May. 13, 2024
The refrigeration process is a crucial component in gas plants, efficiently removing heat from various process streams to control both hydrocarbon and water dewpoints, thereby ensuring the quality of the gas for commercial use.
The target temperature for cooling the gas is dictated by specific dewpoint requirements, which represent the minimum cooling criteria. Lower temperatures than the minimum must be economically justified by analyzing the cost-benefit of extra liquefied petroleum gas (LPG) recovery against increased operational costs. Enhancements in LPG recovery can be achieved through:
At its core, refrigeration involves transferring heat from one medium to another. Heat naturally flows from a warmer medium to a cooler one, and refrigeration systems facilitate this heat transfer. Despite their reliability, these systems may occasionally lose efficiency, warranting an investigation
Fig. 1 depicts standard refrigeration equipment for cooling natural gas. The heat exchanger cools the incoming gas by exchanging heat with the cold gas that has been pre-chilled in a propane chiller.
Fig.1 — Schematic drawing of a typical refrigeration dewpoint control process.
Gas entering refrigeration units is typically water vapor saturated, and cooling it to temperatures below the hydrate point necessitates preventive measures against hydrate formation. This is achieved by adding chemicals like methanol or glycol, with monoethylene glycol (MEG) being the most common.
MEG must be introduced at two points:
Ensuring even distribution of glycol in the gas stream is crucial to prevent freezing. This involves evenly spraying regenerated glycol over the tube sheets in these locations, allowing it to flow through each tube with the gas.
In the chiller, propane or another refrigerant boils at a controlled low temperature, removing heat from the gas stream and condensing part of it. The resulting cold gas, condensate, and MEG mixture flow to a three-phase separator. The condensate is then sent to a fractionation unit, while the sufficiently cooled gas meets both hydrocarbon and water dewpoints.
Rich glycol is separated in a three-phase separator and routed to a regenerator, resulting in a regenerated glycol concentration of about 75-80%, with the remaining percentage being water. The required amount of glycol is calculated based on the Hammerschmidt equation to achieve the necessary hydrate temperature depression.
The refrigeration effect is attained through the vaporization of refrigerants like propane. Propane is particularly suitable due to its low boiling point near ambient temperatures. By controlling the boiling pressure of propane, a desired refrigeration temperature, as low as –40°F, can be achieved.
Incorporating glycol to inhibit hydrate formation in refrigeration necessitates an added glycol regeneration step, increasing both capital and operating costs. These costs might be mitigated using the innovative IFPEXOL process, detailed in Fig. 2.
Fig. 2 — Schematic drawing of typical IFPEXOL dewpoint control process.
IFPEXOL involves using methanol instead of glycol for hydrate suppression, eliminating the need for regeneration. Methanol is generally recovered through distillation, but the IFPEXOL process innovates by allowing most methanol recovery without regeneration.
As shown in Fig. 2, the inlet gas stream is divided into two. One stream countercurrently contacts the rich methanol-water solution in a small contactor, allowing the gas, already water-saturated, to absorb methanol. This methanol-rich gas then reunites with the other stream before entering the gas/gas heat exchanger, with additional methanol added to meet the required temperature depression. Methanol, unlike glycol, does not require precise liquid distribution and condenses with water inside the heat exchanger to inhibit hydrate formation.
Methanol can be lost through the chilled gas exit and condensed hydrocarbon liquids, recoverable via a water wash system. The IFPEXOL process offers simpler equipment and operation compared to glycol-based systems and avoids atmospheric emissions of hydrocarbons like BTEX compounds. However, the continuous methanol supply requirement is a notable drawback. This process is well-suited for offshore operations where space and weight are limited.
Natural gas can be stripped of hydrocarbon liquids by contacting it with lightweight oil. The absorption efficiency increases with decreasing compound volatility at the absorber's pressure and temperature. Higher volatility compounds like methane might have lower absorption rates, while hydrocarbons such as propane and butane achieve higher absorption rates. The absorbed hydrocarbons are recovered upon regenerating the oil.
Fig. 3 illustrates a simple lean oil absorption process. Here, the gas enters the absorption tower bottom, rising through a series of packing or trays, contacting the oil entering from the top, extracting heavier hydrocarbons from the gas.
Fig. 3 — Schematic diagram of lean oil absorption process.
Advanced lean oil absorption operates at lower temperatures using chilled gas to increase absorption efficiency. The design process is complex, often requiring computer-aided calculations to balance material and heat flows intricately.
Developed in the 1960s primarily for ethane recovery, the turbo-expander process treats natural gas for high liquid recovery, achieving significantly low temperatures and condensing a substantial portion of ethane and heavier compounds.
The turbo-expander enhances refrigeration by near isentropic expansion of the gas stream, extracting mechanical energy and thereby reducing gas pressure and temperature far more effectively than Joule-Thomson expansion.
The gas entering a turbo-expander process must be highly dehydrated, typically requiring glycol dehydration followed by molecular sieve units. Fig. 4 shows a basic turbo-expander setup, with variations depending on gas composition and recovery targets.
Fig. 4 — Schematic drawing of turbo-expander equipment.
NGL recovery expander processes can chill gas to –160°F, necessitating drying to an extremely low water dewpoint using molecular sieves. Alternatively, minor methanol additions upstream of the chilling section can prevent freezing issues.
Fig. 5 — Schematic drawing of typical dry desiccant dehydration process.
The design involves detailed heat/material balances and many flash calculations, primarily performed by computer systems.
Cooling natural gas can also be achieved by expanding high-pressure gas to lower pressures through an expansion valve, maintaining constant enthalpy and yielding temperature reductions proportional to pressure ratios and gas composition. This method is practical for low gas rates and fluctuating conditions.
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Fig. 6 illustrates the Joule-Thomson expansion process, showing equipment like expansion valves and inlet separators. High-pressure gas undergoes heat exchange before expansion, condensing certain hydrocarbons and water post-expansion, provided no hydrates form. Hydrate inhibitors like ethylene glycol can be added to prevent this.
Fig. 6 — Schematic drawing of Joule-Thomson expansion equipment.
Natural gas with high CO2 concentration can be treated using regenerative solvents or membrane separation processes. Membranes, made from thin polymeric layers, exploit differences in permeability to separate gas mixtures.
Membranes typically handle polar compounds like CO2, H2S, and H2O more efficiently than hydrocarbons. Nonporous membrane materials, such as those used in the natural gas industry, facilitate this separation by allowing selective diffusion of these compounds.
Membrane processes primarily focus on lowering CO2 concentrations but also address H2S and other impurities although membrane separation alone may not meet stringent H2S specifications. Various strategies, including multi-stage separation and integration with solvent-based systems, help minimize hydrocarbon losses.
Fig. 7 showcases a two-stage membrane separation system designed to reduce hydrocarbon loss by recompressing and recycling permeate streams.
Fig. 7 — Schematic drawing of two-stage membrane separation process equipment.
Designing membrane systems involves preheating gas streams to prevent condensation, and calculating membrane service life based on feed quality and operational care. Computer programs like MemCalc™ by the Gas Technology Institute are invaluable for simulating membrane performance in CO2 removal from natural gas.
Vargas, K.J. 1982. Troubleshooting Compression Refrigeration Systems. Chem. Eng. (22 March): 137.
Minkkinen, A. et al. 1992. Methanol Gas-Treating Scheme Offers Economics, Versatility. Oil & Gas J. (1 June): 65.
Hampton, P. et al. 2001. Liquid-Liquid Separation Technology Improves IFPEXOL Process Economics. Oil & Gas J. (16 April): 54.
Morgan, J.D. 1976. How Externally Refrigerated and Expander Processes Compare For High Ethane Recovery. Oil & Gas J. (3 May): 230.
Dyck, P. and Henderson, D. 1978. Expander Wins For Gas Dewpoint Control. Oil & Gas J. (24 April): 86.
Nielsen, R.B. and Bucklin, R.W. 1983. Why Not Use Methanol for Hydrate Control? Hydrocarbon Processing (April): 71.
Crum, F.S. 1981. There Is a Place For J-T Plants In LPG Recovery. Oil & Gas J. (10 August): 132.
Koros, W.J. 1995. Membranes: Learning a Lesson from Nature. Chemical Engineering Progress (October): 68.
Lee, A.L., Feldkirchen, H.L., and Gomez, J. 1994. Membrane Process for CO2 Removal Tested At Texas Plant. Oil & Gas J. (31 January): 90.
Cook, P.J. and Losin, M.S. 1995. Membranes Provide Cost-Effective Natural Gas Processing. Hydrocarbon Processing (April): 79.
Here are some notable papers in OnePetro that are recommended for further reading:
Visit these external websites for more information related to gas treating and processing:
Looking for a food storage temperature chart? Click the link below to jump ahead and view our item-specific produce storage chart.
How to Store Fruits and Vegetables Chart
There isn't a single way to store fresh produce, just like there's no single way to make a pizza. However, following some universal best practices can help you organize your restaurant's storeroom efficiently:
Ethylene is a plant hormone that promotes seed germination, fruit ripening, and cellular breakdown. Understanding ethylene production and sensitivity helps in extending the shelf life of your produce. Not all plants have the same ethylene production or sensitivity, so it’s essential to store ethylene producers separately from sensitive items.
Commercial kitchen operators often manipulate ethylene to accelerate fruit ripening by placing high ethylene-producing fruits inside a paper bag with items they wish to ripen faster.
Climacteric fruits continue ripening after being picked and produce more ethylene compared to non-climacteric fruits, which do not ripen post-harvest.
These climacteric fruits are high ethylene producers:
We’ve created a downloadable produce storage chart for easy reference in your commercial kitchen.
Download our Ethylene Production and Sensitivity PDF
It's ideal to store fruits and vegetables separately due to fruits’ high ethylene production and vegetables' sensitivity to it. Here’s how to store popular vegetables:
Vegetables typically have low ethylene production. Here are tips to keep your vegetables fresh:
Keep tomatoes away from sunlight in your storeroom. They do not ripen properly in the refrigerator. Store them stem side down in a single layer until fully ripe. Transfer ripe tomatoes to the fridge to extend shelf life.
Store potatoes in a cool, dark, dry spot outside the fridge, as refrigeration converts starch to sugar. Use paper bags, baskets, or large bowls, avoiding plastic bags that trap moisture.
Wrap clean and dry cucumbers in a paper towel and place them inside a plastic bag, leaving the top open for airflow. This method also works for celery and spinach.
Trim carrot tops to prevent moisture extraction. Store carrots in an uncovered container with shallow water in the fridge or an open plastic bag if space is limited.
Here’s how to store popular fruits to ensure ripeness and longevity:
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