Aug-2016

Adjusting gas treatment strategies to resolve methanol issues

It is not uncommon today for producers to introduce methanol into their hydrocarbon systems, either as a means of hydrate inhibition, as an additive blended with H2S scavengers, or for other purposes.

Don O’Brien, Anadarko Petroleum
Jesus Mejorada, Pilot Thomas Logistics
Luke Addington PE, Bryan Research & Engineering

Article Summary

While much of the methanol is recovered in liquid knockout drums, a portion of it remains in the natural gas or natural gas liquids where it eventually makes its way to downstream processing units.

The presence of methanol can have negative consequences for gas processors, such as penalties by pipeline operators for excessive methanol in NGL feeds, problems meeting propane quality specifications in fractionation units, or regulatory issues from methanol emissions.

If a reduction of the methanol content in hydrocarbon streams is desired, it can be done with an aqueous wash stream such as is found in an amine sweetening unit. Methanol will be readily removed by the aqueous amine at first but will steadily accumulate in the amine sweetening unit until some equilibrium is reached, after which the methanol escapes in both the sweetened gas and the acid gas.

Several options are available to the operator when confronted with methanol in amine units, and the correct solution depends on the needs of downstream processes. The methanol can be removed almost entirely through a purge of the regenerator reflux stream. These and other strategies are evaluated with a process simulator, the results of which are compared to vapour-liquid equilibrium and operational data.

In practice, a purge of the liquid reflux to the regenerator is an excellent means of providing an outlet for the methanol. Purge rates and disposal options are evaluated along with the possibility of recovering the methanol as a product for reuse.

Introduction
In recent years, more and more attention has been paid to methanol use in the oil and gas industry. It is a component that is quite common, used for a variety of purposes, and one that has been in use for decades. However, its use, or overuse, is beginning to cause issues for midstream processors as they struggle to meet new natural gas liquids (NGL) product specifications.

Historically, little attention has been paid to the presence of methanol in upstream and midstream facilities. Methanol enters a plant with inlet gas or is injected directly within the process, concentrating in the recovered NGL product with trace amounts present in the residue gas, depending on the process. The methanol then follows propane through fractionation, presenting a challenge for downstream customers as propane demand shifts away from fuel and into petrochemical markets.

To that end, NGL shippers - who had not historically been concerned about methanol - have begun imposing stringent methanol limits and levying penalties on producers for barrels found to be out of compliance. Some typical pipeline specifications for methanol are listed in Table 1.

Sources of Methanol
While methanol is a very common component found in the oil and gas industry, it is not a natural occurring component of petroleum. Its presence is entirely due to intentional addition into natural gas and liquid streams.

Methanol can enter a midstream facility in various fluid phases and from various sources. Understanding what these sources are can be instrumental in forming a complete plan to prevent it from ending up in the finished products.

The primary use for methanol in this industry is hydrate inhibition, a topic that has been well studied. [1] Hydrates are solid crystalline structures that form from a combination of water and hydrocarbons. The formation of hydrates has negative effects on operations, such as plugging of piping, valves, and other safety issues. Once formed, it is often necessary to shut down operations to allow equipment to warm up, a situation that could cost the producer considerable time and money. Inhibiting the formation of these solids is therefore quite important.

Hydrate formation is primarily influenced by temperature, thus, methanol injection is most often used in the winter time, creating a peak “Methanol Season” that most operators define as the period between October and February. Since methanol is also a common carrier solvent, it is not only present during the Methanol Season. It peaks during winter operations and subsides through the summer, but may always be present.

In general, industry has become complacent with regards to methanol injection, deferring to the most conservative mode of operation to reduce exposure to downtime and call-outs. The average cost of methanol in 2015 is on the order of $1.50 per gallon, though the total cost must also factor items such as handling, disposal, and potential off-specification penalties on the order of $1.00 per barrel. Recognising that the use of methanol has become excessive, operators had begun curtailing usage even prior to the enforcement of NGL specifications. For example, timing injection coincidental with plunger operations as opposed to simply keeping injection on while the well is not producing could curtail excess methanol usage.

Tools are available to accurately predict hydrate inhibition with methanol, such as ProMax® [2], and thus the optimal amount of methanol to use can be calculated. Such optimisations have been done with success. [3] Simulating the system could help reduce the use of methanol considerably, helping producers avoid penalties and the additional cost of the excess methanol. With the correct amount of methanol known, producers could still be conservative in their use of the inhibitor without using orders of magnitude more methanol than required.

The Path of Methanol through a Midstream Facility
Understanding the route that methanol takes through a gas plant can be helpful in developing strategies for its removal. When it arrives at the inlet knock-outs, it can typically be found in all three-phases of the plant feed.

While some of the inlet methanol is removed at the front end of an NGL plant, enough can carry through in the hydrocarbon phases to violate pipeline NGL specifications. A good model is required to accurately predict the distribution of methanol within a recovery process, accounting for polar and non-polar properties of methanol. For this study, ProMax’s Peng-Robinson Polar equation of state is used to demonstrate the distribution of methanol between the vapour, hydrocarbon liquid, and aqueous liquid phases at the plant inlet. For systems containing amine solvents, the Electrolytic ELR package is used in combination with the Peng-Robinson Polar package. The results from this package are compared to data from GPA Research Report-149 [4] in Table 2.

As can be seen from these data, the methanol present in the inlet vapour is significant enough that, when concentrated in the NGL stream, it can create off-specification products. Liquid hydrocarbons entering the plant can also contain significant quantities of methanol, as much as 3 to 7 percent by volume. [5]. Liquids collected at the front end knock-outs are typically stabilised, and the overhead vapours are routed back to the NGL recovery plant for processing, carrying methanol with them. So both the inlet vapours and inlet liquids can be a source of methanol feeding into the gas plant.