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Online hydrogen purity analysis 
boosts sulphiding (TIA)

Reactor Resources has utilised the Online Hydrogen Purity Analyzer to assist with the sulphiding of 40 hydrotreating units.

Randy Alexander
Reactor Resources
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Article Summary
Refiners have found that real time analysis of the recycle gas composition speeds up the catalyst sulphiding process while minimising the environmental issues resulting from purging sour gas to the flare or fuel gas system.

During sulphiding, dimethyl sulphide (DMDS) is typically injected into the reactor feed, decomposing to form hydrogen sulphide (H2S). H2S reacts with the metal oxides on the hydrotreating catalyst, forming active metal sulphides on the catalyst surface. The decomposition of DMDS also forms byproduct methane (CH4) that will dilute the recycle gas stream and increase the gas density.

A low hydrogen concentration resulting from H2S and methane formation slows the kinetic rate of the sulphiding reaction. Catalyst manufacturers recommend a minimum hydrogen concentration of 60% in order to maintain an adequate kinetic rate and to minimise coke formation. Increased gas density associated with excess methane make can also cause compressor issues and may lead to compressor trips, depressuring the unit.
In order to avoid these issues, refiners typically take gas samples in ‘sample bombs’ and send them to the lab for analysis. While this approach works in many cases, the time lag waiting on lab results can be 4-6 hours. By this time, adjustments to the gas composition have typically been made based on ‘best guesses’ instead of actual data. These adjustments involve excessive purging of extremely sour gas with fresh make-up hydrogen brought in to the system. However, purging of sour gas can cause environmental issues when excess sulphur is sent to the flare header or fuel gas system.

An Online Hydrogen Purity Analyzer will minimise and possibly eliminate the need to purge sour recycle gas since the real time data the analyser provides make the process much easier to precisely manage. This allows the sulphiding team to maintain hydrogen concentration above the minimum target level. In addition, the analyser provides meaningful safety enhancements since operations personnel will no longer be exposed to hazardous levels of H2S during the sampling procedure.

Case study
A US Midwest refiner utilised Reactor Resources’ Hydrogen Purity Analyzer for the start-up of a ULSD unit. The customer was able to monitor the gas concentration in real time via our cloud-connected system as the recycle gas hydrogen concentration dropped from 100% to below 70% less than four hours after DMDS injection began. With a continuous stream of accurate data, the refiner reacted quickly, purging the optimum amount of sour hydrogen and adding fresh make-up hydrogen only as needed. By using data from the Reactor Resources system, the refiner stayed below the stack sulphur limit of 160 ppm during the start-up.

With the ability to monitor the recycle gas concentration and injection process in real time, the refiner was able to complete the sulphiding process by using only 55.3 tonnes of DMDS, a saving of over €50 000 in chemical costs. It was later learned that the feed used during sulphiding also contained sulphur that was converted to H2S during the start-up. By also measuring the H2S concentration in real time, the sulphur from the feed was accurately accounted for, and the corresponding amount of DMDS was not injected. Instead of ‘following the script’ from the previous start-up of this unit when 72 tonnes of DMDS was injected, the refiner was able to use real time data to inject the optimum amount of spiking agent. Furthermore, issues resulting from over-injection, such as compressor trips and purging of excessively sour gas, were totally avoided.

Reactor Resources’ SmartSkid DMDS Injection System and Online Gas Analyzers have now been used to successfully sulphide over 300 hydroprocessing units across the US, Canada, and Europe.

This short case study originally appeared in PTQ's Technology In Action feature - Q4 2018 issue.

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