Method for withdrawing purge fluid from a column system

Abstract

Title: Method for withdrawing purge fluid from a column system. A method for withdrawing purge fluid from a column system comprising a first column (20) operating at a first pressure and a second column operating at a second pressure lower than the first pressure, and a main heat exchanger located below the column system, wherein: according to a first step, a first valve (31) is opened to withdraw purge fluid from a tank of the second column (30) and no purge fluid is withdrawn from a tank of the first column; and then according to a second step, a second valve (21) is opened to withdraw purge fluid from a tank of the first column (20) and the first valve (31) remains open. Single figure

Description

Title of the invention: Method for withdrawing purge liquid from a column system

[0001] The present invention relates to a method for withdrawing purge liquid from a column system, for example from an air separation apparatus by cryogenic distillation.

[0002] When purging all cryogenic liquids from an air separation device, it is necessary to ensure that liquid cannot be returned to a gas pipe by overflow.

[0003] In the context of a column system with a gas-gas exchanger placed under the column system, the invention consists of sequentially purging the tank of the highest column, then the tank of the lowest column.

[0004] An optional aspect is to send the liquid from the lowest column to the highest column, if enough pressure is available to push the liquid, before purging the liquid from the lowest column.

[0005] This avoids sending liquid into the gas pipe which arrives in the tank of the lowest column.

[0006] According to an object of the invention, there is provided a method for withdrawing purge liquid from a column system, for example from an air separation apparatus by cryogenic distillation, comprising a first column operating at a first pressure and with a second column operating at a second pressure lower than the first pressure, and a main exchanger located under the column system in which: • in a first step, a first valve is opened to draw off a purge liquid from a tank of the second column and no purge liquid is drawn off from a tank of the first column and then • in a second step, a second valve is opened to draw off a purge liquid from a tank of the second column and the first valve remains open.

[0007] According to other optional features: • according to the first step, all the purge valves placed on the circuits operating at the second pressure of the column system are opened and all the purge valves placed on the circuits operating at the first pressure of the column system are left closed. • between the first and second stages, the first column is depressurized by opening a lean liquid return valve and / or an inlet valve condensable at the level of a tank vaporizer of the second column. • after the second stage, all purge valves placed on the low points of the circuit are opened at the second pressure of the column system and all purge valves placed on the low points of the circuit are opened at the first pressure of the column system and also a rich liquid return valve. • at the end of the sequence, all the valves are closed.

[0008] The invention will be described in more detail with reference to the figure.

[0009] [Fig-1] represents an air separation process according to the invention.

[0010] The description is made in the context of an air separation apparatus with a column system comprising a first column 20 operating at a first pressure called the MP column and with a second column operating at a second pressure lower than the first pressure called the LP column, and a main gas-gas exchanger 10 located under the column system. The invention can be extrapolated to other configurations.

[0011] In normal operation, air 1 is cooled in the exchanger 10 by indirect heat exchange with two gases. The air cooled in the exchanger 10 enters the first column 20 where it separates to form rich liquid at the bottom of the column and lean liquid at the top of the column.

[0012] With valve 22 open and valve 21 closed, the rich liquid is sent to the second column 30.

[0013] The valve 23 being open, the lean liquid is sent to the top of the second column 30.

[0014] The second column 30 has a bottom vaporizer heated by nitrogen overhead from the first column 20. The second column 30 tops the first column 20.

[0015] Oxygen gas and nitrogen gas from the second column 30 are sent to the heat exchanger 10.

[0016] When stopping an air separation device, and in particular before defrosting, all cryogenic liquids must be purged.

[0017] The tank of the MP 20 column is purged via the valve 21 to send the purge via 41 to a vaporization system 40, which can be a simple container for example.

[0018] The tank of the LP column 30 (where the vaporizer is also located) is purged via the valve 31 to send the purge via 41 to the vaporization system 40, which can be a simple container for example.

[0019] In general, all the purge valves 21, 31 are opened at the same time. A high flow rate can cause back pressure in the common pipe 41: by the principle of communicating vessels, liquid can be sent from the tank of the LP column to the tank of the MP column. The level in the tank of the MP column can then exceed the level maximum allowed, then the liquid can overflow towards pipe 1 to exchanger 10, which we want to avoid for safety reasons.

[0020] One way to solve this problem is to add a high point on line 1, for example by going as high as the tank of the LP column. This is expensive and increases the pressure drop of circuit 1 in operation, and therefore induces an energy penalty.

[0021] A precise sequence is followed to avoid the return of liquid to the exchanger 10 by overflow. It is then possible to eliminate or limit the size of the high point on the air circuit MP 1.

[0022] Step 1: Valve 31 is opened to purge the tank of the LP column 30. Valve 21 remains closed.

[0023] It is also possible to open all the purge valves placed on the LP circuits of the column system (and leave closed all the purge valves placed on the MP circuits of the column system)

[0024] Step 2: the MP column 20 is depressurized by opening the lean liquid return valve 23 and / or the non-condensable valve at the vaporizer.

[0025] Step 3: Valve 21 is opened to purge the tank of the MP 20 column. Valve 31 remains open.

[0026] Step 4: All the purge valves located on the low points of the LP circuit of the column system are opened and all the purge valves located on the low points of the MP circuit of the column system are opened) and also the rich liquid return valve 22

[0027] At the end of the sequence, all the valves are closed.

Claims

Claims

1. Method for withdrawing purge liquid from a column system, for example from an air separation apparatus by cryogenic distillation, comprising a first column (20) operating at a first pressure and with a second column operating at a second pressure lower than the first pressure, and a main exchanger located under the column system in which: • according to a first step, a first valve (31) is opened to withdraw a purge liquid from a tank of the second column (30) and no purge liquid is withdrawn from a tank of the first column and then • according to a second step, a second valve (21) is opened to withdraw a purge liquid from a tank of the first column (20) and the first valve (31) remains open.

2. Method according to claim 1 in which according to the first step, all the purge valves placed on the circuits operating at the second pressure of the column system (20, 30) are opened and all the purge valves placed on the circuits operating at the first pressure of the column system are left closed.

3. Method according to claim 1 or 2 in which between the first and second steps, the first column (20) is depressurized by opening a lean liquid return valve (23) and / or a non-condensables valve at a bottom vaporizer of the second column.

4. Method according to one of the preceding claims in which after the second step, all purge valves placed on its low points of the circuit are opened at the second pressure of the column system (20, 30) and all purge valves placed on the low points of the circuit are opened at the first pressure of the column system and also a rich liquid return valve (22).

5. Method according to one of the preceding claims in which at the end of the sequence, all the valves (21, 22, 31) are closed.

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