Handbook of petroleum refining processes

ALKYLATION AND

POLYMERIZATION

P ●

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1.3

CHAPTER 1.1

NExOCTANE™ TECHNOLOGY

FOR ISOOCTANE

PRODUCTION

Ronald Birkhoff

Kellogg Brown & Root, Inc. (KBR)

Matti Nurminen

Fortum Oil and Gas Oy

INTRODUCTION

Environmental issues are threatening the future use of MTBE (methyl-tert-butyl ether) in

gasoline in the United States. Since the late 1990s, concerns have arisen over ground and

drinking water contamination with MTBE due to leaking of gasoline from underground

storage tanks and the exhaust from two-cycle engines. In California a number of cases of

drinking water pollution with MTBE have occurred. As a result, the elimination of MTBE

in gasoline in California was mandated, and legislation is now set to go in effect by the end

of 2003. The U.S. Senate has similar law under preparation, which would eliminate MTBE

in the 2006 to 2010 time frame.

With an MTBE phase-out imminent, U.S. refiners are faced with the challenge of

replacing the lost volume and octane value of MTBE in the gasoline pool. In addition, uti￾lization of idled MTBE facilities and the isobutylene feedstock result in pressing problems

of unrecovered and/or underutilized capital for the MTBE producers. Isooctane has been

identified as a cost-effective alternative to MTBE. It utilizes the same isobutylene feeds

used in MTBE production and offers excellent blending value. Furthermore, isooctane pro￾duction can be achieved in a low-cost revamp of an existing MTBE plant. However, since

isooctane is not an oxygenate, it does not replace MTBE to meet the oxygen requirement

currently in effect for reformulated gasoline.

The NExOCTANE technology was developed for the production of isooctane. In the

process, isobutylene is dimerized to produce isooctene, which can subsequently be hydro￾genated to produce isooctane. Both products are excellent gasoline blend stocks with sig￾nificantly higher product value than alkylate or polymerization gasoline.

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1.4 ALKYLATION AND POLYMERIZATION

HISTORY OF MTBE

During the 1990s, MTBE was the oxygenate of choice for refiners to meet increasingly strin￾gent gasoline specifications. In the United States and in a limited number of Asian countries,

the use of oxygenates in gasoline was mandated to promote cleaner-burning fuels. In addi￾tion, lead phase-down programs in other parts of the world have resulted in an increased

demand for high-octane blend stock. All this resulted in a strong demand for high-octane fuel

ethers, and significant MTBE production capacity has been installed since 1990.

Today, the United States is the largest consumer of MTBE. The consumption increased

dramatically with the amendment of the Clean Air Act in 1990 which incorporated the 2

percent oxygen mandate. The MTBE production capacity more than doubled in the 5-year

period from 1991 to 1995. By 1998, the MTBE demand growth had leveled off, and it has

since tracked the demand growth for reformulated gasoline (RFG). The United States con￾sumes about 300,000 BPD of MTBE, of which over 100,000 BPD is consumed in

California. The U.S. MTBE consumption is about 60 percent of the total world demand.

MTBE is produced from isobutylene and methanol. Three sources of isobutylene are

used for MTBE production:

● On-purpose butane isomerization and dehydrogenation

● Fluid catalytic cracker (FCC) derived mixed C4 fraction

● Steam cracker derived C4 fraction

The majority of the MTBE production is based on FCC and butane dehydrogenation

derived feeds.

NExOCTANE BACKGROUND

Fortum Oil and Gas Oy, through its subsidiary Neste Engineering, has developed the

NExOCTANE technology for the production of isooctane. NExOCTANE is an extension

of Fortum’s experience in the development and licensing of etherification technologies.

Kellogg Brown & Root, Inc. (KBR) is the exclusive licenser of NExOCTANE. The tech￾nology licensing and process design services are offered through a partnership between

Fortum and KBR.

The technology development program was initialized in 1997 in Fortum’s Research and

Development Center in Porvoo, Finland, for the purpose of producing high-purity isooctene,

for use as a chemical intermediate. With the emergence of the MTBE pollution issue and the

pending MTBE phase-out, the focus in the development was shifted in 1998 to the conver￾sion of existing MTBE units to produce isooctene and isooctane for gasoline blending.

The technology development has been based on an extensive experimental research

program in order to build a fundamental understanding of the reaction kinetics and key

product separation steps in the process. This research has resulted in an advanced kinetic

modeling capability, which is used in the design of the process for licensees. The process

has undergone extensive pilot testing, utilizing a full range of commercial feeds. The first

commercial NExOCTANE unit started operation in the third quarter of 2002.

PROCESS CHEMISTRY

The primary reaction in the NExOCTANE process is the dimerization of isobutylene over

acidic ion-exchange resin catalyst. This dimerization reaction forms two isomers of

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NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PROCUCTION 1.5

trimethylpentene (TMP), or isooctene, namely, 2,4,4-TMP-1 and 2,4,4-TMP-2, according

to the following reactions:

TMP further reacts with isobutylene to form trimers, tetramers, etc. Formation of these

oligomers is inhibited by oxygen-containing polar components in the reaction mixture. In the

Isobutylene

2

2,4,4 TMP-1

CH2= C - CH3

CH3

CH2 = C - CH2 - C - CH3

CH3 CH3

CH3

CH2 - C = CH2 - C - CH3

CH3 CH3

CH3

2,4,4 TMP-2

NExOCTANE process, water and alcohol are used as inhibitors. These polar components

block acidic sites on the ion-exchange resin, thereby controlling the catalyst activity and

increasing the selectivity to the formation of dimers. The process conditions in the dimer￾ization reactions are optimized to maximize the yield of high-quality isooctene product.

A small quantity of C7 and C9 components plus other C8 isomers will be formed when

other olefin components such as propylene, n-butenes, and isoamylene are present in the

reaction mixture. In the NExOCTANE process, these reactions are much slower than the

isobutylene dimerization reaction, and therefore only a small fraction of these components

is converted.

Isooctene can be hydrogenated to produce isooctane, according to the following reaction:

CH2 – C – CH2 – C – CH3

CH3 CH3

CH3

Isooctene Isooctane

CH2 = C – CH2 – C – CH3 + H2

CH3 CH3

CH3

NExOCTANE PROCESS DESCRIPTION

The NExOCTANE process consists of two independent sections. Isooctene is produced by

dimerization of isobutylene in the dimerization section, and subsequently, the isooctene

can be hydrogenated to produce isooctane in the hydrogenation section. Dimerization and

hydrogenation are independently operating sections. Figure 1.1.1 shows a simplified flow

diagram for the process.

The isobutylene dimerization takes place in the liquid phase in adiabatic reactors over

fixed beds of acidic ion-exchange resin catalyst. The product quality, specifically the distri￾NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION

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bution of dimers and oligomers, is controlled by recirculating alcohol from the product recov￾ery section to the reactors. Alcohol is formed in the dimerization reactors through the reaction

of a small amount of water with olefin present in the feed. The alcohol content in the reactor

feed is typically kept at a sufficient level so that the isooctene product contains less than 10

percent oligomers. The dimerization product recovery step separates the isooctene product

from the unreacted fraction of the feed (C4 raffinate) and also produces a concentrated alco￾hol stream for recycle to the dimerization reaction. The C4 raffinate is free of oxygenates and

suitable for further processing in an alkylation unit or a dehydrogenation plant.

Isooctene produced in the dimerization section is further processed in a hydrogenation

unit to produce the saturated isooctane product. In addition to saturating the olefins, this

unit can be designed to reduce sulfur content in the product. The hydrogenation section

consists of trickle-bed hydrogenation reactor(s) and a product stabilizer. The purpose of

the stabilizer is to remove unreacted hydrogen and lighter components in order to yield a

product with a specified vapor pressure.

The integration of the NExOCTANE process into a refinery or butane dehydrogenation

complex is similar to that of the MTBE process. NExOCTANE selectively reacts isobuty￾lene and produces a C4 raffinate which is suitable for direct processing in an alkylation or

dehydrogenation unit. A typical refinery integration is shown in Fig. 1.1.2, and an integra￾tion into a dehydrogenation complex is shown in Fig. 1.1.3.

NExOCTANE PRODUCT PROPERTIES

The NExOCTANE process offers excellent selectivity and yield of isooctane (2,2,4-

trimethylpentane). Both the isooctene and isooctane are excellent gasoline blending compo￾nents. Isooctene offers substantially better octane blending value than isooctane. However,

the olefin content of the resulting gasoline pool may be prohibitive for some refiners.

The characteristics of the products are dependent on the type of feedstock used. Table

1.1.1 presents the product properties of isooctene and isooctane for products produced

from FCC derived feeds as well as isooctane from a butane dehydrogenation feed.

The measured blending octane numbers for isooctene and isooctane as produced from

FCC derived feedstock are presented in Table 1.1.2. The base gasoline used in this analy￾1.6 ALKYLATION AND POLYMERIZATION

Dimerization Product

Recovery

Hydrogenation

Reaction

Stabilizer

Isobutylene

C4 Raffinate

Alcohol Recycle

Isooctane

Isooctene Hydrogen Fuel Gas

DIMERIZATION

SECTION

HYDROGENATION

SECTION

FIGURE 1.1.1 NExOCTANE process.

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sis is similar to nonoxygenated CARB base gasoline. Table 1.1.2 demonstrates the signif￾icant blending value for the unsaturated isooctene product, compared to isooctane.

PRODUCT YIELD

An overall material balance for the process based on FCC and butane dehydrogenation

derived isobutylene feedstocks is shown in Table 1.1.3. In the dehydrogenation case, an

isobutylene feed content of 50 wt % has been assumed, with the remainder of the feed

NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PROCUCTION 1.7

FCC

DIMERIZATION ALKYLATION

Hydrogen Isooctane

Isooctene

HYDROGENATION

C4 C4 Raffinate

NExOCTANE

FIGURE 1.1.2 Typical integration in refinery.

HYDROGE￾NATION DEHYDRO

Hydrogen

Isooctane

Isooctene

DIMERIZATION

iC4=

NExOCTANE

Butane

HYDROGEN

TREATMENT

RECYCLE

TREATMENT

ISOMERI￾ZATION

DIB

C4 Raffinate

FIGURE 1.1.3 Integration in a typical dehydrogenation complex.

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mostly consisting of isobutane. For the FCC feed an isobutylene content of 22 wt % has

been used. In each case the C4 raffinate quality is suitable for either direct processing in a

refinery alkylation unit or recycle to isomerization or dehydrogenation step in the dehy￾drogenation complex. Note that the isooctene and isooctane product rates are dependent

on the content of isobutylene in the feedstock.

UTILITY REQUIREMENTS

The utilities required for the NExOCTANE process are summarized in Table 1.1.4.

1.8 ALKYLATION AND POLYMERIZATION

TABLE 1.1.1 NExOCTANE Product Properties

FCC C4 Butane

dehydrogenation

Isooctane Isooctene Isooctane

Specific gravity 0.704 0.729 0.701

RONC 99.1 101.1 100.5

MONC 96.3 85.7 98.3

(R
M) / 2 97.7 93.4 99.4

RVP, lb/in2 absolute 1.8 1.8 1.8

TABLE 1.1.2 Blending Octane Number in CARB Base Gasoline (FCC

Derived)

Isooctene Isooctane

Blending BRON BMON (R
M) / 2 BRON BMON (R
M) /

2

volume, %

10 124.0 99.1 111.0 99.1 96.1 97.6

20 122.0 95.1 109.0 100.1 95.1 97.6

100 101.1 85.7 93.4 99.1 96.3 97.7

TABLE 1.1.3 Sample Material Balance for NExOCTANE Unit

Material balance FCC C4 feed, lb/h (BPD) Butane dehydrogenation, lb/h (BPD)

Dimerization section:

Hydrocarbon feed 137,523 (16,000) 340,000 (39,315)

Isobutylene contained 30,614 (3,500) 170,000 (19,653)

Isooctene product 30,714 (2,885) 172,890 (16,375)

C4 raffinate 107,183 (12,470) 168,710 (19,510)

Hydrogenation section:

Isooctene feed 30,714 (2,885) 172,890 (16,375)

Hydrogen feed 581 3752

Isooctane product 30,569 (2,973) 175,550 (17,146)

Fuel gas product 726 1092

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NExOCTANE TECHNOLOGY ADVANTAGES

Long-Life Dimerization Catalyst

The NExOCTANE process utilizes a proprietary acidic ion-exchange resin catalyst. This

catalyst is exclusively offered for the NExOCTANE technology. Based on Fortum’s exten￾sive catalyst trials, the expected catalyst life of this exclusive dimerization catalyst is at

least double that of standard resin catalysts.

Low-Cost Plant Design

In the dimerization process, the reaction takes place in nonproprietary fixed-bed reactors.

The existing MTBE reactors can typically be reused without modifications. Product recov￾ery is achieved by utilizing standard fractionation equipment. The configuration of the

recovery section is optimized to make maximum use of the existing MTBE product recov￾ery equipment.

High Product Quality

The combination of a selective ion-exchange resin catalyst and optimized conditions in the

dimerization reaction results in the highest product quality. Specifically, octane rating and

specific gravity are better than those in product produced with alternative catalyst systems

or competing technologies.

State-of-the-Art Hydrogenation Technology

The NExOCTANE process provides a very cost-effective hydrogenation technology. The

trickle-bed reactor design requires low capital investment, due to a compact design plus

once-through flow of hydrogen, which avoids the need for a recirculation compressor.

Commercially available hydrogenation catalysts are used.

Commercial Experience

The NExOCTANE technology is in commercial operation in North America in the world’s

largest isooctane production facility based on butane dehydrogenation. The project

includes a grassroots isooctene hydrogenation unit.

NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PROCUCTION 1.9

TABLE 1.1.4 Typical Utility Requirements

Utility requirements FCC C4 Butane dehydrogenation

per BPD of product per BPD of product

Dimerization section:

Steam, 1000 lb/h 13 6.4

Cooling water, gal/min 0.2 0.6

Power, kWh 0.2 0.03

Hydrogenation section:

Steam, 1000 lb/h 1.5 0.6

Cooling water, gal/min 0.03 0.03

Power, kWh 0.03 0.1

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CHAPTER 1.2

STRATCO EFFLUENT

REFRIGERATED H2SO4

ALKYLATION PROCESS

David C. Graves

STRATCO

Leawood, Kansas

INTRODUCTION

Alkylation, first commercialized in 1938, experienced tremendous growth during the

1940s as a result of the demand for high-octane aviation fuel during World War II. During

the mid-1950s, refiners’ interest in alkylation shifted from the production of aviation fuel

to the use of alkylate as a blending component in automotive motor fuel. Capacity

remained relatively flat during the 1950s and 1960s due to the comparative cost of other

blending components. The U.S. Environmental Protection Agency’s lead phase-down pro￾gram in the 1970s and 1980s further increased the demand for alkylate as a blending com￾ponent for motor fuel. As additional environmental regulations are imposed on the

worldwide refining community, the importance of alkylate as a blending component for

motor fuel is once again being emphasized. Alkylation unit designs (grassroots and

revamps) are no longer driven only by volume, but rather by a combination of volume,

octane, and clean air specifications. Lower olefin, aromatic, sulfur, Reid vapor pressure

(RVP), and drivability index (DI) specifications for finished gasoline blends have also

become driving forces for increased alkylate demand in the United States and abroad.

Additionally, the probable phase-out of MTBE in the United States will further increase

the demand for alkylation capacity.

The alkylation reaction combines isobutane with light olefins in the presence of a

strong acid catalyst. The resulting highly branched, paraffinic product is a low-vapor-pres￾sure, high-octane blending component. Although alkylation can take place at high temper￾atures without catalyst, the only processes of commercial importance today operate at low

to moderate temperatures using either sulfuric or hydrofluoric acid catalysts. Several dif￾ferent companies are currently pursuing research to commercialize a solid alkylation cat￾alyst. The reactions occurring in the alkylation process are complex and produce an

alkylate product that has a wide boiling range. By optimizing operating conditions, the

1.11

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majority of the product is within the desired gasoline boiling range with motor octane

numbers (MONs) up to 95 and research octane numbers (RONs) up to 98.

PROCESS DESCRIPTION

A block flow diagram of the STRATCO effluent refrigerated H2SO4 alkylation project is

shown in Fig. 1.2.1. Each section of the block flow diagram is described below:

Reaction section. Here the reacting hydrocarbons are brought into contact with sulfu￾ric acid catalyst under controlled conditions.

Refrigeration section. Here the heat of reaction is removed, and light hydrocarbons are

removed from the unit.

Effluent treating section. Here the free acid, alkyl sulfates, and dialkyl sulfates are

removed from the net effluent stream to avoid downstream corrosion and fouling.

Fractionation section. Here isobutane is recovered for recycle to the reaction section,

and remaining hydrocarbons are separated into the desired products.

Blowdown section. Here spent acid is degassed, wastewater pH is adjusted, and acid

vent streams are neutralized before being sent off-site.

The blocks are described in greater detail below:

Reaction Section

In the reaction section, olefins and isobutane are alkylated in the presence of sulfuric acid cat￾alyst. As shown in Fig. 1.2.2, the olefin feed is initially combined with the recycle isobutane.

The olefin and recycle isobutane mixed stream is then cooled to approximately 60°F

(15.6°C) by exchanging heat with the net effluent stream in the feed/effluent exchangers.

1.12 ALKYLATION AND POLYMERIZATION

FIGURE 1.2.1 Block flow diagram of STRATCO Inc. effluent refrigerated alkylation process.

STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS

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STRATCO EFFLUENT REFRIGERATED H2

SO4 ALKYLATION PROCESS 1.13

Since the solubility of water is reduced at lower temperatures, water is freed from the

hydrocarbon to form a second liquid phase. The feed coalescer removes this free water to

minimize dilution of the sulfuric acid catalyst.

The feed stream is then combined with the refrigerant recycle stream from the refrig￾eration section. The refrigerant recycle stream provides additional isobutane to the reac￾tion zone. This combined stream is fed to the STRATCO Contactor reactors.

The use of separate Contactor reactors in the STRATCO process allows for the segre￾gation of different olefin feeds to optimize alkylate properties and acid consumption. In

these cases, the unit will have parallel trains of feed/effluent exchangers and feed coa￾lescers.

At the “heart” of STRATCO’s effluent refrigerated alkylation technology is the

Contactor reactor (Fig. 1.2.3). The Contactor reactor is a horizontal pressure vessel con￾taining an inner circulation tube, a tube bundle to remove the heat of reaction, and a mix￾ing impeller. The hydrocarbon feed and sulfuric acid enter on the suction side of the

impeller inside the circulation tube. As the feeds pass across the impeller, an emulsion of

hydrocarbon and acid is formed. The emulsion in the Contactor reactor is continuously cir￾culated at very high rates.

The superior mixing and high internal circulation of the Contactor reactor minimize the

temperature difference between any two points in the reaction zone to within 1°F (0.6°C).

This reduces the possibility of localized hot spots that lead to degraded alkylate product

and increased chances for corrosion. The intense mixing in the Contactor reactor also pro￾vides uniform distribution of the hydrocarbons in the acid emulsion. This prevents local￾ized areas of nonoptimum isobutane/olefin ratios and acid/olefin ratios, both of which

promote olefin polymerization reactions.

Figure 1.2.4 shows the typical Contactor reactor and acid settler arrangement. A por￾tion of the emulsion in the Contactor reactor, which is approximately 50 LV % acid and

50 LV % hydrocarbon, is withdrawn from the discharge side of the impeller and flows to

the acid settler. The hydrocarbon phase (reactor effluent) is separated from the acid emul￾sion in the acid settlers. The acid, being the heavier of the two phases, settles to the lower

portion of the vessel. It is returned to the suction side of the impeller in the form of an

emulsion, which is richer in acid than the emulsion entering the settlers.

The STRATCO alkylation process utilizes an effluent refrigeration system to remove

the heat of reaction and to control the reaction temperature. With effluent refrigeration, the

hydrocarbons in contact with the sulfuric acid catalyst are maintained in the liquid phase.

The hydrocarbon effluent flows from the top of the acid settler to the tube bundle in the

FIGURE 1.2.2 Feed mixing and cooling.

STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS

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Contactor reactor. A control valve located in this line maintains a back pressure of about

60 lb/in2 gage (4.2 kg/cm2 gage) in the acid settler.

This pressure is adequate to prevent vaporization in the reaction system. In plants with

multiple Contactor reactors, the acid settler pressures are operated about 5 lb/in2 (0.4

kg/cm2

) apart to provide adequate pressure differential for series acid flow.

The pressure of the hydrocarbon stream from the top of the acid settler is reduced to

about 5 lb/in2 gage (0.4 kg/cm2 gage) across the back pressure control valve. A portion of

the effluent stream is flashed, reducing the temperature to about 35°F (1.7°C). Additional

vaporization occurs in the Contactor reactor tube bundle as the net effluent stream removes

the heat of reaction. The two-phase net effluent stream flows to the suction trap/flash drum

where the vapor and liquid phases are separated.

1.14 ALKYLATION AND POLYMERIZATION

FIGURE 1.2.3 STRATCO Contactor reactor.

FIGURE 1.2.4 Contactor reactor/acid settler arrangement.

STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS

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The suction trap/flash drum is a two-compartment vessel with a common vapor space.

The net effluent pump transfers the liquid from the suction trap side (net effluent) to the

effluent treating section via the feed/effluent exchangers. Refrigerant from the refrigera￾tion section flows to the flash drum side of the suction trap/flash drum. The combined

vapor stream is sent to the refrigeration section.

The sulfuric acid present in the reaction zone serves as a catalyst to the alkylation reac￾tion. Theoretically, a catalyst promotes a chemical reaction without being changed as a

result of that reaction. In reality, however, the acid is diluted as a result of the side reac￾tions and feed contaminants. To maintain the desired spent acid strength, a small amount

of fresh acid is continuously charged to the acid recycle line from the acid settler to the

Contactor reactor, and a similar amount of spent acid is withdrawn from the acid settler.

In multiple-Contactor reactor plants, the reactors are usually operated in parallel on

hydrocarbon and in series/parallel on acid, up to a maximum of four stages. Fresh acid and

intermediate acid flow rates between the Contactor reactors control the spent acid strength.

The spent acid strength is generally monitored by titration, which is done in the labo￾ratory. In response to our customer requests, STRATCO has developed an on-line acid ana￾lyzer that enables the operators to spend the sulfuric acid to lower strengths with much

greater accuracy and confidence.

When alkylating segregated olefin feeds, the optimum acid settler configuration will

depend on the olefins processed and the relative rates of each feed. Generally, STRATCO

recommends processing the propylene at high acid strength, butylenes at intermediate

strength, and amylenes at low strength. The optimum configuration for a particular unit

may involve operating some reaction zones in parallel and then cascading to additional

reaction zones in series. STRATCO considers several acid staging configurations for every

design in order to provide the optimum configuration for the particular feed.

Refrigeration Section

Figure 1.2.5 is a diagram of the most common refrigeration configuration. The partially

vaporized net effluent stream from the Contactor reactor flows to the suction trap/flash

drum, where the vapor and liquid phases are separated. The vapor from the suction

trap/flash drum is compressed by a motor or turbine-driven compressor and then con￾densed in a total condenser.

A portion of the refrigerant condensate is purged or sent to a depropanizer. The remain￾ing refrigerant is flashed across a control valve and sent to the economizer. If a depropaniz￾er is included in the design, the bottoms stream from the tower is also sent to the

economizer. The economizer operates at a pressure between the condensing pressure and

the compressor suction pressure. The economizer liquid is flashed and sent to the flash

drum side of the suction trap/flash drum.

A lower-capital-cost alternative would be to eliminate the economizer at a cost of about

7 percent higher compressor energy. Another alternative is to incorporate a partial con￾denser to the economizer configuration and thus effectively separate the refrigerant from

the light ends, allowing for propane enrichment of the depropanizer feed stream. As a

result, both depropanizer capital and operating costs can be reduced. The partial condens￾er design is most cost-effective when feed streams to the alkylation unit are high (typical￾ly greater than 40 LV %) in propane/propylene content.

For all the refrigeration configurations, the purge from the refrigeration loop is treated

to remove impurities prior to flowing to the depropanizer or leaving the unit. These impu￾rities can cause corrosion in downstream equipment. The main impurity removed from the

purge stream is sulfur dioxide (SO2). SO2 is produced from oxidation reactions in the reac￾tion section and decomposition of sulfur-bearing contaminants in the unit feeds.

STRATCO EFFLUENT REFRIGERATED H2

SO4 ALKYLATION PROCESS 1.15

STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS

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The purge is contacted with strong caustic (10 to 12 wt %) in an in-line static mixer and

is sent to the caustic wash drum. The separated hydrocarbon stream from the caustic wash

drum then mixes with process water and is sent to a coalescer (Fig. 1.2.6). The coalescer

reduces the carryover caustic in the hydrocarbon stream that could cause stress corrosion

cracking or caustic salt plugging and fouling in downstream equipment. The injection of

process water upstream of the coalescer enhances the removal of caustic carryover in the

coalescer.

Effluent Treating Section

The net effluent stream from the reaction section contains traces of free acid, alkyl sulfates,

and dialkyl sulfates formed by the reaction of sulfuric acid with olefins. These alkyl sul￾fates are commonly referred to as esters. Alkyl sulfates are reaction intermediates found in

all sulfuric acid alkylation units, regardless of the technology. If the alkyl sulfates are not

removed, they can cause corrosion and fouling in downstream equipment.

STRATCO’s net effluent treating section design has been modified over the years in an

effort to provide more effective, lower-cost treatment of the net effluent stream.

STRATCO’s older designs included caustic and water washes in series. Until recently,

STRATCO’s standard design included an acid wash with an electrostatic precipitator fol￾lowed by an alkaline water wash. Now STRATCO alkylation units are designed with an

acid wash coalescer, alkaline water wash, and a water wash coalescer in series (Fig. 1.2.7)

or with an acid wash coalescer followed by bauxite treating. Although all these treatment

methods remove the trace amounts of free acid and reaction intermediates (alkyl sulfates)

from the net effluent stream, the acid wash coalescer/alkaline water wash/water wash coa￾lescer design and acid wash coalescer/bauxite treater design are the most efficient.

Fractionation Section

The fractionation section configuration of grassroots alkylation units, either effluent refrig￾erated or autorefrigerated, is determined by feed composition to the unit and product spec￾ifications. As mentioned previously, the alkylation reactions are enhanced by an excess

1.16 ALKYLATION AND POLYMERIZATION

FIGURE 1.2.5 Refrigeration with economizer.

STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS

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