0071455914_ar007

CHAPTER 1.7CONOCOPHILLIPS

REDUCED VOLATILITY ALKYLATION PROCESS

(ReVAP)

Mark L. GravleyConocoPhillips

Fuels TechnologyBartlesville, Oklahoma

INTRODUCTION

During the late 1930s, Phillips Petroleum Company researchers discovered the benefits ofusing hydrofluoric acid to catalyze the synthesis of high-octane fuels from a broad rangeof low-value C3, C4, and C5 feedstocks. This research, as well as pilot-plant data, led to thecommercialization of the HF Alkylation process at Phillips’ Borger, Texas, refinery in 1942to provide aviation gasoline during World War II. Since that time, alkylate has been, andcontinues to be, a valuable high-octane blending component for gasoline, as evidenced byits importance in refineries around the world. ConocoPhillips has built 11 HF Alkylationunits in its own refineries and has licensed over 100 grassroots units.

Today, worldwide alkylation capacity exceeds 1.81 million bbl/day, with HF-basedprocesses accounting for approximately 57 percent of the total. Isobutane alkylate is animportant component of modern fuels, due to the high-octane, clean-burning characteris-tics as well as the low vapor pressure and absence of sulfur, olefins, or aromatics.Alkylation is receiving renewed attention by refiners contemplating the replacement ofMTBE in gasoline.

CHEMISTRY

Alkylation occurs when isobutane reacts with olefins in the presence of hydrofluoric acidas the catalyst to produce branched paraffins. In simplest terms, those reactions are

1.79

Source: HANDBOOK OF PETROLEUM REFINING PROCESSES

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2004 The McGraw-Hill Companies. All rights reserved.

Any use is subject to the Terms of Use as given at the website.

Propylene � isobutane → 2,3-dimethylpentane

Isobutylene � isobutane → 2,2,4-trimethylpentane

1-Butene � isobutane → 2,2-dimethylhexane

2-Butene � isobutane → 2,2,4- and 2,3,4-trimethylpentane

Amylene � isobutane → C9H20 (various isomers)

The trimethylpentanes are the preferred reaction products because they generally have thehighest octane value. In practice, however, the reactions are not so simple. Reactionsinvolving isomerization, hydrogen transfer, dimerization, polymerization, �-scission (orcracking), and disproportionation lead to a range of products. Furthermore, these side reac-tions produce substantial quantities of trimethylpentanes even when propylene oramylenes are the olefin feed. Polymerization produces conjunct polymers, which are com-plex, cyclic molecules, and this material is known as acid-soluble oil (ASO).

The reactions are also affected by the dispersion of hydrocarbons in the acid, the reac-tion temperature, the ratio of isobutane to olefin in the reaction zone, and the presence ofwater and ASO in the circulating acid. Since the hydrocarbon feeds are only slightly solu-ble in the HF acid, the reaction is enhanced by dispersing the hydrocarbons in the acid.Improved dispersion, i.e., smaller droplets of hydrocarbon, results in an alkylate productwith more of the desired trimethylpentanes and lower amounts of the undesirable lighterand heavier compounds. Lower reaction temperature also favors the desired reaction prod-ucts. A large excess of isobutene—above the stoichiometric amount—also favors the pro-duction of higher amounts of trimethylpentanes. Thus, the purity of isobutane in therecycle stream has an effect on alkylate quality, and buildup of C5� components in thisstream should be avoided. Small amounts of water enter the alkylation unit in the olefinand isobutane feeds. The water is allowed to accumulate in the acid phase and is found tobe beneficial in that it produces alkylate with higher concentration of C8 components andthus higher octane. Water in the HF is beneficial at levels up to about 3 to 4 wt %.However, water contents above about 2.0 percent generally have a detrimental effect oncorrosion rates in the unit and are avoided.

DESCRIPTION OF THE CONOCOPHILLIPS HFALKYLATION PROCESS

Isobutane reacts with propylene, butenes, and/or amylenes in the presence of hydrofluoricacid to produce a high-octane alkylate for motor gasoline. The reactions produce a varietyof products, primarily C8 branched paraffins, with lesser amounts of C7 and C9 branchedparaffins and small amounts of lighter and heavier paraffins. For best operation, the fol-lowing feedstock contaminant levels are recommended:

Sulfur—20 wt ppm maximum

Water—20 wt ppm maximum

Butadiene—3000 wt ppm maximum

C6�—0.1 LV % maximum

Oxygenates (MTBE, dimethyl ether, etc.)—30 wt ppm maximum

An alkylation unit will operate with feed contaminant at higher than the levels indicatedabove, but the adverse consequences are higher acid consumption, higher production of

1.80 ALKYLATION AND POLYMERIZATION

CONOCOPHILLIPS REDUCED VOLATILITY ALKYLATION PROCESS (ReVAP)



CONOCOPHILLIPS REDUCED-VOLATILITY ALKYLATION PROCESS (ReVAP) 1.81

unwanted by-products, and possible lower octane number of the alkylate product. One feedtreatment to remove butadiene is hydroisomerization, such as the ConocoPhillipsHydrisom Process. Hydroisomerization reduces butadiene (and pentadienes) to very lowlevels and also isomerizes 1-butene to cis- and trans-2-butene. The 2-butene isomers givehigher-octane alkylate in the HF Alkylation Process.

Referring to the flow diagrams in Figs. 1.7.1 and 1.7.2, we see that the olefin and make-up isobutane are typically mixed and then dried. The combined olefin and makeup isobu-tane are mixed with the recycle isobutane and sent to the differential gravity reactor ofPhillips’ proprietary design. This low-pressure reactor has no moving parts, such asimpellers or stirrers, nor are there any pumps to circulate the acid. The feed mixture ishighly dispersed into a moving bed of liquid acid, which circulates because of the differ-ence in density between the acid and the hydrocarbon. Total conversion of olefins to alky-late occurs very quickly.

Operating conditions in the reactor are relatively mild. The temperature will typicallybe about 80 to 110°F (27 to 43°C), or only 5 to 15°F (2.5 to 8°C) above the cooling-watertemperature. The pressure will be only slightly above that required to maintain the hydro-carbons in the liquid phase—usually in the range of 85 to 120 lb/in2 gage (590 to 820 kPa).Each alkylation design case is carefully studied in order to maximize heat recovery andminimize the isobutene/olefin ratio, while producing alkylate of sufficient octane qualityto meet the refiner’s needs. Isobutane/olefin ratios in the range of 8 : 1 to 13 : 1 are typi-cally used.

From the reaction zone, the hydrocarbons and catalyst flow upward to the settling zone(see Fig. 1.7.3). Here, the catalyst separates as a bottom phase and flows, by gravity, on areturn cycle through the acid cooler to the reaction zone, where the reaction cycle is con-tinued. The hydrocarbon phase from the settling zone, containing propane, excess isobu-tane, normal butane, alkylate, and a small amount of HF, is charged to the fractionationsection. Recycle isobutane, essential for favorable control of reaction mechanisms, isreturned to the reactor from the fractionator either as a liquid or as a vapor. In the lattercase, the latent heat of vaporization is recovered in nearby exchangers.

Propane and HF are produced overhead in the fractionator. The HF phase separates inthe overhead accumulator, which is shared with the HF stripper, and is returned to the acidsettler. What HF remains in the propane from the fractionator is removed in the HF strip-per, separates in the overhead accumulator, and is returned to the acid settler. The propaneproduct from the HF stripper contains traces of propyl fluoride, which are removed in thepropane defluorinators. The propane stream is heated and passed over alumina to removethe fluoride, yielding primarily aluminum fluoride and water with a trace of HF. Thepropane is sent through the KOH treater to remove the trace of HF and then to storage. Asimilar set of equipment may be used to treat n-butane, if it is produced as a separate prod-uct stream. The n-butane product may be blended with gasoline for vapor pressure control.

Alkylate is produced as a bottoms product from the fractionation section. The alkylateproduct is suitable for blending in motor gasoline, but may require additional fractionationfor use in aviation gasoline.

To regenerate the system acid, a small slipstream of acid is fed to the acid rerun col-umn to remove the ASO. The HF is stripped from the ASO with hot isobutane. The ASOis washed in the ASO caustic washer to remove free HF, and the ASO is disposed of, typ-ically by burning in the reboiler furnace or blending with fuel oil. Excess water is alsoremoved from the system acid in the acid rerun column.

Auxiliary systems within the alkylation unit include

1. Relief-gas neutralizer to remove HF from gases before being sent to the refinery flare

2. Storage for anhydrous HF during periods when the unit is down for maintenance

3. A neutralizing system for surface drainage and sewer drainage in the acid area




FE

ED

DR

YE

RS

OLE

FIN

FE

ED

ISO

BU

TA

NE

FE

ED

BU

TA

NE

DE

FLU

OR

INA

TO

RS

BU

TA

NE

KO

H T

RE

AT

ER

BU

TA

NE

PR

OD

UC

T

MA

INF

RA

CT

ION

AT

OR

HF

ST

RIP

PE

RR

EA

CT

OR

/R

ISE

R

AC

IDS

ET

TLE

R

AS

OC

AU

ST

ICW

AS

HE

RAC

IDS

TO

RA

GE

ALK

YLA

TE

PR

OD

UC

T

RE

CY

CLE

ISO

BU

TA

NE

AC

IDS

OLU

BLE

OIL

AC

ID S

OLU

BLE

OIL

TO

FU

EL

AC

ID R

ER

UN

CO

LUM

N

HF

AC

ID T

O S

ET

TLE

R

PR

OP

AN

ED

EF

LUO

RIN

AT

OR

SP

RO

PA

NE

KO

H T

RE

AT

ER

PR

OP

AN

EP

RO

DU

CT

NO

RM

AL

BU

TA

NE

AC

IDC

OO

LER

FIG

UR

E 1

.7.1

Flo

w d

iagr

am o

f th

e C

onoc

oPhi

llip

s H

F A

lkyl

atio

n pr

oces

s.

1.82




BU

TA

NE

DE

FLU

OR

INA

TO

RS

DE

PR

OP

AN

IZE

R

HF

AC

ID T

O S

ET

TLE

R

ISO

ST

RIP

PE

RF

EE

DD

RY

ER

S

OLE

FIN

FE

ED

ISO

BU

TA

NE

FE

EDRE

AC

TO

R/

RIS

ER

AC

IDS

ET

TLE

R

AS

OC

AU

ST

ICW

AS

HE

RAC

IDS

TOR

AG

E

RE

CY

CLE

ISO

BU

TA

NE

AC

IDS

OLU

BLE

OIL

AC

ID S

OLU

BLE

OIL

TO

FU

EL

AC

ID R

ER

UN

CO

LUM

N

BU

TA

NE

KO

H T

RE

AT

ER

BU

TA

NE

PR

OD

UC

T

HF

ST

RIP

PE

R

ALK

YLA

TE

PR

OD

UC

T

PR

OP

AN

EK

OH

TR

EA

TE

R

PR

OP

AN

EP

RO

DU

CT

PR

OP

AN

ED

EF

LUO

RIN

AT

OR

S

AC

IDC

OO

LER

FIG

UR

E 1

.7.2

Flo

w d

iagr

am o

f th

e C

onoc

oPhi

llip

s H

F A

lkyl

atio

n pr

oces

s—al

tern

ate

frac

tion

atio

n sc

hem

e.

1.83




4. A change room and storage room for cleaning and storing the protective clothingrequired on occasion by operating and maintenance personnel

5. Wastewater treatment system to remove more than 99 percent of the soluble fluoridein the effluent water

HF Alkylation units are constructed predominately of mild carbon steel. Only the acidregeneration column and some adjacent piping are constructed of nickel-copper alloy 400(Monel).

Other than a small centrifugal pump for charging HF to the acid rerun column, no HFpumping is required in the ConocoPhillips HF Alkylation process. HF unloading fromshipping containers and HF transfers from in-plant storage are accomplished by usingnitrogen or other gas under pressure.


HYDROCARBON FEED IN

COOLING WATER IN

COOLING WATEROUT

REACTOR RISER

ACIDSETTLER

ACIDSTORAGE

ACID OUT

FIGURE 1.7.3 ConocoPhillips HF Alkylation reactor/settler system.




WASTE TREATMENT AND DISPOSAL

Figure 1.7.4 shows the disposition of various waste streams within the alkylation unit.Nonacid gas streams are sent directly to the refinery flare system. HF-containing gases aresent first to the acid relief neutralizer, where the gases are scrubbed with an aqueous solu-tion of sodium hydroxide for removal of HF, and then to the refinery flare. The spent caus-tic solutions from the acid relief neutralizer and the ASO caustic washer are sent to amixing basin, where they are combined with calcium chloride. This mixture then flows tothe precipitation basin, where the calcium fluoride precipitates out of solution. The liquidflows to the refinery wastewater system, and the solid is periodically sent to the landfill fordisposal. The water from the calcium chloride precipitation system contains nominalamounts of sodium chloride and calcium chloride. Spent caustic from the KOH treatersand runoff from drains in the acid area of the plant flow to a neutralization pit and then tothe refinery wastewater system. Water from nonacid drains and sewers goes directly to therefinery wastewater system.

Used alumina—containing aluminum fluoride—from the defluorinators may bereturned to the alumina supplier to be converted back to alumina.

RISK REDUCTION AND SAFETY

The following principles may be used in the HF Alkylation process to minimize risk:

1. Minimize leak potential (few leak sites)

2. Minimize leak rate (i.e., minimum reactor/settler pressures)

3. Minimize leak duration

4. Minimize quantity released

One step to reduce risk is the elimination of any pumps for circulating HF catalystthrough the reactor system. By eliminating rotating equipment, potential packing and sealfailures associated with the equipment were eliminated, along with the dangers of frequentmaintenance exposure. Without the acid pump, isolation valves were no longer required inthe reactor/settler circulation system. Elimination of the acid pump allows the acid settler tooperate at a minimum pressure, which minimizes the leak rate. The result is that for morethan 40 years the reactor circuit design has only welded joints (built to pressure vessel codes)for joining the reactor and acid return pipes to the acid cooler and acid settler. No flanges areused to join these pipes, so these potential leak or failure sites do not exist. This is importantsince more than 90 percent of the HF on-site is contained in this circuit alone.

Risk is further reduced by using such features as remote isolation valves, rapid acid trans-fer (transfer to secure storage in less than 10 minutes), and inventory compartmentalization.For units with multiple acid coolers, the bottom portion of the acid settler is divided intocompartments to reduce the amount of acid that could be released in the event of a majorleak. The compartments segregate the acid in each acid cooler/reactor circuit such that themaximum amount of acid which could be released from a leak in one acid cooler or reactorsection is only slightly more that that contained in each compartment. The rapid transfer ofacid to secure storage is done by gravity flow; i.e., no pumping is required, and it has beenaccomplished in as little as 90 seconds. These features reduce risk by reducing both the dura-tion of a leak and the amount of acid that could be emitted in the event of a leak.

Water spray mitigation systems may also be employed to improve safety. Water sprayscan be used to knock down airborne HF from small leaks and, to some extent, isolatehydrocarbon leaks from ignition sources.





NO

NA

CID

RE

LIE

FN

ON

AC

ID P

UM

P V

EN

TS

NO

NA

CID

GA

S V

EN

TS

HF

SE

RV

ICE

RE

LIE

FH

F S

ER

VIC

E P

UM

P V

EN

TS

VE

NT

GA

S A

BS

OR

BE

R V

EN

T

SP

EN

T C

AU

ST

IC F

RO

MA

SO

WA

SH

ER

SP

EN

T C

AU

ST

IC F

RO

M K

OH

TR

EA

TE

RS

ST

OR

M S

EW

ER

S A

ND

DR

AIN

S IN

TH

E A

CID

-CO

NT

AIN

ING

AR

EA

NO

NA

CID

ST

OR

M S

EW

ER

S A

ND

DR

AIN

S

SP

EN

T A

LUM

INA

FR

OM

ALU

MIN

A T

RE

AT

ER

S

RE

FIN

ER

Y F

LAR

E

AC

ID R

ELI

EF

NE

UT

RA

LIZ

ER

CA

LCIU

M F

LUO

RID

EP

RE

CIP

ITA

TIO

N F

AC

ILIT

Y

RE

FIN

ER

YW

AS

TE

WA

TE

RS

YS

TE

M

SP

EN

T C

AU

ST

ICM

IXIN

G B

AS

IN

LAN

DF

ILL

RE

CY

CLE

TO

MA

NU

FA

CT

UR

ER

(GA

SE

S)

(LIQ

UID

)

(SO

LID

)

(LIQ

UID

)

NaO

HN

aOH

SO

DA

AS

H

SP

EN

TC

AU

ST

ICN

EU

TR

ALI

ZA

TIO

NP

IT

CA

LCIU

MC

HLO

RID

E

FIG

UR

E 1

.7.4

Was

te d

ispo

sal—

Con

ocoP

hill

ips

HF

Alk

ylat

ion

proc

ess.

1.86




A quantitative risk assessment was performed on a large (15,000 BPSD) HF Alkylationunit located in hypothetical rural and urban locations with up to 400,000 people in a 36-mi2 area around the refinery. Risk is site-specific and cannot be easily calculated for a par-ticular location. However, even in the highest population area studied, the current 15,000BPSD design achieved a Societal Risk Index (SRI) of 0.098—well within the Dutch stan-dards limit of 0.2, which is the strictest in the world. Individual risks for fatality due tobeing struck by a falling aircraft are said to be 10,000 times higher than the level of riskthat the Dutch standard calls unacceptable. No U.S. HF alkylation site has as many peopleliving nearby as the case where the process measured the 0.098 SRI value.

The ReVAP (for Reduced-Volatility Alkylation Process) is very similar to convention-al HF Alkylation with the exception that a vapor pressure suppression additive is blendedwith the HF acid. Mobil Oil Corporation and Phillips Petroleum Company developed theReVAP technology jointly in the early 1990s. Based upon bench-scale, pilot-plant, anddemonstration plant tests, each company commercialized the ReVAP technology in 1997in one of its own refineries, where the units continue to operate.

The additive is a nonvolatile, nonodorous, low-toxicity material that is completely mis-cible in the acid phase, but has very limited affinity to other hydrocarbons, including acid-soluble oil. These unique physical properties of the additive reduce the volatility of theacid significantly at ambient conditions. Furthermore, the additive is compatible with themetallurgy of existing HF Alkylation units.

When the additive is mixed with HF acid, it mitigates an acid leak in three ways: by (1)reducing the flash atomization of the acid, (2) reducing the vapor pressure significantly,and (3) diluting the acid. This is a passive mitigation system in that it is always effectiveand requires no intervention by an operator.

Traces of the additive accumulate in the heavier unit products of ASO and alkylate. TheASO is removed from the system acid in the acid rerun column in the normal manner. TheASO is then sent to a simple and efficient recovery system where the ASO and additive areseparated. The additive is returned to the reactor, and the ASO is sent to the ASO causticwasher and treated in the normal manner. Additive is separated from alkylate, recovered,and returned to the reactor. Figure 1.7.5 indicates the modifications required in an existingHF Alkylation unit to convert to the ReVAP technology.

The ReVAP technology has the added benefit of reducing the consumption of HF andcaustic, relative to the conventional HF Alkylation process. ReVAP increases the efficien-cy of separation of ASO and HF, thus reducing the loss of HF, which translates to lowercaustic consumption as well.

YIELD AND PRODUCT PROPERTIES

Based on processing typical butenes produced by fluid catalytic cracking (FCC) and sup-plemental isobutane, Tables 1.7.1 and 1.7.2 give the premises for ConocoPhillips HFAlkylation process economics for a unit with the ReVAP technology producing 6000bbl/day of alkylate.

ECONOMICS

The estimated capital cost for a plant producing 6000 bbl/day of alkylate indicated in theabove material balance, utilizing the flow scheme in Fig. 1.7.1, including the auxiliary sys-tems indicated in the section “Description of ConocoPhillips HF Alkylation Process,” withthe ReVAP technology is $24.8 million. This cost is for a U.S. Gulf Coast location, secondquarter, 2002. Initial catalyst cost, royalty, escalation, and contingency have been excluded.





REACTORSECTION

(EXISTING)

PRODUCTSEPARATION

(EXISTING)

ADDITIVERECOVERY

FROMALKYLATE

(NEW)

ACIDREGENERATION

(EXISTING,POSSIBLE

MODIFICATIONSREQUIRED)

ADDITIVERECOVERY

FROMASO

(NEW)

FEED

RECYCLE ISOBUTANE

ASO TOTREATMENT

ALKYLATE

ADDITIVE EXPORT(IF REQUIRED)

ADDITIVE RECYCLE

FIGURE 1.7.5 Modifications for adding ReVAP to an existing HF Alkylation unit.


Table 1.7.1 Material Balance, BPSD

Olefin Makeup Propane Butane Alkylate Acid-Component feed isobutene product product product soluble oil

Propylene 153 0 0 0 0 0Propane 115 7 146 0 0 0Isobutane 2380 1446 2 38 0 0n-Butane 702 37 0 571 168 0Butenes 3068 0 0 0 0 01,3-Butadiene 20 0 0 0 0 0Pentenes 56 0 0 0 0 0Pentanes 2 0 0 26 42 0Alkylate 0 0 0 0 5790 0Acid-soluble oil 0 0 0 0 0 8

Total 6496 1490 148 635 6000 8




Estimated Utilities Consumptions (Fig. 1.7.1 Flow Scheme), per 1000 bbl ofAlkylate, Including ReVAP Technology

Electricity (operating), kW 77.4

Cooling water, million Btu 15.1

Low-pressure steam (50 lb/in2 gage), million Btu 0.6

Medium-pressure steam (170 lb/in2 gage), million Btu 1.4

Fuel gas (absorbed), million Btu 10.0

Estimated Chemicals Consumptions, per 1000 bbl of Alkylate

Anhydrous HF, lb 70–100

NaOH, lb 47

KOH, lb 9.3

CaCl2, lb 79

Defluorinator alumina, lb 11

ReVAP additive (if used), lb 4

Maintenance and Labor Costs

Operating labor 2 persons per shift

Laboratory labor 1 person per day (8 h)

Maintenance (materials plus labor) 3% of investment per year


Table 1.7.2 Product Properties

Specific gravity at 15°C 0.70Reid vapor pressure, lb/in2 5.0Research octane number, clear 95.6Motor octane number, clear 94.1Olefin 0Sulfur �5 wt ppmASTM D-86 Distillation, °F

Initial boiling point 10410 17650 21290 257Final boiling point 383




BIBLIOGRAPHY

Hutson, Jr., Thomas, and Richard S. Logan: “Estimate Alky Yield and Quality,” HydrocarbonProcessing, September 1975, pp. 107–110.

Lew, Lawrence E., Martyn E. Pfile, and Larry W. Shoemaker: “Meet the Greater Demand for HighOctane Blending Agents with HF Alkylation,” Fuel Reformulation, March/April 1994.

Randolph, Bruce B., and Keith W. Hovis: ReVAP: “Reduced Volatility Alkylation for Production ofHigh Value Alkylate Blendstock: Year 4,” NPRA Annual Meeting, Mar. 17–19, 2002, San Antonio,Tex., Paper AM-02-20.





Documents

0071455914_ar007