Reliable valve operation
Published by Callum O'Reilly,
Senior Editor
Hydrocarbon Engineering,
Molecular sieves, or molsieves, are used in the refining and petrochemical process industries as adsorbents. They are mainly used in scenarios whereby very dry streams are required, helping to remove water and other oxygen sources to avoid catalyst deactivation and potential corrosion problems in the process plant. Molsieves exist in different pore sizes and their use varies between applications.
The drying process typically happens in two stages: adsorption and regeneration. During the adsorption phase, fluids pass through the molsieve beds, adsorbing the water content that is present in the fluid stream. Once the molsieves are saturated, they must be regenerated. In dryers, this regeneration happens by passing hot gas through the molsieve beds which carries the adsorbed water content, thereby re-generating the beds for further adsorption. This is a continuous process involving several adsorption beds.
While there are many process challenges involved in the dryer process, this article will discuss those related to the valves that are required in these dryer units.
Sequencing valve challenges in molsieve dryers
On-off valves used in conjunction with molsieve dryers face certain challenges, which need to be considered during the process of selection.
Firstly, there is a possibility of molsieve dust entering critical areas of the valve. In particular, this is an issue that has been reported by some customers as related to switching valves. Over time, the molsieve pro-duces small particles which can escape from the molsieve beds and eventually enter the pipeline and valves. These particles are not part of the actual process fluid but are a result of normal operation. The challenge is to ensure that the impact of the particles on the equipment and valves is minimised. If particles enter the critical areas of the valve, they can disrupt its operation and hamper overall process efficiency. In addition to this, valves undergo thermal cycling. In the adsorption phase the molsieves adsorb water in the bed, while in the regeneration phase hot gases pass through. Therefore, valves typically experience one set of process conditions with lower temperatures during the ad-sorption phase, and higher temperatures during the regeneration phase. There have been cases where a customer has issues with a valve that cannot be operated during the high temperature phase. As a result, they have to wait for the temperatures to fall.
Thirdly, the molsieve dryer process demands that the valves frequently switch from one phase to another, meaning that valve cycling can be higher than in traditional on-off valves. High cycling can lead to…
Written by Dhanesh Bhaskaran, Valmet, India.
This article was originally published in the April 2023 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/04042023/reliable-valve-operation/
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