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

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352 CHAPTER 8

catalyst. Therefore, in many cases it is desirable to remove the H

S from the recycle

gas. The removal of H

S is performed in a scrubber where the recycle gas in contacted

with an amine (generally MEA or DEA) solution. In this manner, the H

S content of

the recycle gas can be reduced to the parts per million range.

Another method to increase hydrogen purity is membrane separation. This system

removes the hydrogen from the vent gas and recycles it back with the makeup hydro-

gen.

Catalyst contaminants

Temporary

Process variables inﬂuence catalyst life by affecting the rate of carbon deposition on

the catalyst. There is a moderate accumulation of carbon on the catalyst during the

initial days of operation. The rate of increase will be reduced to very low levels under

normal processing conditions. A carbon level of 5 wt% may be tolerated without a

signiﬁcant decrease in desulfurization. However, denitrogenation activity would be

reduced.

The sulfur and nitrogen found in the feed could be considered contaminants to the

extent that they produce hydrogen sulﬁde and ammonia which can react to form

ammonium bisulﬁde. The water injected into the reactor efﬂuent dissolves the am-

monium bisulﬁde and prevents exchanger fouling. Organic nitrogen in the feed, if

present in amounts higher than expected, will require higher reactor temperatures for

processing, and will lead to a reduction in catalyst life.

If the water injection should be stopped for any period of time, the H

S and NH

may accumulate in the recycle gas and result in a sudden loss in catalyst activity.

The activity will return to normal once wash water is reestablished. Catalyst bed

temperatures should not be increased to compensate for the temporary activity loss.

Small amounts of molecular nitrogen, CO and CO

that enter the system with the

makeup gas are not harmful to the catalyst, but must be vented to prevent accumulation

in the recycle gas. Excessive amounts of CO and CO

may have an adverse effect

on catalyst performance, as they may be methanated by the catalyst taking up active

sites and liberating heat. This will raise the outlet temperature and reduce the apparent

catalystactivity. The unit should never be pressured up with high CO +CO

containing

makeup gas as a temperature runaway may result.

Permanent

Permanent loss of catalyst activity is usually caused by the gradual accumulation

of inorganic species picked up from the feed, makeup hydrogen or efﬂuent wash

water. Examples include arsenic, lead, calcium, sodium, silicon, and phosphorus. Low

HYDROTREATING 353

concentrations of these elements (and other alkaline metals) can cause deactivation

over time as they are deposited on the catalyst.

Organic metal compounds are decomposed and typically deposit in the upper section

of the catalyst bed as a metal sulﬁde. The graded catalyst bed, if used, may contain

demetallization catalysts that have a high metals retention capacity. Some of these

catalysts may retain as much as 100 wt% of the fresh catalyst weight as metals from the

feed. These demetallization catalysts typically have a lower activity for desulfurization

and denitrogenation.

Hydrotreating catalysts exhibit a moderate tolerance for metals such as arsenic and

lead. Total metal content of 2–3 wt% of the hydrotreating catalyst have been observed.

However, product analysis frequency should be increased to monitor breakthrough

when calculations show the metals level on the hydrotreating catalyst exceeds 0.5

wt%. Metals cannot be removed by catalyst regeneration. Catalyst replacement should

be considered when metal loading of 1–2 wt% is exceeded on the hydrotreating

catalyst.

Apparent catalyst deactivation may be caused by the accumulation of deposits on

top of the catalyst bed. Solid material, such as corrosion products and coke, will

lead to rapid fouling of the catalyst bed if allowed to enter the reactor. This problem

is remedied by skimming a portion of the catalyst, screening, and reloading. Feed

ﬁltering is quite effective in removing solid material, and as such results in longer

operating cycles due to the lower rate of reactor pressure drop build-up. The use of

feed ﬁltering will depend on the type of feed processed and its source.

Hydrotreaters licensors and catalyst manufacturers

Licensors

Until the 1970s all the types of hydrotreating were offered as licensed processes by

several major licensors. Since that time, more and more of the types of hydrotreating

have been offered without a license and the catalyst used in the process was sold

as a commodity. Today, Chevron, IFP and UOP offer only residue hydrotreating as

a licensed process. A few specialized types of hydrotreating such as some aromatic

saturation units are also offered as licensed units, however, the vast majority of new hy-

drotreaters are sold without a license. Engineering contractors do most of the designs.

Catalyst suppliers

Through the years there have been many manufacturers of hydrotreating catalysts.

Other manufacturers have absorbed some, some have changed names, and some

354 CHAPTER 8

have discontinued hydrotreating catalyst manufacturing. Some suppliers simply sell

catalyst they have re-branded with their name. Following is a list of the current major

suppliers of hydrotreating catalysts: Advanced Reﬁning Technologies LP, Akzo Nobel

Catalysts BV, Axens/Procatalyse SA, Catalysts and Chemicals Industries Co. Ltd.,

Chevron Research and Technology Co., Criterion Catalysts Co. Ltd., Exxon Research

and Engineering Co., Haldor Topsoe AS, Orient Catalyst Co. Ltd., Sud Chemie Inc.,

and UOP.

Chapter 9

Gasoline components

9.1 Motor fuel alkylation

James F. Himes and Robert L. Mehlberg*

9.1.1 Introduction

Motor fuel alkylation in the petroleum reﬁning industry refers to the acid catalyzed

conversion of C

-C

oleﬁns with isobutane into highly branched C

-C

isoparafﬁns

collectively called alkylate, a valuable gasoline blending component. A major con-

stituent of alkylate is 2,2,4-trimethyl pentane which is deﬁned as 100 on the octane

scale.

Alkylation reactions are catalyzed by liquid and solid acids, including H

, AlCl

HCl, HF, HF-BF

-HSO

F (Fluorosulfuric acid), Triﬂuoromethane sulfonic

acid chlorided Pt alumina, BF

on alumina, zeolites, and ion exchange resins. How-

ever, the catalysts and associated processes commercialized during WWII for aviation

gasoline, HF alkylation, and sulfuric acid alkylation, are the focus of this section as

these remain the primary commercial motor fuel alkylation processes. A solid catalyst

alkylation process (UOP Alkylene™) has been developed and is being offered to the

industry.

History

In 1932–6, alkylation was independently discovered by UOP,

Shell, the Anglo Iranian

Oil Company (AIOC), and Texaco whose ﬁrst publications issue in that order. Herman

Pines told the story of UOP’s discovery of the alkylation of ethylene by pentanes in

1932.

At that time leading universities taught that isoparafﬁns were inert except at

high temperatures and pressures. After ﬁnding anomalies in an oleﬁn assay based

upon H

extraction Pines and his mentor V. I. Ipatieff hypothesized that parafﬁns

may not be inert to acids. Despite ridicule, they tested that hypothesis by bubbling

ethylene into chilled pentanes over AlCl

. All the ethylene was converted into saturated

*UOP LLC.

355

356 CHAPTER 9

hydrocarbons. Over the next few years they tested AlCl

-HCl, H

, HF, and HF-

as alkylation catalysts.

Alkylate was found to have excellent aviation gasoline properties. It was the highest

octane fuel component then known, with high motor octane and excellent lead re-

sponse. All of the properties derive from the highly-branched parafﬁns that form its

composition.

Humble Oil built the ﬁrst commercial H

alkylation unit in 1938 at Baytown,

Texas.

Alkylation for aviation gasoline grew rapidly with the Allies war effort. In

1939, six petroleum companies formed a consortium to pool their alkylation tech-

nology and develop both sulfuric acid and HF acid processes for 100 octane aviation

fuel. The ﬁrst commercial HF alkylation unit started up in 1942. During the war 60

alkylation units were built for the Allies’ war effort. Half were built with sulfuric acid

as the catalyst and half with HF.

Following World War II, most alkylation operations were discontinued although a few

reﬁners continued to use the process for aviation and premium automobile gasolines.

In the mid-1950s, use of higher performance automotive engines required the reﬁning

industry to both increase gasoline production and quality. The developmentof catalytic

reforming, such as UOP Platforming™, provided reﬁners with an important reﬁning

tool for production of high octane gasolines. However, the motor fuel produced in

such operations, called reformate, is highly aromatic with a higher sensitivity (the

spread between research and motor octane) and a lower lead response than alkylate.

Many reﬁners expanded their alkylation operations and began to broaden the range

of oleﬁn feeds to both existing and new alkylation units to include propylene and

occasionally even some pentenes along with the butenes.

With the phase-out of leaded gasolines and the advent of environmental gasolines the

lead response of alkylate is no longer valued, but the importance of alkylate and its

production have both grown because of its other properties. Its high unleaded motor

octane, low volatility, low-sulfur, and nearly zero oleﬁns and aromatics make alkylate

critical to the production of quality environmental gasolines. Alkylate can reach 60%

of low-sulfur reformulated premium.

Licensors of motor fuel HF Alkylation processes are UOP LLC and Phillips. Licensors

of H

alkylation processes are Exxon Mobil and Stratco Engineering.

Process chemistry

The reactions taking place in the alkylation reactor are many and relatively complex.

First the oleﬁn reacts with the acid to form an ester; then the ester is alkylated by a

GASOLINE COMPONENTS 357

t-butyl carbenium ion chain mechanism.

The principal overall reactions are described

below.

Primary alkylation reactions

Most of the alkylate product is made by primary alkylation reactions. In these reactions

one mole of oleﬁn reacts with one mole of isobutane to form an isoparafﬁn exactly

4 carbon numbers heavier. Example primary alkylation reactions, showing only the

carbon framework and one or two of the principal product isomers, are shown for

each of the principal feed oleﬁns in equations (1.1)–(1.5).

ALKYLATION OF BUTENE-2

C—C = C—C + C—C

—C → C—C

—C—C

—C

cis-Butene-2 + Isobutane → 2,2,4-Trimethylpentane (1.1)

(100 RON)

ALKYLATION OF BUTENE-1

C = C—C—C + C—C

—C → C—C

—C—C

Butene-1 + Isobutane → 2,3-Dimethylhexane (1.2)

(71.3 RON)

C = C—C—C+C—C

—C → C—C

—C

Butene-1 + Isobutane → 2,3,4-Trimethylpentane (1.3)

(102.7 RON)

ALKYLATION OF ISOBUTYLENE (I-BUTENE)

C=C

—C+C—C

—C → C—C

—C—C

—C

Isobutylene + Isobutane → 2,2,4-Trimethylpentane (1.4)

(100 RON)

358 CHAPTER 9

ALKYLATION OF PROPYLENE

C—C=C+C—C

—C → C—C

—C

—C—C

Propylene + Isobutane → 2,3-Dimethylpentane (1.5)

(91.1 RON)

ALKYLATION OF 2-METHYL-BUTENE-2

C—C = C—C + C

—C—C → C

—C—C—C

—C—C

2-methylbutene-2 +Isobutane → 2,2,4-Trimethylhexane (1.6)

(92 RON)

Complex alkylation reactions

The balance of the alkylate and small amounts of undesirable byproduct conjunct

polymer are formed by more complex reactions of the ionic intermediates. Some of

these reactions are given by equations (2.1)–(2.5). For brevity only a few key steps

and no structures are shown.

HYDRIDE TRANSFER (PROPYLENE EXAMPLE)

+ H

→ C

(Formation of propenium ion)

+ iC

→ iC

+ C

(Hydride transfer from isobutane to

propenium ion)

→ iC

+ H

+ iC

→ iC

Overall

+ 2iC

→ C

+ C

Propylene + 2 Isobutane → Propane + 2,2,4-Trimethylpentane (2.1)

POLYMERIZATION (ISOBUTYLENE EXAMPLE)

+ iC

→ iC

+ iC

→ iC

+ iC

→ iC

+ iC

Overall

2iC

+ iC

→ iC

2 isobutylene + Isobutane → Pentamethylheptane (2.2)

GASOLINE COMPONENTS 359

CRACKING (CRACKING OF HEAVY ENDS FROM ISOBUTYLENE)

2iC

+ iC

→ iC

+ iC

→ iC

(isopentane) + iC

(t-butyl ion)

+ iC

→ iC

+ iC

→ iC

+ iC

(t-butyl ion)

Overall

2iC

+ 2iC

→ iC

+ iC

2 isobutylene + 2 isobutane → isopentane + isoundecane (2.3)

COMPETITION (ISOPENTANE ALKYLATION)

+ C

→ iC

Isopentane + butene-2 → 2,2,5-Trimethlyhexane (2.4)

CONJUNCT POLYMERIZATION

17iC

+ H

→→→ C

+ 4C

isobutylene → conjunct polymer ion (in acid phase) + Heavy alkylate (2.5)

Isomerization

In addition to the listed reactions the acids catalyze isomerization of oleﬁns or their

esters before alkylation as well as isomerization of the alkylate products. Most of the

product isomerizations are methyl shifts of ionic intermediates.

Reaction selectivites—The selectivities of these reactions depend upon the feed-

stock, catalyst, and reaction conditions.

Isobutylene and isopentenes are very reactive and are prone to polymerization to

heavy alkylate (reaction (2.2)) and to conjunct polymer (reaction (2.5)), especially

when catalyzed by H

Propylene is less reactive and tends to make more conjunct polymer, especially for

the H

catalyst.

Butene-1 makes higher yields of low-octane dimethyl hexanes (reaction (1.2)) than

butene-2 in HF, but there is little difference in these oleﬁns in H

. To maximize

octane many reﬁners have added selective hydrogenation units (such as UOP-Huls

SHP™) upstream of the alkylation unit to saturate dioleﬁns reducing the amount

of conjunct polymers as well as isomerizing most of the Butene-1 to Butene-2.

HF enhances hydride transfer reactions (2.1) much more than H

. These re-

actions increase alkylate yield, RVP, and octane. As a result, HF catalyzes higher

yields of isopentane from pentenes than H

; it also produces low amounts of

n-parafﬁns from n-oleﬁns while H

produces none. The additional yield of

hydride transfer reactions naturally comes with increased isobutane consumption.

360 CHAPTER 9

FCC

Alkylate

Product

FCC Feed

Alkylation

Unit

n-Butane

Mixed

Butanes

Butamer

Propane

C3/C4 Olefin

Rich Stream

FCC Products

(Gasoline, etc.)

Figure 9.1.1. Alkylation unit location in reﬁnery.

produces more heavy ends (reaction 2.2) and more conjunct polymer (reac-

tion 2.5) and less isopentane than HF, especially from iso-oleﬁns and propylene. For

all oleﬁns it produces lower yields, higher endpoints, and consumes less isobutane.

Increasing reactor temperature increases cracking and conjunct polymer formation

at the expense of primary alkylation lowering product octane and increasing RVP

and acid consumption.

Increasing isobutane concentration increases primary alkylation and hydride trans-

fer reactions while reducing polymer formation, cracking, and competition which

increase yield and octane and reduce acid consumption.

HF alkylation process ﬂow description

Figure 9.1.1 illustrates how an alkylation unit ﬁts into the reﬁnery in a typical FCC-

Alkylation arrangement. The location of the butane isomerization unit (Butamer™)

is also indicated as many reﬁners have this arrangement.

Figure 9.1.2 is a simpliﬁed process ﬂow diagram for the UOP Propylene-Butene HF

Alkylation process.

Feed pretreatment

In the UOP HF alkylation process oleﬁn-rich feeds from the FCC gas plant are

typically deethanized, Merox treated to remove H

S and mercaptans and dried. Some

reﬁners have also added MTBE or Selective Hydrogenation (SHP) plants upstream

the alky.

GASOLINE COMPONENTS 361

Olefin

Feed

Recycle

i-C

Cooling

Water

Alkylate

Product

Propane

Product

Saturate

Feed

n-C

Product

Isostripper

DeC3

HF Stripper

Reactor

Settler

Figure 9.1.2. UOP HF alkylation.

Reaction

After pretreatment, the oleﬁn feeds are combined with a large excess of recycle isobu-

tane to provide an 6–14 isobutane:oleﬁn molar ratio and injected into the circulating

HF acid catalyst at the shell side inlet to the water-cooled reactor. Cooling water

ﬂows through the reactor tubes to remove the highly exothermic heat of reaction

and to maintain reaction conditions at 80–100

◦

F. The alkylation reaction is very fast

with 100% oleﬁn conversion. The excess isobutane, alkylate product, non-reactive

hydrocarbons (propane, n-butane) in the feeds and the acid catalyst pass on to the set-

tler vessel. The dense acid phase separates from the hydrocarbons rapidly by gravity

and is then pumped back to the reactor. The hydrocarbons containing dissolved HF

ﬂow off the top of the settler to the isostripper.

Fractionation

It consists of an isostripper, a depropanizer, and an HF Stripper. The isostripper is

a large tower with two sidedraws with the primary function recycling isobutane to

support the high isobutane:oleﬁn molar ratio of the reactor. The tower typically has

two reboilers; the upper reboiler typically maximizes the use of relatively low cost low

or medium pressure steam and the lower reboiler uses a heating medium that can give

400–450

◦

F process temperatures. Alkylate is drawn off the bottom of the tower, cooled

in exchangers, and sent to product storage. The next product draw up the tower is the

n-butane sidedraw and above that is the isobutane recycle draw. On most UOP units the

isobutane draw is located below the feed tray to minimize HF in the isobutane recycle.