MERCURY,THE VOLATILE SURPRISE IN GAS PROCESSING

摘要

The concentration of mercury in natural gas varies widely from 450-5000 mug/Nm3 in some fields in North Germany to 180 mug/Nm3 for Groningen in The Netherlands to less than 0.01 mug/Nm3 in some parts of the US and Africa.Mercury can be present in many forms in hydrocarbons,such as elemental,organic,metal organic or inorganic,however,in gas processing plants conditions are such that elemental mercury is the prevailing species.By making use of Shell's Modified and Improved Redlich Kwong equation of state,to calculate the distribution of elemental mercury between gas,condensate and aqueous streams and combining this with a HYSYS simulation of a gas processing plant,the distribution of mercury over the various process streams can be predicted.Using the model a case was generated in which the mercury content of the export gas was set at 30 mug/Nm3,mimicking the current situation in an operating plant.Using the mercury inlet concentrations that generate a mercury concentration in the export gas of 30 mug/Nm3,mercury levels for the produced LPG and condensate that are close to the levels actually measured were predicted,thereby validating the current model.Consequently,we now have the tools to calculate the distribution of mercury in various types of gas processing equipment such as gas-liquid separators,condensate stabilizers and debutanizers.By implementing the results for the individual process units into a HYSYS model of the whole gas processing plant we are capable to predict mercury concentrations in the most important process and product streams by measuring the mercury contents of two gas streams.In the example given in this paper 80 % of the mercury present in the feed gas would report to the export gas,whereas this would be 49 % of the mercury present in the feed condensate.Liquid mercury deposition would first occur in the LTS unit,the unit with the lowest operating temperature in the process flow scheme.