Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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Delayed Coking

In delayed coking, the reactor system consists of a short contact-time

heater coupled to a large drum in which the preheated feed “soaks” on a

batch basis. Coke gradually forms in the drum. A delayed coking unit has

at least a pair of drums. When the coke reaches a predetermined level in

one drum, flow is diverted to the other so that the process is continuous.

Vapors from the top of the drum are directed to the fractionator where

they are separated into gases, naphtha, kerosine, and gas oil. Table 3-3

shows products from a delayed coker using different feeds.

Decoking the filled drum can be accomplished by a hydraulic system

using several water jets under at least 3,000 pounds per square inch gauge.

Operating conditions for delayed coking are 25–30 psi at 480–500°C,

with a recycle ratio of about 0.25 based on equivalent feed. Improved

liquid yields could be obtained by operating at lower pressures. Coking

at approximately 15 psi with ultra low recycle produced about 10% more

gas oil.

Operating at too-low temperature produces soft spongy coke.

On the other hand, operating at a higher temperature produces more coke

and gas but less liquid products. Mochida et al. reviewed the chemistry

and different options for the production of delayed coke.

It is the chem-

istry of the pyrolysis system which controls the properties of the semi

Crude Oil Processing and Production of Hydrocarbon Intermediates 57

Table 3-3

Feeds and products from a delayed coker unit

(using different feeds)

Operating conditions:

Heater outlet temperature, °F 900–950

Coke drum pressure, psig 15–90

Recycle ratio, vol/vol feed, % 10–100

Yields:

Vacuum residue

Middle East of hydrotreated Coal tar

Feedstock vac. residue bottoms pitch

Gravity, ºAPI 7.4 1.3 –11.0

Sulfur, wt % 4.2 2.3 0.5

Conradson carbon, wt % 20.0 27.6 —

Products, wt %

Gas + LPG 7.9 9.0 3.9

Naphtha 12.6 11.1 —

Gas oil 50.8 44.0 31.0

Coke 28.7 35.9 65.1

Chapter 3 1/22/01 10:58 AM Page 57

and final coke structure. Factors that govern the reactions are the coke

drum size, the heating rate, the soak time, the pressure, and the final reac-

tion temperature.

However, if everything is equal (temperature, pres-

sure, soak time, etc.), the quality of coke produced by delayed coking is

primarily a function of the feed quality. Figure 3-3 shows a delayed cok-

ing unit.

Coke produced from delayed coking is described as delayed sponge,

shot, or needle coke depending on its physical structure. Shot coke is the

most common when running the unit under severe conditions with sour

crude residues. Needle coke is produced from selected aromatic feed-

stocks. Sponge coke is more porous and has a high surface area. The

properties and markets for petroleum cokes have been reviewed by

Dymond.

Table 3-4 shows the types of petroleum cokes and their uses.

Fluid Coking

In the fluid coking process, part of the coke produced is used to pro-

vide the process heat. Cracking reactions occur inside the heater and the

fluidized-bed reactor. The fluid coke is partially formed in the heater. Hot

coke slurry from the heater is recycled to the fluid reactor to provide the

heat required for the cracking reactions. Fluid coke is formed by spray-

ing the hot feed on the already-formed coke particles. Reactor tempera-

ture is about 520°C, and the conversion into coke is immediate, with

58 Chemistry of Petrochemical Processes

Figure 3-3. Flow diagram of a delayed coking unit:

(1) coker fractionator, (2)

coker heater, (3) coke drum, (4) vapor recovery column.

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

complete disorientation of the crystallites of product coke. The burning

process in fluid coking tends to concentrate the metals, but it does not

reduce the sulfur content of the coke.

Fluid coking has several characteristics that make it undesirable for

most petroleum coke markets. These characteristics are high sulfur con-

tent, low volatility, poor crystalline structure, and low grindability index.

Flexicoking, on the other hand, integrates fluid coking with coke gasi-

fication. Most of the coke is gasified. Flexicoking gasification produces

a substantial concentration of the metals in the coke product. Figure 3-4

shows an Exxon flexicoking process.

Viscosity Breaking (Vis-breaking)

Viscosity breaking aims to thermally crack long-chain feed molecules to

shorter ones, thus reducing the viscosity and the pour point of the product.

In this process, the feed is usually a high viscosity, high pour point fuel

oil that cannot be used or transported, especially in cold climates, due to

the presence of waxy materials. Wax is a complex mixture of long-chain

paraffins mixed with aromatic compounds having long paraffinic side

chains. Vis-breaking is a mild cracking process that operates at approxi-

mately 450°C using short residence times. Long paraffinic chains break to

Crude Oil Processing and Production of Hydrocarbon Intermediates 59

Table 3-4

Types of petroleum cokes and their end uses

Application Type coke State End use

Carbon source Needle Calcined Electrodes

Synthetic graphite

Sponge Calcined Aluminum anodes

TiO

pigments

Carbon raiser

Sponge Green Silicon carbide

Foundries

Coke ovens

Fuel use Sponge Green lump Europe/Japan space

heating

Sponge Green Industrial boilers

Shot Green Utilities

Fluid Green Cogeneration

Flexicoke Green Lime

Cement

Chapter 3 1/22/01 10:58 AM Page 59

shorter ones, and dealkylation of the aromatic side chains occurs. Table 3-5

shows the analysis of feed and products from a vis-breaking unit.

CATALYTIC CONVERSION PROCESSES

Catalytic conversion processes include naphtha catalytic reforming, cat-

alytic cracking, hydrocracking, hydrodealkylation, isomerization, alkyla-

tion, and polymerization. In these processes, one or more catalyst is used.

A common factor among these processes is that most of the reactions are

initiated by an acid-type catalyst that promotes carbonium ion formation.

Other important catalytic processes are those directed toward improv-

ing the product quality through hydrotreatment. These processes use

heterogeneous hydrogenation catalysts.

Catalytic Reforming

The aim of this process is to improve the octane number of a naphtha

feedstock by changing its chemical composition. Hydrocarbon com-

pounds differ greatly in their octane ratings due to differences in struc-

ture. In general, aromatics have higher octane ratings than paraffins and

cycloparaffins. Similar to aromatics, branched paraffins have high octane

ratings. The octane number of a hydrocarbon mixture is a function of the

octane numbers of the different components and their ratio in the mix-

ture. (See octane ratings of different hydrocarbons in Chapter 2.)

60 Chemistry of Petrochemical Processes

Figure 3-4. Flow diagram of an Exxon flexicoking unit:

(1) reactor, (2) scrubber,

(3) heater, (4) gasifier, (5) coke fines removal, (6) H

S removal.

Chapter 3 1/22/01 10:58 AM Page 60

Increasing the octane number of a low-octane naphtha fraction is

achieved by changing the molecular structure of the low octane number

components. Many reactions are responsible for this change, such as the

dehydrogenation of naphthenes and the dehydrocyclization of paraffins

to aromatics. Catalytic reforming is considered the key process for

obtaining benzene, toluene, and xylenes (BTX). These aromatics are

important intermediates for the production of many chemicals.

Reformer Feeds

The feed to a catalytic reformer is normally a heavy naphtha fraction

produced from atmospheric distillation units. Naphtha from other

sources such as those produced from cracking and delayed coking may

also be used. Before using naphtha as feed for a catalytic reforming unit,

it must be hydrotreated to saturate the olefins and to hydrodesulfurize

Crude Oil Processing and Production of Hydrocarbon Intermediates 61

Table 3-5

Analysis of feed and products from viscosity breaking

Charge inspections Libyan residue

Gravity, °API 24.4

Vacuum Engler, corrected °F

IBP 510

5% 583

10% 608

20% 650

Pour point (max.), °F 75

Visc. SUS @ 122°F 175.8

Product yield, vol %

Gasoline, 100% C

, 330 EP 10.8

Furnace oil, 805°F EP 42.7

Fuel oil 46.3

Gas, C

& Lighter (wt %) 2.1

Properties of products

Furnace oil

Pour point (max.), °F +5

Flash (PMCO), °F 150

Fuel oil

Pour point (max.), °F +40

Flash (PMCC), °F 150

Visc., SFS @ 122°F 67.5

Stability (ASTM D-1661) No. 1

Chapter 3 1/22/01 10:58 AM Page 61

and hydrodenitrogenate sulfur and nitrogen compounds. Olefinic com-

pounds are undesirable because they are precursors for coke, which deac-

tivates the catalyst. Sulfur and nitrogen compounds poison the reforming

catalyst. The reducing atmosphere in catalytic reforming promotes form-

ing of hydrogen sulfide and ammonia. Ammonia reduces the acid sites of

the catalyst, while platinum becomes sulfided with H

Types of hydrocarbons in the feed have significant effects on the oper-

ation severity. Feeds with a high naphthene content are easier to aroma-

tize than feeds with a high ratio of paraffins (see “Reforming reactions”).

The boiling range of the feeds is also an effective parameter. Feeds with

higher end points (≈200°C) are favorable because some of the long-chain

molecules are hydrocracked to molecules in the gasoline range. These

molecules can isomerize and dehydrocyclize to branched paraffins and to

aromatics, respectively.

Reforming Catalysts

The catalysts generally used in catalytic reforming are dual functional

to provide two types of catalytic sites, hydrogenation-dehydrogenation

sites and acid sites. The former sites are provided by platinum, which is

the best known hydrogenation-dehydrogenation catalyst and the latter

(acid sites) promote carbonium ion formation and are provided by an alu-

mina carrier. The two types of sites are necessary for aromatization and

isomerization reactions.

Bimetallic catalysts such as Pt/Re were found to have better stability,

increased catalyst activity, and selectivity. Trimetallic catalysts of noble

metal alloys are also used for the same purpose. The increased stability

of these catalysts allowed operation at lower pressures. A review of

reforming catalysts by Al-Kabbani manifests the effect of the ratio of the

metallic components of the catalyst. A ratio of 0.5 or less for Pt/Re in the

new generation catalysts versus 1.0 for the older ones can tolerate much

higher coke levels. Reforming units can perform similarly with higher

coke levels (20–25% versus 15–20%). These catalysts can tolerate higher

sulfer naphtha feeds (>1 ppm). Higher profitability may be realized by

increasing the cycle length.

Reforming Reactions

Many reactions occur in the reactor under reforming conditions.

These are aromatization reactions, which produce aromatics; isomeriza-

tion reactions, which produce branched paraffins; and other reactions,

62 Chemistry of Petrochemical Processes

Chapter 3 1/22/01 10:58 AM Page 62

which are not directly involved in aromatics formation (hydrocracking

and hydrodealkylation).

Aromatization. The two reactions directly responsible for enriching

naphtha with aromatics are the dehydrogenation of naphthenes and the

dehydrocyclization of paraffins. The first reaction can be represented by

the dehydrogenation of cyclohexane to benzene.

Crude Oil Processing and Production of Hydrocarbon Intermediates 63

This reaction is fast; it reaches equilibrium quickly. The reaction is also

reversible, highly endothermic, and the equilibrium constant is quite

large (6 × l0

@ 500°C).

It is evident that the yield of aromatics (benzene) is favored at higher

temperatures and lower pressures. The effect of decreasing H

partial

pressure is even more pronounced in shifting the equilibrium to the right.

The second aromatization reaction is the dehydrocyclization of paraf-

fins to aromatics. For example, if n-hexane represents this reaction, the

first step would be to dehydrogenate the hexane molecule over the plat-

inum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers,

but cyclization to a cyclohexane ring may occur through a different

mechanism). Cyclohexane then dehydrogenates to benzene.

∆H = +266 KJ/mol

= 7.8 × 104 @ 500°C

This is also an endothermic reaction, and the equilibrium production of

aromatics is favored at higher temperatures and lower pressures.

However, the relative rate of this reaction is much lower than the dehy-

drogenation of cyclohexanes. Table 3-6 shows the effect of temperature

on the selectivity to benzene when reforming n-hexane using a plat-

inum catalyst.

Chapter 3 1/22/01 10:58 AM Page 63

More often than what has been mentioned above regarding the

cyclization of paraffins over the platinum catalyst, the formed olefin

species reacts with the acid catalyst forming a carbocation. Carbocation

formation may occur by abstraction of a hydride ion from any position

along the hydrocarbon chain. However, if the carbocation intermediate

has the right configuration, cyclization occurs. For example, cyclization

of 1-heptene over the alumina catalyst can occur by the following suc-

cessive steps:

64 Chemistry of Petrochemical Processes

Table 3-6

Selectivity to benzene from reforming n-hexane over a

platinum catalyst

Selectivity Selectivity

%toto

LHSV Temp.,°F Conversion Benzene Isohexane

2 885 80.2 16.6 58.0

2 932 86.8 24.1 36.9

2 977 90.4 27.4 23.4

The formed methylcyclohexane carbocation eliminates a proton,

yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehy-

drogenate over the platinum surface or form a new carbocation by losing

–

over the acid catalyst surface. This step is fast, because an allylic car-

bonium ion is formed. Losing a proton on a Lewis base site produces

methyl cyclohexadiene. This sequence of carbocation formation, fol-

lowed by loss of a proton, continues till the final formation of toluene.

Chapter 3 1/22/01 10:58 AM Page 64

It should be noted that both reactions leading to aromatics (dehydro-

genation of naphthenes and dehydrocyclization of paraffins) produce

hydrogen and are favored at lower hydrogen partial pressure.

Isomerization. Reactions leading to skeletal rearrangement of paraf-

fins and cycloparaffins in a catalytic reactor are also important in raising

the octane number of the reformate product. Isomerization reactions may

occur on the platinum catalyst surface or on the acid catalyst sites. In the

former case, the reaction is slow. Most isomerization reactions, however,

occur through formation of a carbocation. The formed carbocation could

rearrange through a hydride-methide shift that would lead to branched

isomers. The following example illustrates the steps for the isomerization

of n-heptane to 2-methylhexane through 1,2-methide-hydride shifts:

Carbocation Formation:

Crude Oil Processing and Production of Hydrocarbon Intermediates 65

Chapter 3 1/22/01 10:58 AM Page 65

Isomerization of alkylcyclopentanes may also occur on the platinum

catalyst surface or on the silica/alumina. For example, methylcyclopen-

tane isomerizes to cyclohexane:

66 Chemistry of Petrochemical Processes

The formed cyclohexane can dehydrogenate to benzene.

Hydrocracking. Hydrocracking is a hydrogen-consuming reaction

that leads to higher gas production and lower liquid yield. This reaction

is favored at high temperatures and high hydrogen partial pressure. The

following represents a hydrocracking reaction:

RCH

Rv + H

RCH

+ RvCH

Bond breaking can occur at any position along the hydrocarbon chain.

Because the aromatization reactions mentioned earlier produce hydrogen

and are favored at high temperatures, some hydrocracking occurs also

under these conditions. However, hydrocracking long-chain molecules

can produce C

, C

, and C

hydrocarbons that are suitable for hydrode-

cyclization to aromatics.

For more aromatics yield, the end point of the feed may be raised to

include higher molecular weight hydrocarbons in favor of hydrocracking

and dehydrocyclization. However, excessive hydrocracking is not desir-

able because it lowers liquid yields.

Hydrodealkylation. Hydrodealkylation is a cracking reaction of an

aromatic side chain in presence of hydrogen. Like hydrocracking, the

Chapter 3 1/22/01 10:58 AM Page 66