Hi, my name is Shane Hale, Product Marketing Manager for Natural Gas at Rosemount Analytical, and today I’d like to discuss a gas chromatograph application for the amine systems in natural gas processing that you may not be familiar with, but can dramatically reduce foaming events and increase production rates.
In natural gas processing, amine systems are frequently used to remove CO2 and H2S from rich gas streams. When hydrocarbon liquids are introduced into the amine contactor, however, foaming can occur which significantly reduces the efficiency of the acid gas extraction. This happens because, when hydrocarbon liquids enter the contactor, they are highly soluble into the amine solution and reduce the surface tension of the aqueous solution. The reduced surface tension then aids in creating bubbles of gas in the amine solution, resulting in foaming. At that point, operators need to reduce flow and may even have to inject foaming inhibitors into the system to regain control.
Avoiding foaming is difficult. The efficiency of amine systems to remove H2S generally increases with lower operating temperatures, and the efficiency at removing carbon dioxide occurs at a specific temperature. However, lower temperatures in the contactor also increase the potential for liquid hydrocarbons to form in the inlet stream and thus increase the potential for foaming to occur.
C9+ Gas Chromatography
Figure 1 – Calculating the Hydrocarbon Dew Point of the inlet stream at the contactor pressure provides an early warning for liquid hydrocarbons in the inlet stream and defines a minimum temperature for the lean amine inlet stream to avoid amine foaming from liquid hydrocarbons.
The solution to the problem is hydrocarbon dewpoint (HCDP) and this is where gas chromatography comes into play. Determining the HCDP of the inlet gas provides the opportunity to (1) avoid hydrocarbon liquids entering the contactor and (2) control the amine temperature to a set-point that optimizes the efficiency of the acid gas extraction while also avoiding the risk of liquid hydrocarbons forming in the contactor. The theoretical HCDP of a gas mixture can be calculated from the gas composition using an equation of state. Typical gas chromatographs used in natural gas applications measure individual hydrocarbons up to n-pentane, and combine all the heavier components as a C6+ value. However, the components that drop out as liquids and cause the foaming issues are the components heavier than C6, so calculating the HCDP with a C6+ analysis (using assumed ratios of C6/C7/C8) will provide inaccurate results that will not be suitable for use in a control strategy. A C9+ gas chromatograph measures the ratio of C6, C7 and C8 components (with heavier components reported as C9+) and provides a much more accurate HCDP calculation that can then be used to optimize the control strategy.
Figure 2 – The process pressure is used to calculate the HCDP at process conditions that provides a two-phase flow early warning for use in the control strategy to avoid foaming in the contactor.
Determining the Phase of the Inlet Gas
When the inlet gas temperature is below the HCDP, the flowing stream is a single phase vapor. As the gas becomes richer with heavy hydrocarbons, the calculated HCDP will approach the stream temperature. When the HCDP reaches the stream temperature, the heavier components will begin to drop out into the liquid phase, increasing the risk of foaming in the amine contactor. By calculating the HCDP at the line pressure with the C9+ gas chromatograph, the difference between the stream temperature and the HCDP can be monitored. As the HCDP approaches the stream temperature an alert can be triggered that enables the operator, or the control strategy, to take actions to reduce the HCDP before liquids form and enter the amine contactor.
Using HCDP for Amine Inlet Temperature Control
The temperature of the amine contactor is typically controlled by cooling the lean amine prior to entry into the contactor to maximize the efficiency of the acid gas removal. However, if the temperature of the contactor is below the HCDP of the inlet gas at the contactor pressure, then liquid hydrocarbons will begin to form as the inlet gas enters the contactor and the risk of amine foaming greatly increases.
As the HCDP varies with pressure, the HCDP of the inlet gas will change as it enters the contactor. By calculating the HCDP of the inlet gas at the pressure of the amine contactor, the minimum temperature of the inlet amine can be determined and used in the control strategy to minimize the risk of amine foaming.
If the determined minimum temperature is too high for the efficient acid gas removal, actions can be taken to reduce the HCDP of the inlet gas while also avoiding the risk of amine foaming.
Emerson has led the industry in the development of gas chromatography for C9+ applications and customized solutions using the 700XA Gas Chromatograph by Rosemount Analytical.
Are you struggling with Amine Foaming in your plant? If so, we’d like to hear from you…
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