GB2165463A - Apparatus for mixing gases - Google Patents

Abstract

Gas supplied by a plant for separating a gas mixture by pressure swing adsorption (said plant including a compressor 2 and adsorption beds 4 and 6) to a first pipeline 8 is mixed with gas supplied to a second pipeline 18 (e.g. from one or more cylinders). The pipelines 8 and 18 both terminate in a constant outlet pipe 20. The pipeline 8 includes an automatically operable stop valve 10 and a device 14 adapted to supply gas at a constant flow rate depending on the pressure downstream thereof even though the upstream pressure fluctuates. The pipeline 18 includes a similar stop valve 26 and constant flow rate device 28. A pressure regulator 22 maintains constant pressure in the pipeline 20, thus determining the flow rates at which the devices 14 and 28 supply gas and thereby ensuring the formation of a gas mixture of constant composition. <IMAGE>

Description

SPECIFICATION Apparatus for mixing gases This invention relates to apparatus for mixing gases. It is particularly concerned with such apparatus for use in mixing two or more gases, wherein at least one gas is supplied from a source of cyclically varying pressure.

Plants for separating by pressure swing adsorption at least one component of the gas mixture from the other components of the mixture are well known. One commercial use of such plants is in the separation of nitrogen from air. The plant typically comprises two pressure vessels each containing a vertical bed of adsorbent although it is also possible to employ just one or more than two separate adsorbent beds. Air is compressed to say 9 atmospheres and then passed into one of the beds.

The adsorbent preferentially adsorbs oxygen, carbondioxide and watervapourfrom the incoming air to leave an unadsorbed gas rich in nitrogen.

While one bed is thus producing product nitrogen, the other bed is being regenerated, that is previously adsorbed gases are desorbed therefrom at a lower pressure. After a chosen period (typically in the order of a minute) the two beds are placed in communication with one another to equalise the pressure therein. The bed previously being regenerated is then used to supply product gas while the other bed is regenerated by placing it in communication with the atmosphere. Typically, the nitrogen product supply pressure is set at a value intermediate the pressure obtaining in the two beds at the end of equalisation step and that to which the incoming air is compressed. The beds of the plant thus constitutes a source of gas of cyclically varying pressure and flow rate.If it is desired to mix the nitrogen produced by the beds with another gas, a conventional gas mixer may be used which typically comprises two pressure regulators in each gas line and a flow meter in each gas line. Such a combination of gas mixer and gas separation plant is we believe unnecessarily complicated in its use of expensive equipment; also the damping of the pressure fluctuates effected by the regulators may be incomplete giving rise to variations in the relative flow rates of the gases to be mixed and then in the composition of the gas mixture.

It is the aim of the present invention to provide a relatively simple gas mixing apparatus in which the gas mixer reliably performs the function of mixing the gas produced by a source of cyclically varying pressure and flow rate with another gas or gases and of smoothing out the variations in pressure.

According to the present invention there is provided apparatus for mixing gases comprising a source of gas of cyclically varying pressure and flow rate, a common outlet pipe for forming the gas mixture, a pressure regulator in said outlet pipe, a first pipeline communicating at one of its ends with the said source and at the other of its ends with the common outlet pipe, at least one second pipeline able to be placed in communication at one of its ends with a source of another gas and at its other end with said common outlet pipe, means in each pipeline adapted to supply gas to the common outlet pipe at a flow rate determined by the pressure in said outlet pipe, an automatically operable stop valve in each pipeline, and means for automatically effecting operation of the stop valves whereby to supply in operation of the apparatus a flow of gas mixture of constant composition from the pressure regulator.

Normally, each pipeline will also include a nonreturn valve, typically intermediate the respective stop valve and the respective constant flow means therein.

The present invention will now be described by way of example with reference to the accompanying drawing, which is a schematic circuit diagram of the gas mixer according to the present invention.

Referring to the accompanying drawing, a plant for the production of nitrogen by the sepa ration of air includes a compressor 2 and two vessels 4 and 6 each containing a vertical bed (not shown) of adsorbent material or materials that preferentially adsorbs water vapour, carbon dioxide and oxygen from incoming air. Vessels 4 and 6 are associated with a first pipeline 8 that takes nitrogen from the outlet of whichever vessel, if any, that is for the time being supplying nitrogen. The operation of such plants for producing nitrogen is well known.

Accordingly, this shall not be described herein in detail and for ease of illustration the necessary onoff valves, typically solenoid operated, that need to be provided are not shown. However, such a plant for producing nitrogen is described in our U.K.

patent specification No. 2042365A.

Essentially during the operation of the plant shown in Figure 1 the adsorbent bed in each vessel continuously performs the following cycle of operations; 1.Admission of compressed air toe bed and delivery of product nitrogen.

2. Equalisation of the pressure in the two beds by placing them in communication with one another.

3. Regeneration of the bed by placing it in communication with the atmosphere.

4. Equalisation of the pressure of the two beds by placing them in communication with one another.

While one bed is performing the first step in this sequence, the other is performing the third step.

During the first step the pressure in the vessel is raised from the level that is attained at the end of equalisation to substantially the pressure to which the compressor 2 supplies air to the vessels 4 and 6.

The pressure in the vessels 4 and 6 thus continuously forms a cycie in which it is raised from the intermediate pressure to substantially the outlet pressure of the compressor, reducing the pressure again to an intermediate pressure and then reducing it further to atmospheric pressure before returning it again to the intermediate pressure. Thus the plant constituted by the compressor 2 and the vessels 4 and 6 constitutes a source of gas of cyclically varing pressure.

The first pipeline 8 has a solenoid valve 10 disposed therein. Downstream of the solenoid vaive 10 is a non-return valve 12 and downstream of the non-return valve 12 is a constant flow device 14 which preferably is a Flostat valve. (Flostat is a trade mark.) The pipeline 8 terminates in a union with the downstream end of a second pipeline 18 and the upstream end of a common outlet pipe 20. A pressure regulator 22 is disposed in the common outlet 20 to maintain a constant pressure in the pipe 20.

The pipeline 18 has a pressure regulator 24 disposed in it near its inlet which is adapted to be connected to one or more gas cylinders (not shown). Downstream of the regulator 24 is a solenoid-actuated stop valve 26, a constant flow device 28 (for example a Flostat valve) is located in the pipeline 18 at a position downstream of the valve 26. Intermediate the valves 26 and 28 is a non return valve 30. If desired, the non-return valve 30 may alternatively be located downstream of the device 28.

The solenoids of the stop valve 10 and 26 are adapted to be actuated by means of a pressure switch 32 sensitive to the pressure in pipeline 8 upstream of the valve 10. The arrangement is that when the pressure sensed by the pressure switch 32 reaches a chosen value the valves 10 and 26 are automatically opened, and when the pressure sensed by the pressure switch 32 falls again to below the chosen value the valve 10 and 26 automatically close. A second pressure switch 34 is provided in communication with a pipeline 18 upstream of the pressure regulator 24. The pressure switch 34 is operatively associated with a lamp 36, the arrangement being such that should the pressure sensed by the pressure switch device 34 fall to a chosen value indicating that the gas cylinder or cylinders are almost empty, the lamp 36 lights up.

Athird pressure switch 38 is located in a region of the common outlet pipe 20 upstream of the pressure regulator 22. In the event that the pressure therein rises a chosen level above the desired "back pressure" set by the regulator 22, the pressure switch 38 is adapted to shut down the compressor 2 of the nitrogen source. (If the nitrogen source alternatively or additionally employs a vacuum pump to provide the pressure swing, the pressure switch 38 may be employed to shut down this vacuum pump).

A manual switch 40 is provided to enable the valve 26 to be kept open independently of the valve 10, if desired.

In operation, the upstream end of the second pipeline 18 is placed in communication with one or more cylinders of compressed gas, say carbon dioxide. The plant may be started up by starting operation of the compressor and by opening the cylinder valve or valves. The valves 10 and 26 are set to open at a pressure greater than the back pressure set by the regulator 22 but less than the maximum pressure that obtains in the vessels 4 and 6 (this maximum pressure being substantially the outlet pressure of the compressor 2). Accordingly, the upstream pressure to which the flow control devices 14 and 28 are respectively subjected will vary. In order to give a constant flow rate from these valves it is therefore necessary to hold constant the pressure upstream of the regulator 22 in the common outlet 20. This function is performed by the regulator 22.It is not essential that there should be continuous delivery of gas from the devices 14 and 28. What is essential is that the composition of the gas mixture formed in the common outlet does not vary. To achieve this end the valves 10 and 26 are opened synchronously and closed synchronously.

Any fluctuations in flow rate of the mixture may be smoothed out in a gas mixture distribution pipeline.

In one typical example, the compressor 2 may have an outlet pressure of 9 atmospheres and the pressure switch 32 be set to open the valves 10 and 26 when the pressure in the upstream end of the first pipeline 8 reaches six atmospheres. The pressure set by the regulator 22 upstream thereof is arranged to be say 5 atmospheres. Accordingly, a suitable pressure difference across the valves 14 and maintained. The valves 14 and 28 are selected to give the desired mixture. For example, a mixture comprising 66% nitrogen and 34% carbon dioxide and may be produced. Full carbon dioxide cylinders contain gas at relatively high pressure, and the pressure regulator 30 reduces the pressure to typically but not necessarily the same pressure as that at which the pressure switch 32 opens the valves 10 and 26. When the carbon dioxide cylinder or cylinders have been substantially exhausted, their pressure will fall rapidly. The pressure switch 34 may therefore be set to operate when the pressure falls to say 6.5 atmospheres and illuminates the lamp 36.

In the event of the requirement of the gas mixture being less than that produced by the plant, the pressure outlet 20 will rise and the pressure switch 38 will switch the air separation plant to stand-by until the product is again required and the pressure falls. This pressure may be selected to be 8 atmospheres in the above example.

The non-return valves 12 and 13 are provided to prevent any backflow of gas through the valves 10 and 26:

Claims (7)

1. Apparatus for mixing gases comprising a source of gas of cyclically varying pressure and flow rate, a common outlet pipe for forming the gas mixture, a pressure regulator in said outlet pipe, a first pipeline communicating at one of its ends with the said source and at the other of its ends with the common outlet pipe, at least one second pipeline able to be placed in communication at one of its ends with a source of another gas at its other end with said common outlet pipe, means in each pipeline adapted to supply gas to the common outlet pipe at a flow rate determined by the pressure in said outlet pipe, an automatically operable stop valve in each pipeline, and means for automatically effecting operation of the stop valves whereby to supply in operation of the apparatus a flow of gas mixture of constant composition from the pressure regulator.

2. Apparatus as claimed in claim 1, in which said source comprises one or more adsorbent beds of a plant for separating a gas mixture by pressure swing adsorption.

3. Apparatus as claimed in claim 1 or claim 2, in which each said pipeline includes a non-return valve.

4. Apparatus as claimed in claim 3, in which each said non-return valve is positioned intermediate the stop valve and the constant flow means in each respective pipeline.

5. Apparatus as claimed in any one of the preceding claims, in which the said automatically operable stop valves are adapted to be operated by means of a pressure switch sensitive to the pressure of said source.

6. Apparatus as claimed in any one of the preceding claims, in which there is a pressure switch in the common pipe upstream of the pressure regulator therein adapted to stop the plant separating gas mixture in the event that the pressure in said common pipe reaches a chosen maximum.

7. Apparatus for mixing gases substantially as herein described with reference to the accompanying drawing.