Natural gas Liquid Production

condensate ngl optimization May 10, 2020

Introduction

Optimizing production capabilities makes dollars and sense. Often improving how we operate and how we control gas processing facilities can greatly increase the profitability and margin of such operations. Near infrared spectroscopy can be used to optimize such facilities in real-time and increase yield of high value products, improve product quality, reduce emissions and increase profitability.

Hydrocarbons, such as ethane, propane, butane etc. differ in value, with the heavier hydrocarbons generally worth more than the lighter hydrocarbons. The fluids produced from a natural gas well (or associated gas from an oil well) will containing varying quantities of these products. Mixed together, these products have less value on the market than if they are separated. This is often referred to as the fractionation spread, or how much money can be achieved by separating the whole inlet fluid into its component products like ethane, propane, butane and C5+ ( sometimes referred to as natural gasoline or condensate). The "frac" spread is an estimate of the profit margin received by a gas processor and is determined by the relative prices of natural gas and natural gas liquids (NGL). 

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Fractionation and Distillation

Fractionation towers operate by controlling the temperature and pressure of the fluids in the tower. I like to use opening a bottle a coke as an example. If we let the pressure off by opening the cap, bubbles come out. If the coke is cold, less bubbles come out than if the coke is hot. Frac towers work the same way, lowering the outlet pressure and increasing the temperature if they want to drive off more gas and lowering the temperature with increasing pressure if they want more light products to stay in the liquid.

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When operating a gas processing facility, whether it be a natural gas production plant, a refinery, or a synthesis gas operation, it is often necessary to monitor and control the fractionation towers. In general, the lighter products that come off the overheads are of less value than the liquids produced from the bottoms of such towers. For many products, there is a specification on the product quality and control of the separation process will affect yields and product quality. Many plants use a "set and forget" philosophy - determining the optimum operating conditions and running there for extended periods of time. However, changes in the inlet composition as new gas sources are brought online, or changes in commodity prices can dramatically affect what operating set point leads to the most profits.

Why The Need To Optimize

In the Canadian marketplace, proper optimization has never been as important as it is today. Excess natural gas capacity has pushed spot prices at plant outlets negative for natural gas, and there is little point in producing a field for its gas production only. The liquid products such as propane and butane and more importantly condensate have retained value but the prices have been on a roller coaster ride. Propane prices dropped dramatically when the direction of flow on the Cochin pipeline was reversed to bring more condensate from the USA into Canada (2015) while stopping the transport of Canadian propane to the USA. Recently, projects like the AltaGas Ridley Island and Pembina Prince Rupert propane export terminals as well as two large chemical plants to make polypropylene from propane have raised hope of increased demand and prices. In the last year, the bottom fell out of the market for butane. In 2018, butane prices collapsed to historical lows, even attracting offers of $0.00 for several days in November . (I will be putting up a future article to discuss process analysis as it applies to butane blending into crude oil and condensate)

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Between changing prices and changing feeds, the optimum operating set-points for fractionation towers should be adjusted since this can increase the production of one product and reduce the production of another by changing separation ratios through control of the temperature and pressure. To do this properly, one needs to have accurate, reliable and maintainable measurement of the hydrocarbon composition and physical properties. This is where online spectroscopy comes into play. For details on the technology, see my post on Fiber Optic Process Technology.

 

Applying Near Infrared Spectroscopy

Our implementation of near infrared spectroscopy operates at full line pressures and temperatures on a short fast bypass line. Usually, this is driven by differential pressure from the process caused by a control valve, an orifice plate, a few pipe elbows or a process filter. Only a few psi of differential is need to drive flow through the short transport loop. The infrared measurement is not pressure, temperature or flow rate sensitive, and no inline filtration or pressure controls are usually needed. This makes the sample transport system extremely simple and reliable. These analyzers can run maintenance free even on dirty,waxy streams like condensate stabilizer bottoms.

One of the big advantages of spectroscopy is that it provides real-time data. Fast response allows the facility to tighten the process control loop and give better control. The question is always - "But how accurate is a spectroscopic analyzer as compared to gas chromatography? " Below are a number of graphs, comparing the results of the near infrared analyzer to laboratory samples pulled in floating piston (constant pressure) cylinders and analyzed by a certified lab. The results are impressive to say the least. The lab methods are compared to the online near infrared - in this case a JP3 Verax CTX sold exclusively in Canada by Insight Analytical.

These results are from the de-methanizer bottoms. Seven lab samples were pulled over the space of four hours. The strong correlation between the lab results and the online near infrared analyzer is readily apparent.

While some of the bar charts appear to show large differences in the bar heights, this is only because the range of the y-axis is small. In general, all results were typically within 0.4 mole% of each other when comparing the lab to the online analyzer. This is very comparable to the repeatability specification for the lab method, which varies from about +/- 0.2 mole% for component concentrations around 5 mole%, to +/- 0.4 mole% for component concentrations around 25 mole % ( based on GPA 2177M). The slide show below summarizes the results.

Similar results are obtained for debutanizer bottoms (note I will cover condensate stabilizers in a separate article). No results are shown for methane, ethane or propane as these were all zero.

Summary and Conclusions

The provided data demonstrates the ability to accurately measure the composition of tower bottoms with a near infrared analyzer. More importantly, we can do this with minimal maintenance and provide real-time results for process control. Such measurements allow for optimization of the tower performance and can directly contribute to improving production volumes and operating margins at gas processing facilities. These measurements have been implemented at more than two dozen separators and fractionation towers in Canada, and many more in the USA.

About Insight Analytical Solutions

Insight Analytical Solutions is a customer focused supplier of analytical equipment, technology and expertise to the oil and gas industry. We have decades of experience in process analytical chemistry and the application of analyzers and sample systems to process optimization. We don’t just sell these products; we offer an integrated package that includes detailed engineering design, custom sample conditioning systems and state of the art analyzers, as fully integrated packages for difficult processes.

We support all segments of the hydrocarbon processing and petrochemical industries, from upstream analysis at the wellhead through gathering systems to mid-streamers and downstream processors. Our expertise lies in identifying opportunities to provide process monitoring and optimization through the application of expertly designed sample systems and implementation of superior analytical methods.

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