Natural Gas Liquids, or NGLs, are ethane, propane, butane and natural ‘gasoline’ (pentanes) associated with oil and natural gas production.
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These hydrocarbons represent a critical income stream for oil and gas operations. They are removed as liquids through condensation during production, and usually stored/shipped in a liquid form. Each NGL is defined by the number of carbons (C2, C3, C4, C5) in its chemical structure and thus each have different sizes and molecular weights. For more than a century, producers have exploited temperature differentials in their boiling point to separate each valuable component via traditional distillation processes.
NGL separation systems deployed at the wellhead, common in fracking sectors, consist of flow control, refrigeration and compression and help reduce flaring off valuable NGL and reduce their release to the environment. Unfortunately, prices for NGLs are volatile. And the expensive costs of these systems create a tradeoff between collection and flaring. There are three critical areas a new technology can focus on to improve the likelihood of NGL collection: 1) operational flexibility to adapt to price volatility, 2) carbon dioxide and hydrogen sulfide tolerance to work with sour gas streams and 3) the reduction of refrigeration to lower the energy penalty of collection.
In 2015 framergy established intellectual property for the adsorptive capacity increase for methane (C1) when combined with heavier hydrocarbons in the presence of Metal Organic Frameworks. In reflecting on the initial experiment, Company COO Ray Ozdemir noted that the heavier hydrocarbons such as propane (C3) left the system in a hierarchical order after methane. This critical observation formed a concept for a new NGL separation system based on Metal Organic Frameworks, or MOFs.
MOFs are crystals used as adsorbents in oil and gas operations. The material has the largest internal surface area known to man. In the framergy’s NGL separation system, the use of surface area and intermolecular interactions, replaces artificial energy inputs such as extreme compression and chilling to a measurable degree. The dispersive retention mechanism that is introduced with MOFs can help retain larger compounds longer and this elution order generally follows the boiling points of the compounds. In this application, framergy uses its AYRSORB™ F250, based on MOFs PCN-250 and MIL-127, which is available for research purposes through Strem Chemicals.
To prove thier concept, Mr. Ozdemir designed and built a gas speciation testing unit durable to wellhead gas pressures, wet and acidic conditions. The Company was supported by the US National Science Foundation, through its small business innovation research progam, which framergy is now a graduate of the highly competitive Phase IIb program. The test unit was designed to capture the test gas with AYRSORB™ F250 in one canister and release the individual gas components into another with the MOF. The operation and data collection were managed by integrating a LabView data acquisition system capable of setting flow control paraments and monitoring gas conditioning paraments in tandem. The initial set of tests were conducted using a simulated wellhead gas stream consisting of methane and NGLs and an Agilent 6850 Gas Chromatograph (GC). The gas consisted of 74.02% methane, 20.01% ethane, 4.96% propane, 0.50% n-butane, and 0.51% nitrogen, which roughly simulates ‘tail gas’, a gas which has had some treatment for impurities.
After control runs of the test unit on the simulated tail gas stream demonstrated that no gas separation occurred with an empty canister, both canisters were packed with approximately 245 grams of dried AYRSORB™ F250 and purged using a combined pressure and vacuum technique. The MOF in Canister 1 was exposed to the tail gas stream for 30 minutes, and then the canister was refilled with the same gas to a pressure of 750 psig to compensate for the pressure drop due to gas adsorption. Running at room temperature, pressure was released from Canister 1 in stages of 50 psig at a time, and a sample was analyzed after each stage from Canister 2. Canister 2 was emptied out every time after two pressure release stages (the canister would reach to 100 psig) to test for separation efficiency of each constituent. After treatment, the source gas was deposited into multiple sink tanks at 100 psig with different hydrocarbon concertation based on the discharged pressures.
The pressure in Canister 1 was decreased from 754 psig to 699 psig in the first run (Canister 2 increased to 40 psi), and in the second run, the pressure in Canister 1 was decreased from 699 psig to 650 psig (Canister 2 increased to 71 psig). A sample from Canister 2 was analyzed at 40 psig (first run) and at 71 psig (second run). After the second sample, Canister 2 was emptied out. The steps were repeated until Canister 1 reached 150 psig. After reaching this pressure, Canister 1 gas was discharged to half its pressure because both canisters reached pressure equilibrium, then Canister 2 was analyzed and discharged after each run. The gas separation test with tail gas showed efficient separation of methane from the rest of the hydrocarbon components. The first gas stream was collected with 95.4 mol% methane and 4.3 mol% ethane. As the gas pressure in Canister 1 decreased, the methane purity decreased. A jump down in methane concentration followed along with a jump up in the ethane concentration. The jump in concentration due to ethane going through a phase change from liquid to gas phase at 540 psig in room temperature when treated with a MOF.
Next, a continuous flow system test was conducted to test the adsorption and separation efficiency of AYRSORB™ F250 continuously rather than batch-mode processing. Using the same setup, a pressure and a mass flow controller were connected to the tail gas stream source gas cylinder to control and regulate the pressure and flow rate with a 5 sccm (standard cubic centimeters per minute) flow rate at ambient pressure. Methane was the first component to exit the column after 23 minutes in a high purity stream of 99.85% purity methane. In a second sample analysis, the treated stream was composed of methane and ethane with concentration slightly above their source concentration and with negligible concentrations of propane and butane. The continuous process demonstrated the capability of separation of tail gas providing high purity methane and separation of NGLs from tail gas.
The single stage and continuous process demonstrated the capability of separation of tail gas streams providing high purity methane and separation of single components of NGLs from tail gas. Both the single stage and continuous process were then applied to a simulated gas stream of 54.90% methane, 22.00% ethane, 13.20% propane, 5.10% butane, 1.00 n-pentane, 0.26% n-hexane, 0.11% n-heptane, 0.51% carbon dioxide and 2.90% nitrogen, which roughly simulates ‘associated gas’, a gas which is more similar to gas coming up from an oil well without treatment. Here a -4 psig vacuum was pulled after Canister 1 reached 0 psig. The gas separation test with the simulated associated gas stream upgraded the methane concentration from 54.9 mol% to 73.1 mol% and the ethane concentration from 20 mol% to 60 mol%. At a very low concentration, methane and ethane were very close in concentrations between 40-50 mol%. These results showed that ethane concentration can be upgraded to a concentration three times higher than the source concentration of ethane using this technology.
The continuous flow system also showed the that with MOF treatment, separation efficiency could be achieved with an associated gas stream. Here, the second sample analysis detected 88% methane with some residual ethane and propane. Hexane concentrations were detected after 150 minutes and Heptane concentrations were not measured during the 270 minute analysis showing that associated gas can upgrade methane concentrations and separate NGLs, while removing heavier hexane and heptane in the presence of acidic gases (carbon dioxide).
NGL processing systems need to reach pressures of ~1000 psi and chilling temperatures of -30 °C (or -107 °C for ethane). The system described above which utilizes MOFs can improve the business case for NGL separation and collection at the well head. The system would only need pressures of ~500 psi and would not require any refrigeration. Once pressure is applied, this initial energy charge can be spread across the entire system, even in when acidic gases are present. Fundamentally, the framergy MOF system should have a huge capital and operating cost advantage when measured against current systems.
Originally authored by O.K.Ray Ozdemir, Carlos Ybanez and Jason Ornstein. Funding support for the US National Science Foundation.