Sep-2020

Increase production rates with Tracerco’s FrothView and Tru-Scan technologies

How Tru-Scan™ and FrothView™ results help increase production, and remove the debutanizer tower, which seemed to be the bottleneck for the unit.

Nicola Porter
Tracerco

Article Summary

Chemical plants and refineries are continuously expanding to meet current and future demands. In most cases, units are being pushed well into and above their design capacity, which can limit output from the unit and product purities. Tray manufactures have developed simulations which can identify their own trays % flood based on vapour/feed rates, ΔP and temperatures throughout vessels. However, if one or more of these values cannot be obtained due to lack of/faulty instrumentation or physical access, accurate modelling may not be obtainable. But what if you were able to accurately measure a trays froth height which corresponds to its % flood without some of these vital modelling parameters?

Tracerco’s patented FrothView™, is a way to quantitively analyse a trays % froth height within vapour/liquid distillation columns. This technique measures each trays froth height and is represented in generating % tray space.

The Field Test
In this case study, Tracerco’s customer was wanting to increase production, but their debutanizer tower seemed to be the bottleneck for the unit. They identified unwanted heavy material in the overhead at high rate conditions. Unfortunately, appropriate DP instrumentation or sample points were not installed, and they were unable measure pressure drop through the vessel. They were able to increase feed over 20-25% of the tray design. However, they could not increase reboiler duty without sending heavy material overhead when reboiler boil up was increased.

Their simulations indicated, at those conditions, the vessel should not be operating near 100% flood.

The customer elected to perform three Tru-Scans™ at low, mid, and high rates on their debutanizer at maximum feed flow rates (20-25% over design). The reboiler duty was increased over the course of the three scans with the final condition at approximately 30% over its design.

The Analysis
The first scan performed at low rates (Figure 1) showed the trays to be holding between 50 and 70% of tray spacing. In-house simulations indicated, at these conditions, the trays should be around 50-60% flood.

The second scan at mid-rates illustrated by the red scanline (Figure 2) showed most of the trays to be operating between 70 and 95% of tray spacing. Inhouse simulations indicated there should have been around 70-80% flood.

The third scan at high rates, illustrated by the black scanline in Figure 3, showed the trays operating at 100% tray spacing indicating flooding, with a light froth height of 22”-24” (56 cm - 61 cm) from tray 21, encroaching on the top tangent line. Modelling showed the trays should be at 80-90% flood.
 
The Conclusion
After the Tru-Scan™ and FrothView™ results were analysed and reported to the customer, they were able to redesign the trays by increasing the hole area on the active panels to reduce pressure drop. They could now reach their new product rates without adversely effecting overhead material purities.

Being able to remove the bottleneck to the rest of the unit by changing the tray design, the customer was able to gain 15-20% additional throughput with help from utilising Tracerco’s FrothView™ technology. The real value in this case study is that our Tru-Scan™ and FrothView™ analysis verified the simulations/modelling were inaccurate.