Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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steam cracking units for olefin production, or as a feedstock for a Cyclar

unit for the production of aromatics.

Residual Fuel Oil

Residual fuel oil is generally known as the bottom product from

atmospheric distillation units. Fuel oils produced from cracking units are

unstable. When used as fuels, they produce smoke and deposits that may

block the burner orifices.

The constituents of residual fuels are more complex than those of gas

oils. A major part of the polynuclear aromatic compounds, asphaltenes,

and heavy metals found in crude oils is concentrated in the residue.

The main use of residual fuel oil is for power generation. It is burned

in direct-fired furnaces and as a process fuel in many petroleum and

chemical companies. Due to the low market value of fuel oil, it is used

as a feedstock to catalytic and thermal cracking units.

Residues containing high levels of heavy metals are not suitable for cat-

alytic cracking units. These feedstocks may be subjected to a demetallization

process to reduce their metal contents. For example, the metal content of

vacuum residues could be substantially reduced by using a selective organic

solvent such as pentane or hexane, which separates the residue into an oil

(with a low metal and asphaltene content) and asphalt (with high metal con-

tent). Demetallized oils could be processed by direct hydrocatalysis.

Another approach used to reduce the harmful effects of heavy metals

in petroleum residues is metal passivation. In this process an oil-soluble

treating agent containing antimony is used that deposits on the catalyst

surface in competition with contaminant metals, thus reducing the cat-

alytic activity of these metals in promoting coke and gas formation.

Metal passivation is especially important in fluid catalytic cracking

(FCC) processes. Additives that improve FCC processes were found to

increase catalyst life and improve the yield and quality of products.

Residual fuels with high heavy metal content can serve as feedstocks

for thermal cracking units such as delayed coking. Low-metal fuel oils

are suitable feedstocks to catalytic cracking units. Product gases from

cracking units may be used as a source for light olefins and LPG for

petrochemical production. Residual fuel oils are also feedstocks for

steam cracking units for the production of olefins.

REFERENCES

Hydrocarbon Intermediates 47

Chapter 2 1/22/01 10:57 AM Page 47

1. Chemical Industries Newsletter, October–December 1998, pp. 9–10.

2. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 70, No. 3,

1991, p. 142.

3. Yepsen, G. and Witoshkin, T., “Refiners Have Options to Deal with

Reformulated Gasoline,” Oil and Gas Journal, April 8, 1991, pp.

68–71.

4. DiCintio, R. et al., “Separate Ethylene Efficiently,” Hydrocarbon

Processing, Vol. 70, No. 7, 1991, pp. 83–86.

5. Chemical and Engineering News, June 29, 1998, pp. 43–47.

6. Hatch, L. F. and Matar, S. “Chemicals from C

,” Hydrocarbon

Processing, Vol. 57, No. 8, 1978, pp. 153–165.

7. Chemical Week, Nov. 16, 1977, p. 49.

8. Fessenden, R. J. and Fessenden, J. S., Organic Chemistry, 4th Ed.,

Brooks/Cole Publishing Co., Pacific Grove, California, 1991, p. 70.

9. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 61, No.

11, 1982, p. 195.

10. Gentry, J. C. and Kumar, C. S. “Improve BTX Processing Economics”

Hydrocarbon Processing, Vol. 77, No. 3, 1998, pp. 69–82.

11. Biesser, H. J. and Winter, G. R., Oil and Gas Journal, Aug. 11, 1975,

pp. 74–75.

12. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 70, No. 3,

1991, pp. 166.

13. Matar, S., Synfuels; Hydrocarbons of the Future, PennWell Publishing

Co., Tulsa, Okla, 1982, p. 10.

14. Barwell, J. and Martin, S. R., International Seminar on Petrochemical

Industries, No. 9 (P-2) Iraq, Oct. 25–30, 1975.

15. Oil and Gas Journal, March 20, 1978, p. 94.

16. Krishna, A. S. et al., “Additives Improve FCC Process,” Hydrocarbon

Processing, Vol. 70, No. 11, 1991, pp. 59–66.

48 Chemistry of Petrochemical Processes

Chapter 2 1/22/01 10:57 AM Page 48

CHAPTER THREE

Crude Oil Processing and

Production of Hydrocarbon

Intermediates

INTRODUCTION

The hydrocarbon intermediates referred to in the previous chapter are

produced by subjecting crude oils to various processing schemes. These

include a primary distillation step to separate the crude oil complex mix-

ture into simpler fractions. These fractions are primarily used as fuels.

However, a small percentage of these streams are used as secondary raw

materials or intermediates for obtaining olefins, diolefins, and aromatics

for petrochemicals production. Further processing of these fractions may

be required to change their chemical composition to the required prod-

ucts. These new products may also be used as fuels of improved qualities

or as chemical feedstocks. For example, reforming a naphtha fraction

catalytically produces a reformate rich in aromatics. The major use of

the reformate is to supplement the gasoline pool due to its high octane

rating. However, the reformate is also used to extract the aromatics for

petrochemicals use. At this point, the production of intermediates for

petrochemicals is not separable from the production of fuels. In this

chapter, the production of hydrocarbon intermediates is discussed in con-

junction with different crude oil processing schemes. These include

physical separation techniques and chemical conversion processes. The

production of olefins is also discussed in the last section.

PHYSICAL SEPARATION PROCESSES

Physical separation techniques separate a mixture such as a crude oil

without changing the chemical characteristics of the components. The

Chapter 3 1/22/01 10:58 AM Page 49

separation is based on differences of certain physical properties of the

constituents such as the boiling and melting points, adsorption affinities

on a certain solid, and diffusion through certain membranes.

The important physical separation processes, discussed here, are dis-

tillation, absorption, adsorption, and solvent extraction.

ATMOSPHERIC DISTILLATION

Atmospheric distillation separates the crude oil complex mixture into

different fractions with relatively narrow boiling ranges. In general, sep-

aration of a mixture into fractions is based primarily on the difference in

the boiling points of the components. In atmospheric distillation units,

one or more fractionating columns are used.

Distilling a crude oil starts by preheating the feed by exchange with

the hot product streams. The feed is further heated to about 320°C as it

passes through the heater pipe (pipe still heater).

The hot feed enters the fractionator, which normally contains 30–50

fractionation trays. Steam is introduced at the bottom of the fractionator

to strip off light components. The efficiency of separation is a function of

the number of theoretical plates of the fractionating tower and the reflux

ratio. Reflux is provided by condensing part of the tower overhead

vapors. Reflux ratio is the ratio of vapors condensing back to the still to

vapors condensing out of the still (distillate). The higher the reflux ratio,

the better the separation of the mixture.

Products are withdrawn from the distillation tower as side streams,

while the reflux is provided by returning a portion of the cooled vapors

from the tower overhead condenser. Additional reflux could be obtained

by returning part of the cold side stream products to the tower. In prac-

tice, the reflux ratio varies over a wide range according to the specific

separations desired. From the overhead condenser, the uncondensed

gases are separated, and the condensed light naphtha liquid is withdrawn

to storage. Heavy naphtha, kerosine, and gas oil are withdrawn as side

stream products. Table 3-1 shows the approximate boiling ranges for

crude oil fractions. The residue (topped crude) is removed from the bot-

tom of the distillation tower and may be used as a fuel oil. It may also be

charged to a vacuum distillation unit, a catalytic cracking or steam crack-

ing process. Figure 3-1 is a flow diagram for atmospheric and vacuum

distillation units.

50 Chemistry of Petrochemical Processes

Chapter 3 1/22/01 10:58 AM Page 50

VACUUM DISTILLATION

Vacuum distillation increases the amount of the middle distillates and

produces lubricating oil base stocks and asphalt. The feed to the unit is

the residue from atmospheric distillation. In vacuum distillation, reduced

pressures are applied to avoid cracking long-chain hydrocarbons present

in the feed.

The feed is first preheated by exchange with the products, charged to

the vacuum unit heater, and then passed to the vacuum tower in an atmos-

phere of superheated steam. Using superheated steam is important: it

Crude Oil Processing and Production of Hydrocarbon Intermediates 51

Table 3-1

Approximate ASTM boiling point ranges for crude oil fractions

Boiling range

Fractions °F °C

Light naphtha 85–210 30–99

Heavy naphtha 190–400 88–204

Kerosine 340–520 171–271

Atmospheric gas oil 540–820 288–438

Vacuum gas oil 750–1,050 399–566

Vacuum residue 1,000+ 538+

Figure 3-1. Flow diagram of atmospheric and vacuum distillation units:

(1,3)

heat exchangers; (2) desalter, (3,4) heater; (5) distillation column, (6) overhead

condenser, (7–10) pump around streams, (11) vacuum distillation heater; (12)

vacuum tower.

Chapter 3 1/22/01 10:58 AM Page 51

decreases the partial pressure of the hydrocarbons and reduces coke

formation in the furnace tubes. Distillation normally occurs at a tem-

perature range of 400–440°C and an absolute pressure of 25–40

mmHg. The top tower temperature is adjusted by refluxing part of the

gas oil product (top product). The size (diameter) of the vacuum dis-

tillation tower is much larger than atmospheric towers because the

volume of the vapor/unit-volume of the feed is much larger than in

atmospheric distillation.

Products obtained as side streams are vacuum gas oil (VGO), lube oil

base stocks, and asphalt. Asphalt may be used for paving roads or may be

charged to a delayed coking unit.

ABSORPTION PROCESS

This process selectively removes a certain gas from a gas mixture

using a liquid absorbent. In the refining industry, this process is used

extensively to free the product gas streams from acid gases (mainly H

either by using a physical or a chemical absorbent. Absorption of acid

gases from natural gas are discussed in Chapter 1.

ADSORPTION PROCESS

Adsorption processes use a solid material (adsorbent) possessing a

large surface area and the ability to selectively adsorb a gas or a liquid on

its surface. Examples of adsorbents are silica (SiO

), anhydrous alumina

(Al

), and molecular sieves (crystalline silica/alumina). Adsorption

processes may be used to remove acid gases from natural gas and gas

streams. For example, molecular sieves are used to dehydrate natural gas

and to reduce its acid gases.

Adsorption processes are also used to separate liquid mixtures. For

example, molecular sieve 5A selectively adsorbs n-paraffins from a low-

octane naphtha fraction. Branched paraffins and aromatics in the mixture

are not adsorbed on the solid surface. The collected fraction containing

mainly aromatics and branched paraffins have a higher octane number

than the feed. Desorbing n-paraffins is effected by displacement with

another solvent or by using heat. The recovered n-paraffins in this range

are good steam cracking feedstocks for olefin production.

Adsorption of n-paraffins (C

-C

) from a kerosine or a gas oil frac-

tion can be achieved in a liquid or a vapor phase adsorption process.

52 Chemistry of Petrochemical Processes

Chapter 3 1/22/01 10:58 AM Page 52

Normal paraffins in this range are important intermediates for alkylating

benzene for synthetic detergents production (Chapter 10). They are also

good feedstocks for single-cell protein (SCP).

The IsoSiv process is an isobaric, isothermal adsorption technique

used to separate n-paraffins from gas oils. The operation conditions

are approximately 370°C and 100 psi.

Desorption is achieved using n-

pentane or n-hexane. The solvent is easily distilled from the heavier

n-paraffins and then recycled.

SOLVENT EXTRACTION

Liquid solvents are used to extract either desirable or undesirable com-

pounds from a liquid mixture. Solvent extraction processes use a liquid

solvent that has a high solvolytic power for certain compounds in the

feed mixture. For example, ethylene glycol has a greater affinity for aro-

matic hydrocarbons and extracts them preferentially from a reformate

mixture (a liquid paraffinic and aromatic product from catalytic reform-

ing). The raffinate, which is mainly paraffins, is freed from traces of eth-

ylene glycol by distillation. Other solvents that could be used for this

purpose are liquid sulfur dioxide and sulfolane (tetramethylene sulfone).

The sulfolane process is a versatile extractant for producing high

purity BTX aromatics (benzene, toluene, and xylenes). It also extracts

aromatics from kerosines to produce low-aromatic jet fuels.

On the other hand, liquid propane also has a high affinity for paraffinic

hydrocarbons. Propane deasphalting removes asphaltic materials from

heavy lube oil base stocks. These materials reduce the viscosity index of

lube oils. In this process, liquid propane dissolves mainly paraffinic

hydrocarbons and leaves out asphaltic materials. Higher extraction tem-

peratures favor better separation of the asphaltic components. Deas-

phalted oil is stripped to recover propane, which is recycled.

Solvent extraction may also be used to reduce asphaltenes and metals

from heavy fractions and residues before using them in catalytic crack-

ing. The organic solvent separates the resids into demetallized oil with

lower metal and asphaltene content than the feed, and asphalt with high

metal content. Figure 3-2 shows the IFP deasphalting process and Table

3-2 shows the analysis of feed before and after solvent treatment.

Solvent extraction is used extensively in the petroleum refining indus-

try. Each process uses its selective solvent, but, the basic principle is the

same as above.

Crude Oil Processing and Production of Hydrocarbon Intermediates 53

Chapter 3 1/22/01 10:58 AM Page 53

CONVERSION PROCESSES

Conversion processes in the petroleum industry are generally used to:

1. Upgrade lower-value materials such as heavy residues to more

valuable products such as naphtha and LPG. Naphtha is mainly

used to supplement the gasoline pool, while LPG is used as a fuel

or as a petrochemical feedstock.

54 Chemistry of Petrochemical Processes

Figure 3-2. The IFP deasphalting process:

(1,2) extractor, (3-6) solvent recovery

towers.

Table 3-2

Typical analysis of light Arabian vacuum resid before and after

solvent treatment using once C

and another C

hydrocarbon solvent

Feed DAO

Solvent — C

Yield, wt % — 70.1 85.5

Sp. gr. 1.003 0.959 0.974

Visc., cSt @ 210°F 345 63 105

Conradson carbon, wt % 16.4 5.3 7.9

Asphaltenes (C

insol.), wt % 4.20 <0.05 <0.05

Ni, ppm 19 2.0 7.0

V, ppm 61 2.6 15.5

S, wt% 4.05 3.3 3.65

, ppm 2,875 1,950 2,170

Chapter 3 1/22/01 10:58 AM Page 54

2. Improve the characteristics of a fuel. For example, a lower octane

naphtha fraction is reformed to a higher octane reformate product.

The reformate is mainly blended with naphtha for gasoline formu-

lation or extracted for obtaining aromatics needed for petrochemi-

cals production.

3. Reduce harmful impurities in petroleum fractions and residues to

control pollution and to avoid poisoning certain processing cata-

lysts. For example, hydrotreatment of naphtha feeds to catalytic

reformers is essential because sulfur and nitrogen impurities poison

the catalyst.

Conversion processes are either thermal, where only heat is used to

effect the required change, or catalytic, where a catalyst lowers the reac-

tion activation energy. The catalyst also directs the reaction toward a

desired product or products (selective catalyst).

THERMAL CONVERSION PROCESSES

Thermal cracking was the first process used to increase gasoline pro-

duction. After the development of catalytic cracking, which improved

yields and product quality, thermal cracking was given other roles in

refinery operations. The three important thermal cracking techniques are

coking, viscosity breaking, and steam cracking.

Steam cracking is of special importance as a major process designed

specifically for producing light olefins. It is discussed separately later in

this chapter.

Coking Processes

Coking is a severe thermal cracking process designed to handle heavy

residues with high asphaltene and metal contents. These residues cannot

be fed to catalytic cracking units because their impurities deactivate and

poison the catalysts.

Products from coking processes vary considerably with feed type and

process conditions. These products are hydrocarbon gases, cracked naph-

tha, middle distillates, and coke. The gas and liquid products are charac-

terized by a high percentage of unsaturation. Hydrotreatment is usually

required to saturate olefinic compounds and to desulfurize products from

coking units.

Crude Oil Processing and Production of Hydrocarbon Intermediates 55

Chapter 3 1/22/01 10:58 AM Page 55

Thermal Cracking Reactions

The first step in cracking is the thermal decomposition of hydrocarbon

molecules to two free radical fragments. This initiation step can occur by

a homolytic carbon-carbon bond scission at any position along the hydro-

carbon chain. The following represents the initiation reaction:

RCH

+ RvC

The radicals may further crack, yielding an olefin and a new free rad-

ical. Cracking usually occurs at a bond beta to the carbon carrying the

unpaired electron.

RCH

+ CH

=CH

Further β bond scission of the new free radical R

can continue to pro-

duce ethylene until the radical is terminated.

Free radicals may also react with a hydrocarbon molecule from the

feed by abstracting a hydrogen atom. In this case the attacking radical is

terminated, and a new free radical is formed. Abstraction of a hydrogen

atom can occur at any position along the chain. However, the rate of

hydrogen abstraction is faster from a tertiary position than from a sec-

ondary, which is faster than from a primary position.

+ RCH

RCH

HCH

Rv + RH

The secondary free radical can crack on either side of the carbon car-

rying the unpaired electron according to the beta scission rule, and a ter-

minal olefin is produced.

56 Chemistry of Petrochemical Processes

Free radicals, unlike carbocations, do not normally undergo isomeriza-

tion by methyl or hydrogen migration. However, hydrogen transfer (chain

transfer) occurs when a free radical reacts with other hydrocarbons.

There are two major commercial thermal cracking processes, delayed

coking and fluid coking. Flexicoking is a fluid coking process in which

the coke is gasified with air and steam. The resulting gas mixture par-

tially provides process heat.

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