Hydrocarbon Processing - April 2021 - 27

Special Focus

Clean Fuels
J. R. PETERSON, D. RANA and R. EWING,
DuPont Clean Technologies, Overland Park, Kansas

Reducing acid consumption: Maximizing sulfuric
acid alkylation unit profitability
Alkylation is a process used to produce highly branched isoparaffins from the reaction of lighter olefins and isobutane in
the presence of sulfuric acid as a catalyst. This highly branched
isoparaffin is called alkylate-a blending component that constitutes approximately 10%-15% of the gasoline pool in the U.S.
Besides the ability to increase octane and lower Reid vapor pressure (RVP) in the gasoline pool, alkylate also reduces vehicle exhaust emissions with zero olefins, zero aromatics and low sulfur.
Alkylate margins have been very healthy worldwide over the
past 10 yr. The gross margins (alkylate value minus feedstock
cost) for the U.S. Gulf Coast have ranged from a low of about
$20/bbl to more than $70/bbl over this period, with an average
of approximately $40/bbl. There are seasonal dips in profitability, and, although the COVID-19 pandemic has taken its toll on
gasoline demand worldwide in 2020 and early 2021, alkylation is
yielding strong margins as gasoline demand increases.
To capitalize on high alkylate margins, refiners have been
maximizing throughput and pushing alkylate production well
beyond design capacity. While units are enjoying increased profitability from increased alkylate production, acid regeneration
costs are also rising. In the spirit of efficiency, operating alkylation units are being asked to make more alkylate with less acid.
Refinery budgets and planning groups are requesting a reduction in acid consumption, while maintaining (or even increasing) alkylate throughput. How to reconcile this conundrum of
more with less? First, we must explore the relationship that acid
consumption has with alkylation unit operating variables.
New alkylation units running at design conditions typically
consume 0.2 lb-0.4 lb of sulfuric acid per gallon of alkylate produced (FIG. 1). However, when units are pushed beyond initial
design capacities, acid consumption rises due to bottlenecks such
as lack of cooling, low isobutane-to-olefin (I/O) ratios and high
space velocities within the reaction zone. In addition, older units
are not typically instrumented well and many lack modern technology design improvements. With overloaded units, the acid
consumption can be two to three times higher than in an equivalent new unit design, and the cost of acid regeneration can surpass 50% of the utility and chemical costs of the alkylation unit.
This article discusses strategies that can be implemented by
refiners to help lower the sulfuric acid consumption of the alkylation unit.

include the olefin feed type, feed contaminants, reaction zone
I/O ratio, diluents, reaction temperature, mixing intensity, acid
entrainment losses and the acid spending range.
Olefin feed type and feed contaminants. Fluid catalytic
cracking (FCC) butylene (especially isobutylene) has the lowest acid consumption among olefins. When refineries decide
to alkylate more propylene, amylene or feeds with higher levels
of contaminants, they experience increases in acid consumption. Butadiene, pentadiene and cyclopentene contaminants
in the olefin feed can double overall acid consumption in the
unit. Other feed contaminants (e.g., sulfur compounds) can increase it, as well.
I/O ratio and diluents. The I/O ratio is another factor that
affects the acid consumption. As the I/O ratio decreases, acid
consumption increases. When fractionation towers reach their
limits, isobutane purities go down and reaction zone diluents
such as propane and n-butane go up, which increases acid consumption and reduces alkylate quality. Increased normal butane
content in the refrigerant reduces reaction zone cooling.
Reactor temperature. Increasing feed rates to the alkylation
unit increases the overall heat of reaction. This heat of reaction
must be rejected within the refrigeration system to maintain

Contributors to acid consumption. Numerous factors con-

FIG. 1. Reaction acid consumption (lb/gal alkylate) for recent
grassroots units and older operating units.

tribute to the acid consumption in an alkylation reaction. These

0.6
Existing units pushed beyond design conditions
(except for Reﬁner 3 and Reﬁner 7) have
> 0.3 lb/gal acid consumption

0.57
0.54
0.51
Acid comsumption, lb/gal

0.48

Grassroots units running at
design conditions have
< 0.3 lb/gal acid consumption

0.45
0.42
0.39
0.36
0.33
0.3
0.27

Reﬁner 9

Reﬁner 10

Reﬁner 7

Reﬁner 8

Reﬁner 5

Reﬁner 6

Reﬁner 4

Reﬁner 3

Reﬁner 2

Reﬁner 1

New unit 7

New unit 6

New unit 3

New unit 5

New unit 1

0.21
0.18

New unit 2

0.24

Reﬁners

Hydrocarbon Processing | APRIL 2021

Hydrocarbon Processing - April 2021

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