Maximising a Propylene yield

In discussions with Aggreko on their refinery’s pain points, a JV Refinery raised the issue of their PP-Splitter not achieving desired yields and being further restricted by seasonal temperatures. Given the high market value of this feedstock and the fact that achieving optimum capacity within the PP splitter is also crucial to avoid flaring when capacity isn’t reached, the customer posed the challenge to Aggreko to engineer a bespoke process solution to overcome it.

A deep understanding of this highly complex process and it’s intricacies was vital. Upgrading the refinery grade propylene (RGP) via thermocompression is a balancing act between the right amounts of liquid and vapour that are introduced to the splitter along with the PGP.

It’s a volatile process which is hugely affected by the liquid element of the process which is cooled water. When this temperature rises by even a few degrees the ripple affect can be complex and costly for refiners.

With the rise in cooled water temperature, overall pressure will increase meaning that the compressor in the production process must accommodate and generate a pressure jump. This however decreases flow rate and the reboiler output is compromised, resulting in a lower grade propane. One option to stop this is a reduction of PGP into the PP-Splitter.

For the customer this was causing a host of problems as the PP-splitter is taken down from its original design capacity, flaring can occur, and the distillation process can be unstable.

Seasonal temperatures were also affecting the cooled water which was as high as 39.5C – Dropping the recovery yield to 60% was forcing the operator to reduce the capacity of the PP-splitter in those instances.

The customer was keen to move away from traditional methods such as the addition of a reboiler or utilisation low-pressure steam and water condensers and with our process engineers understanding that technical changes with cooled water injection could affect yield outcome, we were confident it was a problem we could solve.

The ability to influence the yield meant that there was successful increased delivery of this high value product. With the capacity of the PP-Splitter increased, flaring was avoided. During the most critical hours of the day, propylene was on specification 100% of the time.

The improvement recovery yield increased from 95.6% to 99.8%, meaning that recovery yield, specification and condensation capacity were all enhanced.

The solution was mobilised and commissioned in just 8 weeks, ultimately making this a quick way to maximise revenue.  The refinery has benefited from an additional $17 million USD of Propylene using this process enhancement in 120 days.

The Aggreko solution was a complex process where the smallest tweak in temperature could dramatically change the outcome for the customer. Our expertise was heavily showcased as the temperature changes had to be precise and factor a range of variables including the seasonal change.

Aggreko Process Services (APS) applied their deep technical expertise and understanding of the process to deliver a practical solution in just 8 weeks for the customer. With a proven track record of improving yield efficiencies through analysing current processes, we relished the opportunity to showcase our knowhow and quickly solve this customer’s problem.

Firstly, cooled water via modification in four mobile air cooled chillers was integrated into the production process. Once chilled, the water was reinjected back into the feedline, being mixed with existing hot water – this precise step ensured that steam was created at the crucial 36.5^oC needed.

The specific steam temperature meant that the crucial equilibrium of liquid and vapour in the PP-splitter was achieved. We utilised the cooling water allowing overhead vapour temperatures to be reduced – the propylene vapour can then be condensed to match the exact desired PGP specification.