Optimizing the Dehydration Process with Advanced Process Simulation
Jennifer Dyment
Product Marketing Specialist
Aspen Technology
Sunil Patil
Director of Business Consulting
Asia Pacific

Natural gas from reservoirs usually contains water vapor, the presence of this vapor causes flow assurance issues hence the need to dehydrate the gas and optimize the process. This article illustrates the role of glycol dehydrator unit in the field of natural gas pipelines.

In the operation of natural gas pipelines, a blockage or leak can cause expensive production losses, damaged equipment, and safety hazards. When water is present, for instance, gas hydrates can form creating an icy plug in natural gas mix tures, especially when at low temperatures and high pressures.. With water, carbon dioxide and hydrogen sulfide present, acid gases form and cause corrosion in pipelines which can lead to damaged downstream equipment. In order to create safer and more reliable operations, organizations need to remove free water from the natural gas.

Many governments or agencies regulating shared pipelines maintain restrictions on the water content of sales gas or fungible product. While there are many options to remove excess water, dehydration by a glycol is most commonly used by gas processing facilities with more than 36,000 glycol dehydration units in the United States. Triethylene glycol (TEG) is most frequently used, but other glycols including diethylene gycol (DEG) and monoethylene glycol (MEG) are also utilized.

There are however, still some issues with the dehydrator units as they are often overdesigned, resulting in high capital or operating costs. According to a USEPA report, TEG is recirculated two or more times higher than necessary. In order to ensure design options meet the necessary requirements of saving capital, solvent, or energy costs, thermodynamic modeling and a holistic view of operations is needed.

Understanding the Natural Gas Dehydration Process

A typical glycol dehydrator unit is divided into dehydration and regeneration (Figure 1).For dehydration,first,wet gas enters to the inlet scrubber to remove free water of the gas, and then passesthrough the contactor. The water-lean glycol enters top of the contactor and flows counter-current to the wet gas absorbing water from the gas. Then the water -rich glycol leaves the bottom of the contactor. The rich glycol is used as a condenser cooling medium for the regenerator gas before it enters the flash tank.Acidic gases and light hydrocarbons in the rich glycol stream evaporate at the flash tankand are used as a fuel.

For the regeneration part, the rich glycol stream from the flash tank enters the regenerator, which separates the TEG and water. The glycol is preheated through a heat exchanger by the lean glycol from the reboiler before it enters the regenerator. A stripping gas such as methane and nitrogen can then be injected into the reboiler to help reduce the water contents and reboiler duty. Finally, the regenerated glycol from bottom of the regenerator recycles into the contactor.

Figure 1 Typical glycol dehydration unit

The flash gas which evaporated from flash tank can be used as fuel for the reboiler or as stripping gas. Since the hydrocarbon liquid can cause several problems such as reducing the efficiency of reboiler, if the hydrocarbon liquid exist at the condition, three-phase separation flash drum is needed .Glycol circulation rate determine the water contents in the dry gas and amount and volatile organic components (VOCs) emission from regenerator as well. The higher glycol rate, the lower the water contents and higher the venting emission. Therefore, it is important to optimize the circulation rate of glycol to compensate between two.

In addition to water, another concern for many organizations is the fact that glycol also absorbs methane and BTEX (benzene, toluene, ethyl-benzene, xylene). Theseare considered hazard air pollutants (HAPs) due to the fact that they eventually emit into the atmosphere from the glycol regenerator. The BTEX concentration from the top of the regenerator can be hundred times higher than that in the raw natural gas by concentrating effect of the absorption process. Therefore, although the main purpose of glycol dehydration process is decreasing water content of natural gas, control of HAPs emission is also integral part of glycol dehydration process. Additionally, some light hydrocarbon may not leave from glycol in flash tank , but remain in the glycol and vent from the regenerator.

Modeling Dehydration Process with Advanced Process Simulation Tools

There are number of process conditions that can affect gas dehydration performance and significantly change results- i.e. TEG recirculation and stripping gas flow rate, temperature, pressure and number of equilibrium states. With all these changing variables, organizations need to have a holistic view of operations to better understand the process and help guide decisions to reduce costs and prevent damage to equipment. One way to do this is by utilizingadvanced process simulation tools.

There are currently advanced process simulation tools available today that offer property packages specifically designed and tested for difficult to model processes such as acid gas removal and dehydration (Figure 2).To gain these benefits and more, companies have also implemented a full integrated modeling and simulation engineering environment offering seamless workflows to automatically incorporate equipment sizing, costing, energy networks, and safety systems right in the process model.

For systems involving water, alcohols, and hydrocarbons, predicting the thermodynamic behavior is not as straightforward as in other systems. Various thermodynamic property methods have been used for simulating dehydration, such as Peng Robinson (PR) and Soave-Redlich-Kwong (SRK) Equations of State, and some have been developed specifically for the modeling glycol dehydration. Newer property methods, such as the Cubic Plus Association (CPA) EOS, are gaining popularity for accurately representing not only dehydration systems, but also methanol partitioning and mercury partitioning systems. The CPA EOS is similar to the SRK EOS, but it adds an association term to describe the polar effect between molecules.

Figure 2 Natural gas dehydration flowsheet in Aspen HYSYS simulation

Process Ecology, a Calgary based oil and gas consulting firm has been able to save clients $60,000 per day by reducing emissions and avoiding shutdowns in dehydration unit. By implementing an advanced engineering platform, Process Ecology developed an automated process to generate documentation required to meet environmental regulations. Through several successful projects, the company confirmed that the advanced simulation tools with a property package developed specifically for TEG glycol dehydration, they were using had the ability to accurately predict air emissions in glycol dehydration facilities. As a result, Process Ecology has shown savings up to $30,000 a year for a single dehydrator in energy costs, often through reductions in the glycol circulation rate.

Other companies have started to expand and improve its ability to model dehydration through the integration of the Cubic Plus Association (CPA) Equation of State (EOS) property package with their advanced process simulation tools. With the addition of CPA, organizations can now model dehydration involving TEG, MEG and DEG enabling them to improve accuracy of gas plant models with more options for dehydration solvents.

The Model for Future Success

Gas treating is crucial for meeting regulations. Better process understanding can help guide decisions to reduce costs and prevent costly damage to equipment. As engineers continue to drive improvement in plant operations, it is imperative they have the right simulation tools in place to help make informed decisions and remain competitive. Plant issues arise all too frequently and in order for organizations to ensure the best action is taken, operators need access to an integrated software platform. With the right technology, the engineers can make the right decisions for their plants to ensure profits outweigh costs. Additionally, with these solutions, organizations can minimize risk against unplanned events, as they will have the tools to adjust day-to-day actives to find the most effective and efficient way to run the facility.