Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing

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CATALYTIC REFORMING 221

Heater

Reactor

Separator

Stripper

Fresh

Feed

Recycle Gas

Compressor

Light Ends

Stea

Desulfurized

Product

Hydrogen

Makeup

Compressor

Sour

Water

Wash

Water

Steam

Figure 5.3. Naphtha hydrotreater ﬂow scheme.

the second stage for the hydrogenation of oleﬁns and the removal of sulfur and nitrogen

compounds.

The reformate stream from a catalytic reforming unit is invariably used either as a high-

octane gasoline blending component or as a source of aromatics—BTX (benzene,

toluene, and xylenes), and C

+ aromatics. Reforming for motor fuel applications

still represents the majority of existing reforming capacity. Reformate speciﬁcations

(octane, vapor pressure, end point, etc.) are set to provide an optimum blending prod-

uct. The octane requirement is met through the production of high-octane aromatics,

the isomerization of parafﬁns, and the removal of low octane components by cracking

them to gaseous products. Feedstocks to these units are typically “full range” naph-

thas, consisting of hydrocarbons with 6–12 carbon atoms; however, the initial boiling

point may be varied to limit the presence of benzene precursors.

Reforming units for the production of aromatics are often called BTX reformers.

Naphthas for these units are speciﬁed to contain mostly naphthenes and parafﬁns of

6–8 carbons. The desired reaction is aromatization through dehydrogenation of the

naphthenes, and cyclization and dehydrogenation of the parafﬁns to the analogous

aromatic.

Mercaptans Di–sulfides Thiophene

R ⎯ SH R ⎯ S ⎯ R

C ⎯ C

⏐⏐⏐⏐

⏐

Figure 5.4. Sulfur types.

222 CHAPTER 5

Table 5.2. Reformate composition

mass% liq-vol%

Aromatics

Benzene 3.72 3.39

Toluene 13.97 12.93

Ethylbenzene 3.13 2.90

p-Xylene 3.39 3.14

m-Xylene 7.47 6.91

o-Xylene 4.83 4.47

C9+ Aromatics 36.05 33.30

Total aromatics 72.56 67.04

Total oleﬁns 0.82 1.02

Parafﬁns and naphthenes

Propane 0.00 0.00

Isobutane 0.14 0.20

n-Butane 0.94 1.32

Isopentane 2.52 3.29

n-Pentane 1.74 2.29

Cyclopentane 0.10 0.10

C6 Isoparafﬁns 3.91 4.77

n-Hexane 1.74 2.12

Methylcyclopentane 0.28 0.30

Cyclohexane 0.03 0.03

C7 Isoparafﬁns 7.70 9.02

n-Heptane 2.22 2.60

C7 Cyclopentanes 0.33 0.35

Methylcyclohexane 0.04 0.04

C8 Isoparafﬁns 2.86 3.24

n-Octane 0.62 0.70

C8 Cyclopentanes 0.14 0.14

C8 Cyclohexanes 0.06 0.06

C9 Naphthenes 0.04 0.04

C9 Parafﬁns 0.90 0.99

C10 Naphthenes 0.04 0.04

C10 Parafﬁns 0.24 0.26

C11 Naphthenes 0.00 0.00

C11 Parafﬁns 0.03 0.04

C12 P + N 0.00 0.00

Poly Naphthenes 0.00 0.00

> 200 P + N 0.00 0.00

Total Parafﬁns 25.56 30.84

Total Naphthenes 1.06 1.10

CATALYTIC REFORMING 223

Reformate properties

Table 5.2 shows a typical reformate composition. For motor fuel applications, the

octane number is the dominant parameter of product quality. A higher octane number

reﬂects a lower tendency of the hydrocarbon to undergo a rapid, inefﬁcient detonation

in an internal combustion engine. This rapid detonation is heard as a knocking sound in

the engine, so octane is often referred to as the antiknock quality of a gasoline. Motor

fuel octanes are measured at low engine speeds (research octane number or RON) or

at high engine speeds (motor octane number or MON). In the United States, the octane

values posted on gasoline pumps are the arithmetic average of the MON and the RON.

The acronym RONC, research octane number clear, is used to denote that there are no

additives, such as lead, used to increase octane number. Table 5.3 provides a listing

of the various octanes of pure hydrocarbons according to the American Petroleum

Institute, API (2).

Octane numbers of a hydrocarbon or hydrocarbon mixture are determined by com-

paring its antiknock qualities with various blends of n-heptane (zero octane) and

2,2,4-trimethylpentane, or iso-octane (100 octane). Hydrocarbons may appear to

have different octane numbers when blended with other hydrocarbons of a differ-

ent composition—these are denoted as “blending octanes” and may be signiﬁcantly

different from the actual octane numbers of the individual hydrocarbon components

(Table 5.4) (3).

Other property speciﬁcations of the reformate include volatility or vapor pressure,

often given in terms of the Reid vapor pressure or RVP, end point, color, etc. (3) High-

end point reformates, for example, may not combust well in an internal combustion

engine.

Table 5.3. Examples of research and motor

octanes of pure hydrocarbons

RON MON

Parafﬁns

n-heptane 0 0

2-methylhexane 42.4 46.3

3-ethylpentane 65.0 69.3

2,4-dimethylpentane 83.1 83.8

Aromatics

Toluene 120.1 103.2

Ethylbenzene 107.4 97.9

Isopropylbenzene 113.0 99.3

1-methyl-3-ethylbenzene 112.1 100.0

1,3,5-trimethylbenzene >120 >120

224 CHAPTER 5

Table 5.4. Octane and blending octane numbers by

research method

RON Blending Octane

2,2-dimethyl butane 92.8 89

2-methyl-1-butene 102 146

Cyclopentane 101 141

1,4-dimethylbenzene 117 146

Reformulated gasolines, a requirement of the 1990 Clean Air Act, are the subject

of much legislation. Speciﬁcations require a lower benzene content, lower volatility,

and lower end point. Other speciﬁcations may pertain to the oxygenate content and

other factors that affect the burning characteristics. The gasolines available to the

consumer consist of a mixture of gasoline fractions from many reﬁnery sources,

including: straight run (unprocessed fraction), isomerate, alkylate, reformate, and

FCC fractions, and, on occasion, polymer gasolines.

Reforming reactions

In BTX production, the objective is to transform parafﬁns and naphthenes into ben-

zene, toluene, and xylenes with minimal cracking to light gases. The yield of desired

product is the percentage of feed converted to these aromatics. In motor fuel ap-

plications, octane values of the feed may be raised via aromatization or through

isomerization of the parafﬁns into higher octane branched species without sacriﬁcing

yield. Yield is typically deﬁned as liquid product with ﬁve or more carbons.

Typical catalysts that consist of platinum supported on alumina (with or without other

metals or modiﬁers) are bifunctional in that separate and distinct reactions occur on the

platinum site and on the alumina. The platinum typically performs dehydrogenation

and hydrogenolysis, while the acidic alumina isomerizes, cyclizes, and cracks.

The dehydrogenation of naphthenes to aromatics is probably the most important

reaction. Feeds contain cyclopentanes and substituted cyclopentanes, as well as cy-

clohexanes and their homologues. Six carbon ring cyclohexanes, for example, can be

directly dehydrogenated to produce aromatics and hydrogen.

Dehydrogenation is typically catalyzed by the platinum function on the reforming

catalyst.

CATALYTIC REFORMING 225

Five member ring cyclopentanes must be hydroisomerized to give a cyclohexane

intermediate prior to dehydrogenation to aromatics.

+ 3H

Acid-catalyzed reactions together with the Pt-catalyzed dehydrogenation function are

largely responsible for hydro-isomerization reactions that lead to the formation of

aromatics.

Parafﬁn conversion is the most difﬁcult step in reforming. For that reason, the ability

to convert parafﬁns selectively is of paramount importance in reforming. Parafﬁns

may be isomerized over the acidic function of the catalyst to provide higher octane

branched parafﬁns.

Another acid catalyzed parafﬁn reaction is cracking to lighter products, thus remov-

ing them from the liquid product. Octane is improved through the removal of low

octane parafﬁnic species from the liquid product by their conversion to gaseous,

lower molecular weight parafﬁns.

CCH

Parafﬁns also undergo cyclization to cyclohexanes. This reaction is believed to pro-

ceed through an oleﬁn intermediate, produced by Pt-catalyzed dehydrogenation (4).

The cyclization of the oleﬁn may be catalyzed by the alumina support.

+ H

226 CHAPTER 5

After cyclization, cyclohexane undergoes dehydrogenation to aromatics. Cyclopen-

tanes undergo hydroisomerization to cyclohexane, followed by dehydrogenation to

aromatics. Aromatics are stable species and relatively inert. Reactions of substi-

tuted aromatics involve isomerization, hydrodealkylation, disproportionation, and

transalkylation.

Small amounts of oleﬁns are formed that also undergo a number of isomerization,

alkylation, and cracking reactions. In particular, they appear to play an important role

as an intermediate in cyclization reactions.

The dehydrogenation of naphthenes and parafﬁns is rapid and equilibrium concentra-

tions are established in the initial portions of a catalyst bed. Isomerization reactions

are sufﬁciently fast that actual concentrations are near equilibrium. The observed re-

action rate for dehydrocyclization is reduced by the low concentrations of the oleﬁn

intermediates that exist at equilibrium. Hydrogen partial pressure signiﬁcantly affects

oleﬁn equilibrium concentrations and has a signiﬁcant impact on aromatization and

dehydrocyclization of parafﬁns. Lowering hydrogen partial pressures results in an

increase in the rate of aromatization, a decrease in the rate of hydrocracking, and an

increase in the rate of coke formation.

Table 5.5 provides thermodynamic data for typical compounds in reforming reactions

at a reference temperature of 800

◦

K. Thermodynamic data can be obtained from

Table 5.5. Thermodynamic data for reforming compounds at 800

◦

ideal gas in kcal/mol

Reforming reactions are typically dehydrogenations of the form

A ↔ B + nH

with equilibrium expressed in the form

)

such that they are a strong function of the partial pressure of hydrogen.

H

G

Typical C

’s

n-hexane −48.26 73.08

2-methylpentane −49.68 72.74

3-methylpentane −49.32 73.67

Cyclohexane −37.19 75.94

Methyl cyclopentane −33.73 71.92

Benzene 15.51 52.84

Typical C

’s

n-heptane −54.20 87.43

2-methylhexane −55.91 87.23

3-methylhexane −55.28 87.07

Methyl cyclohexane −45.10 86.15

Toluene 6.65 61.98

CATALYTIC REFORMING 227

standard sources (e.g., 2 and 5). Production of aromatics is favored by the reforming

conditions. Current designs at low hydrogen partial pressures ensure full conversion

to the equilibrium limits.

Catalysts

The platinum must be dispersed over the alumina surface such that the maximum

number of active sites for dehydrogenation is available. Platinum cluster size dimen-

sions are on the order of angstroms, or 10

−10

meters. The interaction of the platinum

with the alumina surface is such that the platinum clusters are relatively immobile and

do not agglomerate during reforming. Sulﬁdation of the platinum is sometimes used

to partially poison the platinum, or reduce its activity; this has the beneﬁcial effect

of reducing a major portion of the hydrogenolysis, or metal-catalyzed cracking reac-

tions. Liquid product yields are improved and the light gas production, particularly

methane, is reduced.

The alumina support is usually in the eta (η) or gamma (γ ) phase, but most often

gamma is used in reforming. Chloride is added to promote acidity. A simpliﬁed

schematic diagram of the alumina functionality is given in Figure 5.5.

Catalysts that are used in reactors where the catalyst bed is not easily removed after

deactivation must have long catalyst life cycles. A typical ﬁxed bed catalyst life cycle

may be a year or longer. Modiﬁers are added to reduce the effect of coke buildup and to

lengthen the catalyst cycle length, either by hydrogenating the coke to a less graphitic

species (6) or by cracking the coke precursors (7). Elements that are commonly added

to the catalysts are rhenium and, to a lesser extent, iridium.

In moving bed units (8) the catalyst ﬂows through the reactors and is regenerated

continually in a sepaarte regeneration vessel that is part of the reactor–regenerator

loop. Process conditions are much more severe, thus shortening catalyst life and

requiring regeneration cycles of only a few days. In moving bed catalysts elements

are added, such as tin and germanium, to increase liquid, aromatic, and hydrogen

yields by reducing the activity of the platinum for hydrogenolysis or metal-catalyzed

cracking reactions. These components also provide some stabilization of the catalyst

relative to Pt alone.

Al Al

Figure 5.5. Alumina schematic.

228 CHAPTER 5

Deactivation mechanisms for reforming catalysts include coking, poisoning, and ag-

glomeration of the platinum. Under normal conditions coke accumulates on the cata-

lyst. In a ﬁxed bed or SR unit, this coke will deactivate the catalyst such that, in time,

the temperature limit of the reforming unit will be reached, or the selectivity to desired

products is too much reduced, or the octane of the liquid product is declining. When

this occurs, the reﬁner will shut down the process unit and regenerate the catalyst to re-

turn it to its original state. In a moving bed unit, some of the catalyst is continually being

regenerated outside the process and returned to the reactors. High selectivity and ac-

tivity are maintained. New, degradation-resistant catalysts allow the reﬁner to operate

continuous regeneration units for more than eight years before removing the catalyst.

The actual chemistry and steps for regeneration for all process units are very similar.

In the following discussion, catalyst regeneration for a SR reformer is described. For

cyclic or continuous reformers, plant shutdown and start-up are unnecessary and the

remaining steps are accomplished in equipment outside the process stream.

The objective of regeneration is to return the catalyst to its initial, fresh state. If

the regeneration is successful, there is no difference between fresh and regenerated

catalyst. To do this, the coke must be burnt off the catalyst, the platinum should be well

dispersed and in a reduced state, and the acidity should be properly adjusted through

chloride adsorption. These needs account for the steps in regeneration; carbon burn,

chloride redispersion and metals reduction.

In order to conduct a regeneration, the heater temperatures and feed rates are reduced

gradually. The circulation of recycle gas is continued to strip hydrocarbons from the

catalyst, leaving only coke. If the coke is to be burnt in the unit, higher temperatures

are maintained and the coke burning procedure is initiated. If the coked catalyst is

to be removed from the unit, temperatures are lowered, to about 100–150

◦

F before

unloading. Since coked catalyst is often pyrophoric, nitrogen blanketing is often used

to protect the catalyst from air contact and combustion.

The coke burning step must be carefully monitored. The combustion of coke to car-

bon dioxide and water is exothermic, and the oxygen concentration must be kept low

to limit the reaction and temperature rise. Excessive temperature can cause agglom-

eration of the platinum or, in more extreme cases, can cause the alumina to change

CATALYTIC REFORMING 229

from the desired phase or crystal structure to a higher temperature phase. The water

produced in the combustion also facilitates sintering of platinum. Due to the need to

gradually burn coke, the carbon burn is usually the most time-consuming part of a

regeneration.

Coke burning is usually done in the range of 400–500

◦

C, and at oxygen concentrations

initially in the 1–2 mol% range. Oxygen content and temperature are often increased

during burning to ensure that all coke has been combusted by the end of the burn.

Oxygen consumption is monitored to determine the total amount of coke combusted

and the extent of the burn.

Since platinum can agglomerate even at relatively moderate exothermic conditions,

the platinum must be redispersed after the carbon burn. The temperature is ﬁrst

increased to approximately 500

◦

C, oxygen content to approximately 5–6 mol%, and

chlorine or an organic chloride that breaks down to HCl and Cl

is injected into the

air/nitrogen stream. Platinum oxychlorides or chlorides form that redisperse platinum

over the alumina surface, ensuring that almost all the platinum is exposed for reaction.

This also adds chloride to the catalyst to enhance its acidity.

Finally, the last step in the regeneration process is the reduction of the metals on the

catalyst and sulﬁding, if necessary. This is done in a dry hydrogen atmosphere. At

the temperatures required for reduction, greater than 350

◦

C, high moisture levels can

lead to platinum agglomeration. Since water is formed in the reduction process as

platinum oxide is reduced to platinum metal, water is drained from the unit during

reduction. The reduction hydrogen is recirculated at as high a rate as possible in order

to minimize moisture content.

Sulﬁding is typically done by injection of H

S or of an organic sulﬁde into the unit at

the end of reduction. Sulﬁding is continued until the speciﬁed sulfur level is reached

or until sulfur is no longer adsorbed by the catalyst and is detected at the outlet of the

catalyst bed.

Reactor performance

The major process variables that affect unit performance are reactor pressure, reactor

temperature, space velocity, H

/HC molar ratio, and catalyst type. The relationship

between the variables and process performance is generally applicable to both SR and

continuous regeneration modes of operation.

The reactor pressure declines across the various reaction stages. The change in reactor

pressure across the unit, known as pressure drop, can be quite high for high-pressure

reforming units, often 50–60 psig or more. The average reactor operating pressure is

generally referred to as reactor pressure. For practical purposes, a close approximation

230 CHAPTER 5

is the last reactor inlet pressure. The reactor pressure affects reformer yields, reactor

temperature requirements, and catalyst stability.

Practical operating constraints have led to a historical range of operating pressures

from 345 to 4,830 kPa (50–700 psig). Decreasing the reactor pressure increases

hydrogen and reformate yields, decreases the required temperature to achieve product

quality, and shortens the catalyst cycle because it increases the catalyst coking rate. The

high catalyst deactivation rate associated with lower operating pressure requires CCR.

The primary control for product quality in catalytic reforming is the temperature of

the catalyst beds. Platforming catalysts are capable of operating over a wide range of

temperatures. By adjusting the heater outlet temperatures, a reﬁner can change the

octane of the reformate and the quantity of the aromatics produced.

The reactor temperature can be expressed as the weighted average inlet temperature

(WAIT). The WAIT is the summation of the product of the fraction of catalyst in

each reactor multiplied by the inlet temperature of the reactor. The weighted average

bed temperature (WABT) is also used to describe catalyst temperature and is the

temperature of the catalyst integrated along the catalyst bed. Temperatures in this

chapter refer to the WAIT calculation. Typically, SR Platforming units have a WAIT

range of 490–525

◦

C (914–977

◦

F). CCR Platforming units operate at a WAIT of 525–

540

◦

C (977–1,004

◦

F). CCR Platforming units operate at even higher temperatures to

produce a more aromatic-rich, high-octane product. The amount of naphtha processed

over a given amount of catalyst over a set length of time is referred to as space velocity.

Space velocity corresponds to the reciprocal of the residence time or time of contact

between reactants and catalyst. When the hourly volume charge rate of liquid naphtha

is divided by the volume of catalyst in the reactors, the resulting quotient, expressed in

units of h

−1

, is the liquid hourly space velocity (LHSV). Typical commercial LHSV

range from 1 to 3 with the volumetric rates measured at standard conditions (60

◦

and 1 atm.abs.).

Alternatively, if the weight charge rate of naphtha is divided by the weight of catalyst,

the resulting quotient, also expressed in units of h

−1

, is the weight hourly space

velocity (WHSV). Whether LHSV or WHSV is used is based on the customary way

that feed rates are expressed at a given location. Where charge rates are normally

expressed in barrels per stream day, LHSV is typically used. Where the rates are

expressed in terms of metric tons per day, WHSV is preferred.

The combination of space velocity and reactor temperature is used to set the octane

of the product. The greater the space velocity, the higher the temperature required to

produce a given product octane. If reﬁners wish to increase the severity of a reformer

operation, they can either increase reactor temperature or lower the space velocity by

decreasing the reactor charge rate.