DE10055321A1 - Method of condensing gases involves condensing gas mixture to allow separation of lower condensation temperature gas - Google Patents

Abstract

The method of condensing a first gas from a gas mixture with higher and lower condensation temperatures involves passing the gases (A,B) through a heat exchanger for cooling (T1). The second higher condensation temperature gas is phase changed to become a liquid and -or solid while the first gas (A) remains gaseous. Downstream of the heat exchanger is a throttle.

Description

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

During the liquefaction process of a gas mixture, the various Contains gases, it happens that the freezing temperature is one of the  Secondary gases is higher than the liquefaction temperature of the primary gas. This leads to ice formation during the liquefaction of the gas mixture Secondary gas in the refrigeration machine's liquefaction chamber. As a result the liquefaction process is severely impaired. To the ice layer in the Removing the liquefaction chamber will either destroy the ice sheet Device or a special defrost period provided.

An example of such a gas mixture is the mixture of nitrogen and Noble gases (argon, helium, etc.), which are separated from the Air is extracted. In this mixture, the pressure is 1 bar Liquefaction temperature of nitrogen at 77 K lower than that Freezing temperature of the argon at 83 K.

In a known method for liquefying a gas, the is mixed with different secondary gases, a chiller comes to Commitment. This is for the liquefaction of the gas with a gas mixed, two different temperature levels are necessary, one for the Phase conversion of the secondary gas and one for the actual one Liquefaction of the primary gas. The two different temperature levels are either by two different chillers or with one two-stage chiller realized. The gas mixture is in the first stage passed where the secondary gas liquefies at a certain temperature. With With the help of a separator, the liquefied secondary gas is removed from the still gaseous gas separated. Then the gas is led to the second stage, in which liquefies it at a certain temperature.

This process has the following disadvantages:

• 1. If the physical properties of the secondary gas thereby are characterized in that the temperature difference between the Condensing temperature and the freezing temperature of the secondary gas is small, it occurs in the first stage of the liquefaction chamber Ice formation. To almost complete liquefaction of the secondary gas  ensure either the wall temperature of the Liquefaction chamber below the freezing temperature of the secondary gas lie, or the design of the heat exchanger adapted accordingly become. Adapting the design of the heat exchanger means that the surface of the heat exchanger should be large enough must, so that the temperature difference can be kept small. Both Chillers where the cooling capacity is only on a small area is available, the enlargement of the surface is structurally complex and uneconomical.
• 2. Despite the above measures, the complete liquefaction of the Secondary gas is not possible, so the gaseous portion of this Gases crystallized in the second stage. These ice crystals act as Insulation layer and deteriorate 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 take place in the refrigeration machine's liquefaction chamber.

This object is achieved in that the Heat exchanger the gas mixture is cooled to a temperature that is above the separation temperature, and that behind the heat exchanger Expansion valve is arranged through which the gas mixture flows and on the Separation temperature is cooled.

Thus, the gases are separated in a simple and safe manner reached different phases without causing ice formation in the Secondary gases or ice does not form in the liquefaction chamber the chiller takes place. The heat exchanger becomes less strong cooled than in the prior art and the final cooling takes place through the expansion valve.

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

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

Fig. 1 is a system diagram of a first embodiment,

Fig. 2 is 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 characteristics:

• a) The gas mixture is compressed from pressure P ₀ to pressure P ₁ . 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 temperature T ₁ . 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 P ₁ to the pressure P ₂ . The temperature drops from T ₁ to T ₂ and the gas B is completely liquefied.
• d) In a separator, the gaseous gas A from the liquefied gas B Cut.
• 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 α should show the expansion course 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 , T ₁ ), the following must be observed:
  • - For the temperature T ₂ , the curve α (expansion curve of the mixture as a function of pressure and temperature) is considered.
  • - 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 _{crit1 is} defined depending on the condensing conditions in stage 1 , which ensures a sufficient safety _distance from Tf _B.
  • - With the help of the temperature T _crit1 , the critical pressure P _{crit1 is} determined from the curve α.
  • - To avoid the liquefaction of gas A in stage 1 , critical pressure P _crit2 and the corresponding critical temperature T _{crit2 must be} defined. This point is the intersection of curves A and α. (Substance A changes its state from gaseous to liquid.)
  • - The pressure P ₁ is selected between the two critical pressures.
    P _crit1 <P1 <P _crit2
  • - The temperature T ₁ is the temperature resulting from the expansion curve α at the pressure P ₁ (point 3 in Fig. 2).
• d) The pressure at operating point 2 is determined by the pressure P ₁ 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. By two-stage liquefaction and by choosing the right one Operating parameters for the liquefaction process ensures that ice formation in the liquefaction chamber is avoided. this makes possible a continuous liquefaction process without the Heat transfer processes are deteriorated by the layer of ice and without the process being interrupted by defrost.
• 2. Because of this two-step process, larger ones can be used Heat transfer surfaces (large heat exchanger) are dispensed with would otherwise be necessary to avoid ice formation.  
• 3. Gas A and Gas B are separated from each other, liquefied and in separate Storage containers collected.

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 T ₁ . The gas mixture of A and B compressed up to the pressure P ₁ is cooled to the temperature T ₁ 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 characteristics:

• a) The gas mixture is _0, compressed by the pressure P to the pressure P _1, where it _comes to a temperature change from T ₀ to T.
• b) The gas mixture is liquefied in the heat exchanger of the refrigerator at the corresponding temperature T ₁ .
• c) The liquefied mixture of A and B is expanded in an expansion valve from the pressure P ₁ to the pressure P ₂ . The gas B crystallizes and the liquid A partially evaporates.
• d) The vaporized portion of gas A is separated from the liquid in a separator Gas and ice separated.
• e) The cold vaporized gas A is used in an additional heat exchanger 2 for pre-cooling 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 The liquefaction 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 considered:

• a) First, a desired final state (T _flA 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 T _flB and the freezing temperature Tf _{B of} the gas B at the pressure P _{2 is} too small, depending on the condensing conditions in the condensing _chamber, a critical temperature T _crit1 must be set to ensure a sufficient safety _distance from Tf _B.
  • - The critical pressure P _{crit2 is} defined at which the liquefaction temperature of gas A is equal to the freezing temperature Tf _B.
  • - The condensing pressure P ₁ is determined as follows.
    P _crit1 <P ₁
    P _crit2 <P ₁
  • - The condensing temperature T ₁ results from the intersection of the condensing pressure P ₁ with the curve A.
• c) The pressure in the operating points 2 and 3 is determined by the pressure P ₁ 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, you can go to larger ones Heat transfer surfaces (large heat exchanger) are dispensed with would otherwise be necessary to avoid ice formation.
• 3. By choosing the right operating parameters for the Liquefaction process ensures that ice formation in the Liquefaction chamber is avoided. This enables one continuous liquefaction process without the Heat transfer processes through the layers of ice are deteriorated and without the process being interrupted by defrost.

The secondary gas may be an impurity gas and the throttle body Expansion valve, a capillary tube, an orifice, an expansion machine or be a turbine.

Claims (3)

1. A process for liquefying a gas (A) which is part of a gas mixture which contains at least a 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 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) is nitrogen and the second gases (B) are noble gases. 3. The method according to claim 1, characterized in that the throttle element an expansion valve, a capillary tube, an orifice, a Relaxation machine or a turbine.