Energy, economy and environment: process simulation’s role in a difficult balancing act
A significant number of glycol dehydration units in gas processing are operated with excessive over-circulation of glycol (EPA, Natural Gas Star). Clearly the main reason behind this practice is the objective from operations personnel to ensure the dry gas water content is well within specs and to stay away from operating problems. However there are a number of undesirable impacts associated with this practice that range from increased energy consumption to increased toxic BTEX emissions. A Canadian report (CETAC-West, 2005) identified significant economic potential for optimization of glycol dehydrator systems with low or no capital investment required. The study estimated that there were 2,400 units suitable for such investigations on the basis that a number of these units are rather small and installed
in remote locations. The potential annual benefit for glycol dehydrators in Canada (assuming 50% success rate in optimizing these units) is approximately 372,000 tonnes of CO2eq eliminated and 180,000 E3M3 of natural gas savings.
The Energy Resources Conservation Board is the regulatory body from the Alberta Government and it has introduced in their requirements the concept of a Dehydrator Engineering and Operations Sheetv(DEOS). It incorporates a graph where the dry gas water content and the total benzene emissions from the unit are plotted as a function of glycol circulation rate (see Figure below).
This graph may be used to identify the glycol circulation rate that will achieve the required dry gas water content and at the same time minimize the emissions of Benzene to atmosphere. Not shown in the graph but also relevant to this discussion are the energy savings achieved in the glycol regeneration step as the circulation rate is reduced. If the unit is operating with a glycol circulation rate of say, 1.5 USGPM, it can be seen from the graph that the dry gas water content is around 1 lb/MMSCF and the benzene emissions are close to 2.4 tonnes/year (in Alberta, new dehydration units must be operated below 1 tonne/year benzene emissions). A reduction in circulation rate to 0.7 USGPM will increase water content by 8% to 1.08 lb/MMSCF but result in a reduction of benzene emissions of around 60% to about 1 tonne/year.
Given the cost and difficulty of directly measuring the emissions on-site, most operators and regulatory agencies have turned to process simulation tools to calculate the emissions. Furthermore, these graphs can be automatically generated by setting up the model in HYSYS and exploiting the extensibility of the simulator to export the results to MS Excel or other software.
This is but one illustration where simulation can be used to support better decision making in both design and operations. By using simulation models together with reporting and analysis tools, it is possible to enhance our understanding of the various trade-offs between energy, economics and environmental goals that arise when optimizing process plants.
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