WO2011083365A2

WO2011083365A2 - Gas dehydration

Info

Publication number: WO2011083365A2
Application number: PCT/IB2010/003480
Authority: WO
Inventors: Satish Kumar; Graeme King
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-01-11
Filing date: 2010-12-31
Publication date: 2011-07-14
Anticipated expiration: 2012-07-11
Also published as: WO2011083365A3

Abstract

For gases that exhibit retrograde water solubility behavior with pressure at a constant temperature, gas compression and dehydration is carried out in a single integrated process using multiple stages of compression followed by a pressure throttling device used mainly for gas cooling purpose, physical separation of water from the gas and a final stage of gas compression. This process is found techno-economically more attractive than commonly used processes like solvent absorption, desiccant adsorption etc. The process does not require the use of closed-cycle mechanical refrigeration systems for cooling the gas, which in general makes it more efficient as compared to those processes which use closed-cycle refrigeration systems. The process is applicable to CO₂ and H₂S, and also to mixtures of these gases with each other or with hydrocarbons.

Description

TITLE OF INVENTION

GAS DEHYDRATION

BACKGROUND OF THE INVENTION

The invention relates to a gas dehydration process and a plant for carrying out such a process.

Gas dehydration is important to a variety of industries that process and transport gases, such as the oil and gas industry, and carbon capture and storage projects that capture carbon dioxide (C0₂) emitted from energy intensive industries burning fossil fuels and emitting large quantities of gaseous C0₂, such as the steel making, aluminum smelting, electrical power generation and concrete manufactoing industries.

Gases are generally saturated with water when they are recovered from reservoirs or captured after combustion. Before transporting them through carbon steel pipelines and facilities sufficient water needs to be removed from them to prevent them from condensing water because water corrodes carbon steel and could also cause flow problems due to ice and hydrate formation.

The water content of C0₂ in pipelines installed in North America is usually kept below 30 lb/MMscf (0.48 g/m³) of C0₂to ensure that water does not condense at the ininimum pipeline operating temperature, which can be as low as -20°C. But in the United Arab Emirates, for example, the minimum ground temperature at pipeline depth is approximately +13°C and it is only necessary to dehydrate C0₂ to a water content of 80 lb/MMscf (1.28 g/m³) of gas in order to prevent water from precipitating inside the pipeline under all possible operating scenarios. It is therefore not necessary to use water content specifications as low as those used in cold countries for C0₂ pipelines. To achieve a water content of less than 30 lb/MMscf (0.48 g/m³) of pipeline quality gas the following approach is commonly used: 1. Partially compress the C0₂ in multiple stages of compression to a suitable pressure for C0₂ dehydration.

2. Use a solvent or adsorbent dehydration process to remove water from the partially compressed C0₂. 3. Further compress the dehydrated C0₂ to the operating pressure of the pipeline that will transport it for use in a downstream project such as enhanced oil recovery or sequestration.

Several types of process are used by industry for gas dehydration in step 2 of this approach. Two of the most common processes are: · Solvent absorption typically using glycol such as Tri Ethylene Glycol (TEG), Mono Ethylene Glycol (MEG), Di Ethylene Glycol (DEG) etc.

• Adsorption with molecular sieve, activated alumina, silica gel or other

desiccant.

Solvent-based dehydration units are expensive both in respect to capital costs and operating costs. Although the solvent removes water from the gas, it also adds amounts of solvent to the gas, so that solvent-based systems lose solvent due to solvent carryover with the dehydrated gas.

Adsorbent-based dehydration units have even higher operating costs than solvent- based ones and also pose problems with waste disposal.

Some processes for gas dehydration use mechanical cooling systems which become less efficient and consume more power as the ambient temperature increases.

Processes that may be efficient in cold regions are less efficient in hot regions, SUMMARY OF INVENTION

The invention provides an alternative method for dehydrating the partially

compressed gas in step 2 above using an open-cycle refrigeration process rather than absorption or adsorption processes. The invention provides a practical dehydration process for any gas that exhibits retrograde water solubility behavior as a function of pressure, such as C0₂ and H₂S, or mixtures of such gases. The mixture may also include other gases such as natural gas as long as the mixture still exhibits retrograde water solubility with increase in pressure. The term "water solubility" refers to the property of gases to retain water in gas phase without releasing a separate liquid water phase. Gases with "normal water solubility behavior" can retain less water at high pressure than at low pressure, whereas gases that exhibit "retrogrades water solubility behavior" can retain more water at high pressure than at low pressure. A gas exhibiting retrograde water solubility typically has normal water solubility at low pressure but as its pressure increases its ability to retain water in gaseous phase reaches a minimum and then as the pressure continues to increase at constant temperature it starts exhibiting retrograde water solubility and the amount of water it can retain starts increasing. For a given constant temperature, the pressure at which the water solubility behavior changes from normal to retrograde behavior is defined as the "minimum water solubility pressure". The minimum water solubility pressure typically increases with increasing temperature.

The invention provides a gas dehydration process comprising the following steps carried out in flow sequence from upstream to downstream: (a) providing a gas that exhibits retrograde water solubility; (b) compressing the gas to a pressure

approximately equal to or higher than its minimum water solubility pressure; (c) expanding and cooling the gas to the desired dehydration temperature and its minimum water solubility pressure, and separating condensed water from the gas ; and (d) compressing the gas to a pressure above its minimum water solubility pressure so that it becomes under- saturated and therefore suitable for transportation in carbon steel pipelines and facilities, subsequent processing or other use. Compressing the gas to a pressure equal to, or higher than, the minimum water solubility pressure in Step (b) above may involve one or more stages of: (i) gas compression, (ii) removing heat from the compressed gas with the help of a cooling medium, such as ambient air, cooling water or refrigerant, and (iii) removing condensed water. Each stage of "initial" compression increases the pressure of the gas and reduces its water content. The final water content of the dehydrated gas output from the process according to the invention is controlled by expanding and cooling the gas in Step (c).

In another aspect, the invention provides a gas dehydration plant comprising: (1) an inlet pipe for supplying a gas that exhibits retrograde water solubility; (2) one or more initial compression stages arranged in series connected to receive the gas from the inlet pipe, where the number of stages of compression is sufficient to compress the gas to a pressure equal to or greater than its minimum water solubility pressure; (3) a pressure throttling device (optionally with a heat exchanger) connected to receive the gas from the last initial compression stage and operable to reduce the temperature and pressure of the gas to a desired dehydration temperature and its rmriimum water solubility pressure at the desired dehydration temperature respectively, and a water separator connected to receive gas from the pressure throttling device and operable to remove condensed water from the gas; and (4) one or more final compression stages connected to receive gas from the water separator and operable to increase the pressure of the gas to a pressure above its minimum water solubility pressure so that it becomes under-saturated and suitable for subsequent transportation, processing or other use.

In embodiments of the invention, each stage of initial compression in (2) above comprises a compressor to increase the pressure of the gas, an after-cooler to reduce the temperature the gas from the compressor and a water separator vessel to physically remove condensed water from the cooled gas. The after-cooler may be cooled by air or water, both of which are simple to implement in terms of initial construction and ongoing maintenance. To improve efficiency, the pressure throttling device used in (3) above may be configured along with a gas-to-gas (feed /effluent) heat exchanger. As the gas passes through the pressure throttling device or orifice its pressure is reduced to the minimum water solubility pressure and the temperature is reduced concomitantly to the desired dehydration temperature due to the Joule-Thomson effect. The gas-to-gas heat exchanger is configured to exchange heat between the gas flowing into the pressure throttling device and the gas flowing out of the pressure throttling device, thereby cooling the gas before it enters the pressure throttling device (expander valve or orifice) and heating the gas after it leaves the water separator vessel. The invention can be used to reduce water content in C0₂ to 30 lb/MMscf which makes it useful in cold countries and it also provides a more efficient way to dehydrate gas in hot environments.

The invention therefore provides a gas dehydration process comprising the following steps carried out in the stated flow sequence from upstream to downstream: (a) providing a gas of the type that has retrograde water solubility behavior as a function of pressure; (b) compressing the gas to at least approximately the minimum water solubility pressure; (c) expanding and cooling the gas using the Joule-Thomson effect and removing water that condenses from the gas; and (d) compressing the gas so that it becomes dry.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings.

Figure 1 shows a gas dehydration plant according to an embodiment of the invention. Figure 2 is a flow diagram of a dehydration process according to an embodiment of the invention.

Figure 3 is a flow diagram showing in more detail one of the initial compression steps Ki of Figure 2.

Figure 4 is a flow diagram showing in more detail the Joule-Thomson cooling step JT of Figure 2.

Figure 5 is a flow diagram showing in more detail one of the final compression steps Fi of Figure 2.

Figure 6 is a graph of water content as a function of pressure for a gas that exhibits retrograde water solubility, and additionally depicts the process steps used in an embodiment of the invention.

DETAILED DESCRIPTION

Figure 1 shows a gas dehydration plant according to an embodiment of the invention.

The gas dehydration plant can be grouped into three main units: an initial compression unit A; a dehydration unit B; and a final compression unit C. An inlet pipe 1 conveys feed gas from another plant (not shown), which generates the gas to be dehydrated as a product or bi-product. The feed gas is of a type that exhibits retrograde water solubility as a function of pressure from the plant to the first stage of initial compression. The inlet pipe 1 leads to a water knockout drum D which serves to prevent any free water from entering the first stage of compression and damaging it in the event the feed gas contains free water. The knock out drum D may be omitted in some embodiments.

The initial compression unit A is composed of a succession of n stages of

compression, Kn, based on compressors Cn. The compression stages Kn collectively serve to increase the pressure of the gas from the inlet pressure at inlet point 1 up to the minimum water solubility pressure or higher by the time it reaches an outlet point 16 at the output of the last of the initial compression stages Kn. Each stage of compression Ki comprises a compressor Ci operable to increase the pressure of the gas, an after-cooler Ei operable to reduce the temperature of the hot gas from the compressor Ci, preferably using air or water as the cooling medium, and a water separator Vi operable to physically separate and remove condensed water from the gas as it leaves the after-cooler Ei, thereby preventing damage to the compressor of the subsequent compression stage. The overall role of the initial compression unit A is therefore to deliver the feed gas to the dehydration unit at a pressure of at least the minimum water solubility pressure. It is therefore conceivable, although unlikely, that in some embodiments an initial compression unit may not be required, namely if the feed gas is already by coincidence at a suitable temperature and pressure.

The dehydration unit B follows the initial compression unit A. The role of the dehydration unit B is to reduce the temperature and pressure of the gas to levels suitable for dehydration to take place, the pressure of the gas preferably being reduced to at or near its miriimum water solubility pressure at the dehydration temperature, and then to separate out the condensed water. The dehydration unit A has as its first component a gas-to-gas heat exchanger HE arranged to receive the gas supplied by the initial compression unit A from point 16, and to cool it with the gas output from the dehydration unit. As an alternative to using the output gas as the cooling medium, a separate cooling medium could be used to effect the cooling, for example air or water. The cooled gas at point 17 is then passed through a water separator S A, which may be omitted in some embodiments. The optional water separator vessel is provided to physically remove liquid water that condenses in the heat exchanger. The gas then passes to a pressure reducing valve TV which serves as a pressure throttling device. The pressure reducing valve TV reduces the gas pressure and causes its temperature to fall due to the Joule-Thomson effect. The reduction in temperature through the pressure reducing valve causes further condensation of water from the gas. The gas flows from the pressure reducing valve to another water separator SB which physically removes the condensed water. The gas then flow's from the water separator SB back past point 18 and through the heat exchanger where it is heated again causing it to become under-saturated before it passes out of the dehydration unit to the final compression unit C.

The final compression unit C is composed of m stages of compression Fj. Each stage Fj has a compressor Pj that serves to compress the dehydrated gas and an after-cooler Xj that serves to cool the gas through heat exchange with a cooling medium such as air or water. The m stages of final compression Fl to Fm collectively serve to increase the pressure of the gas received from the dehydration unit to a pressure at the outlet point 21 that is suitable for supply to a downstream process, a pipeline for

transportation etc. In some embodiments, it may be that the final compression unit can be dispensed with, namely if the gas pressure at the output of the dehydration unit is sufficient for the intended subsequent processing step.

Example - CO? gas dehydration: As an example, the plant shown in Figure 1 can be implemented to dehydrate C0₂ with the following parameters. This example is based on an embodiment of the invention that uses a gas-to-gas heat exchanger as shown in Figure 1. • Gas pressure/temperature at point 1 (inlet): 0.5 bar (50 kPa) and 55°C

• Gas pressure/temperature at point 17 (before the expander): 50 bar (5.0 MPa) and 17°C

• Gas pressure/temperature at point 18 (after the expander):46 bar (4.6 MPa) and 12°C

• Gas pressure/temperature at point 21 (at launch to pipeline): 190 bar (19 MPa) and 58°C

Figure 2 is a flow diagram of a dehydration process according to an embodiment of the invention. Water-bearing gas is supplied from external plant to a compression step KI at a first pressure approximately equal to or greater than its minimum water solubility pressure. The compression steps are repeated 'n' times as illustrated by an i'th compression step Ki and an n'th compression step Kn. The gas is then cooled and expanded in step JT, for example through a Joule- Thomson valve, the temperature being reduced to a pre-specified temperature, and the pressure being reduced to a pressure at or near to the minimum water solubility pressure at the pre-specified temperature. Water that condenses out of the gas following the Joule-Thomson expansion is then removed. The process then continues to a compression step Fl to compress the gas, and subsequent compression steps, the compression steps being repeated 'm' times as illustrated by a j'th compression step Fj and an m'th compression step Fm. The process is now complete and the dry gas is output to a pipeline.

Figure 3 is a flow diagram showing in more detail one of the initial compression steps Ki of Figure 2. Each initial compression step Ki comprises a compression sub-step Ci, followed by an after-cooling sub-step Ei and a water separation sub-step Vi. It will be appreciated that the after-cooling and water separation sub-steps are optional. Figure 4 is a flow diagram showing in more detail the Joule-Thomson cooling step JT of Figure 2. The gas is pre-cooled in a pre-cooling step HE performed in a heat exchanger, and then is processed by a water separation sub-step SA. The gas is then cooled and expanded, for example by passage through a suitable valve, in a sub-step TV and then undergoes a further water separation sub-step SB. It will be appreciated that all these sub-steps are optional except the cooling and expansion sub-step TV. Figure 5 is a flow diagram showing in more detail one of the final compression steps Fi of Figure 2. Each final compression step Fj comprises a compression sub-step Pj, followed by an after-cooling sub-step Xj. It will be appreciated that the after-cooling sub-steps are optional. Figure 6 is a graph of water content W in mg/m³ of C0₂ as a function of operating pressure P in MPa as an example of a gas that exhibits retrograde water solubility. The graph shows water solubility for a family of constant temperature curves at temperatures of 0°C, 10°C, 20°C, 30°C, 40°C, 50°C, and 60°C. An operating pressure range of 2 to 14 MPa is plotted. In this range of temperatures and pressures, the water solubility, i.e. the amount of water the gas can hold without precipitation, is in the range of about 0-6000 mg/m .

Water solubility is defined as the maximum water content that can be absorbed by a fluid at any pressure and temperature without precipitating solid or liquid water. The graph represents a fluid that exhibits the special property of retrograde water solubility. As the gas pressure increases steadily from a low pressure at constant temperature, the water solubility decreases at first but then reaches a minimum value after which it increases again as the pressure continues to increase. The minimum point on each constant temperature line is referred to as the minimum water solubility pressure. C0₂ is an example of a gas that exhibits retrograde behavior with respect to water solubility such that it acquires a greater capacity to hold water as its pressure increases at constant temperature above its minimum water solubility pressure. As can be seen in the graph, the minimum water solubility pressure decreases from around 8MPa at 60°C to around 4MPa at 0°C.

This anomalous behavior is different from most gases (for example, natural gas and nitrogen) which have a water solubility that decreases monotonically with increasing pressure and have no minimum water solubility pressure at normal conditions. The anomalous behavior illustrated in the graph is referred to as retrograde water solubility and is a property exhibited by some gases such as C0₂ and H₂S.

As well as plotting the family of constant temperature curves, the graph shows straight lines illustrating an example of the three main steps of the dehydration process according to the invention. The line showing the initial compression step is labeled A, the line showing the dehydration step is labeled B, and the line showing the final compression step is labeled C.

In this example case, the gas is input to the initial compression unit A at a temperature of 40°C and pressure of 20 bar (2.0 MPa) and the initial compression unit increases its pressure to 65 bar (6.5 MPa). The temperatures and pressures achieved in the initial compression stages are adequate to reduce the water of the gas to 120 lb/MMSCF (1.9 g/m³) at a temperature of 40 C.

The dehydration unit B then reduces the pressure of the gas to 46 bar (4.6 MPa) and its temperature to 12°C. The gas has a minimum water solubility pressure of 46 bar (4.6 MPa) at 12°C and after the precipitated water is removed from the gas the graph shows its water content will be approximately 25 lb/MMscf (4 g/m³)

The final compression unit C increases the pressure to a pressure of 120 bar (12.0 MPa). This final compression stage beyond 46 bar (4.6 MPa) enhances the capacity of the gas to absorb water and reduces its relative humidity to a low value to make it super-unsaturated, thereby eliminating the possibility of water condensing from the gas during any practical operational scenario.

As will be appreciated from the graph, although it will usually be preferable to initially compress the gas in Step A to at least the minimum solubility pressure (the minimum point of the constant temperature curve at any given temperature), the process will still function even if the gas is not compressed fully up to the minimum solubility pressure, but is only compressed to slightly below the minimum solubility pressure. This is because the Joule-Thomson expansion in Step B will often still be able to achieve adequate dehydration, even when the initial compression is not to a pressure of at least the minimum solubility pressure, as is evident from an inspection of the graph. The invention therefore extends to cover a situation in which the gas is compressed only to approximately the minimum solubility pressure, for example to 80% or 90% of the minimum solubility pressure. The precise specifications that will be acceptable will vary from case to case depending on the target dehydration level, the gas being dehydrated, the target gas temperature and other factors. In summary, it has been described how gas compression and dehydration can be carried out in a single integrated process using multistage compressors, coolers and water separator vessels exploiting the retrograde water solubility property of fluids such as C0₂ and H₂S. With the invention it is possible to provide a dehydration plant that does not require an external dehydration unit, as is required by conventional processes such as those based on solvent or adsorbent dehydration. The approach of the invention therefore allows simplification of the gas dehydration process, since solvents and inhibitors are not used and associated waste and contamination is not produced. The process does not require closed-cycle refrigeration, making it more efficient in hot countries, and also simplifying the plant, reducing maintenance and improving reliability.

Implementations of the invention can maximize simplicity by using air cooling to reduce the temperature of the gas after each stage of compression.

The process is applicable not only to C0₂ and H₂S, but also to mixtures of C0₂ and ¾S, mixtures of C0₂ with hydrocarbons, such as natural gas, mixtures of H₂S with hydrocarbons, mixtures of C0₂ & ¾S with hydrocarbons, and any other gas that exhibits retrograde water solubility behavior with pressure.

In summary, the invention provides a gas dehydration process comprising the following steps carried out in the stated flow sequence from upstream to downstream: (a) to provide a gas of the type that exhibits retrograde water solubility behavior as function of pressure, wherein the gas is provided at a first pressure approximately equal to or greater than its minimum water solubility pressure; (b) to reduce the temperature and pressure of the gas, the temperature being reduced to a pre-specified temperature, and the pressure being reduced to a second pressure at or near to the minimum water solubility pressure at the pre-specified temperature, and to remove condensed water from the gas; (c) to compress the gas to a third pressure which is higher than the second pressure such that the relative humidity of the gas falls below 100% and the gas becomes under-saturated; and (d) to supply the compressed under- saturated gas for subsequent processing. In Step (a) the gas may be initially provided at a fourth pressure below the first pressure, and then compressed to the first pressure. The compression from the fourth pressure to the first pressure may be accomplished with one or more stages of: (i) compression of the gas, (ii) heat removal from the compressed gas, and (iii) condensed water removal from the compressed gas. The heat may be removed by cooling with air, water or another cooling medium. The gas may be provided to the process at a wide range of pressures depending on the implementation, for example between 0.1 and 50 bar (10 kPa and 5000 kPa).

The invention also provides a gas dehydration plant comprising: (a) an inlet for conveying a gas of the type that has retrograde water solubility behavior as a function of pressure; (b) an initial compression unit comprising a number of stages of initial compression, composed of at least a first stage of initial compression having an input connected to receive the gas from the inlet, wherein the number of stages of initial compression is specified to compress the gas from the inlet to approximately equal to or greater than its minimum water solubility pressure; (c) a dehydration unit connected to receive the gas output from the initial compression unit and having a throttling device operable to reduce the temperature and pressure of the gas, the temperature being reduced to a pre-specified temperature, and the pressure being reduced to at or near to the minimum water solubility pressure at the pre-specified temperature, the dehydration unit further comprising a water separator connected to receive gas from the throttling device and operable to remove condensed water from the gas; and (d) a final compression unit comprising a number of stages of final compression, composed of at least a first stage of final compression having an input connected to receive the gas output from the dehydration unit, wherein the number of stages of final compression is specified to compress the gas to a pre-specified pressure required by a downstream process at which the relative humidity of the gas is below 100% and the gas is under-saturated, i.e. the gas is dry and will not condense water. It will be understood that the number of compression stages in the initial and final compression units is flexible, and may be one. Moreover, the number of stages needed will depend on factors such as the specification of the compressors and other components of the compression stages, as well as the desired output pressure of the gas.

Claims

CLAIMS What is claimed is:

1. A gas dehydration process comprising the following steps carried out in the stated flow sequence from upstream to downstream:

(a) to provide a gas of the type that exhibits retrograde water solubility behavior as function of pressure, wherein the gas is provided at a first pressure approximately equal to or greater than its minimum water solubility pressure;

(b) to reduce the temperature and pressure of the gas, the temperature being reduced to a pre-specified temperature, and the pressure being reduced to a second pressure at or near to the rninimum water solubility pressure at the pre-specified temperature, and to remove condensed water from the gas;

(c) to compress the gas to a third pressure which is higher than the second pressure such that the gas becomes under-saturated; and

(d) to supply the compressed under-saturated gas for subsequent processing.

2. The process of claim 1 , wherein in Step (a) the gas is initially provided at a fourth pressure below the first pressure, and is compressed to the first pressure.

3. The process of claim 2, wherein Step (a) involves at least one stage of: (i) compression of the gas, (ii) heat removal from the compressed gas, and (iii) condensed water removal from the compressed gas.

4. The process of claim 3, wherein the heat is removed by cooling with air.

5. The process of claim 3, wherein the heat is removed by cooling with water.

6. The process of any preceding claim, further comprising exchanging heat between the gas output from Step (a) and the gas output from Step (b), thereby pre- cooling the gas input to Step (b) with the gas output from Step (b).

7. The process of any preceding claim, wherein in Step (b) the gas has its pressure and temperature reduced by passage through a throttling device.

8. The process of any preceding claim, wherein the gas is predominantly composed of carbon dioxide, hydrogen sulfide, or a mixture thereof.

9. A gas dehydration plant for dehydrating a gas of the type that has retrograde water solubility behavior as a function of pressure, the plant comprising;

an initial compression unit comprising a number of stages of initial compression, composed of at least a first stage of initial compression having an input connected to receive the gas from an external supply, wherein the number of stages of initial compression is specified to compress the gas from the external supply to approximately equal to or greater than its minimum water solubility pressure;

a dehydration unit connected to receive the gas output from the initial compression unit and having a throttling device operable to reduce the temperature and pressure of the gas, the temperature being reduced to a pre-specified temperature, and the pressure being reduced to at or near to the minimum water solubility pressure at the pre-specified temperature, the dehydration unit further comprising a water separator connected to receive gas from the throttling device and operable to remove condensed water from the gas; and

a final compression unit comprising a number of stages of final compression, composed of at least a first stage of final compression having an input connected to receive the gas output from the dehydration unit, wherein the number of stages of final compression is specified to compress the gas to a pre-specified pressure at which the gas is under-saturated.

10. The plant of claim 9, wherein, in the initial compression unit, each stage of initial compression comprises a compressor operable to compress the gas, an after- cooler operable to cool the compressed gas and a water separator operable to remove condensed water from the gas.

11. The plant of claim 10, wherein the after-cooler is cooled by air.

12. The plant of claim 10, wherein the after-cooler is cooled by water.

13. The plant of any preceding claim, wherein the dehydration unit comprises a heat exchanger arranged to pre-cool the gas prior to arriving at the throttling device.

14. The plant of claim 13, wherein the heat exchanger is a gas-to-gas heat exchanger configured so that the gas supplied to the throttling device is cooled that has passed through the throttling device.