Jul-2010

Dealing with mercury in refinery processes

The effects of mercury contamination on operations, and its treatment, are emerging issues for refiners

Raymond Hadden and Tina Moss
Johnson Matthey Catalysts

Article Summary

It is evident that refiners will need to consider a wide range of financial issues in order to ensure the long-term profitability of their operations. One of the factors that could be utilised by refiners to enhance their operating profit is to consider sourcing lower quality crude supplies for their refinery. There are a number of chemical components of crudes that act to decrease the sales price of these feedstocks. For example, there are shipments of mercury- containing crude available at a price differential to mercury-free supplies.

If a refiner chooses to process these feeds then it is vital to understand the fate of mercury within refinery processes, refinery effluent streams and refinery products. Furthermore, as more mercury-containing crude is shipped around the world, the opportunity for cross-contamination of crudes as it is being transported or stored means that many refiners will need to review the impact of this impurity on their operations.

This article considers the possible locations in which mercury- containing crude will be found, it presents an overview of the chemical nature of the mercury compounds that are found in crude oil, and it considers the likely fate of these species across the refinery flowsheet. Finally, operational data will be presented, which demonstrate that Puraspec technology can provide an effective and environmentally assured means of removing mercury from a variety of process schemes.

Where can mercury-containing crude oil be found?
It is reported that belts of mercury lie across the surface of the planet and it is clear that these belts appear to be associated with the boundaries between existing or ancient tectonic plates. In particular, it is reported¹ that the largest mercury deposits in the world are located above areas in which plate subduction has occurred in combination with a degree of volcanic activity. It is observed that mercury most commonly occurs in crude oil that is derived from wells that are located at or near current or historically active plate boundaries (see Figure 1).

Why is mercury of interest to refiners?
The fact that mercury is present in many crudes at trace levels has historically gained little attention in comparison to other metallic contaminants such as arsenic, lead, nickel and vanadium, which are widely recognised as troublesome poisons in downstream catalytic processes. However, mercury poses a very different range of problems for refiners to consider in comparison to the more traditional and well-established metallic contaminants that are contained within crude. These problems arise because of the fact that mercury can be present in crude oil in a range of chemical forms, including its elemental form, which provides a surprising degree of volatility to the contaminant. This factor is revealed by the data (see Figure 2), which show that the high vapour pressure of elemental mercury ensures it is predominantly distributed among the liquefied petroleum gas (LPG) and naphtha product streams during the distillation of crude.

Due to this volatility of the elemental form of mercury, it cannot be assumed that mercury will be exclusively concentrated in the heavy ends from refinery processing. The potential distribution of this contaminant across a broad range of process streams and effluents means that refiners must consider a range of environmental, health, operational and commercial issues if it is known that mercury-laden crudes are being fed to their refinery.

Environmental issues
The United Nations Environmental Programme (UNEP) was established to “provide leadership and encourage partnership in caring for the environment by inspiring, informing, and enabling nations and peoples to improve their quality of life without compromising that of future generations”. This is a wide-reaching brief, but in recent times the UNEP has brought sharp focus upon the issue of the release of mercury to the environment. A number of detailed assessments have been issued by the UNEP that quantify the sources and sizes of emissions of mercury into the environment.^2,3 While it is accepted that natural sources such as volcanoes cause the release of this metal into the environment, it is also noted that anthropogenic sources such as mining and fossil fuel extraction are major sources of much of the mercury that exists within the ecosystems of the earth.

These detailed environmental analyses provide an interesting background to the mercury issue, but the most significant impact upon all hydrocarbon processors is likely to arise because of the global agreement that was reached by an intergovernmental conference during February 2009.4 This UNEP event gained agreement to launch negotiations on an international mercury treaty to deal with worldwide emissions and discharges of mercury, and it was also agreed that the risk to human health and the environment was so significant that accelerated action under a voluntary Global Mercury Partnership was desired while the treaty is being finalised.

The details of the treaty that will eventually arise from these negotiations are, of course, unknown. However, it may be prudent for refiners to take account of the rising global profile of the environmental impact of mercury and to assess the fate of any mercury arising from crude that is processed by their own site.

Health issues
Mercury is a toxic metal with a relatively high vapour pressure. Due to its high density and tendency to agglomerate and bead, it can also be concentrated in drains and other low points of process plants.

Upon opening or cutting any mercury-contaminated pipework or equipment, the potential exists for workers to be exposed to mercury, and hot work would be especially dangerous in this regard. Any such exposure must be limited to ensure conformance with all relevant national or international health regulations. It should also be noted that any mercury-contaminated equipment may require classification as hazardous waste and may not be treatable as ordinary scrap iron. For example, the EU has banned the disposal of waste containing mercury by landfill.⁵

At this time, very little work has been carried out to assess the health impact arising from the mercury that is contained in fuels (such as LPG or naphthas), which are produced by refineries and combusted by consumers (say, for cooking or transportation). However, research has commenced in this area and reports exist6 of mercury emissions into the environment from these sources.