JPS5810125B2 - How to remove sulfur and nitrogen oxides from gas mixtures - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing oxidized sulfur from gas mixtures, in particular using a high pH trickle bed (trickle bed) of activated carbon for this purpose.
cckle bed).

The term "sulfur oxide" as used herein refers to sulfur dioxide (SO2), sulfur trioxide (SO3), and the like.

During the combustion of fossil fuels, sulfur oxides are inevitably produced.

Fuels such as lime and oil consumed by many industrial plants have high sulfur contents, such as 3% by weight or more.

Typical industrial waste gases with large sulfur oxide content are
Chimney gas, gas from torrefaction processes and pulp mill effluents.

Sulfur oxides, particularly sulfur dioxide, pose a serious threat to human health.

Sulfur dioxide causes a variety of acute and chronic illnesses and high concentrations can even lead to death.

Population growth and rapid industrialization create very serious problems of environmental pollution with sulfur oxides and result in the need for strict laws prohibiting the release of these pollutions.

As a result, overcoming the troublesome problem of air pollution from sulfur oxides is of considerable importance.

Much effort has been made to reduce sulfur oxide emissions, and many solutions with various treatment methods are possible.

In the case of sulfur-containing fuels beyond simply choosing low-sulfur fuels, which are becoming less and less available,
Methods have been used to remove most of the sulfur from sulfur-containing fuels.

Other methods of removing sulfur oxides after combustion of fuel have also been developed.

In the latter processing method, a throw-away type (thr
A number of different methods have been developed, such as limestone scrubbing or extraction type, for example various adsorption and reaction methods.

Such methods include adsorption removal by manganese dioxide particles, reaction of molten carbonate with sulfur oxide and oxidation of sulfur dioxide in the presence of a vanadium pentoxide catalyst, followed by hydration to form sulfuric acid.

The process of the present invention is an adsorption and reaction type process and uses a high pH trickle bed of activated carbon.

Previously, examples include U.S. Patent Nos. 2,992,895 and 2,992,065 and Chem, Eng.

Progress Volume 71 No. 5 (May 1975
A paper published by Biscot and Habib in “
It is known to adsorb sulfur oxides by activated charcoal methods such as those described in ``FW-BF Dry Adsorption System''.

The activated charcoal process uses the advantage of activated carbon's ability to catalytically oxidize sulfur dioxide to sulfur trioxide to remove sulfur oxides from gas streams.

Sulfur trioxide can be adsorbed and retained by activated carbon or sulfur trioxide can be combined with water vapor to form sulfur.

This sulfur is also adsorbed and retained by activated carbon.

As such, this method relies on oxygen and water vapor that are normally present in the gas produced by combustion of the fuel.

Once activated carbon reaches its adsorption capacity, it is regenerated at elevated temperatures in a series of reactions.

This reaction reverses what took place during adsorption.

Thus, the adsorbed sulfuric acid is decomposed into water and sulfur trioxide, and the sulfur trioxide is reduced to sulfur dioxide by activated carbon acting as a reducing agent.

However, the activated carbon adsorbent is consumed in the reduction process.

No. 3,486,852 describes a method for regenerating sulfur dioxide and sulfur trioxide by adsorbing them by activated carbon and then washing the activated carbon with water to remove the oxidized sulfur.

The wash liquor can be heated to produce sulfuric acid, which can then be neutralized with an alkaline compound.

Known methods also combine the use of activated carbon and aqueous liquids.

For example, in the method described in U.S. Pat. No. 3,710,548, a gas adsorption column is in the form of particulates that can be kept in contact with an aqueous liquid and is carried by the water, or is used. Activated carbon is used as a heterogeneous catalyst that can serve as a backing for.

US Pat. No. 3,803,804 describes a desulfurization process for exhaust gases.

This method uses activated carbon adsorbent and sprinkles a sulfuric acid solution onto it.

U.S. Pat. No. 3,701,824 discloses removing sulfur compounds having unpleasant odors such as hydrogen sulfide, mercaptans and mercaptan ethers from a gas stream by bringing the gas stream into intimate contact with an aqueous alkaline suspension of activated carbon. It explains how to do it.

A similar method for removing sulfur compounds from hydrocarbons is described in US Pat. No. 1,890.516.

Japanese Patent Application Publication No. 73/79,163 describes a method for desulfurizing waste gas.

In this process, the gas is first sprayed with aqueous sodium hydroxide in a reactor and then passed through an activated carbon-packed column.

U.S. Pat. No. 3,882,221 describes a method for removing sulfur dioxide from waste gas by intimately mixing the waste gas stream with an aqueous solution of caustic that is physically retained on the surface of a silicate support. has been done.

The reaction of sulfur dioxide with caustic produces a water-soluble alkali pyrosulfide salt on the surface of the support.

According to the invention, a bed of granular activated carbon is formed, b. a solution of alkali metal hydroxides and carbonates and a sulfate-forming reagent selected from ammonium hydroxide and ammonium carbonate at a pH of greater than about 70. an aqueous liquid having a concentration of about 95 to 99% saturated with a suitable sulfate corresponding to the specific alkali metal or ammonium selected for the sulfate-forming reagent,
continuously passing the effluent gas stream downward through the grounded bed and passing the effluent gas stream downward through the bed to contact the grounded bed with the effluent gas stream, thereby removing sulfur oxides from the effluent gas stream and removing sulfur oxides from the effluent gas stream. d. recovering the sulfate from the aqueous liquid; .

According to the method of the present invention, oxidized sulfur contained in a mixture with one or more other gases in the gas stream is removed from the gas stream by passing the gas stream through a bed of high pH activated carbon trickle. can do.

The gas mixture for removing sulfur oxides includes gases such as nitrogen, oxygen, water vapor, carbon dioxide, carbon monoxide, sulfuric acid, hydrocarbon gases, helium, argon, neon, krypton, xenon, hydrogen bromide, hydrogen chloride, etc. Consists of one or more types.

Such gases often result from the combustion of sulfur-containing hydrocarbon fuels such as oil, natural gas, and coal, stack gases, and many chemical manufacturing plants such as oil refineries and factories producing sulfuric and nitric acids. It can be found as a typical component of a variety of industrial and commercial effluent gases from sources such as waste gases, smelter gases, stack gases discharged from boilers and furnaces, and coal power plant gas effluents.

Such effluent gases contain large amounts of particulate matter of various types, such as fly ash and soot.

Preferably, such particulate matter is removed by conventional means such as an electrostatic precipitator or simple filter before passing the effluent gas through the high pH activated carbon trickle bed of the present invention.

The space velocity of the effluent gas stream treated by the present invention is:
It can vary over a wide range.

For convenience, such space velocity is referred to herein as
It is measured in units of bed volume per hour (bvh').

The term bed volume refers to the total actual displacement capacity of the activated carbon itself in the bed.

Such bed capacity is easily calculated from the total weight of activated carbon used using the packing density of the particular activated carbon used.

The space velocity of the effluent gas can be as low as 200 bvh' or as high as 20,000 bvh'' or more.

As expected, the size (capacity) of the activated carbon trickle bed itself can vary and apparently accommodates a significant volume of effluent gas flow in absolute terms of standard cubic feet per minute (scfm). A large floor capacity is used for this purpose.

As will be further understood, the space velocity, expressed in terms of bed volume per hour, affects the contact time of the treated effluent gas stream with the activated carbon trickle bed.

This parameter was found to be important in preventing the formation of sulfite ions (SO3-) in the effluent liquid from the trickle bed.

The formation of such sulfite ions in the effluent is undesirable.

In such cases, the effluent liquid should be discarded since sulfite ions are usually an unacceptable contaminant to the effluent water.

Preferably, the contact time is sufficient to virtually completely convert the oxidized sulfur to sodium or other alkali sulfate, as described below.

A contact time of at least about 6 seconds at a bed operating temperature of about 25°C is required to prevent the formation of such sulfite ions in the effluent liquid.

At higher operating temperatures, lower contact times are required in view of faster rates of reaction.

Conversely, lower operating temperatures require longer contact times to ensure the absence of sulfite ions in the effluent liquid.

The process of the invention can be carried out over a fairly wide temperature range without affecting the efficiency of the high pH activated carbon trickle bed.

It has been found that a suitable temperature range for operation of the process of the invention is from about 0 DEG to 100 DEG C. for an activated carbon trickle bed.

Preferably, the temperature range is about 50-90°C.

However, the temperature of the treated effluent gas stream is approximately 15
It can be made as high as 0°C.

As can be seen, the high pH aqueous liquid present in the trickle bed filter imposes certain limitations on the permissible temperature of the effluent gas.

Above about 100° C., the increased risk of vaporization of high pH aqueous liquids must be considered in maintaining the operating efficiency of activated carbon trickle beds.

It has been found that pressure is not a critical parameter in the operation of activated carbon trickle beds, and subatmospheric and supraatmospheric pressures can be used.

However, ambient atmospheric pressure is also advantageous and therefore preferred.

The oxidized sulfur removal efficiency of the high pH activated carbon trickle bed is quite high and can effectively treat effluent gas streams with initial oxidized sulfur concentrations as high as 10,000 p, p, m.
1. The concentration of sulfur oxide in the gas stream released after treatment.
Op, p, m or less.

However, normally the concentration of sulfur oxides in effluent gases, such as gases released from coal combustion plants, is generally about 2,000 parts per million.

As used herein, trickle bed (tri
The term cckle bed merely refers to the term as it is commonly explained in the art.

That is, it shows an activated carbon bed for gas phase adsorption, with the liquid phase being passed continuously through the bed at a slower flow rate than the gas phase being simultaneously passed through the bed.

The construction of the activated carbon trickle bed itself can be of any suitable conventional design.

For example, the bed may be a stationary bed, or the bed may be a moving or pulsed bed.

For a stationary bed, the stream of effluent gas to be treated can flow downward through the bed and the activated carbon is usually in granular form and remains stationary in a fixed position.

In a moving bed, the entire activated carbon bed moves in a direction opposite to the flow direction of the effluent gas stream flowing through the activated carbon bed.

In this way, the bed and exit gas streams move countercurrently to each other.

Backflow operation provides a means of continuously replenishing consumed carbon with fresh carbon in cases where activated carbon has a limited lifetime.

However, in the process of the present invention, the useful activated carbon life is deemed to be long enough to justify a static bed.

Of course, more than one individual high pH activated carbon trickle bed can be used at once.

Thus, several such floors can be used together and in a series or parallel arrangement.

The present invention because the adsorbed and oxidized sulfur oxides react with the high pH aqueous liquid and the reaction products are removed and carried away, leaving the activated carbon free for further adsorption and reaction. The activated carbon trickle bed can be used virtually indefinitely.

However, activated carbon becomes blocked by particulate matter.

This blockage is usually easily removed by a backwash section.

Activated carbon is also blocked by the adsorption of other impurities besides sulfur oxides.

In this case, the activated carbon can be completely replaced by fresh activated carbon, or the activated carbon can be regenerated by known methods, such as thermal or steam regeneration.

The activated carbon adsorbent used in the high pH trickle bed of the present invention can be selected from a number of common activated carbon adsorbents.

This is because most such adsorbents are suitable for the adsorption of sulfur oxides in the gas phase.

Activated carbon is preferably used in granular form and the granules are No., 2U, S, Sieve series (5ieve 5e).
ries) range in size from those that pass through mesh screens to those that are retained by A40U, S, Sieve Series mesh screens.

The primary consideration in such a selection is the resulting pressure drop in the bed.

The activated carbon granules are packed into a suitable container in a known manner to form a bed.

When a bed of activated carbon is used in accordance with the method of the present invention, a stream of high pH aqueous liquid or wash liquor is passed continuously through the bed.

The direction of movement of the aqueous liquid flow or wash liquid flowing through the activated carbon bed can be the same as the flow direction of the effluent gas stream flowing through the same bed.

As a practical matter, high p
Preferably, both the aqueous liquid wash and the effluent gas stream are passed downwardly through the bed.

When passing the effluent gas stream downward through a bed of activated carbon,
A slight positive pressure is maintained by a trickle gas stream and wash stream that trickles an aqueous liquid wash onto the activated carbon bed.

At the outlet of the column, both wash liquid and treated gas are obtained.

The only essential thing is that the flow of the aqueous liquid completely through the activated carbon bed occurs before passing the effluent gas stream through the bed to remove oxidized sulfur.

The effluent gas passes through the activated carbon bed very quickly, while the aqueous liquid drips very slowly without interfering with the gas flow.

The flow rate of the high pH aqueous liquid flowing through the bed of activated carbon is in the range of about 0.1 to 2.0 bed volumes per hour.

Flow rate of approximately 1.0 bed volume per hour (approximately 1.Obvh'
was found to be satisfactory.

The flow rate of the effluent gas stream flowing through the bed of activated carbon is approximately 20
,000bvh' can vary considerably.

Passing both the high pH aqueous liquid wash and the effluent gas downward through the bed is easier to operate than passing them countercurrently to each other.

When flowing both an aqueous liquid wash and a effluent gas from the top of a nearly vertical column containing activated carbon, gravity and a slight positive gas pressure force the gas and aqueous liquid downward through the activated carbon bed. help you move.

On the other hand, if the two flow countercurrently, the gas distribution plate at the bottom becomes substantially clogged, and the movement of gas is hindered, making it impossible to perform a smooth countercurrent operation.

The high pH aqueous liquid used in the method of the invention consists of an aqueous solution of a sulfate-forming reagent selected from the group consisting of alkali metal hydroxides and carbonates and ammonium hydroxides and carbonates.

The pH of the aqueous solution is greater than about 7.0.

Preferably, the pH of the aqueous liquid is at least about 10.0, and most preferably at least about 12.5.

A preferred aqueous liquid is an aqueous solution of sodium hydroxide at a pH of about 13.0.

As can be seen, a faster flow rate of high pH aqueous liquid flowing through an activated carbon bed will result in a higher flow rate of alkali metal hydroxide or carbonate and ammonium hydroxide or carbonate for a given period of pressure response. and thus lower the pH of the aqueous liquid required for a given space velocity of the effluent gas being treated.

It is also understood that the pH and aqueous liquid flow rate are
In the context of providing a sufficient amount of sulfate-forming reagent at the location of the activated carbon reaction, the flow rate of the effluent gas stream and the flow rate of the effluent gas stream are interdependent parameters that together determine the basic amount of sulfate-forming reagent required. do.

Furthermore, although the method of the present invention is primarily designed to obtain removal efficiencies of 99% and greater, the present invention contemplates that variations of the method of the present invention in the manner described herein. It will be appreciated that removal efficiencies of less than 99%, such as 80% or less, can be obtained.

The general method of operation of the high pH activated carbon trickle bed of the present invention to remove oxidized sulfur from the effluent gas stream is theorized to be one of adsorption and reaction.

Thus, for example, sulfur dioxide is adsorbed and oxidized to sulfur trioxide, and sulfur trioxide is directly adsorbed.

The sulfur trioxide retained by the activated carbon is then reacted with the alkali metal hydroxide or carbonate or ammonium hydroxide or carbonate contained in the high pH aqueous liquid. forms ammonium sulfate.

The sulfate is then dissolved in the aqueous liquid and it is carried away from the activated carbon as a result of the aqueous liquid stream or wash flowing through the activated carbon bed.

However, such theoretical considerations are not intended to limit the invention, but are included for illustrative purposes, and the invention is limited only by the scope of the claims.

An important economic advantage of the process of the present invention is the recovery of useful sulfate from high pH aqueous liquids after use in an activated carbon trickle bed.

Such salts have great economic value and can be used to offset the cost of operating the process of the invention.

Collection of these salts can be carried out by any known method suitable for this purpose.

However, since saturated solutions of such sulfates are in readily commercially available form, in addition to the sulfate-forming reagents described above, approximately 95-99% of the sulfate corresponding to the particular alkali metal or ammonium used may be used. It is also advantageous and efficient to use an aqueous liquid solution that is saturated to a degree of .

Such an aqueous liquid solution contains, for example, in addition to sodium hydroxide, a sufficient amount of sodium sulfate to provide a 95-99% saturated solution with respect to sodium sulfate.

As a result, the small amount of sodium sulfate produced during the reaction in activated carbon, as already explained, is combined with a larger amount in a 95-99% sulfate saturated aqueous liquid to completely or almost entirely Gives a saturated solution.

The degree of saturation of the aqueous liquid can be adjusted by calculation or experiment before passing through the activated carbon to give a sulfate saturated solution as the effluent liquid.

As will be appreciated from the foregoing, the nearly saturated aqueous liquid serves as a carrier for the sulfate salts produced by the reaction of the effluent gas stream on the activated carbon bed.

Sulfate recovery is facilitated by using an aqueous liquid that is approximately 95-99% saturated.

Sulfates are more easily recovered from saturated aqueous solutions than from dilute aqueous solutions.

The invention can be better understood from the following examples.

The following examples are given by way of illustration and not by way of limitation.

Example 1 Evaluations are carried out in 1 inch and 5 inch diameter columns containing BPL type activated carbon particles of 4 x 10 mesh size (U, S, sieve series).

This sorbent material is available from Calvon Corporation's Pittsburg Activated Division of Pittsburg, Pennsylvania.

A synthetic effluent gas stream is produced by mixing monitored and conditioned streams of sulfur dioxide and air.

This mixture is passed down through a column of activated carbon.

The alkaline wash solution is pumped at variable speeds and dripped onto the top of the carbon bed.

The inlet and outlet concentrations of sulfur dioxide are measured by detector tubes available from Mine Safety Appliance Company.

The pH of the effluent liquid from the activated carbon column and the pH of the alkaline washing solution are measured by pH paper.

The activated carbon column is first permeated with distilled water at 1 bvh' for 3 hours before contacting with the synthesis effluent gas stream.

The space velocities of the syngas streams are 6,000 and 3,000
0bvh'.

During the evaluation, the effluent liquid from the activated carbon column is recycled.

The column is then packed with fresh activated carbon. The results of the evaluation are shown in the table below.

Example 2 A bed is prepared by packing a 1 inch diameter and 5 inch deep glass column with 60 ml of granular BPL activated carbon of 4 x 10 U, S, Sieve Series mesh size.

The washing solution was 45% by weight NaOH. 50ml of aqueous solution
Prepared by adding to 950 ml of a saturated aqueous solution of a2SO4.

This cleaning solution was synthesized with an effluent gas flow space velocity of 3,000
and 6.000bvh'.

If the syngas flow space velocity is 600 bvh', then
For the cleaning solution, 10ml of a 29% by weight NaOH aqueous solution was mixed with NaOH.
Produced by mixing with 990 ml of a saturated aqueous solution of 2SO4.

Synthesis effluent gas is 802-2.000 ppm% CO2-
with a volumetric composition of 10,6%, 02-8,2% and N2-remaining.

The synthesis effluent gas is saturated with water vapor by scrubbing through two water flappers in series.

This introduction of water vapor into the synthesis effluent gas reduces the amount of N on the carbon bed.
This is done to prevent a2SO4 deposition.

The SO□ concentration in the effluent stream is measured with a detector tube available from Mine Safety Appliance Company.

The effluent stream is also monitored for SO3~ content by adding permanganate to the acidified effluent liquid.

Decolorization of the permanganate solution is taken as an indication of the presence of SO3-.

For comparison, one column is packed with glass beads instead of activated carbon and then treated by the method described above.

The results of these evaluations are shown in the table below.

Claims (1)

Claims: 1. a bed of granular activated carbon is formed; b. a solution of a sulfate-forming reagent selected from alkali metal hydroxides and carbonates and ammonium hydroxide and ammonium carbonate. An aqueous liquid having a pH greater than 0 and saturated to the extent of 95-99% with a suitable sulfate corresponding to the particular alkali metal or ammonium selected for the sulfate-forming reagent is contacted. c. passing the effluent gas stream downwardly through the bed to contact the bed with the effluent gas stream, thereby removing sulfur oxides from the effluent gas stream and removing sulfur oxides from the effluent gas stream; d. recovering the sulfate from the aqueous liquid. 2. The method of claim 1, wherein the pH of the aqueous liquid is at least about 10.0. 3. The method of claim 1, wherein the pH of the aqueous liquid is at least about 125. 4 The flow rate of the aqueous liquid is about 0.1 to about 2.0bvh'
The method according to claim 1. 5. The method of claim 1, wherein the effluent gas stream is contacted with the bed for a period of at least about 6 seconds and the operating temperature of the bed is about 25°C. 6. The method according to claim 1, wherein the sulfate-forming reagent is sodium hydroxide. 7. The method according to claim 1, wherein the bed is a stationary bed. 8. The method according to claim 1, wherein the bed is a moving bed.