Benzene Emissions in Glycol Dehydrators
Summary based on “Control of Benzene Emissions from Glycol Dehydrators”, a Best Management Practices Guide by the Canadian Association of Petroleum Producers (June 2006)
Over the last decade, there has been a trend to reduce benzene emissions from glycol dehydrators. The Alberta Energy and Utilities Board (AEUB) recently introduced a directive to further encourage emissions reductions; non-compliance with AEUB directives could potentially result in suspension of operations. Specific documentation and graphs are required which can potentially identify operational improvements that will result in lower benzene emissions. In addition, capital modifications and process alternatives may also potentially result in vastly reduced benzene emissions.
The upstream oil and gas industry extensively uses glycol dehydrators to remove water from natural gas streams in order to enhance the properties of the raw natural gas as a saleable commodity. The dehydration process also helps to prevent corrosion and hydrate formation in pipelines. Operators employ TEG, DEG or EG for dehydration of natural gas streams.
Benzene occurs naturally in some gas streams. Within the glycol dehydrator system, benzene and other hydrocarbons are absorbed by the glycol in the absorber. The rate of absorption is proportional to the glycol circulation rate. The still column vent is typically the focus of most emission concerns. During heating of the rich glycol in the still column and reboiler, water and hydrocarbons (including benzene) are emitted as vapours from the still column vent. The still column vent can also be the source of methane, ethane and propane emissions from stripping gas and the processed gas.
Operators in the upstream oil & gas industry typically purchase standard sized and equipped glycol dehydrator packages for their operations. In the field, companies operate the dehydrators at conditions that attempt to remove more water to minimize system operating problems. Such practices can increase benzene emissions from glycol dehydrators, due to over-circulation of the glycol and unnecessary use of stripping gas. This is especially common for glycol dehydrators at gas wells where gas production rates decline over time.
A report in 2005 (CETAC-West, 2005) identified significant economic potential for optimization of glycol dehydrator systems with low or no capital investment required. The paper estimated 2,400 units available for such investigations. The potential savings for Canada (assuming 50% success rate in optimizing these units) are of approximately $50MM with 372,000 tonnes of CO2E eliminated and 180,000 E3M3 of natural gas savings.
Minimizing Benzene Emissions from Glycol Dehydrators
Although industry is approaching the limit for benzene emission reductions from dehydrators using the current technology, further reductions may be achieved through:
• Improvements to the design of new or relocated glycol dehydrators;
• Further optimization of gas gathering systems; and
• More efficient operations of glycol dehydrators according to manufacturer’s specifications and operating conditions.
Field operating data can be fed into any of the technical evaluation programs being used to assess the performance of a dehydrator. These programs include GRI-GLYCalc and Rich-Lean methods, or other programs such as Prosim, HYSYS, or other “in-house” commercial simulators. Results of these evaluations can be used in a sensitivity analysis whereby specific components of the system can be identified as the most appropriate place for implementing emission control strategies. Care must be taken to ensure that the simulation models are properly reflecting the actual performance of the units.
Benzene Emissions Key Factors
Of all operating variables affecting benzene emissions, the circulation rate has the greatest impact. This is especially important when considering that operators may have to maintain higher than necessary circulation rates for dehydrating the gas, to overcome some of the inherent physical limitations of the equipment (e.g., the glycol flow distribution across the trays, pump minimum flow requirements etc.). Other important factors are absorber pressure and temperature, use of stripping gas, and existence of a flash tank.
In addition, supplementary or “add-on” emission controls are used to remove or destroy pollutants in the still column vent emissions. The most common practices involve the use of condenser and thermal (flare or incineration) systems, either separately or in series. These emission control options could be considered as an optimization of facility design and are best handled on a site-specific basis.
To encourage emissions reductions, the Alberta Energy and Utilities Board (AEUB) and Alberta Environment have jointly issued Directive 039, entitled Revised Program to Reduce Benzene Emissions from Glycol Dehydrators dated July 10, 2006. This “Dehydrator” Directive came into effect July 10, 2006.
Licensees must comply with the following new requirements by January 1, 2007:
• When evaluating dehydration requirements in order to achieve the lowest possible benzene emission levels, licensees must use the Decision Tree Process recommended by the AEUB and retain appropriate documentation for review by regulatory agencies
• After January 1, 2007, new or relocated dehydration units must have less then 1 tonne/yr benzene emissions.
Completion of the Dehydrator Engineering and Operations Sheet (DEOS)
The DEOS is a listing of the design and operating details of each dehydrator, and a graph showing the relationship between outlet gas water content and benzene emissions from the vent stack versus glycol circulation rate. For new dehydrators most of this data can be assembled and entered by the engineering contractor. The DEOS is a very useful tool for understanding the capability and potential misuse of a dehydrator.
Feedback from Industry has indicated that many dehydrators are operated at a much higher circulation rate than is necessary for adequate removal of water from the inlet gas. The water content rapidly flattens out below the acceptable water content as the glycol circulation increases. Note that the benzene volumes emitted rise rapidly with the circulation rate. Depending on ambient temperatures, and the potential for decreasing underground soil temperatures that promote hydrate formation in pipes, the operator sets a glycol circulation rate. With this operating graph, an operator can pick a glycol circulation rate that is near the beginning of the flattened part of the curve with the confidence that the exit gas will be dry enough to prevent hydrates from forming. This circulation rate is given to the Technical Contact who will update the DEOS with the new benzene emissions volume and send out a laminated DEOS copy for posting near the dehydrator.
There are a number of methods available to estimate or measure glycol dehydrator emissions. These types of programs are useful for assessing existing conditions and evaluating (or predicting) emission reductions associated with various changes to dehydrator operations and/or emission control equipment.
Simulation software such as GRI-GLYCalc™ or HYSYS™ (Aspen Technology) is used to evaluate benzene emissions from glycol dehydrators.
The GRI-GLYCalc™ simulation model, developed by the Gas Research Institute in the U.S, (now called the Gas Technology Institute) has received significant scrutiny throughout the United States. Accurate (C10+) gas analyses or rich/lean glycol analyses are required to run the model. Note however, that some caution must be considered. GRI-GLYCalc allows the use of default values that can sometimes lead to misleading results. Note that GTI is no longer supporting the program and will not address the "computational instabilities" that can lead to misleadingly high emissions from EG units processing lean gas.
How Process Ecology Can Help
Process Ecology can ensure that the requirements of ERCB Directive 039 are met by developing the required documentation and graphs based on operating data, as well as making certain that calculation methods and assumptions used are generating accurate results.
The challenges in fitting the appropriate data to the thermodynamic model used for process simulation cannot be underestimated; the difficulties involved are highlighted by the fact that the commercial simulation tools highlighted in the Best Management Practices Guide do not properly support the property predictions of EG-water mixtures in particular. Process Ecology has the expertise to validate the associated simulation models to ensure accurate results that can lead to true optimal operating conditions.
Process Ecology is currently developing a software utility that incorporates better parameters to represent these systems more accurately as well as opportunities to fit specific unit data to the simulation model. The utility design involves the automated generation of the DEOS report and required graphs. It cannot be overemphasized that the appropriate construction of the thermodynamic model to represent the glycol-water-gas mixtures is an essential element for a successful decision-support tool.
The evaluation of alternatives for benzene reductions can benefit greatly from a simulation study where a sensitivity analysis is included on the main economic parameters that impact the operating cost of these units. Process Ecology has the expertise to support the evaluation of such options and to recommend the optimal arrangements for minimum emissions at minimum cost.
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