AU2018446174A1

AU2018446174A1 - Process and system for tail gas treatment

Info

Publication number: AU2018446174A1
Application number: AU2018446174A
Authority: AU
Inventors: Jun Wu
Original assignee: Pujing Chemical Industry Co Ltd
Current assignee: Pujing Chemical Industry Co Ltd
Priority date: 2018-10-22
Filing date: 2018-10-22
Publication date: 2021-04-01
Anticipated expiration: 2038-10-22
Also published as: AU2018446174B2; WO2020082201A1; RU2705073C1

Abstract

Presented is a process for treating a tail gas having nitrogen oxides and an alkyl nitrite. The process comprises contacting the tail gas with an alkyl alcohol in the presence of oxygen to absorb the alkyl nitrite, contacting the resulting absorbed gas with a reducing gas in the presence of a catalyst to reduce the nitrogen oxides, and washing the absorbed and reduced gas to generate a treated tail gas. Also provided is a system for treating a tail gas having nitrogen oxides and an alkyl nitrite. The treated tail gas has a low nitrogen oxide content and a low alkyl nitrite content.

Description

PROCESS AND SYSTEM FOR TAIL GAS TREATMENT

FIELD OF THE INVENTION

The present invention relates processes and systems for treatment of tail gases comprising nitrogen oxides and alkyl nitrite gases.

BACKGROUND OF THE INVENTION

Nitrogen oxides (NO _x) are mainly used to refer to NO ₂ and NO. NO _x can stimulate lungs and cause damages to the lungs. NO _x can also reduce visibility of the atmosphere. In addition, NO _x react with water in the air to form acid rain, which causes damages to the environment. Methyl nitrite (MN) is a flammable and explosive gas with strong toxicity. MN can form methemoglobin with red blood cells in blood after inhalation, so that the red blood cells lose their ability to carry oxygen. The principle of the toxicity is similar to that of carbon monoxide (CO) , but MN toxicity is much stronger than that of CO. Therefore, it is necessary to treat a tail gas containing NO _x and alkyl nitrites (RONO) by removing NO _x therein to lower its concentration to meet the relevant environmental standard, while simultaneously converting the MN therein before discharging the tail gas.

There remains a need for simple, low energy consumption and effective treatment and equipment for treating nitrogen oxides and alkyl nitrite gas.

SUMMARY OF THE INVENTION

The present invention provides treatment of a tail gas comprising nitrogen oxides and alkyl nitrite gas.

A process for treating a tail gas having nitrogen oxides and an alkyl nitrite is provided. The process comprises (a) contacting the tail gas with an effective amount of an alkyl alcohol in the presence of oxygen to absorb the alkyl nitrite, whereby an absorbed gas is generated; (b) contacting the absorbed gas with a reducing gas in the presence of an effective amount of a catalyst to reduce the nitrogen oxides, whereby an absorbed and reduced gas is generated; and (c) washing the absorbed and reduced gas, whereby a treated tail gas is prepared.

The treated tail gas may have a nitrogen oxide content less than 50 ppm. The treated tail gas may have an alkyl nitrite content less than 10 ppm.

The molar ratio of the nitrogen oxides in step (a) to the oxygen in step (a) may be from 1: 10 to 10: 1. The molar ratio of the alkyl alcohol in step (a) to the alkyl nitrite in step (a) may be from 1: 1 to 100: 1. The alkyl alcohol may be selected from the group consisting of methanol, ethanol, propanol and butanol.

Step (a) may be carried out under an operation pressure of 0.1-1.0 MPa. Step (a) may be carried out at an operation temperature from -30 ℃ to 90 ℃.

Step (a) may be carried out in a first absorption tower. The first absorption tower may be a packing tower, a washing tower or a spraying tower.

The molar ratio of the reducing gas to the nitrogen oxides in step (b) may be from 1: 1 to 10: 1. The reducing gas may comprise H ₂, CO, NH ₃, or a combination thereof. The catalyst may comprise Pd, Pt, Cu, Rh or a combination thereof.

Step (b) may be carried out at a temperature from 50 ℃ to 500 ℃. Step (b) may be carried out under a pressure of 0-10 barG. Step (b) may be carried out at a gas space velocity of 50-10,000 h ^-1.

Step (b) may be carried out in a reactor. The reactor may be a fixed bed reactor.

The absorbed and reduced gas may be washed with water or an alkyl alcohol in step (c) . The alkyl alcohol used in step (c) may be methanol, ethanol, propanol or butanol. The alkyl alcohol used in step (c) may be the same as the alkyl alcohol used in step (a) . The molar ratio of the alkyl alcohol or water in step (c) to the alkyl nitrite in step (a) may be from 1: 1 to 1000: 1.

Step (c) may be carried out in a second absorption tower. The second absorption tower may be a packing tower, a washing tower or a spraying tower.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrating a process and a system for absorbing and removing nitrogen oxides (NO _x) and alkyl nitrites in a gas according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for treating a tail gas having nitrogen oxides (NO _x) and alkyl nitrites (RONO) . This invention was made based on the surprising discovery of novel treatment process by the inventors that removes RONO and NO _x from a tail gas by absorption with an alkyl alcohol and by a catalytic reduction reaction, respectively, such that the treated tail has a NO _x content below 50 ppm and a RONO content below 10 ppm. Also provided is a system for carrying out the treatment process.

A process for treating a tail gas is provided. The tail gas has nitrogen oxides (NO _x) and alkyl nitrite (RONO) . The process comprises contacting the tail gas with an effective amount of an alkyl alcohol in the presence of oxygen to absorb the alkyl nitrite, contacting the resulting absorbed gas with a reducing gas in the presence of an effective amount of a catalyst to reduce the nitrogen oxides, and washing the resulting absorbed and reduced gas. As a result, a treated tail gas is prepared.

Examples of nitrogen oxides include nitrogen monoxide (NO) and nitrogen dioxide (NO ₂) . The tail gas may have an initial nitrogen oxide content greater than 50000 or 100000 ppm. The treated tail gas may have a nitrogen oxide content less than about 60, 55, 50, 45, 40, 35, 30, 25, 20, 15 or 10 ppm, preferably less than about 50 ppm.

Examples of alkyl nitrite include methyl nitrite (MN) and ethyl nitrite. The treated tail gas may have an alkyl nitrite content less than about 20, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2 or 1 ppm, preferably less than about 10 ppm.

In alkyl nitrite absorption step, the molar ratio of the nitrogen oxides to the oxygen may be from about 1: 10 to about 10: 1, preferably from about 1: 5 to about 5: 1, more preferably from about 1: 2 to about 2: 1.

The molar ratio of the alkyl alcohol to the alkyl nitrite may be from about 1: 1 to about 100: 1, preferably from about 2: 1 to about 50: 1, more preferably from about 5: 1 to about 30: 1. The alkyl alcohol may be selected from the group consisting of methanol, ethanol, propanol and butanol.

The alkyl nitrite absorption step may be carried out under an operation pressure of about 0.1-1.0 MPa, preferably about 0.1-0.7 MPa, more preferably about 0.1-0.3 MPa.

The alkyl nitrite absorption step may be carried out at an operation temperature from about -30 ℃ to about 90 ℃, preferably from about -10 ℃ to about 60 ℃, more preferably from about 0 ℃ to about 50 ℃.

The alkyl nitrite absorption step may be carried out in an absorption tower, which may be a packing tower, a washing tower or a spraying tower.

In the reduction step, the molar ratio of the reducing gas to the nitrogen oxides may be from about 1: 1 to about 10: 1, preferably from about 1: 1 to about 5: 1, more preferably from about 1: 1 to about 3: 1. The reducing gas may comprise H ₂, CO, NH ₃, or a combination thereof. The catalyst may comprise Pd, Pt, Cu, Rh or a combination thereof.

The reduction step may be carried out at a temperature of about 50-500 ℃, preferably about 100-450 ℃, more preferably about 150-350 ℃. The reduction step may be carried out under a pressure of about 0-10 barG, preferably about 0-8 barG, more preferably about 0-5 barG. The reduction step may be carried out at a gas space velocity of about 50-10,000 h ^-1, preferably about 70-5,000 h ^-1, more preferably about 100-2,000 h ^-1.

The reduction step may be carried out in a reactor. The reactor may be a fixed bed reactor.

In the washings step, the absorbed and reduced gas may be washed with water or an alkyl alcohol. The alkyl alcohol used in the washing step may be methanol, ethanol, propanol or butanol. The alkyl alcohol used in the washing step may be the same as the alkyl alcohol used in absorption step. The molar ratio of the alkyl alcohol or water used in the washing step to the alkyl nitrite used in absorption step may be from about 1: 1 to about 1000: 1 , preferably from about 2: 1 to about 500: 1, more preferably from about 5: 1 to about 300: 1.

The washing step may be carried out in a second absorption tower. The second absorption tower may be a packing tower, a washing tower or a spraying tower.

A system for treating a tail gas having nitrogen oxides and an alkyl nitrite is also provided. The system comprises a first absorption tower, a reactor and a second absorption towner. In the first absorption towner, the tail gas is exposed to an effective amount of an alkyl alcohol in the presence of oxygen to absorb the alkyl nitrite, whereby an absorbed gas is generated. In the reactor, the absorbed gas is exposed to a reducing gas in the presence of an effective amount of a catalyst to reduce the nitrogen oxides, whereby an absorbed and reduced gas is generated. In the second absorption towner, the absorbed and reduced gas is washed. As a result, a treated tail gas is prepared.

The first absorption tower may be a packing tower, a washing tower or a spraying tower. The reactor may be a fixed bed reactor. The second absorption tower may be a packing tower, a washing tower or a spraying tower.

The term “about” as used herein when referring to a measurable value such as an amount, a percentage, and the like, is meant to encompass variations of ±20%or ±10%, more preferably ±5%, even more preferably ±1%, and still more preferably ±0.1%from the specified value, as such variations are appropriate.

Example 1. Treated Gas 1

A tail gas was treated to generate Treated Gas 1. A gas containing nitrogen oxides (NO _x) and methyl nitrite (MN) was introduced with air into a first washing tower. The molar ratio of the NO _x to the oxygen (O ₂) in the air was 1: 10. Methanol was introduced at a molar ratio of methanol to MN at 1: 1 to adsorb MN. The operation pressure was 0.1 MPa. The operation temperature was -30 ℃. The gas exiting from the top of the adsorption tower was passed together with hydrogen (H ₂) into a fixed bed reactor equipped containing Pt catalyst for catalytic reduction in the presence of a reducing gas having a molar ratio of the reducing gas to the NO _x at 1: 1. The reaction temperature was 500 ℃. The reaction pressure was 0 barG. The gas space velocity was 50 h ^-1. The reduced gas was passed from the fixed bed reactor into a second washing tower for washing with water at a molar ratio of 1000: 1 for the water to the methyl nitrate before treatment and, after washing, the used water was discarded as waste water. The gas exiting from the top of the washing tower was Treated Gas 1. Treated Gas 1 was then subjected to combustion treatment.

Table 1 shows the key process conditions used to prepare Treated Gas 1 and its methyl nitrite content of 8 ppm and NOx content of 43 ppm.

Example 2: Treated Gas 2

A tail gas was treated to generate Treated Gas 2. A gas containing nitrogen oxides (NO _x) and ethyl nitrite was introduced with air into a first packing tower. The molar ratio of NO _x to oxygen (O ₂) in the air was 10: 1. Ethanol was introduced at a molar ratio of ethanol to ethyl nitrite at 100: 1 to absorb ethyl nitrite. The operation pressure was 1.0 Mpa. The operation temperature was 90℃. The gas existing from the top of the filler tower was passed together with CO into a fixed bed reactor equipped with Pd catalyst for catalytic reduction in the presence of a reducing gas having a molar ratio of the reducing gas to the NO _x at 10: 1. The reaction temperature was 50 ℃. The reaction pressure was 10 barG. The gas space velocity was 10,000 h ^-1. The reduced gas was passed into a second packing tower for washing with ethanol at a molar ratio of 1: 1 for the ethanol to the ethyl nitrite in the tail gas before treatment and, after washing, the used ethanol was introduced into the first packing tower to be reused in the absorption step. The gas exiting from the top of the washing tower was Treated Gas 2. Treated Gas 2 was then subjected to combustion treatment.

Table 1 shows the key process conditions used to prepare Treated Gas 2 and its methyl nitrite content of 9 ppm and nitrogen oxide content of 43 ppm.

Example 3. Treated Gases 3-6 and Comparative Gas 1-3

Addition studies were carried out to treat a tail gas containing nitrogen oxide and alkyl nitrite according the process described in Example except the different conditions indicated in Table 1. Table 1 shows the process conditions used for preparing Treated Gases 3-6 and Comparative Treated Gases 1-3 and their methyl nitrite contents and nitrogen oxide contents.

Table 1. Preparation and properties of Treated Gases 1-6 and Comparative Gases 1-3

Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims without departing from the invention.

Claims

A process for treating a tail gas having nitrogen oxides and an alkyl nitrite, comprising:

(a) contacting the tail gas with an effective amount of an alkyl alcohol in the presence of oxygen to absorb the alkyl nitrite, whereby an absorbed gas is generated;

(b) contacting the absorbed gas with a reducing gas in the presence of an effective amount of a catalyst to reduce the nitrogen oxides, whereby an absorbed and reduced gas is generated; and

(c) washing the absorbed and reduced gas, whereby a treated tail gas is prepared.
The process of claim 1, wherein the treated tail gas has a nitrogen oxide content less than 50 ppm.
The process of claim 1, wherein the treated tail gas has an alkyl nitrite content less than 10 ppm.
The process of claim 1, wherein the molar ratio of the nitrogen oxides to the oxygen in step (a) is from 1: 10 to 10: 1.
The process of claim 1, wherein the molar ratio of the alkyl alcohol in step (a) to the alkyl nitrite in step (a) is from 1: 1 to 100: 1.
The process of claim 1, wherein the alkyl alcohol is selected from the group consisting of methanol, ethanol, propanol and butanol.
The process of claim 1, wherein step (a) is carried out under an operation pressure of 0.1-1.0 MPa.
The process of claim 1, wherein step (a) is carried out at an operation temperature from -30 ℃ to 90 ℃.
The process of claim 1, wherein step (a) is carried out in a first absorption tower.
The process of claim 9, wherein the first absorption tower is a packing tower, a washing tower or a spraying tower.
The process of claim 1, wherein the molar ratio of the reducing gas to the nitrogen oxides in step (b) is from 1: 1 to 10: 1.
The process of claim 1, wherein the reducing gas comprises H ₂, CO, NH ₃, or a combination thereof.
The process of claim 1, wherein the catalyst comprises Pd, Pt, Cu, Rh or a combination thereof.
The process of claim 1, wherein step (b) is carried out at a temperature from 50 ℃ to 500 ℃.
The process of claim 1, wherein step (b) is carried out under a pressure of 0-10 barG.
The process of claim 1, wherein step (b) is carried out at a gas space velocity of 50-10,000 h ^-1.
The process of claim 1, wherein step (b) is carried out in a reactor.
The process of claim 17, wherein the reactor is a fixed bed reactor.
The process of claim 1, wherein the absorbed and reduced gas is washed with water in step (c) .
The process of claim 1, wherein the absorbed and reduced gas is washed with the alkyl alcohol in step (c) .
The process of claim 20, wherein the molar ratio of the alkyl alcohol in step (c) to the alkyl nitrite in step (a) is from 1: 1 to 1000: 1.
The process of claim 20, further comprising introducing the alkyl alcohol after step (c) into step (a) .
The process of claim 1, wherein step (c) is carried out in a second absorption tower.
The process of claim 23, wherein the second absorption tower is a packing tower, a washing tower or a spraying tower.