WHITE PAPER / THE ACCELERATION OF ALKYLATION

THE NEW WAVE OF

ALKYLATION ACTIVITY IN THE US BY Geo Stephenson, PE, AND Dominic Varraveto, PE

Octane defi ciencies in U.S. refi neries are being

driven by light naphtha surplus from tight oil

and reduced diluent demand, increased fuel

e! ciency standards and octane loss from

Tier 3 sulfur reductions. These factors are

spurring renewed interest in alkylation.

WHITE PAPER / THE ACCELERATION OF ALKYLATION

© 2017 PAGE 2 OF 4

Light naphtha from tight oil is a suitable gasoline blend

stock, but it has poor blending octane. The octane of light

naphtha can be improved through isomerization, but the

resulting isomerized naphtha has a high vapor pressure.

Higher vapor pressure-blending components limit the

amount of butane that can be blended. The increase in

domestic oil production also has weakened the demand

for heavy Canadian crude, reducing the demand for light

naphtha diluents.

Tighter Corporate Average Fuel Economy standards

have pushed engines to operate at a higher e! ciency,

which requires higher operating temperatures and higher

octane fuels. In turn, the higher octane required by the

newer high-e! ciency engines will lead to the phaseout

of 85-octane gasoline sold in the Mountain West.

FINDING THE BALANCEAn ideal solution is alkylate, which is prized for its

high octane and low vapor pressure. Alkylate is

produced by the reaction of isobutane with light

olefi ns, primarily propylene and butylenes, using a

strong acid catalyst. Developing and implementing

a strategy for increasing alkylate capacity, however,

requires addressing current feedstock, process

confi guration and equipment limitations to meet

growing alkylate demand. But balancing isobutane

availability and fl uid catalytic cracking (FCC) light olefi n

yield with existing alkylation capacity is challenging.

Isobutane feed originates in the refi nery crude oil feed

and is recovered as mixed butanes in a saturated gas

plant and other process units, such as a naphtha reformer

debutanizer and a hydrocracker stabilizer. Depending

on location and availability, additional isobutane can

be imported to the refi nery from natural gas liquids

processing. When internal production is insu! cient

to balance with alkylate demand, the conversion

of normal butane to isobutane in an isomerization

unit is an alternative to importing (see Figure 1).

Butenes are the preferred olefi n, producing the highest

octane alkylate, but propylene and amylene (C5) also can

be alkylated to form high-octane fuel (see Figure 2).

The primary source of olefi n for most alkylation units

is the fl uid catalytic cracking unit (FCCU), where light

olefi ns are formed and recovered. The yield of FCC

light olefi ns can be adjusted by making operational

changes that include varying severity, catalyst

formulations/additives and operating pressure.

Typical light olefi n yield from an FCCU operating

in traditional gasoline mode can range from 8 to 15

percent. Through design changes, the FCCU can be

converted to operate in petrochemical mode, producing

20 percent to more than 35 percent light olefi n.

Other novel confi guration options for alkylate production

include nonrefi nery-based units that import the olefi n

and isobutane and export alkylate product. There is

negligible by-product production in the alkylation

process, which reduces the need to integrate the unit

into a refi nery. On-purpose olefi n can be produced

from natural gas liquids through dehydrogenation

processes to supply stand-alone alkylation plants.

Butane isomerization units also can be incorporated into

stand-alone plants but require a source of hydrogen

for the isomerization process. Dehydrogenation plants

can provide the required hydrogen (see Figure 3).

FIGURE 1: A typical configuration that shows the path of an alkylation unit and a C4 isomerization unit into a single processing unit with a shared deisobutanizer column.

Hydrogen

Normal Butane

IsobutaneNormal Butane

Isobutane

PropyleneButenes

Hydro-Cracker/Import

DIBALKY

FCC

C4 ISOM

Alkylate

IsobutaneNormal Butane

Amylenes

WHITE PAPER / THE ACCELERATION OF ALKYLATION

© 2017 PAGE 3 OF 4

KEY PROCESS VARIABLESTo move forward on this topic of conversation, we’ll

discuss emerging trends in FCC and alkylation units,

including production and recovery of light olefi ns,

high-purity propylene for the petrochemical market,

increased use of amylene as incremental alkylation

feed, and olefi n feed segregation and staging.

In addition to traditional sulfuric acid and hydrofl uoric

acid-catalyzed alkylation processes, solid catalyst and

ionic fl uids provide alternative technologies. The key

process variables that impact the alkylation process are:

• Reaction temperature. The alkylation process

is operated at a low temperature, which favors

higher octane. Higher operating temperatures

cause higher acid consumption and increase

polymerization reactions.

• Acid strength. Higher acid strength favors higher

alkylate quality, but operating at a lower spent

acid strength reduces acid consumption, which

is a major operating cost factor for the process.

• Isobutane concentration. In the alkylation

process, a higher ratio of isobutane to olefi n

(I/O ratio) in the reaction section reduces

polymer formation and acid consumption

but increases the amount of isobutane being

recycled in the process, also increasing operating

costs (see Figure 1).

Within the two predominant technologies that

produce alkylate — sulfuric acid alkylation and

hydrofl uoric acid alkylation — be aware of the

four key di" erences between the two.

Sulfuric acid is generally considered safer than

hydrofl uoric acid. Hydrofl uoric acid will vaporize

when released and form a dangerous acid cloud,

although there are additives that can be added

to the acid to reduce volatility. Sulfuric acid is a

burn hazard but won’t vaporize when released.

The hydrofl uoric acid process regenerates the

acid in the process with only small acid makeup

required. This is caused by contaminants in the

process readily separating from the acid. In the

sulfuric acid process, the acid soluble oils do not

easily separate from the acid and work to weaken it.

The acid must be continuously replaced, resulting in

signifi cant acid replacement and shipping costs.

The sulfuric acid process must operate at a colder

temperature than the hydrofl uoric acid process, where

the reaction heat can be removed using cooling water.

FIGURE 3: Dehydrogenation plants can provide the hydrogen required for the process in butane isomerization units.

IsobutaneNormal Butane

Isobutane

Normal Butane

Alkylate

C3/C4Dehydro

Pipeline/Truck/Retail

ALKY

DIB

ButaneISOM

Olefin

Hydrogen

FIGURE 2: General performance of sulfuric acid (H2SO4) and hydrofluoric (HF) acid-catalyzed alkylation processes based on different olefin feed stocks.

RON RON MON MON

HF H2SO

4HF H

2SO

4

Propene 91–93 89–92 89–91 88–90

Butene-1 90–91 97–98 88–89 93–94

Butene-2 96–97 97–98 92–93 93–94

Isobutene 94–95 90–91 91–92 88–89

Amylene 90–92 90–92 88–89 88–90

WHITE PAPER / THE ACCELERATION OF ALKYLATION

© 2017 PAGE 4 OF 4

In the sulfuric acid process, the reaction heat must

be removed by refrigeration, which is either

provided directly by auto-refrigeration

or indirectly by e# uent refrigeration. Both

systems require mechanical compression.

Both the hydrofl uoric and sulfuric acid processes

require about the same I/O ratio in the reaction

section. In the sulfuric acid process, approximately

half of the isobutane recycle is achieved through the

refrigeration system and the rest through distillation.

In the hydrofl uoric process, all isobutane recycle

is achieved through distillation, which increases

distillation equipment size and operating cost.

PARTNERING TO

EXPLORE OPPORTUNITIESThe abundance of natural gas liquids from shale

gas production, as well as the increasing demand

for alkylate, has presented many opportunities.

These opportunities range from FCC reconfi guration

and alkylation unit revamps for incremental capacity

to fully integrated stand-alone alkylation plants.

An experienced refi nery process engineering company

can provide start-to-fi nish conceptual direction, front-

end planning, and detailed engineering and construction

capabilities to bridge any gaps between the technology

licensor, engineering execution and implementation.

As a technology-neutral company, Burns & McDonnell

will work with the preferred alkylation technology

licensor to develop and optimize the overall alkylation

project, including utilities and o" sites (see Figure 4).

BIOGRAPHIES

GEOFF STEPHENSON, PE, is the process technology

manager for the Process & Industrial Group at

Burns & McDonnell. He holds a bachelor’s degree

in chemical engineering from the University of

California, Santa Barbara and has been involved

in the design of chemical and refi ning facilities for

more than 28 years. He is a licensed professional

engineer in Missouri and Oklahoma.

DOMINIC VARRAVETO, PE, is a refi nery process

manager at Burns & McDonnell. He has 36 years of

refi nery experience, including engineering, process

development, startup and operations support. Varraveto

holds a bachelor’s degree in chemical engineering from

the University of Notre Dame and a master’s degree in

engineering management from the University of Kansas.

He is a licensed professional engineer in California.

ABOUT BURNS & McDONNELLBurns & McDonnell is a family of companies

bringing together an unmatched team of

engineers, construction professionals,

architects, planners, technologists and

scientists to design and build our critical

infrastructure. With an integrated construction and

design mindset, we o" er full-service capabilities with

o! ces, globally. Founded in 1898, Burns & McDonnell

is a 100% employee-owned company and proud to be

on Fortune’s list of 100 Best Companies to Work For.

For more information, visit burnsmcd.com.

Customer Project Location Location/Year

Refi ner Technology evaluation Midwest, ongoing project

Refi ner Grassroots sulfuric acid unit Gulf Coast, ongoing project

Refi ner Sulfuric acid debottleneck Texas, ongoing project

Chemicals producer Sulfuric acid alkylation plant Confi dential, ongoing project

Chemicals producer Sulfuric acid technology evaluation Confi dential, ongoing project

Refi ner HF fractionation Midwest, 2010

Refi ner Coker/VDU OSBL with HF revamp Texas, 2005

Refi ner HF acid leak detection and mitigation Midwest, 2015

Refi ner HF water curtain Midwest, 2015

Refi ner HF acid detection and leak mitigation Midwest, 2015

FIGURE 4: A list of recent Burns & McDonnell alkylation experience.

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