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Apr-2019

Technical collaboration triples NHT cycle length (TIA)

A nine-month cycle once seemed like a victory. But with pressure drop still flat after 27 months, the engineers on a naphtha hydrotreater in the Texas Panhandle realised that Crystaphase had tripled the best cycle length of the previous vendor, and they were playing a whole new game.


John Burwell
Crystaphase Products
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Article Summary
The feedstock slate on this NHT was primarily coker naphtha from storage, and its quality varied between bad and worse. A typical cycle length was about six months. The previous vendor attributed the limitations to the operating conditions. The engineers were not comfortable with such low expectations, but they worked with what they had.

At each shutdown, the vendor would recommend a different combination of 4-5 cm wagon-wheel class materials. Some cycles were longer, but some were shorter. Variances in the feedstock appeared to have more of an effect on the cycle length than the grading system ever did.

One of the engineers was working with Crystaphase on another hydrotreater at the facility. When Crystaphase called for a routine update on the performance of that unit, the engineer mentioned an upcoming turnaround for the NHT.

The engineers provided spent samples of the previous grading system for Crystaphase to analyse in its foulant laboratory. Technicians there found familiar polymerisation in scanning-electron microscopy and energy-dispersive x-ray spectroscopy reports. Coupling that evidence with a particle size distribution analysis, Crystaphase developed an initial recommendation employing CatTrap technology to trap those particles at those sizes.

With just one round of analysis, Crystaphase delivered twice the best cycle length of the other vendor. The unit ran for 18 months with the system installed.

The engineers did not hesitate to reload that system while Crystaphase analysed the next batch of samples. After studying how the CatTrap system performed in that specific unit, Crystaphase would design an optimised system for the following cycle.

This time, Crystaphase was able to measure the accumulation of foulant in different layers of the CatTrap material. With CatTrap 50 and 65 carrying the bulk of the foulant load, it could reduce the volume of CatTrap 10 material and use that space for more CatTrap 50.

The lab was also able to see more clearly the foulant’s morphology. With a higher volume of foulant contained in the spent CatTrap discs than in the wagon-wheel type material, technicians had a sample size sufficient to conduct a more thorough analysis. SEM imaging revealed the distinctive kettle-corn appearance of carbon-bound iron sulphide, a product of coke formation and corrosion in the heating train.

Acting on this discovery, engineers at the refinery worked with Crystaphase to further optimise the plan. The refinery balanced the feedstock slate to minimise the variance in quality, by modulating the coker naphtha it would feed from tankage. Crystaphase developed a loading optimised for that balanced feedstock.

The company also determined the target charge rate and projected life of the catalyst. Using these figures, Crystaphase calculated the volume of foulant the unit needed to trap over the course of the target cycle length of 24 months. That volume, in turn, determined the volume of each size of filtration material it needed to load.

The unit surpassed the 24-month goal, with virtually no rise in pressure drop. At 27 months, the refinery had tripled its previous best run time, and pressure drop was still flat.

After struggling for years under the assumption that if six months was good, then nine months must be great, the engineers had an altogether new scale by which to measure their success.

Today, the object of the game at this refinery is no longer just to get another month or two of cycle length, but to get more uptime out of this vessel – and others at the refinery – than ever before. 

This short case study originally appeared in PTQ's Technology In Action feature - Q1 2019 issue

For more information contact: john.burwell@crystaphase.com
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