Natural gas processing

Hydrate formation prevention strategies

A detailed simulation study was performed to determine the likelihood of hydrate formation in a subsystem including pipelines and a wellhead. In case the risk associated with hydrates was considered to be high, a mitigation strategy would have to be developed. The Process Ecology Hydrate Extension was used to determine the requirements for either line heaters or methanol injection for those locations that showed potential for hydrate formation. Start-up conditions for wells were a particular concern as temperature gradients could be quite large; temperature at startup was lower than the minimum design temperature of pipes and could also result in brittle failure.

Strategies for hydrate inhibition such as line heating, methanol injection, and water removal were proposed and reviewed.

It is well known that the formation of hydrates in natural gas processing facilities and pipelines is a critical issue as hydrates can plug equipment, instruments, and restrict or interrupt flow in pipelines. Hydrates will form when the temperature is at or below the hydrate formation temperature, normally with “free” water present, and depending on gas composition and pressure. In general, hydrates can be prevented by:

  1. Maintaining the system temperature above the hydrate formation temperature by using a heater and/or insulation.
  2. Dehydration of the gas to prevent the condensation of a free water phase.
  3. Injection of thermodynamic inhibitors to suppress the hydrate formation temperature in the free water phase.

For wellsites, line heating or dehydration is often not practical or economically feasible, and the injection of hydrate inhibitors is an effective method for preventing hydrate formation. Methanol (MeOH) is widely used as an inhibitor in natural gas pipelines, particularly in cold climate facilities (e.g., Canada). In these difficult environments, methanol injection is usually the most economical solution for preventing hydrate formation and is often the only option.


Determination of MeOH Injection Rate

The determination of methanol injection rate can be a challenging task mainly because of methanol partitioning considerations: the injected MeOH may partition into three phases: (a) the aqueous phase, (b) the vapor phase and (c) the hydrocarbon phase. The amount of MeOH to be injected must be sufficient to suppress hydrate formation in the aqueous phase, and also to replace methanol “losses” to the equilibrium vapor and hydrocarbon liquid phases. In this project, a combination of the GPSA manual and process simulation were used to overcome these challenges and calculate the required methanol injection rate.  This information is crucial for safe pipeline operations.

Process Ecology has worked closely with Aspentech and their HYSYS simulation environment on several methanol partitioning studies through the years.  It is crucial to choose an appropriate fluid property package (PP) to determine the methanol injection rate. Some commonly used property packages, like Peng-Robinson (PR) will largely overpredict MeOH in the hydrocarbon phase. In 2015 Aspentech released HYSYS v8.8 with the addition of the Cubic-Plus-Association (CPA) fluid property package. This updated CPA PP can more accurately model methanol phase behavior, especially the liquid-liquid equilibria (LLE) including the prediction of the partitioning of methanol between water and the hydrocarbon phase.


Case study

A simplified PFD of a portion of the system is shown below:



The results of the CPA package are promising, showing a marked improvement in methanol inhibitor predictions in particular, when compared to alternative methods. For example, the amount of methanol required to suppress hydrate formation temperature in a specified fluid from 20 °C to 0 °C is as follows:



Compared to the rate predicted by the Peng Robinson PP, the CPA PP in HYSYS results in a MeOH injection rate that is much closer to the number calculated by GPSA.


Conclusions

In summary, while process simulators like HYSYS bring significant benefits and convenience to operating and engineering companies, results must always be carefully evaluated and selection of the correct property package when performing calculations is essential.For methanol injection rate calculations in HYSYS, we believe that the Peng Robinson property package vastly overpredicts the amount of methanol partitioned, or “lost” into the hydrocarbon liquid phase. We have demonstrated that the new CPA property package gives a much better prediction of required MeOH rates for hydrate suppression.