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

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362 CHAPTER 9

The isostripper overhead vapor is a propane-enriched isobutane stream and HF which

is condensed and separated into settling drum. The HF phase is pumped back to the

reactor section. The HF saturated hydrocarbon phase is charged to the depropanizer.

The depropanizer and its associated HF stripper remove propane from the isobutane

recycle. The depropanizer bottoms is returned to the reactors as part of the recy-

cle isobutane. The depropanizer overhead containing the propane product and HF

are condensed and separated in the overhead receiver. The acid phase is returned to

the reactor section and the acid-saturated propane is stripped free of acid in the HF

Stripper column. The HF stripper bottoms is an acid-free propane product which is

treated with hot alumina to remove organic ﬂuorides, cooled and treated with KOH

pellets to remove traces of HF and water.

Acid regeneration

A key advantage of HF alkylation over sulfuric is the ability to recover the acid from

the byproduct polymer, water, and other contaminants. In the UOP HF Alkylation

Process, a small stream of circulating acid is stripped with superheated isobutane in

a small monel tower called the Acid Regenerator. The regenerator overhead is HF

and isobutane that are recycled to the reactor; the regenerator bottoms is polymer and

the HF–water azeotrope which are neutralized with aqueous KOH. The neutralized

polymer has good fuel value. The amount of polymer produced is generally only 1 to

2 barrels per 1,000 barrels of alkylate product.

KOH regeneration

The UOP process minimizes chemical costs by regenerating the KOH used to treat

products and all waste and storm water. This KOH regeneration is accomplished using

lime. As the lime is mixed into the KOH, regeneration of the KOH takes place by the

following reaction:

2KF + Ca(OH)

→ 2KOH + CaF

The calcium ﬂuoride forms a precipitate and can be easily separated from the regen-

erated KOH.

Process variables in HF alkylation

Key variables are reactor temperature, isobutane to oleﬁn molar ratio, acid strength,

and acid to hydrocarbon volume ratio.

Reaction temperature is one of the more important process variables as it has a

signiﬁcant inﬂuence on the octane number of the product. Almost all HF alkylation

reactors are operated below 100

◦

F. At higher temperatures a decrease in alkylate

octane number will occur. Above 120

◦

F polymerization and cracking side reactions

become excessive reducing alkylate quality. In many cases, acid regeneration capacity

of the HF alkylation unit would not be able to maintain proper control of the acid

strength at temperatures above 110

◦

F. Extremely low reaction temperatures may cause

incomplete alkylation. Thus reaction temperatures below 80

◦

F are typically not used.

GASOLINE COMPONENTS 363

Isobutane to oleﬁn molar ratio is generally the most important variable that the reﬁner

has the most control over within limitations of isostripper fractionation loadings. As

the isobutane to oleﬁn molar ratio is increased, octane increases; thus the ﬂow of

isobutane recycle is usually kept at a practical maximum at all times, up to the capac-

ity of the isostripper. The reasons to reduce the recycle ratio are for the conservation

of fractionation energy or for a reduction in the hydride transfer reaction. The higher

energy consumption, the greater consumption of isobutane (due to the hydride trans-

fer) and the increased production of propane (due to the hydride transfer) must be

justiﬁed economically against higher product quality. Another practical limitation of

isobutane circulation is possible entrainment of acid from the reactor section acid

settler to the isostripper because of inadequate settling time.

Acid strength is usually kept between 85 and 95 mass% HF. Maintenance of this

strength level results from a balance between the performance of the unit feed treating

systems for sulfur and water removal and acid regeneration operation. In some cases

oxygenate removal systems or diene removal systems are also used on the feed where

there are known to be high oxygenates (such as downstream of an MTBE unit, or

high dioleﬁns (from severe FCC conditions). The action of the acid on reactions is a

complex phenomenon and is dependent on the type as well as the amount of diluents.

The fresh acid is supplied by acid manufacturers at 99.0

wt% HF. This purity is too

high for optimum performance of the HF alkylation process. As the water content

of the circulating acid increases, carbon steel that is not attacked by anhydrous HF,

becomes less resistant to acid attack.

The acid to hydrocarbon volume ratio used in the reactor of the UOP process is

generally around 1:1. At some point below 0.8–1, excess polymerization occurs. In

the most extreme cases, alkylate production could stop.

Two other variable related topics are reaction time and pressure. As the reaction time

decreases, the combined ﬂuorides leaving the reactor section will increase. However,

any reduction in time is limited by the settling capacity and there is generally little

effect within the permissable operating ranges of a particular unit. Excessive velocities

in the settler will cause free (above saturation level) acid to be carried over into the

isostripper. This carryover may result in corrosion of the upper trays of the isostripper

and discolored alkylate as heavier contaminants in the acid that is carried over drop

into the alkylate product. Pressure is not really a process variable as long as it is kept

high enough that all of the hydrocarbon and acid in the reactor section remain in the

liquid state.

HF feed contaminants

As with many reﬁning processes, the control of contaminants coming into the unit with

the normal feedstocks is critical to the long and dependable operation of the HF alky-

lation unit. Above the recommended maximum levels of feed stock contaminants, acid

consumption, acid regeneration requirements, and in some cases unit corrosion and

364 CHAPTER 9

product quality are all measurably affected. Generally all contaminants are kept as

low as possible within the capabilities of the feed treating systems. The major feed

contaminants normally found in alkylation feeds are water or oxygenates, sulfur com-

pounds, nitrogen compounds, non-condensibles, and dioleﬁn.

HF alkylation maintenance

Because HF is highly corrosive to most materials, careful control and maintenance of

equipment metallurgy and condition is required. Carbon steel is the primary material

used for vessels and piping and it can be used only because of a corrosion barrier layer

of iron ﬂuoride that forms on any carbon steel surface exposed to HF. The iron ﬂuoride

layer is tenacious and serves as a barrier against further carbon steel corrosion as

long as the deposit remains undisturbed. Under certain conditions, such as when wet

acid is in the unit, this iron ﬂuoride scale can soften and break off leading to fouling

and corrosion issues. In severe cases this can lead to considerable unscheduled feed

outages as well as clear safety issues. Most reﬁners aggressively monitor their equip-

ment’s remaining corrosion allowance and use regularly scheduled valve and ﬂange

replacement to head off any problems. For maintenance during an FCC turnaround,

the normal time most alkylation turnarounds are maintained, many reﬁners choose

to dissolve all the iron ﬂuoride scale by using a chemical cleaning company.

There are other areas within an alkylation unit where conditions are too severe for

carbon steel and monel is the primary metallurgy for such areas as monel has good

resistance even for high water content HF. Temperature is also an important variable

for corrosion rate of both carbon steel and at higher temperatures for monel. Another

use of monel is for moving parts where use of carbon steel would cause cementing of

parts together a iron ﬂuoride scale formed on each steel surface.

Sulfuric acid alkylation

Today there are two processes for H

alkylation the Cascade process licensed by

Exxon Mobil and MW Kellogg and the Stratco efﬂuent refrigerated process.

Figure 9.1.3 shows the cascade alkylation process.

Feed pretreatment

It usually consists of deethanizing and Merox-treating of FCC oleﬁns. Some reﬁners

have added selective hydrogenation units (SHP) to saturate dienes and reduce acid

consumption. Feeds are generally not dried.

Reaction

The FCC oleﬁns are chilled and coalesced to remove water and injected through

sparge rings to 3–6 agitated reaction zones in a large horizontal reactor/settler vessel.

Recycle isobutane from the deisobutanizer and the refrigeration system and recycle

acid from the settler are fed to a pre-ﬂash zone and “cascade” from one zone through

GASOLINE COMPONENTS 365

Propane

Refrigerant

~~ ~~ ~~ ~~

Settler

FCC

Olefins

Acid to

Reactors

Fresh

Acid

Alkaline

Water

Mixed

Butanes

Alkylate

n-Butane

Coalesers

Acid Recycle

Compressor

Recycle

i-Butane

Reactor

Figure 9.1.3. Cascade auto-refrigerated alkylation process.

specially designed weirs from which the process name derives. Typical isobutane

oleﬁn ratios are 8–12 for the process. The ﬁrst zone in the cascade reactor has the

lowest operating temperature and the highest isobutane concentration and produces

the highest octane alkylate. As additional oleﬁn is injected in subsequent zones the

temperature increases and isobutane concentration decreases and successively lower

octanes are produced. Because isobutane and H

are highly immiscible, each

zone requires a mixer with high power inputs to produce a tight emulsion. After

the ﬁnal reaction zone the emulsion is allowed to settle. The settler acid phase is

pumped back to the lead zone and the hydrocarbon phase efﬂuent is pumped to efﬂuent

treating.

Refrigeration

The heat of reaction is removed by “auto-refrigeration” at reaction temperatures of

35–65

◦

F. While refrigeration is often viewed as costly, in this process it conserves the

heat of reaction to distill 4–5 moles of isobutane recycle per mole of oleﬁn alkylated

and concentrates propane. Isobutane and propane vaporized from the reactor are

compressed, and condensed with cooling water and recycled as “refrigerant” to the

reactors. A fraction of the refrigerant is charged to the depropanizer to remove propane

contained in the feeds from the unit.

Efﬂuent treating

The hydrocarbon efﬂuent containing alkylate and excess isobutane is warmed by

chilling recycle isobutane and feed and treated to remove traces of entrained acid and

366 CHAPTER 9

ester reaction intermediates. Treating systems include washing with fresh acid and

aqueous caustic (as shown). Caustic and water washes, bauxite, and KOH pellets have

also been used.

Fractionation

After efﬂuent treating the balance of reactor isobutane requirement is distilled from

the Alkylate and n-butane deisobutanizer tower (DIB). Most reﬁners charge saturated

butanes from other units to the DIB for isobutane/n-butane splitting. N-butane is

distilled from the Alkylate for control of product RVP in a debutanizer, and in some

cases an n-butane vapor draw from the DIB. Finally, in a few units, aviation alkylate

is produced by removing heavy ends in a Rerun column.

Stratco effluent refrigerated alkylation process

In the Stratco process (Figure 9.1.4.), the principal differences from the Cascade

process are in the reactor and refrigeration design and that the reaction is carried out

without vaporization.

Reaction

Treated feeds and recycle isobutane are ﬁrst chilled and coalesced to remove water

and charged to several Stratco contactors. Feed and isobutane from the DIB and

n-Butane

Propane

Refrigerant

Flash

Effluent

Flash

Refrigerant

Settler

Acid Recycle

STRATCO Contactors

Effluent

Compressor

FCC

Olefins

Recycle

i-Butane

Coalescer

Acid to

Contactors

Acid

Wash

Alkaline

Water

Wash

Mixed

Butanes

Alkylate

Figure 9.1.4. Stratco efﬂuent refrigerated alkylation process.

GASOLINE COMPONENTS 367

refrigeration and recycle acid from the settler are emulsiﬁed together by the high

power impeller of the Stratco Contactor. After reaction and chilling the emulsion

passes to the settler located above the contactors for acid separation. The acid phase

is recycled by gravity to the contactor impeller. And the hydrocarbon phase (efﬂuent)

routed in the tube-side of the contactor heat exchanger.

Refrigeration

The heat of reaction is removed by chilling the emulsion in shell side of the contactor

heat exchanger by partially vaporizing settler efﬂuent on the tube side. Refrigerant

vapor is separated from the efﬂuent liquid in a ﬂash drum, compressed, and condensed.

A portion of the condensed refrigerant is routed to a depropanizer. The balance

of the refrigeration and depropanizer bottoms are ﬂash cooled and returned to the

reactors.

Efﬂuent treating and fractionation

These steps are essentially the same as in the Cascade process.

Process variable in H

alkylation

Temperature

Maximum octanes for butenes and pentenes are obtained at about 35–40

◦

F and for

propylene at about 50

◦

F. Typically, octane decreases at about 0.04–0.06 octanes per

◦

F at higher temperatures.

Isobutane concentration

Reaction selectivity and octane increase with isobutane concentration. Typical re-

sponses are about 0.1 octane per 1 vol% isobutane in efﬂuent.

Acid strength

Octane is maximized at about 93 wt% acidity. It declines at about 0.2–0.3 RON per

1% acidity decline below that. The economics of acid cost usually require lower ﬁnal

spending strengths (as low as 88 wt%), so multi-reactor alkylation units typically

feed fresh acid to several reactors in parallel operating at 93% then reuse the 93% as

makeup to other reactors in series.

Space velocity

Octane decreases with the charge rate of oleﬁn. Alkylation reactors are typically

designed at 0.15–0.30. LHSV deﬁned as the volume of oleﬁn charged per hour per

volume of acid. Operations at much higher throughputs are possible but low octanes

and high acid consumption generally result.

368 CHAPTER 9

Acid fraction

Octane increases with acid fraction and peaks at about 60 vol% acid in emulsion.

Power per unit volume

Octane depends upon the mass transfer rate of isobutane from the hydrocarbon phase

into the acid phase. According to Sprow

reaction selectivity and octane increase with

interfacial area which depends upon power per unit volume to the 0.25 power.

Sulfuric acid consumption and regeneration

Sulfuric acid alkylation uses about 100 times as much acid as HF alkylation because

cannot be stripped from the conjunct polymer, water, and contaminants. Sul-

furic acid is typically spent at 88–91 wt% H

, below which it is too weak to carry

out alkylation. The spent acid is not a waste. It is regenerated by burning the organics

which reduces the H

to SO

and water vapor. The SO

is then dried and oxidized

to SO

and scrubbed with water to produce 98.5–99.5% H

For sulfuric alkylation, aside from the direct hazards of contacting the acid, there are

transportation safety issues around the need to ship and regenerate 20–500 tons of

daily. Several reﬁners have built dedicated acid plants to regenerate spent acid

from the alkylation unit or pipelines to and from an acid manufacturer.

Acid consumption is generally expressed as the pounds of acid diluted from 99%

to 90% by conjunct polymer, water, and other contaminants per gallon or barrel of

alkylate. Consumption can be as low as 16 lbs per barrel alkylate for diene-free,

MTBE rafﬁnates run at ideal process conditions. Acid consumption can exceed 40 lb/

bbl of alkylate when running high levels of isobutylene, pentenes, and propylene at

high temperatures.

contaminants

Dienes

Butadiene and pentadienes are a major contaminants affecting acid consumption. Di-

enes and can account for 10–30% of the acid consumed. Dienes can dilute 5–20 lbs

of 99% acid down to 90% per lb of diene. Some reﬁners use dioleﬁn selective hydro-

genation units (such as UOP-Huls SHP™) to eliminate this source of acid consump-

tion.

Water and oxygenates

Water can also account for 10–20% of acid consumption. Wet feeds, wet isobutane

recycle, and high reactor temperatures are the main sources. Oxygenates including

acetone from the FCC, methanol and dimethyl ether from MTBE and TAME units

also consume acid. Water and oxygenates dilute 10 lbs of 99% acid to 90 per lb of

contaminant.

GASOLINE COMPONENTS 369

Sulfur and nitrogen compounds

Dilute about 20 lbs of 99% acid to 90% per lb of sulfur or nitrogen.

C6+

Dilutes 2 lbs of 99% acid to 90% per lb of contaminant. This can be a signiﬁcant

contributor to acid consumption when alkylating pentenes.

Ethylene

Dilutes 20 lbs of 99% acid to 90% per lb of contaminant by forming stable sulfates

that stay in the acid.

Ethane and lighter

While inert, light ends increase compressor discharge pressure for total condensation.

These indirectly increase acid consumption when limited by horsepower by increasing

reactor temperature.

Table 9.1.1. Alkylation Product Properties/Yields

HF Alkylation

Volume Yields per Butene only with

vol oleﬁn FCC Propene/Butene FCC Butene only Butene Isomerization

Isobutane Consumed 1.28 1.15 1.15

C5 plus Alkylate Produced 1.78 1.77 1.77

C5 plus Alkylate Properties

Speciﬁc Gravity 0.693 0.697 0.697

RON-0 93.3 95.5 96.5

MON-0 91.7 93.5 94.5

RVP (PSI) 2.8 2.7 2.7

Alkylation

Volume Yields per

vol oleﬁn FCC Propene/Butene FCC Butene only

Isobutane Consumed 1.2 1.12

C5 plus Alkylate Produced 1.72 1.72

C5 plus Alkylate Properties

Speciﬁc Gravity 0.693 0.697

RON-0 92 96

MON-0 90.4 94

RVP (PSI) 2.8 2.7

370 CHAPTER 9

Alkylate properties

Table 9.1.1 illustrates typical product properties for alkylate from HF and H

alkylation units for propylene-butylene and butylene only units. Alkylate is composed

of primarily C

’s and C

parafﬁns with a relatively narrow boiling range. Motor fuel

alkylate has desirable T50 and T90 distillation properties in addition to the long

recognized low endpoint. RON is in the mid to low 90s and MON is 1–2 units

lower. Its high MON is an important property of alkylates. The reid vapor pressure of

debutanized alkylate can be as low as 5–6 PSIA except when alkylating pentenes. Its

low RVP and very low sulfur and the absence of oleﬁns and aromatics make alkylate

a valuable component of reformulated gasolines by reducing Evaporative, NO

and

toxics emissions. These properties (or rather lack thereof ) cause alkylate to trade at

a premium over its already high octane value.

Almost all recently designed new alkylation units have been butylenes only or butylene

with some portion of the pentenes. In most cases for butylene only units, the feedstock

is pretreated in a selective hydrogenation unit to both remove the dioleﬁns which

increase acid passivation tar precursors and for HF alkylation units to isomerize most

of the low octane producing 1-butene to the more desirable 2-butene isomers. Such a

pretreating unit can raise the octane of HF alkylate product by one 1 Road ON as can

also be seen in the third column of Table 9.1.1.

Recent developments

Motor fuel alkylation has recently received a boost from the planned phase-out of

MTBE in gasoline blending formulations. Without MTBE, the best conventional

components for today’s reformulated gasolines are high octane parafﬁnic components

like alkylate.

While environmental legislation has increased demand for alkylate, its catalysts

and HF came under intense environmental scrutiny in recent years. While

both acids are corrosive and toxic, HF is especially hazardous since it has the po-

tential to form toxic aerosols, which can travel signiﬁcant distances downwind of an

accidental release. This scrutiny challenged the petroleum industry to enhance safety

and even to seek new alkylation catalysts.

UOP-Texaco and Mobil-Phillips developed cosolvents called Alkad™ and Re-

vap™, respectively, to reduce HF aerosoling.

A consortium of reﬁners and HF manufacturers lead by Amoco developed HF

dispersion models and water spray mitigation with extensive, large-scale testing to

design of release control systems. Together, cosolvents and water spray mitigation

can reduce downwind HF concentrations by over 95% should a release occur.

GASOLINE COMPONENTS 371

Through the American Petroleum Institute, the industry developed a comprehensive

document for safe operation of HF alkylation units entitled ‘Recommended Practice

for Safe Operation of Hydroﬂuoric Acid Alkylation Units’, API Recommended

Practice 751, ﬁrst published in June, 1992 and reissued in February, 1999.

Finally the efforts to ﬁnd new catalysts for alkylation have yielded solid cata-

lyst processes such as UOP’s Alkylene™ process that are ready for commerciali-

zation.

Conclusions

Motor fuel alkylation using liquid hydroﬂuoric or sulfuric acids is one of the oldest

catalytic processes used in reﬁning and petrochemical operations. The liquid acid

processes remain important despite concerns with safety and environmental properties

of the liquid acids. Solid catalyst alkylation using processes such as Alkylene™ (UOP

LLC) are emerging technology.

References

1. V. M. Ipatieff and A. V. Grosse, J Am. Chem. Soc., 1935 37 1616.

2. Pines, Herman, ChemTech, March 1982 150.

3. Hooper, John H. D. Chemistry and Industry, 1986 20, 683.

4. Jernigan, E. C., Gwyn, J. E. and Claridge, E. L. Chem. Eng. Prog. 1965 61(11) 94.

5. Sprow, F. B., Ind. Eng. Chem. Process Des. Dev., 1969 8(2), 254.

6. V. N. Ipatieff and R. E. Schaad, Mixed polymerization of butenes by solid phosphoric acid

catalyst. Ind Eng Chem (May 1938) 30 596–599.

7. A. Chauvel and G. Lefebvre, Petrochemical Processes.Vol1,Synthesis-Gas Derivatives

and Major Hydrocarbons, ed. Technip (1989) 183–187.