NGL Recovery Technologies
In order to understand NGL recovery technologies, you must first understand their role in the processing of natural gas. Natural gas may contain ethane, propane, methane and other valuable heavier hydrocarbons. Depending upon market conditions, the recovery of these condensable hydrocarbons, known as NGLs or hydrocarbon condensates may be quite lucrative.
Due to the capital and operating expenses typically required to setup and run such a project, careful planning and economic valuation is mandated. The purpose of this document is to evaluate common NGL recovery technologies. Throughout the following series we will provide the necessary information to begin a preliminary analysis of a potential NGL recovery project. Ideally, a producer, engineer or consultant should be able to use the information provided here to quickly and inexpensively evaluate and determine whether a condensate recovery project is worthy of further consideration.
Determination of Condensates Available for Recovery
A quick estimate of the available condensates available for recovery should provide an approximation of the potential income that could be generated upon recovery.
The amount of hydrocarbon condensates available for recovery can be determined by multiplying the natural gas flow rate by the total GPM of the sample. To obtain the GPM figure, a typical analysis of natural gas provided by a laboratory will consist of a column containing the GPM or gallons of a particular condensate contained in 1,000 cubic feet of natural gas analyzed in the sample provided.
It should be noted that the characteristics of the produced NGL stream should be carefully considered. In the current economic climate ethane sells at a much lower price than does butane and heavier hydrocarbons.
Recovery Rate Estimation
Although the amount of hydrocarbon condensate available for recovery may be known, it is important to realize that no process is capable of recovering 100% of this amount. Depending upon the NGL recovery process chosen and the operating parameters set, the total recovery and recovery amount for each component may change dramatically. Additionally, the same equipment processing a stream containing a higher concentration of NGLs will have a higher recovery rate than when processing a leaner stream.
As a rule of thumb, recovery rates for condensate recovery options are roughly:
Joule-Thomson (JT) Plant
- Ethane Recovery <20%
- Propane Recovery: 60%
- Butane and Heavier Recovery: 80%
- Ethane Recovery: 40%
- Propane Recovery: 80%
- Butane and Heavier: 90%
- Ethane Recovery: 90% or Greater
- Propane Recovery: 95% or Greater
- Butane and Heavier: 99%
Heating Value and Dew Point Concerns
In many instances, the NGL and condensate composition of natural gas must be lowered in order to meet heating value specifications set by pipelines or to ensure that liquids do not form within a natural gas transportation pipeline. In these instances, the purpose of NGL recovery may be a necessary step in order allow the gas to be sold instead of for NGL recovery.
The heating value and dew point of a natural gas stream may be estimated if the composition is known. Estimation procedures are detailed in GPSA publications and will be discussed in a separate document.
NGL Recovery Technologies
Joule-Thomson (JT) plants takes advantage of the Joule-Thompson affect that causes a gas to usually cool when its pressure is reduced through an insulated valve. JT plants typically lower the temperature of the incoming gas to approximately -20 degrees Fahrenheit.
- Equipment is the least inexpensive of available options.
- Requires minimal, if any utilities or maintenance.
- May be operated unattended in rural areas.
- JT plant itself typically has no moving parts.
- Ideal if wellhead pressure of gas is very high and must be reduced to meet pipeline pressures.
- Expensive compression is required if inlet gas is not at high pressure.
- Lowest NGL and condensate recovery of available options.
- Pressure fluctuations will affect NGL recovery.
- Not usually attractive for larger gas flows ( >2.5 MMSCFD).
Mechanical Refrigeration natural gas processing enables the collection of condensates in the absence of high pressure gas sources. The chiller in a mechanical refrigeration unit cools associated gas below -20 degrees Fahrenheit which increases the pressure range over which condensates can condense into liquids. Mechanical refrigeration is flexible in the pretreatment required, but may leave a significant percentage of condensates unremoved depending on gas composition.
- Higher recovery rates than with standalone JT plants.
- No pressure loss is incurred on the gas stream.
- Potentially allows unit to be installed within existing process with minimal difficulty.
- Compression costs are lower than with JT plants
- Much less expensive than cryogenic separation processes.
- May be modular, allowing for rapid relocation.
- A single unit may be able to handle a large range of incoming gas flow rates.
- Condensate recovery may be ‘tuned’ by adjusting the operating pressure.
- Higher pressures will cause increased recovery of propane, ethane
- Typically requires electricity or a dedicated generator.
- Requires more maintenance than JT plant due to pumps, compressors and other integral components.
- Higher cost than JT plant.
- Unlike JT plant, will produce substantial noise, which may be a problem if located near dwellings.
Cryogenic separation is capable of removing C3+ like other condensate removal methods, but is also able to remove other more problematic components such as ethane and nitrogen. Refrigeration is used to chill a pressurized gas/condensate stream. Expansion of the refrigerated stream causes the temperature to reach sub -120 Fahrenheit. Under these conditions, ethane, propane and heavier hydrocarbons will typically condense.
- Nearly complete recovery of all condensates is possible.
- Nitrogen removal is possible.
- Single unit is capable of processing very large amounts of gas (> 100 MMSCFD).
- Not economically viable unless the amount of gas feed is substantial, typically (> 10 MMSCFD).
- Requires extensive maintenance program, typically with a dedicated full time staff.
- Installation is very large, typically requiring its own building or facility.
- Installation costs and site preparations are typically substantial.
- Substantial utilities and support infrastructure are generally required.
- With some rich streams, heavier hydrocarbons must be removed before entering the cryogenic unit to prevent the formation of solids.
This concludes our look at common NGL recovery technologies. In our next post, we’ll teach you how to easily estimate the value provided by each of these systems and calculate the return from any NGL recovery efforts. In the meantime, you might be interested in reading our white paper on enhanced condensate recovery. See below for more details.