The Catalyst Review July 2020 - 7

SPECIAL FEATURE
Established Alkylation Catalysts and Processes

Figure 5. Flow diagrams for HF and SA alkylation processes.

Alkylation is an established refining technology. The majority of the operating
alkylation capacity was installed over 70 years ago. During World War II,
the military needed more efficient, higher-octane fuels to power advanced
airplane engines and field vehicles. The introduction of FCC technology
enabled the production of higher-octane gasoline, jet fuel and byproduct
light olefins. Early alkylation technologies used strong liquid-acid catalysts.
Hydrofluoric (HF) or sulfuric acid (SA) were the preferred catalysts to alkylate
C4 paraffins and other light olefins (Figure 5).
HF and SA alkylation processes are acid-hydrocarbon emulsion reactions.
Both processes use corrosive materials and have considerable process safety
and operational concerns. Due to the corrosive liquid catalysts, maintenance
Source: US EIA, 2013..
and inspections are required more often and incur higher expenses. The
storage, transport and regeneration of liquid
acid increase operating costs. As summarized
Table 3. Pros and cons of established alkylation technologies.
in Table 3, both alkylation technologies have
benefits and risks that individual refiners must
evaluate. In general, SA is considered the safer
catalyst. SA will not vaporize and create a vapor
cloud when released. However, SA is a burn
hazard, especially with spills and leaks.
HF systems regenerate the catalyst in process
and only require a small acid makeup. In
the HF process, contaminants are readily
separated from the acid catalyst. Conversely,
SA alkylation creates acid-soluble oils (ASOs)
that are not easily removed from the acid
and weaken it. Because of ASO formation, SA
must be continuously replaced, thus adding to
transportation and storage issues. SA-based
alkylation processes have higher operating
costs for catalyst makeup and yield losses.
HF-based processes recycle all unreacted
iC4 through the distillation unit. The large
Source: Author.
distillation train involves higher initial capital
cost. However, the larger distillation capacity becomes a benefit in revamp projects, as this column can be reused. Conversely, the
SA process recycles half of the unreacted iC4 through a refrigeration unit and the remainder through a distillation unit (Table 3). The
refrigeration unit adds to the complexity and cost of unit operations.
Present capacity. Global alkylate capacity is over 2 million bpd (2 MMbpd). The US is the largest gasoline market and has an installed
alkylation capacity of approximately 1.337 MMbpsd.10 In addition, the US imports approximately 600,000 bpd (600 Mbpd) of highoctane blendstock; almost 30% of the imports are alkylate.11
Globally, approximately 700 refineries are in
Table 4. Capacity and technology licenses for SA and HF processes.
operation. Over 300 alkylation units use HF- or
SA-based technologies.12 Half of the present
alkylation capacity is SA technology, and the
remainder are HF-based operations.12,13 Over
one-third of the licensed alkylation units are
located in the U.S.12,13 Fifty US refineries operate
HF alkylation units.13 HF alkylation technology
Source: Author.
remains the most economically attractive route
to produce alkylate.14 The refining industry and American Petroleum Institute (API) have established rigorous guidelines under API
Recommended Practice (RP) 751-Safe Operation of Hydrofluoric Acid Units. This standard covers hazard management, maintenance
and inspection, operating procedures, metallurgy, design and construction requirements, mitigation systems and more. Applying API
The Catalyst Review

July 2020

The Catalyst Review July 2020

Table of Contents for the Digital Edition of The Catalyst Review July 2020