Flash-gas (petroleum)

In an oil and gas production, flash-gas is a spontaneous vapor that is produced from the heating or depressurization of the extracted oil mixture during different phases of production.^[1] Flash evaporation, or flashing, is the process of volatile components suddenly vaporizing from their liquid state. This often happens during the transportation of petroleum products through pipelines and into vessels, such as when the stream from a common separation unit flows into an on-site atmospheric storage tank. Vessels that are used to intentionally “flash” a mixture of gas and saturated liquids are aptly named "flash drums." A type of vapor-liquid separator. A venting apparatus is used in these vessels to prevent damage due to increasing pressure, extreme cases of this are referred to as boiling liquid expanding vapor explosion (BLEVE).

The composition of the gas that is flashed is dependent on many factors, therefore it is suggested that all extractions be analyzed to determine accurate compositional values. As a generality, this definition applies to the nature of flashing

The production of flash-gas and its release into the atmosphere, via venting and improper handling during production, is of concern to environmental efforts due to the presence of

(VOC) which have been suggested to have harmful long-term environmental impacts. Various efforts by organizations around the world have been made to develop appropriate guidelines for handling flash gas as well as tools for evaluating flash emissions through model based calculations.

Natural Gas Liquids/Liquid Petroleum Gas

Natural gas liquids (NGL) are the fraction of the

Capturing NGL’s has shown an uptick in their economic share of U.S. production levels the past few years,[6] sparking interest for advancement in recovery techniques for petroleum extraction. Uses of NGLs vary, from other combustible fuels like autogas and natural gasoline, to the manufacturing of plastics. The apparent value of these materials has put the level of products developed from NGLs at an all-time high in the United States in 2015.^[6] Despite its value and use, many of the components that make up NGL and LPG become considered “waste gas” during production and are flared on-site or vented into the atmosphere as flash gas.^[7] This can be due to a lack of infrastructure to capture and store and/or transport these liquids to market. Profitability under current models is highly dependent on the access to pipeline infrastructure, gas volume produced, and the number of/distance between production facilities (batteries).^[8]

Environmental Concerns

Flash gas is the emissions from flash evaporation, breathing emissions, and working emissions. Breathing, or standing emissions, occur naturally as liquid sits in a vessel and undergoes normal evaporation into the atmosphere. Working emissions are created when the liquid is agitated, such as during transportation, emptying/filling of tanks, sampling, etc.^[9] Contained within all of these emissions are materials that have been deemed hazardous to humans and the environment by regulatory agencies and scientific bodies worldwide.^[10][11] The complex nature of chemical interactions in flare, flash, and other production vapors have spawned many efforts to describe and control how these hydrocarbons interact with other materials in the atmosphere to create air pollution.^[12] The study of these interactions is encompassed by the field of atmospheric chemistry.

Hazardous Air Pollutants (HAP)

The environmental impact of emissions can be quantified in terms of the amount of

There are currently 187 different chemical compounds that are classified as HAP which are declared in the

The EPA, as of 2015, has public record of two [2005/2011] National Air Toxics Assessments (NATA) which present the effects of air toxic emissions on a national-scale.^[17] In addition to identifying and prioritizing air toxins, the assessment determines specific locations that are at risk so that corrective measures may be implemented. Reports also go into detail about potential cancers and health problems associated with the level of reported toxins.

Greenhouse Gases (GHG)

It is estimated that 90% of GHG emissions from

.

One study estimated a 40% increase in the atmospheric concentration of the GHG carbon dioxide (CO₂) is due directly to human activities since 1750.^[21][22] GHGs other than CO₂ which are produced during petroleum operations are also a concern to the environment. On average, 43.6% of waste gas from a typical plant is methane.^[23] Methane has 28-36 times the amount of global warming potential (GWP) as CO₂ because of its ability to absorb more radiation energy.^[24] Nitrous oxide has a GWP of 265-298 times that of CO₂ and is known to also be released during flaring and combustion of fossil fuels.^[25]

Volatile Organic Compounds (VOC)

In 1977, the EPA released “Recommended Policy on Control of

Photochemical Reactions

Much of the negative impact associated with

Mechanistic description for the formation of Photochemical Smog

${\displaystyle {\ce {{{\mathit {N}}O2}+{\mathit {h}}v->{{\mathit {N}}O}+{\mathit {O}}}}}$ (1)

Nitrogen dioxide reacts with UV radiation to form nitrogen oxide and an oxygen radical.

${\displaystyle {\ce {{\mathit {O}}+{\mathit {O}}_{2}->{{\mathit {O}}_{3}}}}}$ (2)

The radical oxygen combines with O2 gas in the atmosphere to form ozone.

${\displaystyle {\ce {{{\mathit {O}}_{3}}+{\mathit {N}}O->{{\mathit {N}}O_{2}}+{\mathit {O}}_{2}}}}$ (3)

Ozone reacts with nitrogen oxide to form nitrogen dioxide and oxygen.

${\displaystyle {\ce {{{\mathit {R}}H}+{\mathit {O}}H{.}->{{\mathit {R}}{.}}+{\mathit {H}}_{2}O}}}$ (4)

A species of VOC bound to hydrogen reacts with hydroxide to form a VOC free-radical and water vapor.

${\displaystyle {\ce {{{\mathit {R}}{.}}+{\mathit {O}}_{2}->{{\mathit {R}}O_{2}}}}}$ (5)

The free-radical VOC reacts with oxygen to form a VOC bound with oxygen gas.

${\displaystyle {\ce {{{\mathit {R}}O2}+{\mathit {N}}O->{{\mathit {R}}O^{-}{.}}+{\mathit {N}}O_{2}}}}$ (6)

The VOC oxygen bond is broken in the presence of nitrogen oxide, forming a VOC-oxygen anion and nitrogen dioxide to further participate in reaction 3. Where hv is UV radiation and R is a volatile organic compound (VOC).^[38] Additionally, some organic compounds (notably aldehydes) can photolyze in the atmosphere to form radicals which participate in atmospheric reactions. [46] VOCs or oxidant precursors are emitted to the atmosphere from both natural and man-made sources. Globally, natural emissions appear to outweigh anthropogenic emissions. However, it is the high concentration of anthropogenic sources of volatile organics together with NO_x emissions from combustion processes in urban areas which increase the risk of the urban ozone problem. Wind and other climatological activities (transport mechanisms) then carry the formed oxidant into rural areas.

VOC Diagnostics

Studies have been done to try to find if VOCs can be directly related to cancers in the human body. One such example is a group that collected breath samples from a lung cancer patient to determine the VOC content of the lungs before and after surgery.^[39] This study suggests that there may be certain biomarker VOCs that indicate the presence of a disease or cancer in a patient. Additional researchers have also begun trying to identify ties between the onset of cancer and the levels of VOCs in the environment.^[40] A database to gather information on cancer and other aspects of VOCs in human cell lines, the human volatilome, has been started by researchers.^[41]

Secondary Organic Aerosols (SOA)

The contribution of VOCs to the levels of SOA in the atmosphere have been modeled using simulation chamber experiments to gain a better understanding of the chemistry involved.^[47] SOAs are said to be a major causative factor to the mortality rates associated with the exposure to harmful fine particulate air pollution however there is an absence of in vitro particle exposure techniques to test for toxicity. There is research currently being done to find the interaction between SOA’s and lung cells to see their potential harmful effects.^[48]

Methods for Calculating and Estimating Flash Gas Composition and Flash Loss

Flash Gas Calculations

Calculation of flash gas content is often based on the principle of vapor-liquid equilibrium (VLE) and combines theories like Raoult’s Law with component mass/energy balances, similar to those used in distillation unit operations.^[49] Since the mixture is of multiple components, more complex equations like the Rachford-Rice equation are applicable in ideal situations and have frequently been used in chemical plants and refineries.^[50] It is important to note that these equations are based on the ideal gas law and liquids at thermodynamic equilibrium, while hydrocarbons in oil and gas production are considered to be non-ideal or real, and may not be in equilibrium. Additional relationships, such as the Van Der Waals equation and other equations of state (EOS) can be applied to account for these discrepancies and can be used to provide a better estimation of flash gas content. Higher molecular weight hydrocarbons show even further deviation from ideal behavior and need additional computational adjustments, such as Sutton’s^[51] modification to Steward et al.^[52]

Chemical Simulation

More realistic modelling techniques encompass variable conditions that can occur on-site during

(GOR) using precise measurement techniques. However, this only provides insight on the sample of flash gas and does not account for real time fluctuations of all on-site sources of flash gas, including working and standing losses.

Estimations Based on Volatile Organic Compounds

Another acceptable technique in flash estimation is the Vasques-Beggs Equation (VBE)

Equations of State

An equation of state (EOS) is a thermodynamic equation relating state variables which describes the state of matter under a given set of physical conditions.^[56] EOS are used to make calculations about the predicted phase-behavior of hydrocarbons before extraction from a reservoir.^[57] Soave-Redlich-Kwong and Peng-Robinson are examples of two commonly used EOS in industry for determining critical properties of multicomponent mixtures.^[58] These equations can be useful for the prediction and estimation of flash loss when used in conjunction with sampling and reservoir data.

Reduction of Flare Gas

Vapor Recovery Units

Vapor Recovery Units (VRU’s) have commonly been used to capture vented gas and other waste gas (

The gas portions can be used to fuel on-site operation, be transported to storage tanks for further separation, can be piped out directly to another facility, or be injected into a compressor; the liquid portions (NGLs) that commonly form are normally sent to the water tank. If they are sent to oil storage they will re-vaporize and go through the VRU, creating an endless cycle.

VRU’s can create revenue through the resale and use of the captured vapors that would have been vented into the environment and lost. The revenue produced is dependent on the amount of vapor captured and sold into a pipeline. Analysis has been done to show the economic impacts from emission reduction and capture using VRU and other technology.[60] These units also reduce the amount of emissions of VOCs associated with oil and natural gas production, again by preventing them from being vented directly into the atmosphere.

There have been concerns raised about the efficiency of VRU’s.^[61] It has been pointed out that vapor recovery units (VRU’s) cannot recover lighter hydrocarbons that have been dissolved into the liquid phase because the units are designed to recover what is already in the vapor phase. Also, when the gas in put under high pipeline pressure, NGLs are condensed out and are returned to the water tank (in rare instances they are processed and sold as NGL’s).

Thief Hatch Alternatives/Vapor Recovery Towers

Thief hatch, also known as a gauge hatch, is a term given to a close-able aperture on a tank or vessel used in the oil and gas industry.^[62] These hatches are placed in atmospheric or low pressure tanks in order to allow for level or sample measurements and to provide vacuum relief to the vessel. Thief hatches are also used in gauge tanks and in transport vehicles where they must be opened to the atmosphere to take measurements and samples. Flash gas is readily leaked whenever these hatches are opened and therefore they can pose a problem in terms of emissions. There have also been reports of fatalities caused by manual tank gauging and sampling via thief hatches.^[63]

Vapor recovery towers (VRT) have been posed as a potential solution for vapor loss via thief hatches. VRTs are a way to reduce the vapor loading on storage tanks by removing some of the flash gas before it gets to the oil tanks. The VRT can improve safety by reducing the potential for issues relating to high vapor evolution from the oil tanks.

Heater Treaters

Heater treaters are often used in industry for the following: To break up emulsions to separate the oil from produced water, stabilize the crude oil or condensate by separating volatile, lighter hydrocarbon fractions, (C1-C8) from the heavy, less volatile fraction (C9+) for safety reasons.^[64] Vent gas from heater treaters is known as a significant source of flash gas and consequential VOCs. It is possible to re-route this vent gas to a VRU to capture some of the stream or other processing units in order to reduce emissions and make additional revenue.

Oil and Gas Separators/Pressure Optimization

Three phase separators are used in oil and gas production to separate water, oil, and gas into isolated streams.^[65] A separator, in general, is a pressurized vessel that is used to divide a product based on principles of chemical separation such as phase and density. These vessels are also known as de-liquidizers or degassers because they either remove liquid from the bulk gas stream or remove gas from the bulk liquid stream during processing. Pressure optimization based on sample compositions taken upstream can provide a solution to inefficient separations and can help to reduce the overall amount of flash gas produced.^[66]

See also

• Flash-gas (refrigeration)
• Vapor recovery

References