MEG Dehydration Ability in MEG Injection Plant

In order to continue the last tip of the month’s discussion on MEG injection plant, in this “Tip of the Month”, we will focus on two more questions:

1. Does MEG have any dehydration ability at the three phase cold separator condition of a typical mechanical refrigeration plant?
2. What is the dehydration ability of MEG if the mechanical refrigeration in a typical MEG injection plant goes out of service?

As described in the last tip of the month, a typical mechanical refrigeration process is 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. In short, 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. 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 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. For more detail, refer to chapters 6 and 16 of Gas Conditioning and Processing, Volumes 1 and 2, [1, 2] respectively.

Question 1: Does MEG have any dehydration ability at the three phase cold separator condition of a typical mechanical refrigeration plant?

In order to answer this question, first we determine the water dew point temperature without any MEG injection and compare the results with the case of 80 weight percent lean MEG injection. Let’s assume a typical natural gas, cold separator pressure of 40 bara and -20°C [580 psia & -4°F] with 10 weight dilution (i.e. rich MEG concentration of 70 weight %). By performing computer simulation using ProMax [3], the water dew point temperature:

• without MEG injection is -22.7°C (-8.7°F) corresponding to water content of 30.72 kg/106 std m3 [1.94 lbm/MMSCF]
• with MEG injection is -29.6°C (-21.2°F) corresponding to water content of 17.6 kg/106 std m3 (1.11 lbm/MMSCF)

Therefore, the water dew point temperature depression is 6.9°C (12.4°F). Similarly, a water dewpoint temperature depression of 7.8°C [14°F] was obtained for the case of 5 weight percent MEG dilution (i.e. rich MEG concentration of 75 weight %). These results indicate that even at low temperature, in addition to the hydrate inhibition effect, MEG has the ability to do partial dehydration. It should be noted that for this gas the hydrate formation temperature at 40 bara [580 psia] is 14.9°C [58.7°F].

Question 2: What is the dehydration ability of MEG if the mechanical refrigeration in a typical MEG injection plant unexpectedly goes out of service?

Let’s assume the same gas as in question 1 is passing through a mechanical refrigeration system with the same chiller temperature of -20°C [-4°F]. Let’s also assume that due to the break down of mechanical refrigeration system (lack of chilling) the cold separator temperature reaches 21.1°C [70°F]. Again, we used ProMax to perform the simulations and the calculated results are plotted in Figures 1 (A&B) and 2 (A&B). Figure 1 (A&B) presents the effect of lean MEG circulation rate on water dew point temperature and water content. Figure 2 (A&B) indicates that for a 10 weight % dilution, about 4350 kg MEG solution per 106 std m3 [270 lbm MEG solution per MMSCF] of gas is required. Figure 2 (A&B) also indicates that for this amount of dilution, the water dew point temperature drops from 21.1°C [70°F] to about 12.2°C [54°F] and the corresponding water content drops from 567 to 325 kg/106 std m3 [35 to 21 lbm/MMSCF]. Again, it can be seen that the MEG can dehydrate natural gas partially at higher temperature. It is also interesting to see from Figures 1 and 2 that further increase in lean MEG solution circulation rate, beyond 4350 kg/106 std m3 [270 lbm/MMSCF], does not reduce the water dewpoint temperature considerably and; therefore, it justifies the rule of thumb for 10 weight % dilution.

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 Dr. Mahmood Moshfeghian

References:

1. Campbell, J. M. “Gas conditioning and processing, Volume 1: Basic Principles,” 8th Ed., John M. Campbell and Company, Norman, Oklahoma, USA, 2001.
2. Campbell, J. M. “Gas conditioning and processing, Volume 2: The Equipment Modules,” 8th Ed., John M. Campbell and Company, Norman, Oklahoma, USA, 2000.
3. ProMax, version 1.2, Bryan Research & Engineering Inc, Bryan, Texas, 2005.