Hydrocarbon Processing - April 2021 - 28

Clean Fuels

FIG. 2. Turbulence dissipation rates of standard three-blade vs.
next-generation impellerb design.

optimal reaction temperatures. When units are pushed beyond
the refrigeration system capacity, the reactor temperature will
rise above the optimum value of 5.6°C-7.2°C (42°F-45°F).
At higher temperatures, acid consumption and corrosion can
increase dramatically. Most refiners set a maximum reaction
temperature limit [e.g., 15°C (60°F)] to maintain acceptable
corrosion rates. To lower the reaction zone temperature, first
ensure that the existing refrigeration system is running at its
full potential. Then, consider reactor enhancements (such as
tube inserts, 0.75-in. tube bundles and proprietary internal
modifications a) and/or refrigeration upgrades (such as supplemental chillers or compressor modifications).
Acid entrainment. Acid settlers without coalescing media
experience higher acid entrainment losses in the hydrocarbon
effluent, especially as alkylation unit throughput increases.
Acid carryover can also lead to corrosion of downstream
equipment. To minimize acid entrainment losses, coalescing
media can typically be retrofitted on older acid settlers. Higher
acid inventories in the reaction zone promote negative side reactions that degrade the alkylate product quality and increase
the acid consumption. Most modern units have smaller acid
settlers with coalescing media that allow for both reduced acid
inventory and less acid carryover.
Acid spending range. Acid spending that ranges from
99.2% fresh acid strength down to 90% spent acid strength
makes it possible to extract maximum value from the acid before it is sent off to regeneration. Modern unit instrumentation,
along with good lab practices, are essential to ensure that the
differences between actual and target acid strengths are minimized to reduce acid demand. Units with best-in-class monitoring systems can safely reduce spent acid strength below 90
wt%, resulting in significant acid savings.
Mixing intensity. Units with less mixing intensity (hp/
bbl alkylate) typically consume more acid and produce lowerquality alkylate. If impellers are worn or the speed is reduced,
unit performance degrades. Reaction temperature can increase
with less mixing due to lower heat transfer, as well. Add-ons,
such as next-generation impellersb and proprietary internal
modifications a (FIGS. 2 and 3), increase turbulence and mixing,
and reduce acid consumption.
Refiners' options to reduce acid consumption. The following are ways that refiners can reduce acid consumption
within their alkylation units.
Spent acid strength. Many refiners operate at a spent acid
strength higher than the design or target spent acid strength,
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APRIL 2021 | HydrocarbonProcessing.com

thus " giving away " acid. Operations personnel almost always
err on the side of caution to avoid an acid runaway, as there are
typically minimal consequences in wasting acid, but significant
consequences for a low acid strength excursion.
Much of this acid waste has to do with the delay time and
inaccuracy of acid strength lab results. If the results for an acid
sample come back and are falsely low, what is the operations
team to do? It will generally crank up the fresh acid rate and retest the samples. The longer it takes to get a sample back, and the
less accurate the lab results, the more acid is wasted.
Since acid costs can be very significant, it makes sense to
spend effort on streamlining acid sampling, delivery and lab procedures to achieve a quick and accurate turnaround. Many refinery labs do not centrifuge the acid samples, which contributes to
misleading results-typically 0.5 wt%-1 wt% lower than actual.
Sometimes, labs will let the samples sit for a couple of hours to
decant the hydrocarbon. Decanting is less effective than centrifuging-possibly allowing more time for humidity to contaminate the sample and, thus, delay the reporting of the results.
Operations should be diligent in challenging its lab personnel for quicker and more accurate results, since so much money
is at stake. It is also a good idea to periodically " blind test " the lab
with identical triplicate samples to find the standard deviation
(SD). For example, the authors' lab's SD is less than 0.03 wt%
for triplicate samples. A refinery does not typically require this
level of accuracy; however, reducing SD to about 0.2 wt% can
allow refiners to confidently spend closer to the target strength
and realize significant acid cost savings. A centrifuge and good
lab techniques are very inexpensive by comparison.
Reduce feed contaminants. Dienes (butadiene, pentadiene, etc.) are common contaminants in the alkylation unit feed
stream. If the total diene concentration within the olefin feed
is greater than 0.5 wt%, or if acid costs are especially high, consider sending the olefin feed to a selective hydrogenation unit to
remove these contaminants.
High water content in the olefin feed can also impact acid
consumption but can be removed by a properly designed feed
preparation section (feed/effluent exchanger and feed coalescer). The feed should be cooled as much as possible [typically
down to approximately 13°C (55°F)] in the feed/effluent exchanger to reduce the solubility of water in the hydrocarbon
phase. This allows more water to drop out in the downstream
feed coalescer. Modern units with dry alumina treating and dry
recycle isobutane typically do not need a feed coalescer, as there
is no free water to remove.
Process optimization. Process optimization is the first step
toward reducing acid consumption, and it offers multiple variables that can be adjusted. In general, it is the temperature and
I/O ratio that have the biggest impact on acid consumption.
The first strategy is to maximize heat removal from the system to lower the reaction temperature closer to 7.2°C (45°F).
Due to refrigeration limitations, it is not always possible to reduce the temperature for units operating over design capacity.
However, there are often " low-hanging fruit " refrigeration issues that have been overlooked. Before spending money on improvements, make sure that the refrigeration system is running
as efficiently as possible. Items to review include:
* Ensuring that the compressor anti-surge valve is
completely closed with no bypassing. Check that the

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Hydrocarbon Processing - April 2021

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