Hydrocarbon Processing - April 2021 - 29

Clean Fuels

FIG. 3. Improvements in the proprietary contactor flow regime over a standard design.

downstream side of the valve is at ambient temperature.
Anti-surge flow wastes significant compressor power.
* Each incremental 1 psi of line loss between the suction trap
and the compressor inlet raises the reaction temperature
by approximately 1.4°C (2.5°F). Verify that the mist
eliminator in the suction trap was designed for a low
pressure drop (about 0.1 psi) and is operating properly.
* For units with fixed-speed compressors, make sure
that the suction throttle valve is 100% open.
This valve should be full line size with low pressure
drop (approximately 0.25 psi).
* For units with variable-speed compressors, ensure
that the driver can operate at full design speed.
Sometimes, this requires cleaning of the steam turbine
and piping to remove salts.
* Check that the cold sides of the condensers are clean
and operating as designed. These exchangers are critical
and should receive special attention. Hot vapor bypasses
should be closed to minimize condensing pressure.
* Trend and optimize the refrigerant propane content
to find out at what value the refrigeration system works
best. This is typically a trial-and-error process.
* Refrigeration systems can be poorly designed.
Review your system and its controls with an expert.
Consider a system revamp to increase heat removal.
It is important to remember that there is no one-size-fits-all
strategy to improve a unit's performance or capacity. The refiner's goals should be understood in terms of maximizing alkylate
capacity or quality, or minimizing operating costs such as acid
consumption.
Case studies. A series of case studies were used to determine
economics for the various aftermarket options that many refiners use to increase refrigeration.
Tube inserts. Tube inserts increase the heat transfer by distributing equal amounts of refrigerant to each tube in a tube
bundle. They typically lower the reactor temperatures by 2.2°C
(4°F) at a fixed olefin feed rate. When two-phase effluent enters
the tube bundle channel head without inserts, the vapor separates and causes some tubes to operate hotter due to higher vapor content. This results in reduced overall heat transfer. Tube
inserts eliminate vapor separation in the channel head by maintaining enough pressure on the effluent to keep it liquid until it
flashes within the tube inserts.

0.75-in. tube bundle. A 0.75-in. tube bundle provides approximately 35% more heat transfer area compared to a 1-in.
tube bundle. The additional heat transfer area reduces acid consumption by reducing the reactor temperature approximately
3.3°C (6°F) at a fixed olefin feed rate.
Tube inserts and 0.75-in. tube bundle. Combining a 0.75in. tube bundle with tube inserts reduces the reaction temperatures by about 4.4°C (8°F) at a fixed olefin rate.
Proprietary internal modifications a. These design changes improve the flow regime within the reactor (FIG. 3). In the
standard design, emulsion flow leaves the annulus between the
reactor shell wall and circulation tube, then turns 180° to flow
across the tube bundle 3 ft-4 ft (approximately 1 m) in front of
the tube sheet.
Results from computational fluid dynamic studies showed
that the standard design has low velocities and, therefore, low
heat transfer near the tube sheet. To remedy this, the circulation
tube was extended, and a flow distributor was added for better
use of the entire tube bundle heat transfer surface area. This improves heat transfer and lowers the reaction temperature by approximately 1.7°C (3°F) at a fixed olefin rate.
Feed and refrigerant chillers. Several refiners have supplemented their existing refrigeration system with packaged chillers
that remove incremental heat from the reaction zone. Typically,
these chilling units cool glycol, which then cools the reactor feed
and/or the condensed refrigerant. They are not as efficient as a
properly designed primary compressor, especially if an intermediate heat transfer fluid is used. However, these chilling units can
be rented (or permanently installed), and they offer a relatively
easy path to increased alkylate capacity.
A series of unit simulations were completed to demonstrate
the economics of various unit options. The results are detailed
in TABLE 1. The following are the details of each simulation case:
* Design Case: This was for a 16,000-bpd alkylation
unit operating at the ideal design condition reaction
temperature [7.2°C (45°F)].
* Operating Case: The same unit pushed to produce
more than 21,000 bpd of alkylate, while staying within
the refiner's reaction temperature limit of 15.6°C (60°F).
Acid costs increase dramatically, but unit profitability
increases by almost an order of magnitude more.
Therefore, refiners typically push their alkylation units.
* Performance Optimized Operating Case: This case
is the previous case with extra attention devoted to
Hydrocarbon Processing | APRIL 2021 29

Hydrocarbon Processing - April 2021

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