed hydration of isobutylene produces ter-butyl alco-

hol. The reaction occurs in the liquid phase in the presence of 50–65%

at mild temperatures (10–30°C). The yield is approximately 95%:

Olefins and Diolefins-Based Chemicals 253

ter-Butyl alcohol (TBA) is used as a chemical intermediate because a

tertiary butyl carbocation forms easily. It is also used as a solvent in

pharmaceutical formulations, a paint remover, and a high-octane gaso-

line additive. The alcohol is a major by-product from the synthesis of

propylene oxide using tertiary butyl hydroperoxide. Surplus ter-butyl

alcohol could be used to synthesize highly pure isobutylene for MTBE

production by a dehydration step. The reaction conditions, the catalyst

used in a pilot-scale unit, and the yield are reviewed by Abraham and

Prescott.

It was concluded that MTBE conversion increases from 8

wt% to 88 wt% as the temperature increases from 400°F to 600°F at

about 40 LHSV (liquid hourly space velocity). At a lower space velocity

(≈20 LHSV), conversion increased from 12 wt% to 99 wt% for the same

temperature range. Figure 9-5 shows the effect of temperature and

LHSV on the conversion

Chapter 9 1/22/01 11:07 AM Page 253

254 Chemistry of Petrochemical Processes

Figure 9-5. Effect of temperature and liquid hourly space velocity on

conversion.

Figure 9-6. A simplified flow diagram of a tertiary butyl alcohol pilot plant.

Chapter 9 1/22/01 11:07 AM Page 254

Figure 9-6 is a simplified flow diagram of a TBA dehydration pilot unit.

Olefins and Diolefins-Based Chemicals 255

The addition of carbon monoxide to isobutylene under high pressures

and in the presence of an acid produces a carbon monoxide-olefin com-

plex, an acyl carbocation. Hydrolysis of the complex at lower pressures

yields neopentanoic acid:

Neopentanoic acid (trimethylacetic acid) is an intermediate and an ester-

ifying agent used when a stable neo structure is needed.

DIMERIZATION OF ISOBUTYLENE

Isobutylene could be dimerized in the presence of an acid catalyst to

diisobutylene. The product is a mixture of diisobutylene isomers, which

are used as alkylating agents in the plasticizer industry and as a lube oil

additive (dimerization of olefins is noted in Chapter 3).

CHEMICALS FROM BUTADIENE

Butadiene is a diolefinic hydrocarbon with high potential in the chem-

ical industry. In 1955, it was noticed that “the assured future of butadi-

ene (CH

=CH-CH=CH

) lies with synthetic rubber . . . the potential of

butadiene is in its chemical versatility . . . its low cost, ready availabil-

ity, and great activity tempt researchers.”

Butadiene is a colorless gas, insoluble in water but soluble in alcohol.

It can be liquefied easily under pressure. This reactive compound poly-

merizes readily in the presence of free radical initiators.

Chapter 9 1/22/01 11:07 AM Page 255

Butadiene is mainly obtained as a byproduct from the steam cracking

of hydrocarbons and from catalytic cracking. These two sources account

for over 90% of butadiene demand. The remainder comes from dehydro-

genation of n-butane or n-butene streams (Chapter 3). The 1998 U.S. pro-

duction of butadiene was approximately 4 billion pounds, and it was the

36th highest-volume chemical. Worldwide butadiene capacity was nearly

20 billion pounds.

Butadiene is easily polymerized and copolymerized with other

monomers. It reacts by addition to other reagents such as chlorine,

hydrocyanic acid, and sulfur dioxide, producing chemicals of great com-

mercial value.

ADIPONITRILE (NC(CH

)

CN)

Adiponitrile, a colorless liquid, is slightly soluble in water but soluble

in alcohol. The main use of adiponitrile is to make nylon 6/6.

The production of adiponitrile from butadiene starts by a free radical

chlorination, which produces a mixture of 1,4-dichloro-2-butene and 3,4-

dichloro-l-butene:

2CH

=CH—CH=CH

+2Cl

ClCH

CH=CHCH

+CH

=CHCHClCH

The vapor-phase chlorination reaction occurs at approximately

200–300°C. The dichlorobutene mixture is then treated with NaCN or

HCN in presence of copper cyanide. The product 1,4-dicyano-2-butene is

obtained in high yield because allylic rearrangement to the more thermo-

dynamically stable isomer occurs during the cyanation reaction:

ClCH

CH=CHCH

Cl+CH

=CHCHClCH

Cl + 4NaCN

2NCCH

CH=CHCH

CN+4NaCl

The dicyano compound is then hydrogenated over a platinum catalyst

to adiponitrile.

NCCH

CH=CHCH

CN + H

NC—(CH

)

—CN

Adiponitrile

Adiponitrile may also be produced by the electrodimerization of acry-

lonitrile (Chapter 8) or by the reaction of ammonia with adipic acid fol-

lowed by two-step dehydration reactions:

256 Chemistry of Petrochemical Processes

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