0071455914_ar004

CHAPTER 1.4UOP HF ALKYLATION

TECHNOLOGY

Kurt A. Detrick, James F. Himes, Jill M. Meister, and Franz-Marcus Nowak

UOPDes Plaines, Ilinois

INTRODUCTION

The UOP* HF Alkylation process for motor fuel production catalytically combines lightolefins, which are usually mixtures of propylene and butylenes, with isobutane to producea branched-chain paraffinic fuel. The alkylation reaction takes place in the presence ofhydrofluoric (HF) acid under conditions selected to maximize alkylate yield and quality.The alkylate product possesses excellent antiknock properties and high-octane because ofits high content of highly branched paraffins. Alkylate is a clean-burning, low-sulfur, low-RVP gasoline blending component that does not contain olefinic or aromatic compounds.

The HF Alkylation process was developed in the UOP laboratories during the late1930s and early 1940s. The process was initially used for the production of high-octaneaviation fuels from butylenes and isobutane. In the mid-1950s, the development and con-sumer acceptance of more-sophisticated high-performance automotive engines placed aburden on the petroleum refiner both to increase gasoline production and to improve motorfuel quality. The advent of catalytic reforming techniques, such as the UOP Platforming*process, provided an important tool for the production of high-quality gasolines availableto refiners. However, the motor fuel produced in such operations is primarily aromatic-based and is characterized by high sensitivity (that is, the spread between research andmotor octane numbers). Because automobile performance is more closely related to roadoctane rating (approximately the average of research and motor octanes), the productionof gasoline components with low sensitivity was required. A natural consequence of theserequirements was the expansion of alkylation operations. Refiners began to broaden therange of olefin feeds to both existing and new alkylation units to include propylene andoccasionally amylenes as well as butylenes. By the early 1960s, the HF Alkylation processhad virtually displaced motor fuel polymerization units for new installations, and refinershad begun to gradually phase out the operation of existing polymerization plants.

The importance of the HF Alkylation process in the refining situation of the 2000s hasbeen increased even further by the scheduled phase-out of MTBE and the increased

1.33

*Trademark and/or service mark of UOP.

Source: HANDBOOK OF PETROLEUM REFINING PROCESSES

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emphasis on low-sulfur gasoline. The contribution of the alkylation process is critical inthe production of quality motor fuels including many of the “environmental” gasolineblends. The process provides refiners with a tool of unmatched economy and efficiency,one that will assist refiners in maintaining or strengthening their position in the productionand marketing of gasolines.

PROCESS CHEMISTRY

General

In the HF Alkylation process, HF acid is the catalyst that promotes the isoparaffin-olefinreaction. In this process, only isoparaffins with tertiary carbon atoms, such as isobutane orisopentane, react with the olefins. In practice, only isobutane is used because isopentanehas a high octane number and a vapor pressure that has historically allowed it to be blend-ed directly into finished gasolines. However, where environmental regulations havereduced the allowable vapor pressure of gasoline, isopentane is being removed from gaso-line, and refiner interest in alkylating this material with light olefins, particularly propy-lene, is growing.

The actual reactions taking place in the alkylation reactor are many and are relativelycomplex. The equations in Fig. 1.4.1 illustrate the primary reaction products that may beexpected for several pure olefins.

In practice, the primary product from a single olefin constitutes only a percentage ofthe alkylate because of the variety of concurrent reactions that are possible in the alkyla-tion environment. Compositions of pilot-plant products produced at conditions to maxi-mize octane from pure-olefin feedstocks are shown in Table 1.4.1.

Reaction Mechanism

Alkylation is one of the classic examples of a reaction or reactions proceeding via the car-benium ion mechanism. These reactions include an initiation step and a propagation stepand may include an isomerization step. In addition, polymerization and cracking steps maybe involved. However, these side reactions are generally undesirable. Examples of thesereactions are given in Fig. 1.4.2.

Initiation. The initiation step (Fig. 1.4.2a) generates the tertiary butyl cations thatwill subsequently carry on the alkylation reaction.

Propagation. Propagation reactions (Fig. 1.4.2b) involve the tertiary butyl cationreacting with an olefin to form a larger carbenium ion, which then abstracts a hydridefrom an isobutane molecule. The hydride abstraction generates the isoparaffin plus anew tertiary butyl cation to carry on the reaction chain.

Isomerization. Isomerization [Eq. (1.4.12), shown in Fig. 1.4.2c] is very important inproducing good octane quality from a feed that is high in 1-butene. The isomerizationof 1-butene is favored by thermodynamic equilibrium. Allowing 1-butene to isomerizeto 2-butene reduces the production of dimethylhexanes (research octane number of 55to 76) and increases the production of trimethylpentanes. Many recent HF Alkylationunits, especially those processing only butylenes, have upstream olefin isomerizationunits that isomerize the 1-butene to 2-butene.

1.34 ALKYLATION AND POLYMERATION

UOP HF ALKYLATION TECHNOLOGY



UOP HF ALKYLATION TECHNOLOGY 1.35

Equation (1.4.13) is an example of the many possible steps involved in the isomeriza-tion of the larger carbenium ions.

Other Reactions. The polymerization reaction [Eq. (1.4.14), shown in Fig. 1.4.2d]results in the production of heavier paraffins, which are undesirable because theyreduce alkylate octane and increase alkylate endpoint. Minimization of this reaction isachieved by proper choice of reaction conditions.

The larger polymer cations are susceptible to cracking or disproportionation reactions[Eq. (1.4.15)], which form fragments of various molecular weights. These fragments canthen undergo further alkylation.

Isobutylene

CH3-C = CH2+CH3-CH-CH3 CH3-C-CH2-CH-CH3

CH3 CH3 CH3

CH3 CH3

Isobutane

Isobutane

Isobutane

Isobutane

(Isooctane)2,2,4-Trimethylpentane

(1.4.1)

CH3-CH = CH2 + CH3-CH-CH3

CH3

CH3-CH-CH-CH2-CH3

CH3CH3

Propylene 2,3-Dimethylpentane

(1.4.4)

2-Butene

CH3-CH = CH-CH3 + CH3-CH-CH3 CH3- C-CH2-CH-CH3

CH3 CH3

CH3

2,2,4-Trimethylpentane

CH3

or CH3-CH-CH-CH-CH3

CH3CH3CH3

2,3,4-Trimethylpentane

(1.4.3)

CH2 = CH-CH2-CH3 + CH3-CH-CH3

CH3

CH3-CH-CH-CH2-CH2-CH3

CH3CH3

1-Butene 2,3-Dimethylpentane

(1.4.2)

FIGURE 1.4.1 HF alkylation primary reactions for monoolefins.





TABLE 1.4.1 Compositions of Alkylate from Pure-Olefin

Feedstocks

Olefin

Component, wt % C3H6 iC4H8 C4H8-2 C4H8-1

C5 isopentane 1.0 0.5 0.3 1.0C6s:

Dimethylbutanes 0.3 0.8 0.7 0.8Methylpentanes — 0.2 0.2 0.3

C7s:2,3-Dimethylpentane 29.5 2.0 1.5 1.22,4-Dimethylpentane 14.3 — — —Methylhexanes — — — —

C8s:2,2,4-Trimethylpentane 36.3 66.2 48.6 38.52,2,3-Trimethylpentane — — 1.9 0.92,3,4-Trimethylpentane 7.5 12.8 22.2 19.12,3,3-Trimethylpentane 4 7.1 12.9 9.7Dimethylhexanes 3.2 3.4 6.9 22.1

C9� products 3.7 5.3 4.1 5.7

C-C = C + HF

C

(1.4.5)

(1.4.8)

(1.4.7)

(1.4.6)

C-C-C C-C-C

C C

+

C-C = C-C + HF C-C-C-C

F

F

C-C-C-C

+ +

+

+

+

+

iC4

iC4

iC4

C-C-C-C + C-C-C

C

C = C-C-C + HF C-C-C-C

F

C-C-C-C C-C-C-C + C-C-C

C

C = C-C + HF C-C-C

F

C-C-C C-C-C + C-C-C

C

FIGURE 1.4.2a HF alkylation reaction mechanism—initiation reactions.





C=C-C-C + C-C-C =

C

(1.4.9)

(1.4.11)

(1.4.10)

C-C-C-C-C-C + C-C-C

C C

+

Dimethylhexane

C + iC4

iC4

iC4

+

C-C = C-C + C-C-C

C

C-C-C-C-C + C-C-C

C C

+Trimethylpentane

C

+

+C

C-C = C + C-C-C

C

C-C-C-C-C + C-C-C

C C

+Trimethylpentane

C

+

+C

C

FIGURE 1.4.2b HF alkylation reaction mechanism—propagation reactions.

C=C-C-C (1.4.12)

(1.4.13)

2, 2, 4 -TrimethylpentaneiC4

iC4

iC4

1-Butene

C-C = C-C

2-Butene

C-C-C-C-C

C

C

+

C

C-C-C-C-C

C

C

+

C

C-C-C-C-C

C

C

+

C

2, 3, 4 -TrimethylpentaneC-C-C-C-C

C

C + C

C-C-C-C-C

C

+

CC

C-C-C-C-C

C

+

CC

C-C-C-C-C

C

+

C

C

2, 3, 3 -Trimethylpentane

FIGURE 1.4.2c HF alkylation reaction mechanism—isomerization.




Hydrogen Transfer. The hydrogen transfer reaction is most pronounced withpropylene feed. The reaction also proceeds via the carbenium ion mechanism. In thefirst reaction [Eq. (1.4.16)], propylene reacts with isobutane to produce butylene andpropane. The butylene is then alkylated with isobutane [Eq. (1.4.17)] to formtrimethylpentane. The overall reaction is given in Eq. (1.4.18). From the viewpoint ofoctane, this reaction can be desirable because trimethylpentane has substantiallyhigher octane than the dimethylpentane normally formed from propylene. However,two molecules of isobutane are required for each molecule of alkylate, and so thisreaction may be undesirable from an economic viewpoint.

PROCESS DESCRIPTION

The alkylation of olefins with isobutane is complex because it is characterized by simpleaddition as well as by numerous side reactions. Primary reaction products are the isomer-


(1.4.14)

(1.4.15)

+

C-C-C-C-C + C-C = C-C C12+

C

C=C-C + C-C-C

C3H6 + 2iC4H10

Trimethylpentane

C3H8 + Trimethylpentane

C C C16+ etc.

Polymerization

Cracking-Disproportionation

C12+ C5

+ + C7+

Hydrogen Transfer

C-C-C + C-C=C

CC

(1.4.16)

(1.4.17)C-C=C + C-C-C

CC

Overall Reaction: (1.4.18)

FIGURE 1.4.2d HF alkylation reaction mechanism—other.




ic paraffins containing carbon atoms that are the sum of isobutane and the correspondingolefin. However, secondary reactions such as hydrogen transfer, polymerization, isomer-ization, and destructive alkylation also occur, resulting in the formation of secondary prod-ucts both lighter and heavier than the primary products.

The factors that promote the primary and secondary reaction mechanisms differ, asdoes the response of each to changes in operating conditions or design options. Not all sec-ondary reactions are undesirable; for example, they make possible the formation of isooc-tane from propylene or amylenes. In an ideally designed and operated system, primaryreactions should predominate, but not to the complete exclusion of secondary ones. For theHF Alkylation process, the optimum combinations of plant economy, product yield, andquality are achieved with the reaction system operating at cooling-water temperature andan excess of isoparaffin and with contaminant-free feedstocks and vigorous, intimate acid-hydrocarbon contact.

To minimize acid consumption and ensure good alkylate quality, the feeds to the alky-lation unit should be dry and of low sulfur content. Normally, a simple desiccant-dryingsystem is included in the unit design package. Feed treating in a UOP Merox* unit for mer-captan sulfur removal can be an economic adjunct to the alkylation unit for those applica-tions in which the olefinic feed is derived from catalytic cracking or from other operationsin which feedstocks of significant sulfur content are processed. Simplified flow schemesfor a typical C4 HF Alkylation unit and a C3-C4 HF Alkylation unit are shown in Figs. 1.4.3and 1.4.4.

Treated and dried olefinic feed is charged along with recycle and makeup isobutane(when applicable) to the reactor section of the plant. The combined feed enters the shell ofa reactor–heat exchanger through several nozzles positioned to maintain an even tempera-ture throughout the reactor. The heat of reaction is removed by heat exchange with a largevolume of coolant flowing through the tubes having a low temperature rise. If coolingwater is used, it is then available for further use elsewhere in the unit. The effluent fromthe reactor enters the settler, and the settled acid is returned to the reactor.

The hydrocarbon phase, which contains dissolved HF acid, flows from the settler andis preheated and charged to the isostripper. Saturate field butane feed (when applicable) isalso charged to the isostripper. Product alkylate is recovered from the bottom of the col-umn. Any normal butane that may have entered the unit is withdrawn as a sidecut.Unreacted isobutane is also recovered as a sidecut and recycled to the reactor.

The isostripper overhead consists mainly of isobutane, propane, and HF acid. A dragstream of overhead material is charged to the HF stripper to strip the acid. The overheadfrom the HF stripper is returned to the isostripper overhead system to recover acid andisobutane. A portion of the HF stripper bottoms is used as flushing material. A net bottomstream is withdrawn, defluorinated, and charged to the gas concentration section (C3-C4splitter) to prevent a buildup of propane in the HF Alkylation unit.

An internal depropanizer is required in an HF Alkylation unit processing C3-C4 olefinsand may be required with C4 olefin feedstocks if the quantity of propane entering the unitis too high to be rejected economically as previously described. The isostripper overheaddrag stream is charged to the internal depropanizer. Overhead from the internal depropaniz-er is directed to the HF stripper to strip HF acid from the high-purity propane. A portion ofthe internal depropanizer bottoms is used as flushing material, and the remainder is returnedto the alkylation reactor. The HF stripper overhead vapors are returned to the internaldepropanizer overhead system. High-purity propane is drawn off the bottom of the HF strip-per, passes through a defluorination step, and is then sent to storage.

A small slipstream of circulating HF acid is regenerated internally to maintain acidpurity at the desired level. This technique significantly reduces overall chemical con-






FIG

UR

E 1

.4.3

UO

P C

4H

F A

lkyl

atio

n pr

oces

s.

1.40




1.41

FIG

UR

E 1

.4.4

UO

P C

3-C

4H

F A

lkyl

atio

n pr

oces

s.




sumption. An acid regenerator column is also provided for start-ups after turnarounds orin the event of a unit upset or feed contamination.

When the propane or normal butane from the HF unit is to be used as liquefied petro-leum gas (LPG), defluorination is recommended because of the possible breakdown ofcombined fluorides during combustion and the resultant potential corrosion of burners.Defluorination is also required when the butane is to be directed to an isomerization unit.After defluorination, the propane and butane products are treated with potassium hydrox-ide (KOH) to remove any free HF acid that might break through in the event of unit misop-eration.

The alkylation unit is built almost entirely of carbon steel although some Monel is usedfor most moving parts and in a few other limited locations. Auxiliary neutralizing andscrubbing equipment is included in the plant design to ensure that all materials leaving theunit during both normal and emergency operations are acid-free.

ENGINEERING DESIGN

The reactor and distillation systems that UOP uses have evolved through many years ofpilot-plant evaluation, engineering development, and commercial operation. The overallplant design has progressed through a number of variations, resulting in the present con-cepts in alkylation technology.

Reactor Section

In the design of the reactor, the following factors require particular attention:

● Removal of heat of reaction● Generation of acid surface: mixing and acid/hydrocarbon ratio● Acid composition● Introduction of olefin feed

The proper control of these factors enhances the quality and yield of the alkylate product.Selecting a particular reaction system configuration requires careful consideration of

the refiner’s production objectives and economics. The UOP reaction system optimizesprocessing conditions by the introduction of olefin feed through special distributors to pro-vide the desired contact with the continuous-acid phase. Undesirable reactions are mini-mized by the continuous removal of the heat of reaction in the reaction zone itself. Theremoval of heat in the reaction zone is advantageous because peak reaction temperaturesare reduced and effective use is made of the available cooling-water supply.

Acid Regeneration Section

The internal acid regeneration technique has virtually eliminated the need for an acidregenerator and, as a result, acid consumption has been greatly reduced. The acid regener-ator has been retained in the UOP design only for start-ups or during periods when the feedhas abnormally high levels of contaminants, such as sulfur and water. For most units, dur-ing normal operation, the acid regenerator is not in service.

When the acid regenerator is in service, a drag stream off the acid circulation line at thesettler is charged to the acid regenerator, which is refluxed on the top tray with isobutane.





The source of heat to the bottom of the regenerator for a C3-C4 HF Alkylation unit is super-heated isobutane from the depropanizer sidecut vapors. For a C4 HF Alkylation unit, thestripping medium to the acid regenerator is sidecut vapors from the HF stripper bottoms.The regenerated HF acid is combined with the overhead vapor from the isostripper andsent to the cooler.

Neutralization Section

UOP has designed the neutralization section to minimize the amount of additional efflu-ents such as offensive materials and undesirable by-products. Releasing acid-containingvapors to the regular relief-gas system is impractical because of corrosion and odor prob-lems as well as other environmental and safety concerns. The system is composed of therelief-gas scrubber, KOH mix tank, circulating pumps, and a KOH regeneration tank.

All acid vents and relief valves are piped to this relief section. Gases pass up throughthe scrubber and are contacted by a circulating KOH solution to neutralize the HF acid.After the neutralization of the acid, the gases can be safely released into the refinery flaresystem.

The KOH is regenerated on a periodic basis in the KOH regeneration tank by usinglime to form calcium fluoride (CaF2) and KOH. The CaF2 settles to the bottom of the tankand is directed to the neutralizing basin, where acidic water from acid sewers and smallamounts of acid from the process drains are treated. Lime is used to convert any fluoridesinto calcium fluoride before any waste effluent is released into the refinery sewer system.

Distillation System

The distillation and recovery sections of HF Alkylation units have also seen considerableevolution. The modern isostripper recovers relatively high-purity isobutane as a sidecutthat is recycled to the reactor. This recycle is virtually acid-free, thereby minimizing unde-sirable side reactions with the olefin feed prior to entry into the reactor. A small rectifica-tion section on top of the modern isostripper provides for more efficient propane rejection.

Although a single high-pressure tower can perform the combined functions of isostrip-per and depropanizer, UOP’s current design incorporates two towers (isostripper anddepropanizer) for the following reasons:

● Each tower may be operated at its optimum pressure. Specifically, in the isostripper forthis two-tower design, the relative volatilities between products increase, and the num-ber of trays required for a given operation are reduced in addition to improving separa-tion between cuts.

● This system has considerably greater flexibility. It is easily convertible to a butylene-only operation because the depropanizer may be used as a feed splitter to separate C3sand C4s. The two-tower design permits the use of side feeds to the isostripper column,should it be necessary to charge makeup isobutane of low purity. This design also per-mits the production of lower-vapor-pressure alkylate and a high-purity sidecut nC4 forisomerizing or blending and the ability to make a clean split of side products.

● The two-tower design permits considerable expanded capacity at low incremental costby the addition of feed preheat and side reboiling.

● Alkylate octane increases with decreasing reaction temperature. During cooler weather,the unit may be operated at lower isobutane/olefin ratios for a given product octane,because the ratio is fixed by the product requirement and not by the fractionationrequirements. The commensurate reduction in utilities lowers operating costs.





● Because of the low isostripper pressure in a two-tower system, this arrangement permitsthe use of steam for reboiling the isostripper column instead of a direct-fired heater,which is necessary in a single-tower system. In most cases, a stab-in reboiler system issuitable even for withdrawing a sidecut. Using a steam reboiler can be a considerableadvantage when refinery utility balances so indicate, and it also represents considerableinvestment-cost savings.

● The two-tower system has proven its performance in a large number of operating units,and its flexibility has been proven through numerous revamps for increased capacity onexisting units.

● The two-tower system also requires less overhead condenser surface, which lowers theinvestment required for heat exchange.

● Clean isobutane is available for flush, whereas only alkylate flush is available in the sin-gle-column operation. This clean-isobutane stream is also available to be taken to stor-age and is a time saver during start-ups and shutdowns.

● Although fewer pieces of equipment are required with the single tower, the large num-ber of trays and the high-pressure design necessitate the use of more tons of material andresult in a somewhat higher overall cost than does the two-tower system.

● The regenerator column contains no expensive overhead system, and the internal HFregeneration technique results in improved acid consumption.

● Because a high-temperature differential can be taken on most cooling water, cooling-water requirements for the two-tower system are only about two-thirds those of the sin-gle-tower system.

COMMERCIAL INFORMATION

Typical commercial yields and product properties for charging various olefin feedstocks toan HF Alkylation unit are shown in Tables 1.4.2 and 1.4.3. Table 1.4.4 contains the detailedbreakdown of the investment and production costs for a pumped, settled acid-alkylationunit based on a typical C4 olefin feedstock.

ENVIRONMENTAL CONSIDERATIONS

The purpose of operating an HF Alkylation unit is to obtain a high-octane motor fuelblending component by reacting isobutane with olefins in the presence of HF acid. In theUOP HF Alkylation process, engineering and design standards have been developed andimproved over many years to obtain a process that operates efficiently and economically.This continual process development constitutes the major reason for the excellent productqualities, low acid-catalyst consumption, and minimal extraneous by-products obtained bythe UOP HF Alkylation process.


TABLE 1.4.2 HF Alkylation Yields

Olefin Required vol. Vol. alkylatefeedstocks iC4/vol. olefin produced/vol. olefin

C3-C4 1.28 1.78Mixed C4 1.15 1.77




As in every process, certain minor process inefficiencies, times of misoperation, andperiods of unit upsets occur. During these times, certain undesirable materials can be dis-charged from the unit. These materials can be pollutants if steps are not taken in theprocess effluent management and product-treating areas to render these by-product mate-rials harmless.

In a properly operated HF Alkylation unit, the amount of additional effluent, such asoffensive materials or undesirable by-products is minimal, and with proper care, thesesmall streams can be managed safely and adequately. The potentially offensive nature ofthe streams produced in this process as well as the inherent hazards of HF acid has result-ed in the development of effluent management and safety procedures that are unique to theUOP HF Alkylation process. The following sections briefly describe these procedures andhow these streams are safely handled to prevent environmental contamination. The refin-er must evaluate and comply with any pertinent effluent management regulations. An over-all view of the effluent management concept is depicted in Fig. 1.4.5.

Effluent Neutralization

In the Alkylation unit’s effluent-treating systems, any neutralized HF acid must eventual-ly leave the system as an alkali metal fluoride. Because of its extremely low solubility in


TABLE 1.4.3 HF Alkylation Product Properties

Propylene-Property butylene feed Butylene feed

Specific gravity 0.693 0.697Distillation temperature, °C (°F):

IBP 41 (105) 41 (105)10% 71 (160) 76 (169)30% 93 (200) 100 (212)50% 99 (210) 104 (220)70% 104 (219) 107 (225)90% 122 (250) 125 (255)EP 192 (378) 196 (385)

Octanes:RONC 93.3 95.5MONC 91.7 93.5

Note: IBP � initial boiling point; EP � endpoint; RONC � researchoctane number, clear; MONC � motor octane number, clear.

TABLE 1.4.4 Investment and Production Cost Summary*

Operating cost $/stream day $/MT alkylate $/bbl alkylate

Labor 1,587 0.016 0.176Utilities 6,609 0.066 0.734Chemical consumption, laboratory 5,639 0.056 0.627allowance, maintenance, taxes,and insurance

Total direct operating costs 13,835 0.138 1.537Investment, estimated erected cost (EEC), first quarter 2002 $27,800,000

*Basis: 348,120 MTA (9000 BPSD) C5 � alkylate.Note: MT � metric tons; MTA � metric tons per annum; BPSD � barrels per stream-day.




water, CaF2 is the desired end product. The effluent containing HF acid can be treated witha lime [CaO-Ca(OH)2] solution or slurry, or it can be neutralized indirectly in a KOH sys-tem to produce the desired CaF2 product.

The KOH neutralization system currently used in a UOP-designed unit involves atwo-stage process. As HF acid is neutralized by aqueous KOH, soluble potassium fluo-ride (KF) is produced, and the KOH is gradually depleted. Periodically, some of the KF-containing neutralizing solution is withdrawn to the KOH regenerator. In this vessel, KFreacts with a lime slurry to produce insoluble CaF2 and thereby regenerates KF to KOH.


FIGURE 1.4.5 UOP HF Alkylation process effluent management.




The regenerated KOH is then returned to the system, and the solid CaF2 is routed to theneutralizing basin.

Effluent Gases. The HF Alkylation unit uses two separate gas vent lines to maintainthe separation of acidic gases from nonacidic gases until the acidic gases can bescrubbed free of acid.

Acidic Hydrocarbon Gases. Acidic hydrocarbon gases originate from sections ofthe unit where HF acid is present. These gases may evolve during a unit upset, duringa shutdown, or during a maintenance period in which these acidic gases are partiallyor totally removed from the process vessels or equipment. The gases from the acidvents and from the acid pressure relief valves are piped to a separate closed reliefsystem for the neutralization of the acid contained in the gas. The acid-free gases arethen routed from this acid-scrubbing section to the refinery nonacid flare system,where they are disposed of properly by burning.

The acidic gases are scrubbed in the acid neutralization and caustic regeneration system,as shown Fig. 1.4.6. This system consists of the relief-gas scrubber, KOH mix tank, liquid-knockout drum, neutralization drum, circulating pumps, and a KOH regeneration tank.

Acidic gases, which were either vented or released, first flow to a liquid-knockout drumto remove any entrained liquid. The liquid from this drum is pumped to the neutralizationdrum. The acidic gases from the liquid-knockout drum then pass from the drum to thescrubbing section of the relief-gas scrubber, where countercurrent contact with a KOHsolution removes the HF acid. After neutralization of the HF acid, the nonacidic gases arereleased into the refinery flare system.

The KOH used for the acidic-gas neutralization is recirculated by the circulationpumps. The KOH solution is pumped to the top of the scrubber and flows downward tocontact the rising acidic gas stream and then overflows a liquid-seal pan to the reservoirsection of the scrubber. In addition, a slipstream of the circulating KOH contacts the acidicgas just prior to its entry to the scrubber. The circulating KOH removes HF through thefollowing reaction:

HF � KOH → KF � H2O (1.4.19)

Maintaining the circulating caustic pH and the correct percentage of KOH and KFrequires a system to regenerate the caustic. This regeneration of the KOH solution is per-formed on a batch basis in a vessel separate from the relief-gas scrubber. In this regenera-tion tank, lime and the spent KOH solution are thoroughly mixed. The regenerated causticsolution is pumped back to the scrubber. The CaF2 and any unreacted lime are permittedto settle out and are then directed to the neutralization pit. The regeneration of the spentKOH solution follows the Berthollet rule, by which the insolubility of CaF2 in water per-mits the complete regeneration of the potassium hydroxide according to the followingequation:

2KF � Ca (OH) 2 → 2KOH � CaF2 (1.4.20)

Nonacidic Hydrocarbon Gases. Nonacidic gases originate from sections of theunit in which HF acid is not present. These nonacidic gases from process vents andrelief valves are discharged into the refinery nonacid flare system, where they aredisposed of by burning. The material that is vented or released to the flare is mainlyhydrocarbon in nature. Possibly, small quantities of inert gases are also included.

Obnoxious Fumes and Odors. The only area from which these potentiallyobjectionable fumes could originate is the unit’s neutralizing basins. To prevent thedischarge of these odorous gases to the surroundings, the neutralizing basins aretightly covered and equipped with a gas scrubber to remove any offensive odors. The





FIG

UR

E 1

.4.6

Aci

d ne

utra

liza

tion

and

cau

stic

reg

ener

atio

n se

ctio

n.

1.48




gas scrubber uses either water or activated charcoal as the scrubbing agent. However,in the aforementioned neutralizing system, odors from the basin are essentiallynonexistent because the main source of these odors (acid regenerator bottoms) ishandled in separate closed vessels.

Liquid Effluents. The HF Alkylation unit is equipped with two separate sewersystems to ensure the segregation of the nonacid from the possibly acid-containingwater streams.

Acidic Waters. Any potential HF containing water streams (rainwater runoff in theacid area and wash water), heavy hydrocarbons, and possibly spent neutralizing mediaare directed through the acid sewer system to the neutralizing basins for theneutralization of any acidic material. In the basins, lime is used to convert theincoming soluble fluorides to CaF2.

The neutralizing basins consist of two separate chambers (Fig. 1.4.7). One chamber isfilled while the other drains. In this parallel neutralizing basin design, one basin has theinlet line open and the outlet line closed. As only a few surface drains are directed to theneutralizing basins, inlet flow normally is small, or nonexistent, except when acid equip-ment is being drained. The operator regularly checks the pH and, if necessary, mixes thelime slurry in the bottom of the basin.

After the first basin is full, the inlet line is closed, and the inlet to the second basin isopened; then lime is added to the second basin. The first basin is mixed and checked withpH paper after a period of agitation; if it is acidic, more lime is added from lime storageuntil the basin is again basic. After settling, the effluent from the first basin is drained.

Nonacidic Waters. The nonacid sewers are directed to the refinery water disposalsystem or to the API separators.


FIGURE 1.4.7 Neutralizing basin.




Liquid Process Effluents (Hydrocarbon and Acid). Hydrocarbon and acideffluents originate from some minor undesirable process side reactions and from anyfeed contaminants that are introduced to the unit. Undesirable by-products formed inthis manner are ultimately rejected from the Alkylation unit in the acid regenerationcolumn as a bottoms stream.

The regeneration-column bottoms stream consists mainly of two types of mixtures.One is an acid-water phase that is produced when water enters the unit with the feedstreams. The other mixture is a small amount of polymeric material that is formed duringcertain undesirable process side reactions. Figure 1.4.8 represents the HF acid regenera-tion circuit.

The first step in the disposal of these materials is to direct the regenerator bottoms tothe polymer surge drum, where the two mixtures separate. The acid-water mixture formsan azeotrope, or constant boiling mixture (CBM), which is directed to the neutralizingdrum (Fig. 1.4.8) for neutralization of the HF acid. The acid in this CBM ultimately endsup as insoluble CaF2 (as described previously). The polymer that remains in the polymersurge drum is then transferred to the tar neutralizer, where the HF acid is removed. Thepolymer has excellent fuel oil properties and can then be disposed of by burning as longas applicable regulations allow such. However, by the mid-1980s, technology and specialoperating techniques such as internal acid regeneration had virtually eliminated this liquid-effluent stream for many units.

Solid Effluents

Neutralization Basin Solids. The neutralization basin solids consist largely ofCaF2 and unreacted lime. As indicated previously, all HF-containing liquids that aredirected to the neutralizing basins ultimately have any contained soluble fluoridesconverted to insoluble CaF2. The disposal of this solid material is done on a batchbasis. A vacuum truck is normally used to remove the fluoride-lime sludge from the


FIGURE 1.4.8 HF acid regeneration circuit.




pit. This sludge has traditionally been disposed of in a landfill after analysis to ensureappropriate properties are met.

Another potential route for sludge disposal is to direct it to a steel manufacturing com-pany, where the CaF2 can be used as a neutral flux to lower the slag melting temperatureand to improve slag fluidity. The CaF2 may possibly be routed back to an HF acid manu-facturer, as the basic step in the HF-manufacturing process is the reaction of sulfuric acidwith fluorspar (CaF2) to produce hydrogen fluoride and calcium sulfate.

Product-Treating Solids. The product-treating solids originate when LPGproducts are defluorinated over activated alumina. Over time, the alumina loses theability to defluorinate the LPG product streams. At this time, the alumina isconsidered spent, and it is then replaced with fresh alumina. Spent alumina must bedisposed of in accordance with applicable regulations or sent to the alumina vendorfor recovery.

Miscellaneous Solids. Porous material such as wiping cloths, wood, pipecoverings, and packings that are suspected of coming into contact with HF acid areplaced in specially provided disposal cans for removal and are periodically burned.These solids may originate during normal unit operation or during a maintenanceperiod. Wood staging and other use of wood in the area are kept to a minimum. Metalstaging must be neutralized before being removed from the acid area.

MITIGATING HF RELEASES—THECHEVRONTEXACO AND UOP ALKAD PROCESS

Growing environmental and public safety concerns since the mid-1980s have heightenedawareness of hazards associated with many industrial chemicals, including HF acid.Refiners responded to these concerns with the installation of mitigation systems designedto minimize the consequences of accidental releases. ChevronTexaco and UOP developedthe Alkad* technology1 to assist in reducing the potential hazards of HF acid and to workin conjunction with other mitigation technology.

HF Acid Concerns and Mitigation

Although HF alkylation was clearly the market leader in motor fuel alkylation by the mid-1980s, growing concerns about public safety and the environment caused HF producersand users to reassess how HF acid was handled and how to respond to accidental releases.In 1986, Amoco and the Lawrence Livermore National Laboratory conducted atmospher-ic HF release tests at the Department of Energy Liquefied Gaseous Fuels Facility inNevada. These tests revealed that HF acid could form a cold, dense aerosol cloud that didnot rapidly dissipate and remained denser than air. In 1988, another set of tests, the Hawktests, was conducted to determine the effect of water sprays on an HF aerosol cloud. Thesetests indicated that a water/HF ratio of 40/1 by volume would reduce the airborne HF acidby about 90 percent.2 As a result of these investigations, many refiners have installed, orare planning to install, water spray systems in their HF alkylation units to respond to acci-dental releases.

Other mitigation technology installed by refiners includes acid inventory reduction, HFdetection systems, isolation valves, and rapid acid transfer systems. These mitigation sys-






tems can be described as external, defensive response systems because they depend on anexternal reaction (for example, spraying water) to a detected leak.

ChevronTexaco and UOP chose to develop a system that would respond prior to leakdetection. Such a system could be described as an internal, passive response systembecause it is immediately effective, should a leak occur. In 1991, ChevronTexaco and UOPbegan to work together to develop an additive system to reduce the risk associated with theHF alkylation process. The objective was to develop an additive that would immediatelysuppress the HF aerosol in the event of a leak but would not otherwise interfere with thenormal performance of the HF unit.

Aerosol Reduction

ChevronTexaco screened a large number of additive materials for aerosol reduction capa-bility in its R&D facilities in Port Arthur, Texas. The most promising materials that sig-nificantly reduced aerosol and maintained adequate alkylation activity were tested in alarge-scale release chamber in Oklahoma.3

Release tests with additive demonstrated the potential reduction of airborne HF acid atvarious additive concentrations. This reduction was determined on the basis of the weightof material collected relative to the weight of material released. The aerosol reductionachieved is described in Fig. 1.4.9. As shown, reductions of airborne HF acid of up to 80percent may be possible, depending on the additive concentration level at which a refineris able to operate. Employing the Alkad technology in conjunction with water sprays mayresult in more than 95 percent reduction of the airborne HF acid.

Process Development

ChevronTexaco and UOP conducted a trial with the most interesting additive material inthe older of two alkylation units at the former Texaco refinery in El Dorado, Kansas, in1992. During the trial, the alkylation unit operated well, with no changes as a result of thepresence of additive in the acid. Following this successful trial, UOP designed facilities torecover the acid-additive complex from the acid regenerator bottoms stream and recyclethis material to the reactor section.


FIGURE 1.4.9 Aerosol reduction results.




The recovery process has been further optimized following the operations from 1994through 1998. The addition of the recovery process to an HF alkylation unit or design gen-erally requires a new column, separator, and associated equipment. The HF acid regener-ator column is still used for the removal of water and light polymer from the process. Asimplified flow scheme is shown in Fig. 1.4.10.

A slipstream of circulating acid is sent to the additive stripper column. The additivestripper sends acid, water, and light acid-soluble oils overhead and on to the acid regener-ator. Heavy acid-soluble oils and the concentrated HF-additive complex are sent to theadditive stripper bottoms separator. From this separator the polymer is sent to neutraliza-tion, and the HF-additive complex is recycled to the reactor section. The acid regeneratorremoves water and light acid-soluble oils from the additive stripper overhead stream. Thewater is in the form of a constant boiling mixture of water and HF.

Commercial Experience

After construction of the modular additive recovery section was completed, Texaco beganoperating the Alkad technology in September 1994. The immediate observation when theadditive was introduced was an increase in product octane and a reduction in alkylate end-point. Research octane was 1.5 or more numbers higher than the baseline operation (Fig.1.4.11). A comparison of operations with and without additive is shown in Table 1.4.5,which breaks down two alkylate samples from equivalent operating conditions. An analy-sis of the alkylate components has shown that the increased octane is partially due to a sig-nificantly higher octane in the C9� material. Increased paraffin branching in the C7 andlighter fraction is also a contributor to the octane boost. As shown in Fig. 1.4.12, initialdata indicated that the alkylate 90 percent distillation point had decreased 14 to 19°C (25to 35°F) and the endpoint had dropped 17 to 22°C (30 to 40°F). As gasoline regulationschange, this distillation improvement may allow refiners to blend in more material fromother sources and still meet regulatory requirements in their areas and effectively increasegasoline pool volume. Texaco installed this additive-recovery system for approximately $7million U.S.


CirculatingAcid

AdditiveStripperColumn

Polymer toNeutralization

iC4 iC4

HF-Additive toReactor Section

Light Acid-Soluble Oilsand CBM to

Neutralization

AcidRegenerator

Column

HF to Reactor orFractionaction Section

FIGURE 1.4.10 UOP HF Additive Recovery Process.





FIGURE 1.4.11 Alkylate octane.

FIGURE 1.4.12 Alkylate distillation.

TABLE 1.4.5 Alkylate Composition Comparison

No additive With additive

Alkylate RONC (measured) 90.8 92.2Composition, LV %:

C6 2.84 3.58C7 14.15 19.06C8 45.24 44.35C9� 17.49 16.28

Calculated C9 � RONC 81.6 89.5Dimethylbutane/methylpentane 1.7 2.5Dimethylpentane/methylhexane 51.0 77.1

Note: LV % � liquid volume percent.




The Alkad process significantly reduces the hazards associated with an accidentalrelease of HF acid and minimizes the refiner’s further investment in motor fuel alkylationmitigation technology.

REFERENCES

1. J. C. Sheckler and H. U. Hammershaimb, “UOP Alkylation Technology into the 21st Century,” pre-sented at the 1995 UOP Refining Technology Conferences.

2. K. W. Schatz and R. P. Koopman, “Effectiveness of Water Spray Mitigation Systems for AccidentalReleases of Hydrogen Fluoride,” summary report and volumes I–X, NTIS, Springfield, Va., 1989.

3. K. R. Comey, III, L. K. Gilmer, G. P. Partridge, and D. W. Johnson, “Aerosol Reduction fromEpisodic Releases of Anhydrous HF Acid by Modifying the Acid Catalyst with Liquid OniumPoly (Hydrogen Fluorides),” AIChE 1993 Summer National Meeting, Aug. 16, 1993.





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