It’s always interesting to have a little historical trivia attached to our industries, and this article provides a good example: refineries started adding alkylation units during World War II to increase production of aviation gasoline for all those piston-engine aircraft. The process is still in use today, with a few unresolved challenges.
Hydrofluoric acid (HF) alkylation is a very corrosive process, which Lydia Miller address in the April 2018 issue of Hydrocarbon Engineering, Providing a Guide. The de-propanizing tower is a critical part of the process unit, and precise control is required.
A typical de-propanizing tower is approximately 200 ft tall and has around 80 trays. There is a tray about two-thirds of the way up the tower that is much deeper than the others and is used as the liquid collection point, or draw tray. Some of the liquid is returned to the tower as reflux, while the balance is recycled back to the reactor. The liquid level in the draw tray must be monitored and controlled since it is the determining factor for how the liquid is sent back. This seemingly simple measurement can cause many difficulties and challenges for a variety of reasons.
One of those reasons is the highly corrosive nature of HF. Within a refinery, there isn’t much that is more toxic and corrosive than HF, so any equipment routinely exposed to it must be able to resist being eaten away. This means super-alloys and high-performance polymers for any wetted parts, including level instrumentation used to control the draw tray. A failure of control is fatal to the process.
The HF alkylation process needs precise regulation to perform properly. The ratios of feedstocks and acid are critical for maximum efficiency, and to avoid reactions creating undesirable polymers. If acid strength is not high enough, it loses the ability to act as a catalyst. This allows polymer production to increase, which further reduces acid strength in a condition called ‘acid runaway’. If this downward spiral of acid loss takes hold, the unit must be taken offline, inspected and the acid completely replaced.
Effective control requires knowing the liquid level in the draw tray so operators can control how much of it is sent back as recycle feedstock and how much is left in the tower as reflux. Draw tray level is measured via a bridle on the outside of the tower. Various level measuring technologies have been tried over the years, but one has emerged as the most practical: guided-wave radar (GWR), such as the Rosemount 5300 Level Transmitter. Exposure to HF meant that it had to be made out of a special alloy, but the design is adaptable to such “specials.” Solving the challenge was not without its hiccups due to HF shortening the life of polymer gaskets, but as the article describes, a minor modification remedied the problem.
This new assembly was installed more than two years ago and has presented no sign of issues. Since then the alkylation unit has been stable, maintaining production with high-quality alkylate products, in large part due to the reliable and accurate level measurement delivered by the GWR instrument.
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