4 Advice to Choose a MEG Plant

May. 13, 2024

Ethylene Glycol vs. Propylene Glycol: Differences & Uses

This article has been updated. It is accurate as of Feb. 8, 2022

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For low-temperature hydronic systems, systems in which chillers and AHUs are located outdoors, or other equipment used in low-temperature processes, some form of glycol is a critical ingredient. It works to lower the fluid’s freezing temperature, enabling lower-temperature operation and preventing freezing.

Basically, if there’s any risk of your fluid-containing equipment being exposed to freezing temperatures, it needs some form of glycol. There are two main types of glycol used for this function: ethylene and propylene. Some important differences exist between the two, and they should be understood before making a decision.

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What is ethylene glycol?

Ethylene glycol (CH₂OH₂), also known as 1,2-ethanediol, is an alcohol-based organic compound often used as an anti-freezing agent in HVAC equipment and vehicle systems, among others. Its an odorless, colorless, viscous liquid, known to have a sweet taste.

In its pure form, ethylene glycol freezes at around -10°F, but when mixed with water, it can remain liquid at much lower temperatures. For example, a mixture consisting of 40% water and 60% glycol can withstand temperatures close to -50°F before freezing.[1]

For glycols used as heat transfer media, viscosity – a fluid’s resistance to flow – is a critical property, affecting flow rate, friction loss, and ultimately thermal performance. Relative to propylene glycol, which we’ll cover later in this post, ethylene glycol is less viscous. For heat transfer applications, lower viscosity is beneficial. Higher viscosity means higher friction loss – i.e. more energy is required to move a more viscous substance through the system. Applications that require glycol also often include turbulators as well, which help minimize the impact of glycol's viscosity by creating turbulent flow.

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When should I use ethylene glycol?

Between ethylene and propylene glycol, ethylene glycol is the more thermally conductive of the two (see table below). Therefore, ethylene glycol is a good choice for applications in which thermal performance is the top priority.

Essentially, if none of the circumstances in the next section describe your application, ethylene glycol is likely the better option.

When shouldn’t I use ethylene glycol?

Ethylene glycol is toxic to humans, causing a number of physiological issues when ingested, including death (the Centers for Disease Control estimate a lethal dose to be between 1,400 and 1,600 mg/kg). As a result, ethylene glycol should not be used in applications in which contamination of drinking water is possible. It should also not be used for heating or cooling systems at facilities like food processing plants or other operations creating products for consumption.

The reason for this is two-fold. Ethylene glycol is harmful to terrestrial and aquatic animals, and when deposited into waterways, takes roughly 10 to 30 days to biodegrade. Glycols biodegrade through aerobic biodegradation, during which the breakdown of glycols is enacted by bacteria, which require oxygen to perform this function. This action depletes the oxygen levels of the effected waterways, which can have devastating effects if the amount of glycol and rate of biodegradation are great enough.

What is propylene glycol?

Propylene glycol (C₃H₈O₂), also called Propane-1,2-diol, is a synthetic liquid used for a litany of purposes across dozens of industries. It’s a viscous, colorless, nearly odorless liquid that possesses a faintly sweet taste.

Like ethylene glycol, propylene glycol is combined with water in variable concentrations to depress the freezing temperature of a working fluid in heat transfer applications.

The eutectic temperature, or the lowest possible freezing temperature attainable by any ratio of the two substances (propylene glycol + water) is -76°F at a concentration of 60% propylene glycol and 40% water. However, commercial products typically have that ratio inverted, at 40% propylene glycol and 60% water, the freezing point of which is roughly -7°F.[2]

When should I use propylene glycol?

The answer to this question also answers the question of “when shouldn’t I use propylene glycol?” as well. Relative to ethylene glycol, propylene glycol’s higher viscosity and friction loss combined with its lower heat capacity typically limit its usage to applications with safety concerns.

There are extremely few, if any, instances where propylene glycol would be chosen for its thermal performance. It’s simply less effective than ethylene glycol for its intended function. But given ethylene glycol’s toxicity, several applications exist for which propylene glycol must be used, such as those we covered earlier in this post, namely food production, applications in which water contamination is possible, and the HVAC systems at facilities of these types.

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Propylene glycol is considered to be relatively safe for humans. It’s a common ingredient in a variety of beauty products, pharmaceuticals, and indirect food additives. While less toxic than ethylene glycol, propylene glycol can pose some environmental concerns.

Propylene – like ethylene glycol – is broken down by aerobic means, but where ethylene takes approximately 10 to 30 days to biodegrade, propylene glycol does so in 20 to 30 days or more.

Still not sure which type of glycol is best for your application? Give Super Radiator Coils a call, and let’s talk.

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[1] Siegfried Rebsdat; Dieter Mayer. "Ethylene Glycol". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH

[2] “Properties of Some Particular Solutions” (PDF). Portal del DMT. Retrieved 2020-09-22

MEG Injection vs. TEG Dehydration

In this “Tip of the Month”, we will focus on the question of: Which technology should you choose? The answer, of course, is “It depends.” It depends on what you are trying to accomplish, the constraints imposed on your system and the relative economics.

A Rule of Thumb is “Use MEG injection if you have to cool the gas for NGL recovery anyway.” Like all Rules of Thumb, there are exceptions. But let’s explore the basics of each technology.

Let’s begin by defining our terms. See Figure 1 for a typical mechanical refrigeration process used for hydrocarbon dew point control and moderate NGL recovery that uses MEG injection to prevent hydrate formation. Warm inlet gas is cross-exchanged with the cold dry sales gas and then flows to the gas chiller. To prevent hydrates from forming, MEG is injected in the tubes at the warm end of both exchangers. The temperature of the chiller is adjusted to condense liquids from the feed gas. The cold gas exiting the chiller together with the rich MEG solution and condensed hydrocarbons enters the cold three-phase separator. The rich MEG is sent to the regeneration section of the unit where the water is removed. The resulting lean MEG is sent back to the process.

Figure 1. Typical mechanical refrigeration plant with glycol injection system [1]

In this flow diagram, two things are taking place: temperature reduction of the process gas to condense both water and hydrocarbons; and, MEG injection and subsequent regeneration to prevent hydrates from forming. Inspection of Figure 1 reveals the majority of the equipment, including the refrigeration compressors, etc. which are not shown, is employed to reduce the temperature. Besides mechanical refrigeration, other options to achieve the required gas cooling include JT – valve expansion or use of a turboexpander. For either of these options, the MEG injection and regeneration portion of this plant is minor by comparison.

In this scheme, the sales gas exiting the gas-to-gas exchanger has a water and hydrocarbon dew point determined by the operating temperature of the cold separator. The CAPEX of this system is essentially driven by the gas cooling equipment, including the refrigeration system. The key point to remember here is that the water is being removed from the gas by low temperature condensation. The purpose of the injected MEG is not to “dehydrate” the gas but to prevent formation of hydrates. At the MEG concentrations normally used in these systems, approximately 80 – 85 wt%, the MEG absorbs only a small amount of water vapor from the gas.

Let’s now look at a typical circulating TEG system. See Figure 2. The same rich, water saturated natural gas stream flows to a properly sized inlet separator to remove liquids. The gas then enters a glycol contactor equipped with either structured packing or bubble cap trays. As the gas rises, the water is removed by the falling TEG. The concentration of the lean glycol entering the top of the contactor is the main variable that determines the water dew point specification that can be made. The rich glycol that leaves the glycol contactor is sent to a flash drum and then to a regeneration section. The lean glycol leaving the regenerator is then returned to the contacting tower.

In this system, we are only making water dew point specification gas. The NGL content/hydrocarbon dew point of the sales gas is the same as that of the feed gas. Circulating TEG systems are therefore used only for dehydration. A significant cost item for the circulating TEG system is the high pressure contacting tower.

Now let’s explore how we can compare and contrast these two technologies.

If your objective is to make only pipeline water specification gas, you will most likely choose a circulating TEG system. This is intuitively obvious from a comparison of the two flow diagrams cited above. Assume, for example, that you want to dehydrate a lean natural gas stream that is water saturated at 70 bar and 40°C. A quick comparison of Figures 1 and 2 shows that there is much more equipment associated with chilling the feed gas (Figure 1 + the refrigeration compressors, etc. that are not shown) then there is with a circulating TEG system (Figure 2). Hence, for dehydration only to pipeline water specifications, a circulating TEG system will almost always be selected.

On the other hand, if your objective is to recover hydrocarbons and remove water simultaneously, then a low – temperature process with MEG injection may be the best choice. Assume you have a rich natural gas stream that is water saturated at 70 bar and 40°C. Assume a mechanical refrigeration process is selected for hydrocarbon liquids recovery with a cold temperature of -35°C. We have two options to consider: we can dehydrate the gas with a circulating TEG system to a water dew point of -35°C and then send the dehydrated gas to an LTS plant consisting of a gas-to-gas exchanger, chiller, refrigeration system, etc., but with no MEG injection/regeneration system; or, we can send the feed gas directly to the LTS plant which has an MEG injection system retrofitted to prevent hydrates from forming.

Figure 2. Basic glycol dehydration unit [2]

Since the underlying equipment required to recover NGL’s is the same in both options, the cost comparison is essentially between the circulating TEG system and the MEG injection system. The TEG system will use less circulating rates then the MEG system, but will likely have a higher regeneration duty. Achieving the large dew point depression of 75°C with a circulating TEG system will be challenge and will add to the system cost. The key difference, however, is the circulating TEG system requires a high pressure contactor while the MEG injection system does not. In this situation, the most likely choice will be to go with the MEG Injection system.

For more information about dehydration and hydrate inhibition, the reader should refer to JMC books and enroll in our G4 (Gas Conditioning and Processing) and G5 (Gas Conditioning and Processing – Special) courses.

By Harvey M. Malino and Mark Bothamley

References:

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Campbell, J. M. “Gas conditioning and processing, Volume 1: Basic Principles,” 8th Ed., John M. Campbell and Company, Norman, Oklahoma, USA, 2001.
Campbell, J. M. “Gas conditioning and processing, Volume 2: The Equipment Modules,” 8th Ed., John M. Campbell and Company, Norman, Oklahoma, USA, 2000.