Controlling emissions

The sulphur paradox is resulting in refiners being squeezed from both sides by tighter product specifications and increasing emissions legislation.

Although there are several technologies that take out the sulphur and nitrogen molecules from the hydrocarbons, these molecules then end up in some of the streams that must be cleaned up. Sulphur, mostly as hydrogen sulphide (H2S) is removed from the above streams using treating solutions. The treating solutions are then regenerated, thereby creating a H2S rich stream that must be handled.

Refiners are not permitted to just send this stream to an incinerator, burn it to SO2 and emit it into the atmosphere. They must convert the H2S to something else; a by-product that can be used, such as sulphur, or disposed of as non-hazardous waste. How can refiners manage all of this?

A range of investments through application of the Shell Global Solutions Pentagon Model can help refiners meet future regulation and environmental issues that deal with handling sulphur as a by-product. A portfolio of technologies as part of Shell Global Solutions’ Sulphur Technology Platform can help remove refinery emissions, enabling refiners to manage the sulphur paradox.

First pentagon: Handling more H2S

With refiners faced with doing more with less, the first step is to make sure they are getting the maximum use out of the assets they have before they start making new investments or wholesale changes to refinery operations. This process of going back and looking at refinery assets based on current and anticipated concerns is known as the first pentagon. It is the starting point in the journey to improved operations, which continuously adapt to changing product specifications and emissions standards.

Reducing emissions plays a key role in decision-making for refinery operators. To ensure, for example, that the refinery amine treating system is operating at optimum, maintenance of amine hygiene is very important. The Sulphur Technology Platform enables proper amine hygiene, ensuring that the treating solvent a refiner is using matches what the system was designed for. It also ensures that the type of amine molecule and the amount of amine circulating solution is correct, so that all the amine molecules are able to pick up the acid gas components.

Most gas streams contain some contaminants that degrade amine over time and form, for example, organic acids. These organic acids tie up the amine molecule and, as a result, refiners don’t get as much out of the amine as they would normally.

What Shell does in its own refineries every one to two years is to clean up the amine, bringing in outside truck-mounted systems to take out these contaminants so that the amine can be restored to its full capacity.

Applying the first pentagon can help refiners get five to 10 per cent more capacity out of the assets that they already have without making significant equipment changes. Before making a new investment, refiners that thrive are making sure they are getting the maximum out of what they have while they plan for any future additions and operational changes.

Second pentagon: Debottlenecking the amine system

When applying the second pentagon, refiners implement short-term solutions or revamps, allowing investments to be made during a normal turnaround.

Many new amine solvents available today are more energy-efficient and have higher capacity. As a result, change out of a solvent during a turnaround can help a refiner get more capacity in the system. For instance, a plant in the northern part of the US was using diethanolamine (DEA) as the solvent, and the inlet gas rates were about to increase by about 30 to 35 per cent.

Shell Global Solutions studied the facility and recommended changing the refining process to the Sulfinol-X1 process. The plant was able to boost capacity by 30 per cent and save energy for regeneration without making any hardware changes. The plant was able to lower its circulation rate due to the high capacity of Sulfinol-X and the smaller heat of regeneration requirements.

Another refinery was adding a new hydrotreater unit (DHT) and wanted help with optimising their amine system. They needed amine for the recycle hydrogen scrubber in the new DHT, and wanted the same amine for use in other parts of the refinery. The facility still needed to meet the sulphur specifications on the low-pressure fuel system. Shell Global Solutions evaluated the system and changed the monoethanolamine (MEA) process that they were using to the Shell Global Solutions’ ADIP-M (aqueous MDEA) process.

In addition, Shell Global Solutions recommended acid aided regeneration additive that was needed to keep the fuel gas H2S specification low. This helped increase the sulphur handling capacity from 167 t/d to 290 t/d (almost double the capacity of the system), supply amine to new DHT recycle gas scrubber and maintain low fuel gas H2S specifications without adding a whole new unit.

Even simple, small changes and investments can get refiners anywhere between 15 to 30 per cent more capacity in the existing assets.

Third pentagon: Achieving very low sulphur emissions

Refiners ultimately look to implement long-term solutions or schedule phased investments as part of the third pentagon. The Sulphur Technology Platform provides a range of technologies that can help refiners focus on long-term improvements while meeting product and emissions standards at the same time.

For example, the Claus, SuperClaus and EuroClaus units can take a refinery up to 96 to about 99.3 per cent sulphur recovery. However, if a refinery needs to go beyond this, they would need to add tail gas treating to the tail gas coming from the sulphur plant. The tail gas coming from the sulphur plant contains many sulphur species. In a tail gas treating plant, all these sulphur species are converted to H2S, which is then picked up by an amine system and recycled to the front end of the Claus unit.

The Shell Claus Offgas Treatment unit (SCOT) is a low sulphur, low-temperature tail gas treating unit that allows refiners to achieve > 99.9 per cent sulphur recovery. This particularly helps refiners meet the fluid catalytic cracking (FCC) consent decree, whereby refineries are expected to reduce sulphur emissions, or at least hold the sulphur emissions at the levels at which they were at before, whilst the feed to the FCC is increasing in sulphur content.

In the new Shell Global Solutions designs, the amine unit, the sulphur plant and the tail gas treating units are all completely integrated so refiners can thrive by achieving very high (>99.9 per cent) sulphur recoveries and low emissions. Shell Global Solutions, along with its various alliances, offer the extremely robust Sulphur Technology Platform that covers all facets of refinery operations where sulphur or nitrogen is present in any form, allowing refiners to respond quickly and effectively to changing environmental specifications.