WO2002039038A1

WO2002039038A1 - Gas liquefaction method

Info

Publication number: WO2002039038A1
Application number: PCT/EP2001/007811
Authority: WO
Inventors: Martin Altenbokum; Ulrich Kaulfuss; Sergiy Tkachuk
Original assignee: Gea Happel Klimatechnik Gmbh
Priority date: 2000-11-08
Filing date: 2001-07-07
Publication date: 2002-05-16
Also published as: DE10055321A1; AU2001272528A1

Abstract

The invention relates to a method for liquefying a gas (primary gas) which is part of a gas mixture that contains at least one second gas (secondary gas). Said second gas has a higher liquefaction temperature than the first gas. The gas mixture is especially cooled down by means of a heat exchanger. The second gas is subjected to phase transition at a separation temperature at which the first gas is gaseous and the second gas is liquid and/or solid, or at which the first gas is gaseous or liquid and the second gas is solid. The heat exchanger cools down the gas mixture to a temperature that lies above the separation temperature. Downstream of the heat exchanger a throttle element is disposed through which the gas mixture flows and is cooled down to the separation temperature.

Description

Process for liquefying a gas

The invention relates to a method for liquefying a gas (primary gas) which is part of a gas mixture which contains at least one second gas (secondary gas), the second gas having a higher liquefaction temperature than the first gas, the gas mixture being cooled in particular by a heat exchanger and the second gas undergoes a phase transition at a separation temperature in which the first gas is gaseous and the second gas is liquid and / or solid, or in which the first gas is gaseous or liquid and the second gas is solid.

During the liquefaction process of a gas mixture that contains various gases, it happens that the freezing temperature of one of the secondary gases is higher than the liquefaction temperature of the primary gas. This leads to ice formation of the secondary gas in the liquefaction chamber of the refrigerator during the liquefaction of the gas mixture. As a result, the liquefaction process is severely affected. In order to remove the ice layer in the liquefaction chamber, either an ice layer-destroying device or a special defrosting period is provided.

An example of such a gas mixture is the mixture of nitrogen and noble gases (argon, helium, etc.), which is obtained from the air by non-cryogenic separation. In this mixture, the pressure is 1 bar Liquefaction temperature of nitrogen at 77 K lower than the freezing temperature of argon at 83 K.

A refrigeration machine is used in a known method for liquefying a gas which is mixed with various secondary gases. Two different temperature levels are required for the liquefaction of the gas which is mixed with a gas, one for the phase change of the secondary gas and one for the actual liquefaction of the primary gas. The two different temperature levels are realized either with two different chillers or with a two-stage chiller. The gas mixture is passed to the first stage, where the secondary gas liquefies at a certain temperature. With the help of a separator, the liquefied secondary gas is separated from the still gaseous gas. The gas is then passed to the second stage, where it liquefies at a certain temperature.

This process has the following disadvantages:

1. If the physical properties of the secondary gas are characterized in that the temperature difference between the liquefaction temperature and the freezing temperature of the secondary gas is small, ice formation occurs in the first stage of the liquefaction chamber. To ensure almost complete liquefaction of the secondary gas, either the wall temperature of the liquefaction chamber must be below the freezing temperature of the secondary gas, or the design of the heat exchanger must be adapted accordingly. The adaptation of the design of the heat exchanger means that the surface of the heat exchanger must be designed to be large enough so that the temperature difference can be kept small. In the case of chillers, where the cooling capacity is only available in a small area, the increase in the surface area is structurally complex and uneconomical.

2. Despite the above measures, the complete liquefaction of the secondary gas is not possible, so that the gaseous portion of this Gases crystallized in the second stage. These ice crystals act as an insulation layer and worsen the heat transfer in the second stage.

The object of the invention is to avoid the formation of ice in secondary gases or not to allow them to take place in the liquefaction chamber of the refrigerator.

This object is achieved according to the invention in that the gas mixture is cooled by the heat exchanger to a temperature which is above the separation temperature, and in that an expansion valve is arranged behind the heat exchanger, through which the gas mixture flows and is cooled to the separation temperature.

A separation of the gases into different phases is thus achieved in a simple and reliable manner without the formation of ice in the secondary gases or the formation of ice not taking place in the liquefaction chamber of the refrigerator. The heat exchanger cools down less than in the prior art and the final cooling takes place only through the expansion valve.

It is particularly advantageous if the first gas is nitrogen and the second gases are noble gases. It is preferably proposed that the throttle element is an expansion valve, a capillary tube, an orifice, an expansion machine or a turbine.

Two embodiments of the invention are shown in the drawings and are described in more detail below. Show it

1 is a system diagram of a first embodiment,

2 shows a diagram with the vapor pressure curves of the first embodiment, Fig. 3 is a system diagram of a second embodiment and

Fig. 4 is a diagram with the vapor pressure curve of the second embodiment.

In the first method shown in FIGS. 1 and 2, the gas mixture from the first gas A to be obtained and the secondary gas B is liquefied with the aid of a refrigeration machine and thereby avoids the formation of ice in the secondary gas B. As part of this process, the refrigeration capacity of the refrigeration machine is determined by means of constructive measures provided at two different temperature levels. An expansion valve is used between the two stages. Pressure and temperature conditions in the stages are defined based on the physical properties of the gases.

The process has the following features: a) The gas mixture is compressed from pressure P ₀ to pressure Pi. An increase in temperature of the temperature T ₀ to the temperature T _to take place. b) After optional pre-cooling, the compressed gas mixture is passed into the first liquefaction stage and cooled down to the temperature Ti. The gas B is partially liquefied. c) After stage 1, the gas mixture and the liquefied gas B are expanded in an expansion valve from the pressure Pi to the pressure P ₂ . The temperature drops from Ti to T ₂ and the gas B is completely liquefied. d) The gaseous gas A is separated from the liquefied gas B in a separator. e) In the second stage, the gas A is liquefied at the pressure P ₂ and at the temperature T ₃ (T ₃ <Tfl _A ).

The system diagram of the process is shown in FIG. 1. The selection of the suitable process parameters is to be made using the vapor pressure curves of the two gases shown in FIG. 2.

Curves A and B represent the vapor pressure curves of gas A and gas B. The curve is intended to represent the course of expansion of the gas mixture A + B.

When selecting process parameters (working pressures and temperatures), the following should be considered: a) First, a desired final state 5 (T ₃ and P ₂ ) should be defined in which the gas A is liquefied. b) A temperature T ₂ (Tf _B <T ₂ <Tfl _B ) should be selected based on the physical properties of gas A and gas B (for example the enthalpy of vaporization of gas B, concentration), at which the complete liquefaction of gas B at the pressure P _{2 is} guaranteed. This corresponds to point 4 of Fig. 2. c) To determine the operating point 3 (P ₂ , Ti), the following must be observed:

- The curve (expansion curve of the mixture as a function of pressure and temperature) is considered for the temperature T ₂ .

- If the difference between the condensation temperature Tfl _ß and the freezing temperature Tf _{B of} the gas B at the pressure P _{2 is} too small, a critical temperature T _crit ι is defined depending on the liquefaction conditions in stage 1, which ensures a sufficient safety distance from Tf _B.

- The temperature T _krtt ι is used to determine the critical pressure P _kr itι from the curve.

- In order to avoid the liquefaction of gas A in stage 1, critical pressure P _kr i _t2 and the corresponding critical temperature Tkr. 2 can be defined. This point is the intersection of the curves A and <α. (Substance A changes its state from gaseous to liquid.) - The pressure Pi is selected between the two critical pressures.

Pcrit. <P1 <Pkrit2

- The temperature T-i is the temperature resulting from the expansion curve α at the pressure Pi (point 3 in Fig. 2). d) The pressure at operating point 2 is determined by the pressure Pi determined in point 3. This pressure corresponds to the final compression pressure. The temperature in point 2 depends on the change in state between points 1 and 2 during the compression. e) Point 1 corresponds to the initial state of the gas mixture from A and B.

Advantages of the process:

1. The two-stage liquefaction and the selection of the correct operating parameters for the liquefaction process ensure that ice formation in the liquefaction chamber is avoided. This enables a continuous liquefaction process without the heat transfer processes being impaired by the ice layer and without the process being interrupted by defrosting.

2. Due to this two-stage process, there is no need for larger heat transfer surfaces (large heat exchangers) that would otherwise be necessary to avoid ice formation.

3. Gas A and Gas B are separated from each other, liquefied and collected in separate storage containers.

In the second method shown in FIGS. 3 and 4, the gas mixture of gas A and B is liquefied with the aid of a refrigerator and thereby avoids the formation of ice in the liquefaction chamber of the refrigerator. In this process, the cooling capacity of the chiller is made available at a defined temperature Ti. The gas mixture of A and B compressed up to the pressure Pi is cooled to the temperature T ^ in the liquefaction chamber of the refrigerator. Both gases are liquefied. The liquefied gases are expanded in an expansion valve up to the pressure P ₂ .

The process has the following features: a) The gas mixture is compressed by the pressure P ₀ at the pressure Pi, thereby it _comes to a change in temperature from To to T. b) The gas mixture is liquefied in the heat exchanger of the refrigerator at the corresponding temperature Ti. c) The liquefied mixture of A and B is expanded in an expansion valve from the pressure Pi to the pressure P ₂ . The gas B crystallizes and the liquid A partially evaporates. d) The evaporated portion of gas A is separated from the liquid gas and the ice in a separator. e) The cold vaporized gas A is used in an additional heat exchanger 2 for precooling the main stream of the mixture of A and B. Here, the main flow of A and B from the initial temperature T _on until cooled to the temperature T _gw.

The process flow is shown in Fig. 3.

In this process, the property of the gases is used that the condensing temperature of the gases increases with increasing pressure, but the freezing temperature remains almost constant.

The changes in state of the liquefaction process are shown in FIG. 4. Curves A and B are the corresponding vapor pressure curves for gases A and B. When selecting the process parameters (working pressures and temperatures), the following should be observed: a) First, a desired final state (T _f | _A and P ₂ ) should be defined, in which gas A is liquefied. b) To determine operating point 4 (P1, T1), the following must be observed: - If the difference between the condensing temperature Tfl _B and the freezing temperature Tf _{B of} the gas B at the pressure P _{2 is} too small, a critical temperature T _k π _t i must be determined depending on the condensing conditions in the condensing chamber in order to ensure a sufficient safety margin of Tf _B to ensure.

- The critical pressure P _kr .t2 is defined at which the liquefaction temperature of gas A is equal to the freezing temperature Tf _B.

- The condensing pressure Pi is determined as follows.

P rftt <P ^ Pkrit2 <Pi

- The condensing temperature Ti results from the point of intersection of the condensing pressure Pi with the curve A. c) The pressure in the operating points 2 and 3 is determined by the pressure Pi determined in point 4. This pressure corresponds to the final compression pressure. The temperature in point 3 T _gw is the temperature after pre-cooling in the counterflow heat exchanger, and the temperature in point 2 T _an depends on the change in state between points 1 and 2. d) Point 1 corresponds to the initial state of the gas mixture from A and B.

The advantages of the method according to the invention are:

1. The liquefaction process is possible with only one temperature level.

2. Despite the one-step process, there is no need for larger heat transfer surfaces (large heat exchangers) that would otherwise be necessary to avoid ice formation.

3. The selection of the correct operating parameters for the liquefaction process ensures that ice formation in the liquefaction chamber is avoided. This enables a continuous liquefaction process without the heat transfer processes being impaired by the ice layers and without the process being interrupted by defrosting. The secondary gas can be an impurity gas and the throttle element can be an expansion valve, a capillary tube, an orifice, an expansion machine or a turbine.

Claims

Expectations

1. A process for liquefying a gas (A) which is part of a gas mixture which contains at least one second gas (B), the second gas having a higher liquefaction temperature than the first gas, the gas mixture being cooled in particular by a heat exchanger and that second gas undergoes a phase transition at a separation temperature, in which the first gas is gaseous and the second gas is liquid and / or solid, or in which the first gas is gaseous or liquid and the second gas is solid, characterized in that the heat exchanger Gas mixture is cooled to a temperature which is above the separation temperature, and that a throttle element is arranged behind the heat exchanger, through which the gas mixture flows and is cooled to the separation temperature.

2. The method according to claim 1, characterized in that the first gas (A) nitrogen and the second gases (B) are noble gases.

3. The method according to claim 1, characterized in that the throttle member is an expansion valve, a capillary tube, an orifice, an expansion machine or a turbine.