JPH01260085A - Process for air oxidation of aqueous solution containing sodium sulfide with counter-current packed column and apparatus therefor - Google Patents

Abstract

PURPOSE:To perform oxidation of white liquor, etc., with an inexpensive apparatus having a simple structure with low electric power consumption, under normal pressure, by counter-currently contacting white liquor, etc., with air in an oxidation column furnished with a gas-liquid contacting stage composed of a packed bed and a chimney tray. CONSTITUTION:The upper part of an oxidation column 3 is provided with at least one stage (two stages in the figure) of a gas-liquid contacting stage composed of a combination of a packed bed 4 packed with a filler to promote the gas-liquid contact and a chimney tray 5 placed below the packed bed. A white liquor feeding pipe and an exhaust pipe are connected to the column top and a black liquor discharge pipe is connected to the column bottom. An air feeding pipe 10 is connected to the upper part of a black liquor reservoir 8 of the column bottom and a white liquor discharge pipe 11 is connected to the chimney tray of the lowest stage. A black liquor ejection part 7 is placed between the white liquor discharge pipe and a connecting part of the air feeding pipe 10. The carbon dioxide gas in the air is absorbed in the shower of the black liquor and the falling white liquor is oxidized by contacting with the ascending flow of the treated air.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an air oxidation method and apparatus for an aqueous solution containing sodium sulfide (white liquor, green liquor, black liquor) obtained during the production of sulfate pulp.

[Prior art] Sulfate pulp is produced by adding a mixture of sodium hydroxide and sodium sulfide to the pulp raw material and steaming it under pressure, but the steaming waste obtained here is generally called "black liquor". After concentrating and burning this waste liquid, it becomes a soda melt, which is then dissolved in water to form what is called ``green liquor'', which is further causticized with quicklime to produce an alkaline solution, which is generally called ``white liquor.'' . This white liquor is a mixture of sodium hydroxide and sodium sulfide, so in the production of sulfate pulp, it is customary to reuse it in the steaming of the pulp raw material. It is also used as an alkali source for the bleaching process.

However, since the white liquor produced by the sulfate method contains sodium sulfide, when using it for pulp bleaching, the sodium sulfide in the white liquor must be removed or reduced in advance. This is because soda damages the pulp. To remove or reduce the sodium sulfide contained in white liquor,
Conventionally, the following air oxidation method has been used.

One of them is to keep white liquor in a vertical tank and blow air from the bottom of the tank to oxidize the sodium sulfide in the white liquor as described below.

2Na, S÷20. + H, O−+ 2NaOH+
Na, S, 0°, and the other is one in which a mixture of white liquor and air is supplied to a reactor filled with a solid catalyst to air-oxidize sodium sulfide through a catalytic chemical reaction.

[Problems to be Solved by the Invention] Among the white liquor air oxidation methods conventionally employed, the tank method described above requires air to be blown into the tank with a pressure that overcomes the static pressure of the white liquor. In order to prevent this, a Roots-type air blower is used, but this blower is not only expensive, but also has the disadvantage of consuming a large amount of power. Furthermore, in order to sufficiently oxidize the sodium sulfide in the white liquor using this method, it is necessary to blow air in for a long period of 8 to 9 hours, so the sodium hydroxide in the white liquor is There is also the inconvenience that carbon dioxide gas reacts as described below and causes the color to decrease.

2NaOH" CO3→Na=C0a "H*0 On the other hand, in the catalytic reactor method, the reaction between sodium hydroxide and carbon dioxide gas is suppressed by the action of a catalyst, and only the oxidation reaction of sodium sulfide can proceed selectively. It is excellent in terms of points, but about 30
It requires a reaction pressure as high as kg/ajG. The air blowing compressor used for this purpose is more expensive than the air blower used in the tank type, and consumes even more power. In addition, this method has the problem that calcium precipitates on the surface of the catalyst, resulting in a decrease in the activity of the catalyst. In other words, because quicklime is used in the causticizing process to obtain white liquor from green liquor, the white liquor contains calcium carbonate equivalent to the saturated solubility, but since this gradually precipitates on the catalyst surface, The catalytic activity decreases over time. Therefore, in the catalytic reactor system, there is the trouble of having to periodically pickle the catalyst, and consideration must also be given to the treatment of the acidic liquid discharged at that time.

The same applies to the air oxidation of green liquor and black liquor; for example, when black liquor is air oxidized as a preliminary treatment before being processed in an evaporator, or when green liquor is used to clean acidic gases generated at a bulk factory. When green liquor is air oxidized as a preliminary treatment before use, conventional air oxidation methods are not always effective.

[Means for Solving the Problems] The present invention provides a new countercurrent packed column type air oxidation method and its apparatus which can solve all the above-mentioned problems pointed out in the conventional methods at once.

That is, the method of the present invention includes at least one gas-liquid contact stage in the upper region of the column, which is composed of a combination of a packed bed filled with gas-liquid contact packing and a chimney tray located below the packed bed, and White liquor or green liquor is supplied to the top of a tray column having a supply nozzle for unoxidized white liquor or green liquor and a discharge nozzle for waste air at the top of the column, and for the air supplied below the contact stage. After being brought into contact with the ejected droplets of black liquor to cause carbon dioxide in the air to be absorbed therein, the substantially carbon dioxide-free air is brought into contact with the white liquor or green liquor flowing down the contact stage. The white liquor or green liquor is oxidized, and the waste air that has finished contacting with the liquor is discharged from the above-mentioned discharge nozzle to the outside of the column, and the oxidized white liquor or green liquor is discharged from the chimney tray of the lowest contact stage to the column. It is characterized by being extracted outside.

1. An apparatus suitable for carrying out the method of the present invention includes at least one set of gas, consisting of a packed bed containing gas-liquid contact packing and a chimney tray below the packed bed, in the upper region of a substantially vertical column. A liquid contact stage is installed, a white liquor or green liquor supply nozzle and a waste air discharge nozzle are installed at the top of the column, and a chimney tray at the bottom is used to extract white liquor or green liquor from the oxidation method from the column. A discharge nozzle is provided, and a nozzle for spraying black liquor downward is provided below the lowest chimney tray, and a black liquor discharge nozzle is provided at the bottom of the tower to prevent air between these two nozzles. The configuration includes a supply nozzle.

In the present invention, when the object of air oxidation is black liquor, the black liquor is supplied only to the top of the column.

There are no eruptions in the area below the tower.

[Function] The present invention will be explained with reference to FIG. 1, taking air oxidation of white liquor as an example. Unoxidized white liquor in a tank 1 is supplied to the top of a white liquor oxidizing tower 3 by a pump 2. The white liquor oxidation tower 3 has at least one gas-liquid contact stage composed of a packed bed 4 containing gas-liquid contact packing in its upper region and a chimney tray 5 located below the packed bed 4. In one drawing, an example is shown in which two gas-liquid contacts are provided. A spray nozzle 7 for black liquor supplied through a line 6 is provided below the lowest chimney tray 5, and the black liquor sprayed downward from here passes through a liquid-liquid reservoir 8 to the outside of the tower. taken out. A portion of this black liquor can be circulated again to the spray nozzle 6 if necessary. The oxidizing air is supplied from the air blower 9 via the line 10 to the oxidizing tower 3 at an intermediate position between the spray nozzle 7 and the liquid-liquid collecting section 8, and rises inside the tower.

The unoxidized white liquor supplied to the top of oxidation tower 3 is:

While flowing down the surface of the packing in the packed bed in the form of a thin film, it comes into contact with the air rising in the column in a countercurrent manner, but in the present invention, the air supplied into the column does not come into contact with the unoxidized white liquor. First, it comes into contact with droplets of black liquor sprayed from the nozzle 7 to absorb and remove carbon dioxide from the air. Although not shown, it is also an effective method to install a gas-liquid contact stage similar to that in the upper region of the tower in order to improve the efficiency of contact between air and black liquor in this region. By contact with black liquor.

Since the carbon dioxide contained in the air is absorbed by the black liquor, the air rising to the gas-liquid contact stage in the upper region of the tower does not substantially contain carbon dioxide, and therefore comes into contact with this air in a self-flowing manner. Sodium sulfide in the unoxidized white liquor is 2Na, S + 20. + H2O→2NaOH+
It is only oxidized like Na, 5203, and the sodium hydroxide in the liquid is not neutralized by carbon dioxide gas like 2NaOH+CO*→Na, GOa+H,0.

The white liquor air-oxidized in this way is taken out of the tower from the lowest chimney tray, and is collected in the oxidized white liquor tank 12 via a line 11. This oxidized white liquor can be circulated to the oxidation tower 3 via the pump 13 and line 14 as required. Further, a part of the black liquor that absorbs carbon dioxide gas in the air and is discharged from the bottom of the tower can be added to the unoxidized white liquor as a catalyst for air oxidation of the white liquor.

In the present invention, as the packing used in the gas-liquid contact stage of the white liquor oxidation tower 3, any of various gas-liquid contact packings such as Raschig rings, Paul rings, Tellerettes, McMahon backings, etc. can be used. However, according to the inventor's experiments, Sumitomo/Sulzer Backing (trade name) or Sumitomo/Sulzer Merapak (trade name) manufactured by Sumitomo Heavy Industries, Ltd. is the most effective filler for use in the present invention. Are suitable.

[Effects of the Invention] The oxidation tower of the present invention can be operated at normal pressure, and the pressure loss within the tower is extremely small, so the power consumption of the air blower can be reduced to about 1710 mph compared to the conventional method. In addition, since the carbon dioxide gas in the air is removed with the black liquor before the air and the white liquor come into contact, the sodium hydroxide contained in the white liquor will not be lost during air oxidation. Since the present invention does not use a solid catalyst, there is no need to worry about calcium carbonate adhering to the filler.

Furthermore, the air oxidation method and apparatus of the present invention can be used not only when air oxidizing white liquor, but also when air oxidizing black liquor as a preliminary treatment before treating black liquor with an evaporator. In addition, when green liquor is used for cleaning acidic gas generated in a pulp mill, it can also be used as an air oxidation method for green liquor as a preliminary treatment.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing one embodiment of the present invention. 1: Unoxidized white liquor tank 2: Pump 3 White liquor oxidation tower 4
: Filled bed 5 : Chimney tray 6 : Black liquor supply line 7
:Ejection nozzle 8:Liquid reservoir 9:Air blower 11
= Oxidized white liquor line 12 Dioxide white liquor tank 13: Pump 14: Oxidized white liquor circulation line

Claims (1)

[Scope of Claims] 1. At least one gas-liquid contact stage is provided in the upper region of the column, consisting of a combination of a packed bed packed with packing material for enhancing gas-liquid contact and a chimney tray located below the packed bed. The white liquor or green liquor is supplied to the top of a packed column having a white liquor or green liquor supply nozzle and a waste air discharge nozzle at the top of the column, and the air supplied below the contact stage is After being brought into contact with the spray droplets of black liquor to absorb carbon dioxide gas in the air, the air substantially free of carbon dioxide gas is brought into gas-liquid contact with the white liquor or green liquor flowing down the packed bed. The white liquor or green liquor is oxidized, and the waste air that has completed gas-liquid contact with the white liquor or green liquor is discharged to the outside of the tower from the discharge nozzle, and the oxidized white liquor or green liquor is transferred to the bottom contact A countercurrent packed tower air oxidation method for white liquor or green liquor, which is characterized by extracting the white liquor or green liquor from the chimney tray of the stage to the outside of the tower. 2. Equipped with at least one gas-liquid contact stage in the upper region of the column, which is composed of a packed bed containing packing materials and a chimney tray located below the packed bed to enhance gas-liquid contact, and a black plate on the top of the column. Black liquor is supplied to the top of a packed bed having a liquid supply nozzle and a waste air discharge nozzle, and the air supplied below the contact stage is brought into gas-liquid contact with the black liquor flowing down the packed bed. The waste air that has completed gas-liquid contact with the black liquor is discharged to the outside of the tower from the discharge nozzle, and the oxidized black liquor is discharged from the chimney tray of the lowest contact stage to the outside of the tower. A countercurrent packed column air oxidation method for black liquor, which is characterized by the extraction of black liquor. 3. Install at least one packed bed packed with gas-liquid contact packing in the upper area of the almost vertical tower, and a chimney tray below each packed bed, and supply white liquor or green liquor to the top of the tower. A nozzle and a waste air discharge nozzle are provided, and a discharge nozzle is provided in the lowest chimney tray to draw out the oxidized white liquor or green liquor to the outside of the column. A countercurrent packed tower type air oxidation device that is equipped with a nozzle for spraying liquid downward and an air supply nozzle between the bottom end of the tower and the main nozzle. 4. At least one packed bed filled with gas-liquid contact packing is installed in the upper area of the almost vertical tower, and a chimney tray is installed below each packed bed. An air discharge nozzle is provided, and a discharge nozzle is provided on the lowest chimney tray to extract the oxidized black liquor from the tower, and an air supply nozzle is provided between the lowest chimney tray and the lowest end of the tower. A countercurrent packed tower type air oxidation equipment is installed.