Mercury has always been present in oil, natural gas, produced water, condensate and soot from coal fired electricity generation but until recently it has not been widely known or acknowledged except in relation to coal. Accurate characterization and effective removal of mercury from these streams is becoming an emerging issue in the treatment of condensate and produced water generated during production of LNG (liquefied natural gas) and oil, and in removal of mercury from the fuels themselves.
There are a number of factors driving this emerging awareness and urgency, some of which are increased production, expansion of population, impact on flora, fauna and detrimental effects to process and user equipment. The numbers involved are large. There are about 99 billion barrels of produced water generated per year for example, at an average contamination rate of 1 ppm. Produced water alone contributes 37 million lbs. of mercury per year to the environment.
The pernicious and dangerous nature of Hg contamination is illustrated by explosions in Moomba, Australia and Skikda, Algeria (Figures 1 and 2) caused by mercury embrittlement. Customers are beginning to refuse fuel based on mercury content and projects are being delayed due to the presence of Hg. A joint venture in Australia between BHP and Exxon was recently pushed back at least two years with cost overruns in the billions due to discovery of mercury in the gas.
Mercury is a liquid metal and unique in its behavior in fuels, air and water.
The failure to recognize and characterize the unique states and behavior of mercury and treatment and analysis methodologies based on facile and unsubstantiated assumptions is the basis of the problematic nature of Hg remediation.
Analytical chemistry has lagged behind in this area as well. Nearly all Hg analysis is destructive-AAS (atomic absorption spectrometry) or ICP (induction coupled plasma)-and therefore will only indicate the presence but not the state of Hg. Knowledge of state is the critical factor in designing effective remediation technology. The behavior of Hg is extremely state dependent. One illustration is the case of LNG. Both the Moomba and Algerian explosions were caused by the presence of Hg. Typically the LNG processing consists of gross water removal, coalescence of suspended liquid hydrocarbons (at STP), (condensate also has Hg present), molecular sieve for trace water removal, sulfide or bromide impregnated granular media (usually carbon) for Hg removal, cooling and liquefaction of LNG fraction (C2-C5), and reverse osmosis, which recovers the retentate and burns off the permeate (methane).
Even though there are usually hundreds of thousands of
tons of Hg removal media deployed, multiple pounds of Hg pounds per week
are typically burned off with the methane. How is this possible for Hg
to make it through all of these process steps and why should it take hundreds
of tons of media to remove mere pounds of Hg? The answer lies in the implicit
assumption as to the state of the Hg in LNG. The assumption has been that
the Hg must be in a gaseous state and would therefore instantly react
with sulfide to form HgS
A second illustration is the case of Hg in produced water. Carbon or modified carbon (sulfide impregnated or other) is used to remove Hg from produced water. Despite this there are platforms which have been shut down due to Hg visibility beading out on the metallic structures. Again, incomplete characterization is at the root of the problem. In water, Hg can exist in ionic, organically bound colloidal or elemental states. Carbon will remove ionic mercuric salts but is not effective in removal of elemental or organic colloidal forms. Obviously and confirmed by testing, not all of the Hg in produced water is in the ionic form.
LNG and oil production companies are currently not able to generate accurate composition state analysis of the species of Hg in particular streams. The exact reasons are complex but have to do with changes and errors which occur during sampling and the inability of the analysis to discriminate between states.
Depending on concentrations, this device can be deployed in a side stream for minutes or days and the flow monitored. Each of the components can be destructively tested to yield the compositional breakdown as they have been pre-segregated in the trapping process.
This approach and device has been used to evaluate produced water and LNG streams with the following results:
Although we are dealing with Hg in both cases, it is obvious that the treatment approach for those two streams have to be weighted completely differently in order to accommodate the disparate species. Currently, in both cases, modified granular media (which is only appropriate for the ionic form) is used almost exclusively.
This analysis explains the poor results for use of granular media in LNG applications and the marginal results of granular media in the treatment of produced water. Hg treatment in LNG should be exclusively focused on conversion of aerosolized elemental Hg to a liquid state using patented MyCelx capillary technology or other types of coalescence.
Modified granular media will be effective in removing
ionic Hg from produced water but the treatment train must also include
components capable of handling elemental and organic forms of Hg, essentially
a scaled version of the characterization device itself.