Vapor-recovery unit passes South Texas field test

April 14, 2003
Performance testiccng of a vapor-recovery unit developed by Comm Engineering USA, Lafayette, La., has shown the technology capable of collecting 64 MMscf/year of vent gases with a value of about $350,000.

Performance testing of a vapor-recovery unit developed by Comm Engineering USA, Lafayette, La., has shown the technology capable of collecting 64 MMscf/year of vent gases with a value of about $350,000. In this application, such a return produces a payback period of about 4 months.

Testing of the Environmental Vapor Recovery Unit (EVRU) took place at a TotalFinaELf SA exploration and production facility in South Texas by the US Environmental Protection Agency's (EPA) Office of Research and Development as part of its Environmental Technology Verification (ETV) program to facilitate deployment of new technologies through voluntary performance verification and information dissemination.

The Greenhouse Gas Technology Center, managed by EPA's partner verification organization Southern Research Institute, Research Triangle Park, NC, is one of several verification organizations operating under the ETV program.

Mitigation technologies for the gas industry include various types of seals and gas-recovery technologies and are of particular interest to the center. These mitigation technologies are also of interest to members and affiliates of EPA's Office of Air and Radiation (OAR) Natural Gas STAR Program.

For this reason, the STAR Program provided funding and technical review and input in support of this verification.

The technology

The vapor-recovery unit (Fig. 1) is a nonmechanical eductor or jet pump that recovers vent gas by using high-pressure motive gas to entrain hydrocarbon vapors from low-pressure sources. The facility's existing dehydrated high-pressure natural gas pipeline supplies motive gas.

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A pressure sensor monitors the motive-gas pressure; its output signal controls the valve and regulator that maintain gas flow at the design pressure, typically 600-850 psig. The motive gas flows through a venturi orifice situated in a mixing chamber and creates a differential pressure within the unit's jet pump.

The mixing chamber contains a port that allows low-pressure vent gas (0.1-0.3 psig) to be drawn into the chamber due to the partial vacuum created by the motive gas as it expands through the eductor nozzle. The low-pressure vent gas drawn into the eductor's mixing chamber combines with the motive gas and leaves the eductor's discharge line at an intermediate pressure (~ 40 psig).

The gas can serve as fuel on site or be repressurized with a booster compressor and injected into a natural gas transmission line for sale.

It is a closed-loop system designed to reduce or eliminate emissions of greenhouse gases (methane and carbon dioxide), volatile organic compounds, hazardous air pollutants (HAPs), and other constituents present in vent gas.

Comm Engineering recommends the vapor-recovery unit for application to low-pressure hydrocarbon vent sources such as storage tanks, heater treaters, gas-dehydration units, water-polishing operations, low-pressure separators, and compressors.

Facility; test setup

Testing was at TotalFinaELf's El Ebanito exploration and production facility 30 miles northwest of McAllen, Tex. It handles separation of natural gas and crude oil condensate product, gas compression, and gas dehydration from wells within a 5-mile radius. In a typical year, daily crude oil production ranges between 900 and 1,200 b/d.

Existing vent lines bring the vapors from all seven tanks, five condensate storage tanks, and two gun-barrel tanks into a common, 6-in. OD header connected to the 2-in. suction line of the unit. The manifold and vapor-recovery unit's skid were near these tanks.

A 2-in. pipeline supplies motive gas to the unit. The motive-gas line includes a pressure regulator and a flow controller to maintain a design pressure of 850 psig. A 4-in. pipeline, operating at about 40 psia, conveyed the discharge gas to the facility's booster gas compressor, about 25 ft away.

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The booster compressor pressurized the gas and injected it into the sales-gas pipeline. Fig. 2 shows a schematic of the unit's installation and the center's measurement instruments; Fig. 3 shows the installed system.

The installed test vapor-recovery unit at TotalFinaElf's South Texas production site (Fig. 3).
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The vapor-recovery unit's primary purpose is to collect and transfer low-pressure vent gas for use or sale. The quality and quantity of gas evolving from each stock tank vary according to many factors, including condensate-production rate and composition, how frequently the inventory turns over, separator operating conditions, ambient temperature, and atmospheric pressure.

The secondary purpose of the unit is to control stock-tank pressure changes caused by flash gas being released from the stored condensate and working and standing losses due to condensate transferring activities. The tanks must be maintained at a slightly positive pressure to avoid air contamination or over-pressurization during all phases of a typical tank's duty cycle.

Each tank is equipped with a pressure relief vent (PRV) that automatically activates when the tank pressure exceeds about 4 oz (or 0.25 psig) above atmospheric pressure. The gas vents directly to the atmosphere until the pressure decreases to the specified level.

The unit maintains internal tank pressures between 0.10 and 0.30 psi over local atmospheric pressure. A pressure sensor, inside one of the five stock tanks, continuously monitors the tank's operating pressure. The unit's programmable logic controller (PLC) interprets this pressure reading and uses it to control the two separate eductors.

These eductors are configured in parallel and designed to recover 300 Mscfd (standard conditions of 60° F. at 14.7 psia) or 208 scfm vent gas. The primary eductor, with a capacity of 200 Mscfd (139 scfm), operates continuously to maintain tank pressures less than 3.2 oz (0.20 psig).

When a new inventory of condensate enters the tanks and the pressure begins to build to more than 3.2 oz, a secondary eductor begins operating. This eductor, with a capacity of 100 Mscfd or 69 scfm, recovers gas to maintain the tank pressure at less than 4.8 oz (0.33 psig). It turns off automatically when the tank pressure drops to less than 1.6 oz (0.10 psig), and the primary eductor is the only unit operating.

The design motive-to-vent gas ratio for the test site is about 5.2 scfm/1 scfm or 2.8 lb/1 lb. A PLC controls the unit's eductors and continuously monitors critical operating parameters. In the event the unit cannot maintain the required tank pressure and the pressure begins to build, some of the vent gas will escape through the PRVs to the atmosphere or flare.

Performance verification

Before installation, all tank hatches were sealed and standard industry leak checks ensured the tanks and the entire gas piping system were leak tight.

The test strategy consisted of collecting continuous flow measurements averaged in 1-min increments for the motive-gas flow rate, discharge-gas flow rate, the vapor-recovery unit's suction pressure, discharge-gas temperature and pressure, and ambient conditions for a period of 8 days. The center's data-acquisition system recorded all continuously monitored performance data.

Gas samples of the discharge and vent gas, collected throughout the test period, were sent to a laboratory for compositional and heating-value analysis. Fig. 2 shows the locations of all measurement instruments and gas sampling ports.

We maintain quality assurance in all results by rigorous adherence to data-quality indicators and data-quality objectives set before testing. Data-quality objectives must be met for each verification parameter for conclusions to be drawn. In this test, all data-quality objectives were met.

Gas recovery rate. A Rosemount Model 3095 integral orifice meter measured the motive-gas flow rate, and an American Meter Co. Model GTS-4 rotary turbine measured the discharge-gas flow rate. The difference between the two readings represented the gas- recovery rate of the EVRU.

Gas samples, collected from the vent-gas stream (unit suction) and the unit's discharge stream, were analyzed according to American Society of Testing and Materials (ASTM) Method D1945 and Gas Processors Association (GPA) Method 2286 to determine concentrations of , non-CH4 hydrocarbons, and BTEX (benzene, toluene, ethylbenzene, and xylenes).1

ASTM Method 3588 determined gas density, compressibility, and heating value analyses.2 CH4

The discharge-gas heating values were used to assign an industry-accepted monetary value of the gas recovered and subsequently sold. Four gas samples from the discharge stream and four samples from the vent-gas stream were collected during the test.

Annual gas savings, emission reductions. The 1-min gas-recovery-rate measurements also determined the total volume of gas recovered by the vapor-recovery unit over the verification period. It was calculated as the integral of individual 1-min flow measurements over the testing time.

To estimate annual gas savings, the total gas recovered was extrapolated for a period of 1 year following the verification period. In consultation with the host site operator, we decided to assume that the oil production rate and other operating conditions that existed during the field testing would persist over the year. It was further assumed that unit's operational availability would remain unchanged

Annual gas savings are simply a comparison between the estimated annual gas recovered with the EVRU system and two baseline scenarios.

The first baseline scenario assumes that no recovery system is in place and the vent gas is simply released to the atmosphere. In this case, annual gas savings will be equal to the annual gas recovered.

The second baseline scenario evaluates the annual gas savings for the test site where a conventional VRU unit had been used to recover vent gas. The center consulted with site operators to define the percent of time the VRU system had been down. The annual gas savings for the test site results from multiplying the annual gas recovered by 10% down time. In this case, the use of the unit will eliminate the gas previously vented to the atmosphere during VRU downtime.

Annual emission-reduction estimates for methane and HAPs were determined by multiplying the annual gas savings by the pollutant concentrations determined from vent-gas sample analysis. Emission estimates are reported as maximum potential reductions, assuming all the recovered gas vents directly to the atmosphere. Emission-reduction estimates are also reported for the site condition in which incremental savings incurred during downtimes of conventional VRU are realized by use of the EVRU system.

Value of recovered gas. Determining the annual value of the recovered gas involved multiplying the lower heating value of the volume of gas recovered annually by the market price in effect at the time of testing, $2.85/MMbtu.

Total installed cost. Comm Engineering and site operators supplied capital costs and included all equipment and accessory items attributed to the installation. Labor hours associated with the installation, setup, and shakedown of the EVRU were also verified. The total installed cost is the sum of the capital equipment, accessory items, and labor costs. Costs associated with center's measurement instruments are not included in this figure.

Test results

Tables 1-3 show test results and cost details.

Gas-recovery rate

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Table 1 shows daily average discharge gas, motive gas, and vent-gas flow rates. The overall average vent-gas- recovery rate for the testing period was 175 Mscfd. The daily average motive gas required to recover the vent gas varied between 635 and 775 Mscfd.

The overall average motive-to-vent-gas volume ratio during the test period was 4.2 scfm/1 scfm (2.2 lb/1 lb), and daily averages ranged from a low of 3.1 to a high of 5.7 scfm/1 scfm. The overall average is less than the 5.2 (by volume) or 2.8 (by mass) design ratio reported by Comm Engineering.

We had expected the vent-gas-recovery rate to be positively correlated with daily total oil production, but the relatively short testing period paired with a relatively constant oil production rate resulted in no observable trends.

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We had also expected a positive correlation between ambient temperature and gas-recovery rate because of the higher rate of generation of flash gases and working losses. The 3-day data set in Fig. 4 shows the expected correlation clearly.

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Fig. 5 shows more information on the unit's operation by showing motive-to-vent gas ratio as a function of vent-gas pressure, and Fig. 6 shows vent-gas-recovery rate as a function of the vapor-recovery unit's suction pressure.

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The data in these figures are based on 1-min measurements collected during the +900-psig motive-gas pressure condition and after the motive-gas pressure was stabilized to 850 psig. The figures show a systematic relationship between the unit's suction pressure, gas-recovery rate, and motive-gas flow rate.

The relationship also depends upon whether the primary eductor operates alone or in conjunction with the secondary eductor. Note that the secondary eductor never operated alone during the tests.

At the 850-psig condition, motive-gas flows generally varied 350-380 scfm with one eductor operational and 530-565 scfm with both units operating. The EVRU recovered 100-140 scfm gas with the primary eductor operating and 140-180 scfm with both eductors operating. These recovery capacities are consistent with the design values described previously.

When the system is operating at design motive-gas pressures, the vent-gas- recovery rate is consistent with design flow capacities and clearly responsive to the cycling of the second eductor unit. The average motive-to-vent-gas ratio was 3.2 scfm/1 scfm with one eductor operating and 3.5 scfm/1 scfm with two eductors operating.

At the 950-psig condition, the motive-gas flow rate generally varied 340-350 scfm with one eductor operational and 500-530 scfm with both eductors operating. With the primary eductor operating, the motive-to-vent-gas ratio was 3.2 scfm/1 scfm as in the 850-psig condition.

When both eductors are in operation, however, the motive-to-vent-gas ratio increases dramatically to 6.1 scfm/1 scfm, which equates to significantly less gas being recovered. In this case, the gas-recovery rate was relatively low and variable.

These observations make clear that, at higher motive-gas pressures, the system is unable to recover the volume ratio of vent gas. Eventually the system is unable to perform as designed and the unit's suction pressure, which is directly related to the tank pressure, begins to build and pressurize the entire piping network.

The verification test revealed that continuous and proper control of the motive-gas pressure is essential to achieving the best performance from the EVRU system.

Annual savings; emission reductions

The unit was estimated to have an availability of 99.9% for the purpose of these calculations. The VRU that had been in operation previously was estimated to have an availability of 90%. Had this particular facility been uncontrolled before installation of the test unit, the gas savings would amount to almost 64 MMscf/year. Compared to the VRU that was in place, the savings would be about 6.4 MMscf/year.

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Table 2 shows annual reductions of methane, HAPs, and other hydrocarbons compared to both base cases.

Value of recovered gas

Analyses showed the average lower heating value of the vent gas to be 1,919 btu/1 scf. At the average vent-gas production rate of 174,855 scfd, this would amount to 335 MMbtu/day recovered. The host site valued the various blends of gas it supplied to customers during June 2002 at an average $2.85/MMbtu. The recovered gas therefore was estimated to have a market value of $956/day or $349,318/year.

Total installed cost

The total capital, labor, and materials costs for purchase and installation of the EVRU are $107,958. Table 3 presents a breakdown of the costs.

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With the annual value of recovered gas calculated at $349,318, a simple payback period of 0.3 years is estimated for this site if no recovery system had been in place.

References

1. Standard Test Method for Analysis of Natural Gas by Gas Chromatography. ASTM-D1945-96ei, American Society for Testing and Materials, West Conshohocken, Pa.; 2001.

2. Standard Practice for Calculating Heat Value, Compressibility Factor, and Relative Density of Gaseous Fuels. ASTM-D3588-98, American Society for Testing and Materials, West Conshohocken, Pa.; 2001.

The authors

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David A. Kirchgessner (kirchgessner.david@ epa.gov) has been a senior research scientist for the US Environmental Protection Agency's Office of Research and Development in Research Triangle Park, NC, for 27 years. The last 10 years have been spent on research directed toward the quantification and mitigation of greenhouse gases from the fossil fuel industries. Kirchgessner received a bachelor's in economics and a master's in geology from the University of Buffalo. He also holds a PhD in geology from the University of North Carolina and a master's in public health administration. He is a registered professional geologist in North Carolina.

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Sushma Masemore ([email protected]) is a senior engineer at Southern Research Institute, also in Research Triangle Park, and a member of the Greenhouse Gas Technology Center. She holds a BS in chemical engineering from the University of Maryland, Baltimore County, and has experience in chemical process design, air-pollution-control technology evaluations, emissions-measurement methods development, and advanced energy performance testing.

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William Chatterton ([email protected]) is a senior scientist at Southern Research Institute and a member of the Greenhouse Gas Technology Center. He holds a BS in environmental science from the State University of New York at Plattsburgh and has more than 18 years' experience in air-pollution measurement and control and evaluation of pollution-control equipment.

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Robert Richards (richards@ sri-rtp.com) is a senior engineer with more than 9 years' experience in environmental engineering and 4 years' experience in manufacturing engineering. He drafts verification test plans and reports, manages, and conducts field operations, commissions a wide variety of test equipment, and implements QA activities. Richards holds a BS in industrial technology (welding engineering) from Utah State University, a 2-year certificate from the Montana College of Technology (diesel mechanics), and a BS in Art (ceramics) from the University of Wisconsin. He is a licensed professional engineer in Montana.

The installed test vapor-recovery unit at TotalFinaElf's South Texas production site (Fig. 3).