Loss into gain in high capacity trays - Part 1: excursion

Systematic troubleshooting diagnosed and solved two complex and independent problems that limited the capacity of an atmospheric crude distillation tower

NEASAN O’SHEA (ret’d) and DAN CRONIN, Phillips 66 Whitegate refinery

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Article Summary

A crude tower at the Whitegate refinery in Ireland had been retrayed (by others) with modern high capacity trays, but upon start-up achieved less than 80% of its previous capacity. Troubleshooting identified two bottlenecks: premature flooding on some of the heavy naphtha-diesel fractionation trays, and a phenomenon that we termed ‘excursion’ in which sudden temperature reductions around the diesel draw, accompanied by distillate yield loss, occurred upon rate increase.

This article focuses on the excursion bottleneck. Systematic field tests showed conclusively that the excursions occurred when the diesel draw rate exceeded about 2000 t/d. This was lower than the diesel flow rate that could be drawn prior to the retray. The new bottleneck was diagnosed to result from a seemingly benign modification to the downcomer that transported diesel to its draw pan. This modification halved the degassing time in the downcomer, leading to aerated liquid reaching the diesel draw pan. The excursions occurred when the diesel draw nozzle and rundown line reached their self-venting flow limit. The excursion reflected the switch from self-venting flow to choked flow in the diesel rundown line.

Based on this diagnosis, some of the diesel was drawn out of an available kerosene draw nozzle further up in the tower. This unloaded the diesel draw, keeping it in the self-venting mode. This mode of operation also helped the premature flooding, which was the second bottleneck in the tower (and will be described in the following article in PTQ Q3 2016). As a result, drawing kerosene permitted the tower to recover almost all the capacity loss without any hardware modification. The longer term solution was to replace the diesel draw by a seal-welded chimney tray and to increase the sizes of the diesel draw nozzle and rundown line. These modifications completely eliminated the excursions, permitting higher than design capacities to be achieved.

Improvements in trays and packings
The last 2-3 decades saw tremendous improvements in high capacity trays and packings. Modern trays and packings have permitted towers to be debottlenecked to throughputs and separations not previously envisioned. Yet, occasionally, a high capacity tray or packing debottleneck falls short of achieving its design targets.

A common reaction is to blame the trays or packing, and to seek a sledgehammer solution, such as abandoning the new trays and replacing them with structured packing, or even building a new tower. Solutions like this are not only costly, but are also highly counter-productive and may have a negative environmental impact.1 Often they may obscure simple solutions that can be implemented immediately (rather than at the next turnaround) and minimise losses, down time, and are environmentally friendly.

In most situations where high capacity trays or packings fail to achieve their design expectations, it is not that the trays or packings are ‘no good’. Rather, it is seemingly minor hydraulic details that cause the premature bottlenecks. The correct strategy to address these is troubleshooting that seeks the root cause and eliminates the real bottleneck.

Good troubleshooting is systematic and thorough. It combines interacting with the operating staff to collect operating experience and obtain a good problem definition, conducting well-targeted field tests, applying process diagnostics (such as gamma scans and surface temperature surveys), simulation, and hydraulic analysis. A vital element is good testing of each theory and hypothesis with valid field tests. One good field test can humble a thousand expert opinions. This approach was adopted by our team and will be demonstrated in this article.

The article describes a case history where a high capacity retray of a crude fractionator (by others) led to a capacity loss. A thorough and systematic investigation utilising extensive field tests identified and diagnosed the causes of two completely independent bottlenecks that caused the revamp to fall short of its expectations. The diagnosis led to a very simple solution that circumvented the need to shut the crude unit down, overcame both bottlenecks, involved no hardware changes, cost little to implement, and recovered most of the losses almost immediately. Longer term, the diagnosis led to correct, low cost internals modifications that produced capacities well above the design.

In the 1990s, Irish Refining Company operated a 52500 b/d (7070 t/d) crude unit processing North Sea crudes. (The refinery is now owned by Phillips 66.) Products from the unit included light and heavy naphthas, diesel, and atmospheric residue. The diesel was produced to a cloud point specification of 0°C while the heavy naphtha was produced to an ASTM D86 95% point specification of 200°C.

The atmospheric tower contained two pumparounds, bottom (BPA) and top (TPA) and two active side draws. The upper side draw from Tray 24 was heavy naphtha; the lower side draw from Tray 11 was diesel. The diesel draw was steam-stripped in a side stripper.

The naphtha end point was maintained using a temperature control (TRC) just below Tray 23. This TRC adjusted the TPA duty. The TPA was drawn from a sump just under the downcomer from Tray 21 to Tray 20, so that any liquid not drawn overflowed onto Tray 20 and became reflux to the heavy naphtha-diesel fractionation section.

The diesel cloud point was maintained by another TRC just above Tray 10. This TRC adjusted the BPA duty. The BPA was drawn from a total draw chimney tray just below Tray 9. The level control on the chimney tray manipulated the wash-back, which was the reflux to the packed wash bed below.

Initially, the four-metre inside diameter atmospheric crude tower contained 34 single-pass jet tab trays. At a later time, the tower (see Figure 1) was revamped to increase fractionation efficiency and to permit the refinery to intermittently take a kerosene side-cut while maintaining the same throughput of 52500 b/d (7080 t/d). To achieve this, Trays 11-20 (the trays in the heavy naphtha diesel fractionation section) were replaced by single pass, high capacity Nye trays. Structured packings were installed in the wash and stripping sections. A new kerosene side draw was installed at the inlet to Tray 16, approximately mid-way between the heavy naphtha and diesel draws. A side stripper was added to strip the kerosene. Initially, the kerosene rundown system was incomplete so the kerosene side draw was not operated.

After the retray, the tower was not able to achieve the throughput it had previously achieved. At throughputs exceeding 38000 b/d (about 5130 t/d) the tower would start to flood with a rise in pressure drop and the heavy naphtha product going off-specification with heavy components. Initial gamma scans confirmed flood initiating near the – as yet unused – kerosene draw (around Trays 14-16) and building up till Tray 23. The pumparound Trays 21-23 were jet tab trays and were not changed in the revamp.

In addition, a severe instability that we termed an ‘excursion’ would set in at about the same rates as the flooding initiated. A relatively small increase in BPA duty or reduction in coil outlet temperature (COT) would trigger a massive reduction in Tray 10 temperature, usually accompanied by a heavy naphtha yield loss, with the lost yield ending up in the atmospheric residue. This first article focuses on this excursion phenomenon and its troubleshooting.

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