The present invention relates to the use of mixed solvents in gas or oil processing systems to depress the freezing point of said systems.

Background to the Invention

Freezing (the formation of ice or solid solvent) or hydrate formation is a major problem in oil and gas production and processing systems such as dew point control in the transport of gas, various oil and gas processing systems in which a cooling medium is used and contacting units for drying gas. In the context of the present application, freezing will be used to describe the formation of ice, the formation of pure solid solvents at low temperature such as solid alcohol or glycol or the formation of mixed solid complexes, for example the solid monoethylene glycol - water complex at low temperatures. The most common method to help prevent freezing is by the addition of a thermodynamic inhibitor (a freezing point inhibitor) such as an alcohol (e.g. methanol or ethanol) or a glycol (e.g. monothylene glycol or triethylene glycol). By using the correct amount of inhibitor compared to the amount of water, it is possible to reduce the freezing point in the gas and oil processing systems to as low as -30 to -40 ^< €.

During low temperature processing it is important to have enough freezing point inhibitor present to avoid freezing. However, if the concentration of the inhibitor is too high this may lead to freezing of the inhibitor itself. Pure triethylene glycol, for example, has a freezing point of only -5°C while pure monothylene glycol has a freezing point of only -13Ό. Many of the most important procedures in gas and oil processing systems occur at lower temperatures than this and formation of the solid inhibitor can only be avoided by making sure that the liquid phase is a mixture of the inhibitor and water. This narrows the operation range significantly. For example, in a dew point control unit currently in operation, the injected monothylene glycol has a concentration of 80 wt% monothylene glycol and 20 wt% water, and after the process it is 78 wt% monothylene glycol and 22 wt% water. Outside this narrow concentration range water or monoethylene glycol can form solid.

An option would be to use an alcohol such as methanol or ethanol. However, they have much higher vapour pressures than the glycols and this leads to significant loss of inhibitor to the gas phase and the possible contamination of gas or liquid products later in the process.

The problem with the existing technology is that the best inhibitors, glycols such as monoethylene glycol or triethylene glycol used to prevent freezing during gas processing have high freezing points and if the concentration of the inhibitor is too high, solid inhibitor may form and plug the equipment.

This can be understood further by considering the thermodynamic background. If two different components A and B have different freezing points Ta, Tb, a mixture of them will in general have a lower freezing point than the pure components.

Component A might have, for example, a freezing point of -5°C while component B might have a freezing point of -15°C. If one starts with pure A, and then starts to mix in B, the freezing temperature is reduced. The same happens if we start with B and mix in some A. At some specific ratio of A and B, the two freezing curves meet. Below this point, only solid is present.

The reason why the freezing point is reduced is mainly because the components are diluted, i.e. the mole fraction is reduced. The slope of the freezing curve is determined by the enthalpy of freezing for the various solids. In addition, there are specific interactions between the molecules of the different types of chemicals.

The phase diagram can be more complex if A and B can form new components, for example AB, A ₂ B, or AB ₂ . An example of this is the use of monothylene glycol as an antifreeze agent in water. The phase diagram for this is even more complex because an intermediate solid complex phase is being formed. The formation of these intermediate solid complex phases together with the limitations generated by the hydrate lines and the formation of solid inhibitor once the specific ratio of

water:monoethylene glycol is reached results in only a narrow safe area within which it is possible to operate.

The formation of an intermediate solid complex phase has been known for monothylene glycol as antifreeze agent for some time. Our own internal work has also experimentally verified that a similar behaviour is exhibited for triethylene glycol - water mixture, i.e. when triethylene glycol instead of monothylene glycol is used as antifreeze agent. Our own internal work has also experimentally verified that such a complex is not formed in a mixture of the two inhibitors, i.e. monoethylene and triethylene glycol. US 3,857,686 discloses two applications of glycols (and other hydrate inhibitors): 1 ) to inject with gas as it is cooled to prevent hydrate formation in condensed water; and 2) as a cooling medium. The focus of this patent is that the viscosity of glycols becomes very high at low temperatures. According to the disclosure of the patent, by adding butyrolacetone the viscosity of the glycol/water mixture is significantly reduced. There is a passing reference to the use of butyrolacetone in combination with more than one glycol. However, the emphasis of the application relates to butyrolacetone plus a single glycol and all of the examples are to a single glycol plus butyrolacetone. There is no disclosure or suggestion that the butyrolacetone should be omitted or that a combination of glycols alone would be of use in depressing the freezing point of a gas system.

Because freezing (the formation of ice or solid solvent) is a major problem in oil and gas production and processing systems such as dew point control in the processing of gas, various oil and gas processing systems in which a cooling medium is used and contacting units for drying gas is such a major concern, and there are such significant limitations in existing means to address this problem (e.g. freezing point inhibitors such monothylene glycol or ethanol), there is a considerable need in this field for a new means to overcome these problems. The present inventors have found a simple and yet very effective means to overcome the problems encountered in the methods of the prior art by adoption of the method of the present invention.

Description of the Invention

The present inventors have addressed the problem of freezing in gas or oil processing systems through the discovery that mixed solvents can be used to depress the freezing point much further in said gas or oil processing systems, compared to the use of a single inhibitor.

Thus, according to the present invention there is provided a method for the depression of the freezing point in a gas or oil processing system by the use of a freezing point inhibitor, characterised in that said freezing point inhibitor consists solely of a mixture of two different glycols.

With a pure glycol acting as a freezing point inhibitor (e.g. monothylene glycol) or with a mixture of glycols (e.g. monothylene glycol and triethylene glycol), this has only a minor effect on the hydrate formation temperature, as long as the total amount of inhibitor (in mole fraction relative to water) is the same in each case. However, because the freezing temperature of the inhibitor itself is reduced compared to either of the inhibitors alone, it is possible to operate at a higher inhibitor concentration without solid inhibitor being formed. When the inhibitor concentration is increased, the ice formation temperature and hydrate formation temperature is reduced. What is also of great importance is that a mixture of monoethylene - triethylene glycol inhibits the formation of solid complex phases in the presence of water (complexes found to occur for the binary systems MEG - water and TEG - water) which might have posed a low temperature limit of this process for a given concentration. The justification is that our own experimental work (and also simulations) did not identify formation of solid phases for a ternary water - monoethylene glycol - triethylene glycol system at mixed solvent concentration range of interest when reducing temperature down to -55°C. This allows significantly improved depression of what is currently considered as low temperature processing of gas or oil processing systems compared to that which has previously been achievable with known inhibitors.

As a result, the safe operational area for natural gas-water-inhibitor systems can be extended to lower temperatures and higher inhibitor concentrations by use of a mixed solvent inhibitor compared to the use of single inhibitors, giving a much wider safe operational window. Lower temperatures, because we avoid the formation of solid complexes and higher inhibitor concentrations because we are depressing the inhibitor solidus line.

To summarize, the key points of the present invention are as follows:

• A freezing point inhibitor that consists solely of a mixture of two different glycols in accordance with the present invention will work more or less identically as a pure inhibitor consisting of just one glycol with respect to hydrate inhibition for the same total amount of inhibitor, with the freezing temperature of hydrate being practically unaffected.

• A freezing point inhibitor that consists solely of a mixture of two different glycols in accordance with the present invention has a lower freezing point and it is therefore possible to increase the inhibitor concentration without it freezing out as a pure solid (monoethylene- or triethylene glycol) or intermediate solid phase in the presence of water and hence operate at lower temperatures.

Preferred methods of the present invention include the method according to the present invention as defined above, wherein said freezing point inhibitor consists solely of a mixture of two different glycols selected from the group consisting of monothylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol and tetraethylene glycol; more preferably it is selected from monothylene glycol, diethylene glycol and triethylene glycol; and most preferably the freezing point inhibitor consists of a mixture of monothylene glycol and triethylene glycol.

In a first particularly preferred aspect of the present invention as defined above, there is provided a method according to present invention wherein said gas or oil production system is a system for low temperature gas processing. Preferably, said freezing point inhibitor that consists solely of a mixture of two different glycols inhibits the formation of ice and hydrates in said system for the low temperature processing of gas.

In the context of the present invention, low temperature gas processing is the processing of natural gas at low temperatures to remove water or liquid hydrocarbon (so-called dew point control), e.g. at temperatures of -20 °C to -30 °C under the application of pressure. Before gas can be exported to customers, the composition of the gas must be adjusted so that the condensation of water or a liquid hydrocarbon phase does not occur, even if the gas is exposed to very low temperatures. A typical requirement for export gas is be that its dew point is below -10°C at 5,000 kPa. One option to obtain this is to cool the gas to -10°C at 5,000 kPa and remove whatever condenses at this temperature. At -10°C freezing is easy to avoid by, for example, the addition of monothylene glycol. However, in practice, the dew point control is done at a much higher pressure to save energy, and it is then necessary to go to much lower temperatures. In a specific dew point control unit in operation, the gas is cooled to -26°C at 6,700 kPa. This operational temperature is very close to the limits of hydrate formation (too little monothylene glycol) or solid monothylene glycol formation (too high monothylene glycol concentration). To be able to operate at this low temperature, the monothylene glycol concentration is only allowed to vary between 78-80 wt%. This means that 80 wt% monothylene glycol -water mixture is injected before cooling, and due to water condensation, the monothylene glycol concentration is reduced to 78 wt%.

In accordance with the method of the first preferred aspect of the present invention, a single freezing point inhibitor such as monoethylene glycol is replaced with a freezing point inhibitor that consists solely of a mixture of two different glycols such as a mixture that consists of monoethylene glycol and triethylene glycol. We have determined that a monoethylene glycol-triethylene glycol mixture consisting of approximately 75 mole% monoethylene glycol and 25 mole% triethylene glycol has a freezing point of around -30 °C. As this freezing temperature is below the

temperature to which the gas is cooled in the dew point control of the aforementioned unit, by way of example (-26°C at 6,700 kPa), this mixture can therefore be used during gas processing in accordance with the first preferred aspect of the present invention without any danger of the mixture of glycols freezing out. Adjustment of the relative amounts of the two glycols allows mixtures with different freezing points to be obtained which are suitable for the desired temperature to which the gas is cooled. Furthermore, when the mixture of glycols in accordance with the present invention mixes with condensed water, the freezing point of the ternary system will be further reduced.

Particularly preferred embodiments of this first particularly preferred aspect of the method of the present invention, which may be used in combination as well as individually, include the following.

First, a method according to the above first particularly preferred aspect of the present invention, wherein use of said freezing point inhibitor consisting solely of a mixture of two different glycols enables a higher concentration of said inhibitor to be used at a lower temperature than can be used when either of said glycols is used as an inhibitor on its own, without formation of solid inhibitor due to the inhibitor freezing.

Second, a method according to the above first particularly preferred aspect of the present invention, wherein said freezing point inhibitor that consists solely of a mixture of two different glycols which inhibits the formation of ice and hydrates in said system is selected from the group consisting of monothylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol and tetraethylene glycol; more preferably it is selected from monothylene glycol, diethylene glycol and triethylene glycol; and most preferably the freezing point inhibitor is a mixture of monothylene glycol and triethylene glycol.

Third, a method according to the above second particularly preferred embodiment of the first particularly preferred aspect of the present invention, wherein the mole ratio of monothylene glycohtriethylene glycol is from 80:20 to 20:80, more preferably from 50:50 to 70:30, and most preferably from 65:35. Fourth, a method according to the above first particularly preferred aspect of the present invention, wherein the use of said mixture of two different glycols prevents the formation of a solid intermediate comprising water and one or both of said glycols.

Finally, a method according to the above first particularly preferred aspect of the present invention, wherein the mol ratios of the two different glycols:water, and in particular monothylene glycohtriethylene glycohwater in said gas are 20-40:30-20:50- 40 respectively.

In a second particularly preferred aspect of the present invention, there is provided a method according to the present invention wherein said gas or oil processing system is a low temperature gas or oil processing system operating at temperatures of at least -50 'Ό in which a cooling medium is used, said cooling medium comprising a mixture of water and said freezing point inhibitor that consists solely of a mixture of two different glycols. Preferably, the gas or oil production system is a gas system.

Mixtures of water and a glycol such as water-monothylene glycol or water-triethylene glycol have frequently been used in the art as a cooling medium. The water- monothylene glycol system has a minimum liquid temperature of approximately - 50 'Ό and a water-triethylene glycol system has a minimum liquid temperature of approximately -65 °C. However, these low temperatures can only be obtained in a narrow concentration range.

The present inventors have found that if a single glycol in accordance with the prior art is replaced by a mixture consisting solely of a mixture of two different glycols, the freezing point is reduced significantly and the concentration range is significantly wider. If, for example, a mixture of glycols is used consisting of monothylene glycol and triethylene glycol in which the monothylene glycol/triethylene glycol mixture contains 50-75 mole% monothylene glycol, the present inventors have found that the temperature of the resulting cooling medium can be reduced to as low as -70 to -85 'Ό. Thus, by replacing the monothylene glycol or triethylene glycol in a cooling medium system with a mixture of glycols such as a mixture consisting solely of a mixture of monothylene glycol/triethylene glycol, the cooling medium can be used to significantly lower temperatures in the system. Particularly preferred embodiments of this second particularly preferred aspect of the method of the invention, which may be used in combination as well as individually, include the following.

First, a method according to the above second particularly preferred aspect, wherein said freezing point inhibitor that consists solely of a mixture of two different glycols is selected from the group consisting of monothylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol and tetraethylene glycol; more preferably it is selected from monothylene glycol, diethylene glycol and triethylene glycol; and most preferably the freezing point inhibitor consists of a mixture of monothylene glycol and triethylene glycol.

Second, a method according to the above first particularly preferred embodiment of the above second particularly preferred aspect of the present invention, wherein the mol ratio of monothylene glycohtriethylene glycol is from 80:20 to 20:80, more preferably from 50:50 to 70:30, and most preferably from 65:35.

Third, the above second particularly preferred aspect, wherein the mol ratios of water:monothylene glycohtriethylene glycol in said cooling medium are in the range 60-40:25-35:15-25 respectively, preferably in the range 55:30:15 to 45:35:20 and most preferably the mol ratio is 50:32.5:17.5.

In a third particularly preferred aspect of the present invention, there is provided a method according to the present invention wherein said gas or oil processing system is a contacting unit for drying gas, wherein said freezing point inhibitor that consists solely of a mixture of two different glycols is used to remove water from said gas.

Gas is dried before it is exported. A common method to dry gas is to use what is a contacting unit. A particularly common example of this in the art is called a triethylene glycol contactor. Gas is mixed with triethylene glycol, normally in a counter flowing column. Close to pure triethylene glycol is added at the top of the column and as it comes into contact with the gas it extracts water out of the gas. The triethylene glycol is then taken out at the bottom of the column and regenerated by heating so that the water evaporates. This is possible because the boiling point of triethylene glycol is 285 °C while that of water is 100°C. The dry regenerated triethylene glycol is then reused and circulated back to the contactor. Typical operation conditions for a contactor are 30 'Ό. Typical requirements for a triethylene glycol drying process are that the water content of the gas is so low that water will not start to condense before below -18 ^< € at 6,900 kPa.

Being able to obtain the required dew point of - ^" Ι δ'Ό at 6,900 kPa requires that the triethylene glycol has very high purity when the contactor is operated at a

temperature as high as 30 °C. By operating the contacting unit at a lower

temperature, water can be removed more efficiently. A limitation relating to lowering the temperature limit for operation of a triethylene glycol contacting unit would be the freezing point of triethylene glycol. If the triethylene glycol is replaced by the said freezing point inhibitor being a mixture of triethylene glycol and monoethylene glycol, the contacting unit can be operated at lower temperatures and thereby obtain a more efficient removal of water.

In cases where the natural gas to be treated is at low temperature, the gas has to be heated before entering the triethylene glycol contactor unit. The energy requirement for heating the gas can be high, and it will be positive if the contacting temperature can be reduced. The use of a mixture of monoethylene- and triethylene glycol will enable the operation of the counter flow contacting unit at very low temperatures.

Particularly preferred embodiments of this third particularly preferred aspect of the method of the invention, which may be used in combination as well as individually, include the following.

First, a method according to the above third particularly preferred aspect, wherein said freezing point inhibitor that consists solely of a mixture of two different glycols is selected from the group consisting of monothylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol and tetraethylene glycol; more preferably it is selected from monothylene glycol, diethylene glycol and triethylene glycol; and most preferably the freezing point inhibitor consists of a mixture of monothylene glycol and triethylene glycol.

Second, a method according to the above first particularly preferred embodiment of the above third particularly preferred aspect of the present invention, wherein the mol ratio of monothylene glycohtriethylene glycol is from 50:50 to 1 :99.

Third, a method according to the above third particularly preferred aspect, wherein the use of a freezing point inhibitor that consists solely of a mixture of two different glycols to remove water from said gas in said contacting unit enables the water to be removed at a lower temperature than is possible with either of the glycols alone when used as the freezing point inhibitor for this purpose.

Finally, a method according to the above third particularly preferred aspect, wherein said contacting unit is a counter flowing column.

Detailed Description of the Invention

The present invention may be explained further with reference to the following drawings:

Figure 1 is a solid-liquid diagram for a binary system of two components A and B (plot of temperature v. mol%);

Figure 2 is a solid-liquid diagram for monoethylene glycol-water binary mixtures (plot of temperature v. wt%);

Figure 3 is the same diagram as Figure 2, but this time including typical natural gashydrate line;

Figure 4 is a solid-liquid diagram for triethylene glycol-water binary mixtures (plot of temperature v. mol%);

Figure 5 is a solid liquid diagram for triethylene glycol-monoethylene glycol binary mixtures (plot of temperature v. mol fraction);

Figure 6 is a theoretical phase diagram for hydrate inhibitor-water, wherein the hydrate inhibitors are triethylene glycol, monoethylene glycol (alone and in combination) (plot of temperature v. water mol. fraction);

Figure 7 is a theoretical calculated phase diagram of the solid-liquid diagram for hydrate inhibitor-water, wherein the hydrate inhibitors are triethylene glycol, monoethylene glycol (alone and in combination) (plot of temperature v. water mol. fraction); and

Figure 8 is a theoretical calculated phase diagram for hydrate inhibitor-water, wherein the hydrate inhibitors are triethylene glycol, monoethylene glycol (alone and in combination) (plot of temperature v. water mol. fraction).

Figure 9 is a simplified process diagram for a dew point control unit. Figure 10 is a simplified drawing of a counter flowing absorber column.

Freezing, as explained above, is a major problem during oil and gas production and processing. The most common method to date to avoid freezing is to add an antifreeze agent such as an alcohol (e.g. methanol or ethanol) or a glycol

(monoethylene glycol or triethylene glycol). By using the correct amount of inhibitor compared to the amount of water, it is possible to reduce the freezing point as low as -30 to -40 °C. A typical application of a mixture of water and one antifreeze agent is for example the cooling system in a car engine. This is illustrated in Figure 1 , where the lines with square dots show the freezing curves (i.e. solid liquid equilibrium of Solid A with the liquid mixture, or solid B with the liquid mixture). Pure A freezes out at -5°C while component B has a freezing point of -Ι δ'Ό. If one starts with pure A, and then starts to mix in B, the freezing temperature is reduced, e.g. a mixture of A and B with 20 mol% concentration of component B in the liquid phase depresses the freezing point of pure solid A to -15°C (see the solid line in Figure 1 ). At some specific ratio of A and B (in the case shown in Figure 1 , 60% B), the two freezing curves meet. Below this point, here at approx -45 °C, only solid is present.

The phase diagram can be more complex if A and B can form new components, for example AB, A ₂ B, or AB ₂ . One example where this can occur is with the use of monothylene glycol as an antifreeze agent. The phase diagram is even more complex compared to that shown in Figure 1 , because an intermediate solid complex phase is being formed. This is illustrated in Figure 2, which shows the freezing diagram of the binary monothylene glycol - water system. The solid complex phase with a temperature maximum at approximately 50% monothylene glycol

concentration in the liquid phase is shown.

As explained above, the use of an antifreeze agent (such as monothylene glycol or triethylene glycol) is a common practice is gas and oil processing such as offshore processing platforms where gas temperature is reduced to sub-zero values, e.g. in order to condense and separate heavy hydrocarbons and meet the dew point specification (the process commonly known as dew point control, see Figure 9). In Figure 9, natural gas (1 ) is transported via a pipeline and arrives to the processing facilities at 100 bar and 10°C. A glycol-water mixture (2) (e.g. an 80 wt. %

monoethylene glycol-water mixture) is injected to the natural gas prior to dew point control. The resulting mixture then flows through a JT valve or an expander (3) into the dew point control unit widely called a "cold separator" (4). The cold separator (4) is maintained at a pressure of 60-70 bar and a temperature of -20 to -30 resulting in liquid phases consisting of the glycol (e.g. monoethylene glycol), water and heavy hydrocarbons to separate out (6) while the purified gas is routed to the export pipeline (5).

During low temperature processing it is important to have enough inhibitor present to avoid freezing. However, having too much inhibitor (too high a concentration) may lead to freezing of the inhibitor itself (moving, for example, towards the freezing curve of solid B in Figure 1 ). Pure triethylene glycol has a freezing point of only -δ'Ό and monothylene glycol has -13°C. Many processes occur at lower temperatures than this and the formation of the solid inhibitor can only be avoided by making sure that the liquid phase is a mixture of the inhibitor and water. This narrows the operation range significantly. For example, in a specific gas field in operation, the injected monothylene glycol has a concentration of 80 wt% monothylene glycol, 20 wt% water, and after the process it is 78 wt% monothylene glycol, 22 wt% water. The narrow operating range results in unplanned shutdowns due to solid formation.

The narrow operating widows are further illustrated in Figure 3. The line on the right hand side of the diagram (solid black line with open squares) is the freezing curve of pure monothylene glycol solid. The dashed line with closed black squares on the left hand side is the hydrate formation curve of a typical natural gas system. That forms a triangle between those lines which provides the safe operating window (see densely spotted area). This illustrates how tightly the safe operational area is limited by the relatively low freezing points, the hydrate formation lines and the formation of the intermediate solid complex.

The equivalent experiments to those for monoethylene glycol-water were repeated to produce a phase diagram for triethylene glycol-water mixtures. The purpose of studying this mixture was two-fold: a) to generate reliable experimental data and evaluate whether a solid intermediate phase similar to that for monoethylene glycol- water system is formed, and b) perform theoretical thermodynamic simulations of the phase behaviour and develop reliable solid models for the calculations.

The results are presented in Figure 4. The findings of these experiments are that: a) an intermediate solid complex phase does exist (between 25 and 60 mol% triethylene glycol as illustrated in Figure 4); and b) good simulation results can be achieved. It is important to note that prior to verifying the existence of the

intermediate phase and based on existing open literature data only, one would expect a phase diagram similar to Figure 1 . This means that an unforeseen solid phase and blockage of the process would occur, if triethylene glycol was applied as the antifreeze agent in the processing units.

Having evaluated the mixtures of water and a single antifreeze agent (including careful checking for the existence of an intermediate phase), work was performed to determine the freezing diagram of monoethylene glycol— triethylene glycol binary systems. Data that were actually obtained by measurement of the binary system at different mol fractions of the two glycols were gathered (points) as well as theoretical calculations (solid line) and the results are presented in Figure 5. As can be seen, it was discovered that: a) no intermediate phase is formed and this mixture can be used as the cooling medium to very low temperatures [for example a mixture of monoethylene glycol— triethylene glycol with 60 mol% monoethylene glycol will depress the freezing point of pure triethylene glycol (normal freezing point of -5°C) down to -32°C]; and b) a reliable theoretical solid model has been developed which can be applied to the system of water utilizing two antifreeze agents (a mixture consisting solely of two glycols).

Having demonstrated the drawbacks of the current use of a single freezing point inhibitor such as monoethylene glycol or triethylene glycol to depress the freezing point in a gas or oil processing system, studies were performed to determine how these drawbacks could be overcome. The performance of a mixture of two glycols, monoethylene glycol and triethylene glycol, as discussed above and illustrated in Figure 5, provided the present inventors with the starting point for the present invention.

On the basis of the solid model developed, theoretical thermodynamic calculations were performed to determine the phase diagram for water with a mixture of monoethylene glycol and triethylene glycol as the freezing point inhibitor, initially ignoring any intermediate solid complex formation. The purpose of this exercise was to find the optimum concentration of the monoethylene glycol and triethylene glycol antifreeze mixture to be used. The results are presented in Figure 6. In the same figure the binary monoethylene glycol-water and triethylene glycol-water phase diagrams are presented with the characteristic intermediate solid complexes.

As can be seen from those calculations, a mixture of 50-50% mole monoethylene glycol and triethylene glycol enables trouble-free operation at much lower temperatures compared to the case that only one antifreeze agent is used, as the dashed (short and long dashes) black lines for pure monoethylene glycol or pure triethylene glycol freezing have been shifted to much lower temperatures (reference thick solid line on the left hand side of the figure). The optimum concentration is a mixture of 65-35% mol monoethylene glycol— triethylene glycol respectively, which starts freezing at around -35°C.

An outstanding question so far is the following: Do we get a solid complex formation of any combination of a) monoethylene glycol, triethylene glycol.water molecular structure or the verified monoethylene glycol.water or triethylene glycol.water solid structures?

In order to address this issue theoretical simulations were performed, based on the assumption that the known monoethylene glycol.water solid phase or triethylene glycol.water solid phase can still be formed. The reason for choosing those intermediate complexes is that their existence is experimentally proven. The results are presented in Figure 7.

In Figure 7, the curved lines in the middle of the diagram correspond to an assumed solid complex phase at different given monoethylene glycol-triethylene glycol concentrations. As can be seen, it was found that: a) a mixture of monoethylene glycol-triethylene glycol inhibits the formation of the solid triethylene glycol.water phase (i.e. intermediate complex phase freezing formation temperatures are depressed); b) if such an intermediate solid complex phase was to form, it would not be thermodynamically stable at any concentrations of interest for operation (ranging from 50-50% to 65-35% monoethylene glycol-triethylene glycol respectively), because a pure solid phase would freeze out at higher temperature than the solid complex and hence be the stable phase, c) for the concentration range of 50-50% to 65 - 35% monoethylene glycol-triethylene glycol, it has theoretically been shown as a result of this work that the thin and thick solid MEG-TEG lines (50-50 mol% and 60- 40 mol% respectively) in Figure 7 correspond to the operating limit of mixed solvents with respect to solid formation.

Experiments have been performed to verify these theoretical calculations. Utilizing a mixture of monoethylene glycol-triethylene glycol (50-50% mol %) with water corresponding to the following total composition of the ternary mixture: 25% monoethylene glycol, 25% triethylene glycol and 50% water and cooled down up to - 55 'Ό (apparatus limit), no freezing out of either water, the glycols or the formation of intermediate sold complex was detected. This finding was in excellent agreement with the theoretical calculations discussed above and illustrated in Figures 6 and 7, which show that a freeze out of glycol would be expected at approximately -57°C. Note that in Figure 7, if an intermediate phase is to be formed, the freezing temperature is suppressed approximately to -75 'Ό (thin solid curved line) and will not be stable, as freeze out of glycol would have occurred first.

Furthermore, experiments were performed to determine the phase diagram for hydrate inhibitor-water, where the hydrate inhibitor is monoethylene glycol and triethylene glycol (alone or in various combinations) at different water mole fractions. The results are as presented in Figure 8.

In Figure 8, the long-dashed line shows the phase diagram for water-monoethylene glycol, which is the same as in Figure 6. The densely spotted triangle is the same as in Figure 3 which shows the safe operating area for a water-monoethylene glycol- hydrate system. However, when the pure monoethylene glycol inhibitor is replaced by a 60-40 mixture of monoethylene glycol - triethylene glycol, the triangle is expanded to the broader rhombus by the addition of the area marked out with the area that is more sparsely dotted. The hydrate line is indicated as the thick solid line.

Thus it can be seen that the operating envelope for safe operation (verified by experiment and not only theoretical calculations) has been expanded compared to Figure 3, while experimental results are fully consistent with theoretical calculations.

Figure 10 shows a simplified drawing of a counter flowing absorber column. This is used to dry the gas before it is exported. In this column natural gas (7) is fed via an inlet into the bottom of the counter current absorber column (8). A typical prior art example of such an absorber column is a so-called triethylene glycol contactor, in which the gas is mixed with triethylene glycol which is fed into the counter current absorber column via inlet (9) at the top of the column. Close to pure triethylene glycol is added at the top of the column and as it comes into contact with the gas it extracts water out of the gas. The triethylene glycol is then taken out at the bottom of the column via outlet (1 1 ) and regenerated by heating so that the water evaporates, while the dehydrated natural gas is removed via outlet (10) at the top of the absorber column. This is possible because the boiling point of triethylene glycol is 285 °C while that of water is 100 °C. The dry regenerated triethylene glycol is then reused and circulated back to the contactor. Typical operation conditions for a contactor are 30 'Ό. Typical requirements for a triethylene glycol drying process are that the water content of the gas is so low that water will not start to condense before below -18°C at 6,900 kPa. Being able to obtain the required dew point of -Ι δ'Ό at 6,900 kPa requires that the triethylene glycol has very high purity when the contactor is operated at a

temperature as high as 30 °C. By operating the contacting unit at a lower

temperature, water can be removed more efficiently. A limitation relating to lowering the temperature limit for operation of a triethylene glycol contacting unit would be the freezing point of triethylene glycol. If the triethylene glycol is replaced by a freezing point inhibitor comprising a mixture of glycols such as a mixture of triethylene glycol and monoethylene glycol in accordance with the invention, the contacting unit can be operated at lower temperatures and thereby obtain a more efficient removal of water.

In cases where the natural gas to be treated is at low temperature, the gas has to be heated before entering the triethylene glycol contactor unit. The energy requirement for heating the gas can be high, and it will be positive if the contacting temperature can be reduced. The use of a mixture of monoethylene- and triethylene glycol will enable the operation of the counter flow contacting unit at very low temperatures.

It can be concluded that whether hydrate inhibition is done with a pure inhibitor (e.g. monoethylene glycol) or using a mixture (e.g. monoethylene glycol and triethylene glycol), this will only have minor effect on the hydrate formation temperature, as long as the amount of inhibitor (in mole fraction relative to water) is the same. However, because the freezing temperature of the inhibitor is reduced, it is possible to operate at a higher inhibitor concentration without getting freeze out of solid inhibitor. When the inhibitor concentration is increased, the ice formation temperature and hydrate formation temperature is reduced. Furthermore, it has been discovered by using these freezing point inhibitors consisting solely of a mixture of two different glycols, no intermediate solid complex phase is formed. The result is that it is possible to operate at lower temperatures, and the safe operating area is increased.

The present invention is not limited by the examples described above and further extensions of this to other areas within the broader scope of the claimed invention will be readily apparent to the person skilled in the field of gas or oil processing systems operational at low temperatures such as dew point control in the transport of gas, various oil and gas processing systems in which a cooling medium is used and contacting units for drying gas.