Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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are specially used when coproduct propylene, butadiene, and the butenes

are needed. The advantage of using ethane as a feed to cracking units is

a high ethylene yield with minimal coproducts. For example, at 60% per

pass conversion level, the ultimate yield of ethylene is 80% based on

ethane being recycled to extinction.

The following are typical operating conditions for an ethane cracking

unit and the products obtained:

Conditions:

Temperature, °C 750–850

Pressure, Kg/cm

1–1.2

Steam/HC 0.5

Yield wt %

Hydrogen + methane 12.9

Ethylene 80.9

Propylene 1.8

Butadiene 1.9

Other* 2.5

* Other: Propane 0.3, butanes 0.4, butenes 0.4, C

1.4

Propane cracking is similar to ethane except for the furnace tempera-

ture, which is relatively lower (longer chain hydrocarbons crack easier).

However, more by-products are formed than with ethane, and the sepa-

ration section is more complex. Propane gives lower ethylene yield,

higher propylene and butadiene yields, and significantly more aromatic

pyrolysis gasoline. Residual gas (mainly H

and methane) is about two

and half times that produced when ethane is used. Increasing the severity

Crude Oil Processing and Production of Hydrocarbon Intermediates 97

Table 3-15

Ultimate yields from steam cracking various feedstocks

Feedstock

Yield, wt % Ethane Propane Butane Naphtha Gas oil Saudi NGL

+ CH

13 28 24 26 18 23

Ethylene 80 45 37 30 25 50

Propylene 2.4 15 18 13 14 12

Butadiene 1.4 2 2 4.5 5 2.5

Mixed butenes 1.6 1 6.4 8 6 3.5

1.6 9 12.6 18.5 32 9

Chapter 3 1/22/01 10:58 AM Page 97

of a propane cracking unit increases ethylene and residual gas yields and

decreases propylene yield. Figure 3-13 shows the influence of conversion

severity on the theoretical product yield for cracking propane.

Cracking n-butane is also similar to ethane and propane, but the yield

of ethylene is even lower. It has been noted that cracking either propane

or butanes at nearly similar severity produced approximately equal liquid

yields. Mixtures of propane and butane LPG are becoming important

steam cracker feedstocks for C

–C

olefin production. It has been fore-

casted that world LPG markets will grow from 114.7 million metric

tons/day in 1988 to 136.9 MMtpd in the year 2000, and the largest por-

tion of growth will be in the chemicals field.

Cracking Liquid Feeds

Liquid feedstocks for olefin production are light naphtha, full range

naphtha, reformer raffinate, atmospheric gas oil, vacuum gas oil, resi-

dues, and crude oils. The ratio of olefins produced from steam cracking

of these feeds depends mainly on the feed type and, to a lesser extent, on

the operation variables. For example, steam cracking light naphtha pro-

duces about twice the amount of ethylene obtained from steam cracking

vacuum gas oil under nearly similar conditions. Liquid feeds are usually

98 Chemistry of Petrochemical Processes

Figure 3-13. The influence of conversion severity on the theoretical product yield

for the cracking of propane. Acetylene, methyl acetylene, and propadiene are

hydrogenated and both ethane and propane are recycled to extinction (wt%).

Chapter 3 1/22/01 10:58 AM Page 98

cracked with lower residence times and higher steam dilution ratios than

those used for gas feedstocks. The reaction section of the plant is essen-

tially the same as with gas feeds, but the design of the convection and the

quenching sections are different. This is necessitated by the greater vari-

ety and quantity of coproducts. An additional pyrolysis furnace for crack-

ing coproduct ethane and propane and an effluent quench exchanger are

required for liquid feeds. Also, a propylene separation tower and a methyl

acetylene removal unit are incorporated in the process. Figure 3-14 is a

flow diagram for cracking naphtha or gas oil for ethylene production.

As with gas feeds, maximum olefin yields are obtained at lower

hydrocarbon partial pressures, pressure drops, and residence times. These

variables may be adjusted to obtain higher BTX at the expense of higher

olefin yield.

One advantage of using liquid feeds over gas feedstocks for olefin pro-

duction is the wider spectrum of coproducts. For example, steam crack-

ing naphtha produces, in addition to olefins and diolefins, pyrolysis

gasoline rich in BTX. Table 3-16 shows products from steam cracking

naphtha at low and at high severities.

44, 48

It should be noted that opera-

tion at a higher severity increased ethylene product and by-product

methane and decreased propylene and butenes. The following conditions

are typical for naphtha cracking:

Temperature °C: 800

Pressure Atm.: Atmospheric

Steam/HC Kg/Kg: 0.6–0.8

Residence time sec: 0.35

Steam cracking raffinate from aromatic extraction units is similar to

naphtha cracking. However, because raffinates have more isoparaffins,

relatively less ethylene and more propylene is produced.

Cracking gas oils for olefin production has been practiced since 1930.

However, due to the simplicity of cracking gas feeds, the use of gas oil

declined. Depending on gas feed availability and its price, which is

increasing relative to crude prices, gas oil cracking may return as a poten-

tial source for olefins. Gas oils in general are not as desirable feeds for

olefin production as naphtha because they have higher sulfur and aromatic

contents. The presence of a high aromatic content in the feed affects the

running time of the system and the olefin yield; gas oils generally produce

less ethylene and more heavy fuel oil. Although high sulfur gas oils could

be directly cracked, it is preferable to hydrodesulfurize these feeds before

cracking to avoid separate treatment schemes for each product.

Crude Oil Processing and Production of Hydrocarbon Intermediates 99

Chapter 3 1/22/01 10:58 AM Page 99

100 Chemistry of Petrochemical Processes

Figure 3-14. Flow diagram of an ethylene plant using liquid feeds.

Chapter 3 1/22/01 10:58 AM Page 100

Processes used to crack gas oils are similar to those for naphtha.

However, gas oil throughput is about 20–25% higher than that for naph-

tha. The ethylene cracking capacity for AGO is about 15% lower than for

naphtha. There must be a careful balance between furnace residence

time, hydrocarbon partial pressure, and other factors to avoid problems

inherent in cracking gas oils.

Table 3-17 shows the product composi-

tion from cracking AGO and VGO at low and high severities.

44,48,50

Figure 3-15 shows the effect of cracking severity when using gas oil on

the product composition.

PRODUCTION OF DIOLEFINS

Diolefins are hydrocarbon compounds that have two double bonds.

Conjugated diolefins have two double bonds separated by one single

bond. Due to conjugation, these compounds are more stable than mono-

olefins and diolefins with isolated double bonds. Conjugated diolefins

also have different reactivities than monoolefins. The most important

industrial diolefinic hydrocarbons are butadiene and isoprene.

Crude Oil Processing and Production of Hydrocarbon Intermediates 101

Table 3-16

Products from steam cracking naphtha at high severities

44,48

Cracking severity

Products** Low High

Methane 10.3 15

Ethylene 25.8 31.3

Propylene 16.0 12.1

Butadiene 4.5 4.2

Butenes 7.9 2.8

BTX 10 13

17 9

Fuel oil 3 6

Other*** 5.5 6.6

Feed:

Sp. gr 60/60°F 0.713

Boiling range °C 32–170

Aromatics 7

**Weight percent

***Ethane (3.3 and 3.4%), acetylene, methylacetylene, propane, hydrogen.

Chapter 3 1/22/01 10:58 AM Page 101

102 Chemistry of Petrochemical Processes

Figure 3-15. Component yields vs cracking severity for a typical gas oil.

Table 3-17

Product composition from cracking atmospheric gas oil

and vacuum gas oil

44,48,50

AGO VGO

Severity Severity

Products* Low High Low High

Methane 8.0 13.7 6.6 9.4

Ethylene 19.5 26.0 19.4 23.0

Ethane 3.3 3.0 2.8 3.0

Propylene 14.0 9.0 13.9 13.7

Butadiene 4.5 4.2 5.0 6.3

Butenes 6.4 2.0 7.0 4.9

BTX 10.7 12.6

-205°C** 10.0 8.0 18.9 16.9

Fuel oil 21.8 19.0 25.0 21.0

Other*** 1.8 2.5 1.4 1.8

***Weight %.

***Other than BTX.

***Acetylene, methylacetylene, propane, hydrogen.

Chapter 3 1/22/01 10:58 AM Page 102

Butadiene (CH

= CH-CH = CH

)

Butadiene is the raw material for the most widely used synthetic rub-

ber, a copolymer of butadiene and styrene (SBR). In addition to its util-

ity in the synthetic rubber and plastic industries (over 90% of butadiene

produced), many chemicals could also be synthesized from butadiene.

Production

Butadiene is obtained as a by-product from ethylene production. It is

then separated from the C

fraction by extractive distillation using furfural.

Butadiene could also be produced by the catalytic dehydrogenation of

butanes or a butane/butene mixture.

=CH-CH=CH

+ 2H

The first step involves dehydrogenation of the butanes to a mixture of

butenes which are then separated, recycled, and converted to butadiene.

Figure 3-16 is the Lummus fixed-bed dehydrogenation of C

mixture to

butadiene.

The process may also be used for the dehydrogenation of

mixed amylenes to isoprene. In the process, the hot reactor effluent is

quenched, compressed, and cooled. The product mixture is extracted: un-

reacted butanes are separated and recycled, and butadiene is recovered.

Crude Oil Processing and Production of Hydrocarbon Intermediates 103

Figure 3-16. Flow diagram of the Lummus process for producing butadiene:

(1) reactor, (2) quenching, (3) compressor, (4) cryogenic recovery, (5) stabilizer,

(6) extraction.

Chapter 3 1/22/01 10:58 AM Page 103

The Phillips process uses an oxidative-dehydrogenation catalyst in the

presence of air and steam. The C

mixture is passed over the catalyst bed

at 900 to 1100°C. Hydrogen released from dehydrogenation reacts with

oxygen, thus removing it from the equilibrium mixture and shifting the

reaction toward the formation of more butadiene. An in-depth study of

the oxidative dehydrogenation process was made by Welch et al. They

concluded that conversion and overall energy costs are favorable for

butadiene production via this route.

In some parts of the world, as in Russia, fermented alcohol can serve

as a cheap source for butadiene. The reaction occurs in the vapor phase

under normal or reduced pressures over a zinc oxide/alumina or magne-

sia catalyst promoted with chromium or cobalt. Acetaldehyde has been

suggested as an intermediate: two moles of acetaldehyde condense and

form crotonaldehyde, which reacts with ethyl alcohol to give butadiene

and acetaldehyde.

Butadiene could also be obtained by the reaction of acetylene and

formaldehyde in the vapor phase over a copper acetylide catalyst. The

produced 1,4-butynediol is hydrogenated to 1,4-butanediol. Dehydration

of 1,4-butanediol yields butadiene.

104 Chemistry of Petrochemical Processes

Isoprene (2-methyl 1,3-butadiene) is the second most important con-

jugated diolefin after butadiene. Most isoprene production is used for the

manufacture of cis-polyisoprene, which has a similar structure to natural

rubber. It is also used as a copolymer in butyl rubber formulations.

Chapter 3 1/22/01 10:58 AM Page 104

Production

There are several different routes for producing isoprene. The choice

of one process over the other depends on the availability of the raw mate-

rials and the economics of the selected process.

While most isoprene produced today comes from the dehydrogenation

of C

olefin fractions from cracking processes, several schemes are used

for its manufacture via synthetic routes. The following reviews the

important approaches for isoprene production.

Dehydrogenation of Tertiary Amylenes (Shell Process)

t-Amylenes (2-methyl-1-butene and 2-methyl-2-butene) are produced

in small amounts with olefins from steam cracking units. The amylenes

are extracted from a C

fraction with aqueous sulfuric acid.

Dehydrogenation of t-amylenes over a dehydrogenation catalyst pro-

duces isoprene. The overall conversion and recovery of t-amylenes is

approximately 70%.

The C

olefin mixture can also be produced by the reaction of ethyl-

ene and propene using an acid catalyst.

Crude Oil Processing and Production of Hydrocarbon Intermediates 105

The C

olefin mixture is then dehydrogenated to isoprene.

From Acetylene and Acetone

A three-step process developed by Snamprogetti is based on the reac-

tion of acetylene and acetone in liquid ammonia in the presence of an

alkali metal hydroxide. The product, methylbutynol, is then hydro-

genated to methylbutenol followed by dehydration at 250–300°C over an

acidic heterogeneous catalyst.

Chapter 3 1/22/01 10:58 AM Page 105

From Isobutylene and Formaldehyde (IFP Process)

The reaction between isobutylene (separated from C

fractions from

cracking units or from cracking isobutane to isobutene) and formalde-

hyde produces a cyclic ether (dimethyl dioxane). Pyrolysis of dioxane

gives isoprene and formaldehyde. The formaldehyde is recovered and

recycled to the reactor.

106 Chemistry of Petrochemical Processes

From Isobutylene and Methylal (Sun Oil Process)

In this process, methylal (dimethoxymethane) is used instead of

formaldehyde. The advantage of using methylal over formaldehyde is

its lower reactivity toward 1-butene than formaldehyde, thus allowing

mixed feedstocks to be used. Also, unlike formaldehyde, methylal does

not decompose to CO and H

The first step in this process is to produce methylal by the reaction of

methanol and formaldehyde using an acid catalyst.

Chapter 3 1/22/01 10:58 AM Page 106