MXPA99011276A

MXPA99011276A - Black water flash and vapor recovery process and apparatus

Info

Publication number: MXPA99011276A
Application number: MXPA/A/1999/011276A
Authority: MX
Inventors: S Wallace Paul; Anderson Johnson Kay; D Fair Delome
Original assignee: Texaco Development Corporation
Priority date: 1997-06-06
Filing date: 1999-12-06
Publication date: 2001-05-17

Abstract

La invención es un proceso para desgasificar y enfriar una suspensión de agua negra caliente. La suspensión de agua negra caliente se obtiene de depuradores de gassin. La suspensión de agua negra se enfría y desgasifica exponiendo la suspensión de agua negra a un vacío bajo condiciones suficientes para separar los gases disueltos de la suspensión de agua negra. Esos gases son entonces removidos de la suspensión de agua negra. Esos gases pueden ser reciclados de manera ventajosa en el proceso de gasificación.

Description

EVAPORATION SYSTEM OF BLACK WATER AND STEAM RECOVERY FIELD OF THE INVENTION • This invention relates in a general manner to the cooling and degassing of sewage water obtained from gassing.

BACKGROUND OF THE INVENTION Synthetic gas, or gassin, can be produced by reacting solid or liquid carbonaceous fuels with gases such as air, enriched air, or oxygen, in the optimal presence of steam or water in a gasification reactor. The gassin obtained is extracted from the gasification reactor and subjected to several cleaning operations to free it from various pollutants which are formed or released from solid or liquid carbonaceous fuels during the gasification operation. These pollutants can easily become environmental pollutants if they are not properly treated during the operation of gasification. For example, materials often found in gassin include hydrogen sulfide, ammonia, cyanides, phenols, various halogens and particles in the form of carbon, ash and coal, as well as trace metals.

The elimination and control of these contaminants must be > 'managed in a satisfactory manner to make gasification a viable process without suffering the concomitant contamination problems. When the gas is discharged the gasifier is usually subjected to multiple cooling and cleaning operations that involve a purification technique where the gas is introduced into at least one scrubber and is put in contact with a sprinkler of water which cools the gas and condenses condensable materials such as tar, petroleum or organic compounds. The water used for the purification operation becomes what is commonly known as "black water", since it is contaminated with coal. This black water may also contain soluble gases. This black water can be subjected to a variety of steps which can include the decantation of the solids containing carbon, the partial concentration of solids in the suspension, the separation of gases such as hydrogen sulfide, ammonia, and also the steps solvent extraction to remove carbon and dissolved carbon-containing compounds such as phenols and cyanides. The particulate solids, ie carbon, soot and ash, introduced into the flow of hot raw gas from a partial oxidation gas generator are removed by rapidly cooling the flow of hot gas directly into water in a cooling drum and purifying with water in a gas purification zone. By this means, a clean gas flow and a dispersion of particulate solids, i.e. carbon and ash, are produced. It is economical to recover the water in the aforementioned dispersion by removing the solid particles and gaseous impurities. However, in the recovery operation, aqueous emulsions difficult to pump in the system and which have to be removed are formed. The recovered water can then be recycled to the zone of rapid cooling and gas cleaning. The prior art used an evaporation column for the recovery of gray water. Gray water is water that has had a substantial fraction of the carbon and other solids removed. These systems are not capable of removing a sufficient amount of harmful gases to allow the open atmospheric treatment of degassed black water.

BRIEF DESCRIPTION OF THE INVENTION The invention is a process for degassing and cooling a suspension of hot black water. The hot black water suspension is obtained from gas scrubbers.

The black water suspension is cooled and degassed by exposing the black water suspension to a vacuum under conditions sufficient to separate the dissolved gases from the black water suspension. Those gases are then removed from the black water suspension. The gases can be recycled advantageously in the gasification process.

DETAILED DESCRIPTION OF THE INVENTION As used herein, the term "vacuum" means a pressure less than atmospheric pressure, that is, an absolute pressure less than about 101 KPa. The degree of vacuum is defined here by the absolute pressure, that is, a pressure of 0 KPa is a perfect vacuum. As used herein, the term "gases" means those molecules that are in a gaseous state at the pressure and temperature conditions that exist at that point, and may include vapors that would condense at room temperature or even at higher temperatures. Water vapor is a gas, as the term is used here. In the partial oxidation process to produce mixtures of gases comprising hydrogen and carbon monoxide, the raw process gas flow contains introduced particulate solids, i.e. carbon and ash. The gas also contains polluting gases, particularly carbon dioxides, ammonia and hydrogen sulfide. The gas may also contain salts, including sodium and calcium salts. The particular solids and a large fraction of the polluting gases are removed by cooling or purification or both, with water. In the process of gas purification that is generated in a gasification reactor, a suspension of hot black water containing particulate carbon and ash is produced, where the water contains carbon dioxide, hydrogen sulfide, ammonia, and possibly other gases, and may contain soluble salts, in particular calcium salts. Sulfides, including hydrogen sulfide, can be solidified by the addition of ferrous salts, that is, ferrous sulfate, and subsequently treated as a solid in black water. Alternatively, sulfides can be treated as a volatile gas. The hot black water suspension contains from about 0.3 to about 10% by weight of suspended solids. The hot black water suspension is usually around 150 degrees Celsius to about 300 degrees Celsius, although the temperature is not critical. This suspension of black water is evaporated in a vacuum to remove dissolved gases, volatile compounds and water vapor. Evaporation in a vacuum involves exposing the suspension of hot black water to a vacuum under conditions where gases and vapors can be released, leaving or leaving the suspension of hot black water. In general, significant agitation or gas-liquid contact, such as that obtained by means of plates in an evaporation tower or by introducing the liquid of a door above the gas-liquid contact, is sufficient. Evaporated gases, which include vapors such as water vapor, are then separated by gravity from the suspension in a quiet region of the vacuum vapor, and the gases leave the drum above while the cold degassed suspension leaves the vapor through below . Evaporation also necessarily removes water vapor, thereby cooling the gas flow. The amount of cooling depends on the amount of vacuum generated and the amount of water vapor removed. For a pressure of approximately 52 KPa, the suspension should be cooled to approximately 82 degrees Celsius. The greater the vacuum, the more complete the removal of gases and vapors. However, the black water suspension contains gases that were dissolved in it at high pressure, that is, frequently at pressures of an absolute pressure of 15,000 KPa or more. Therefore, a high pressure of about 50 to about 100 KPa will generally remove most dissolved gases. A pressure of an absolute pressure of about 35 to about 50 KPa will remove most of the dissolved gases in general, and a vacuum of an absolute pressure of about 10 to about 35 KPa will remove essentially all the dissolved gases. In a process where there is sufficient gas-water contact and a vacuum of an absolute pressure of between about 35 to about 75 KPa is maintained, the resulting black water suspension may contain less than about 10 parts per million by weight of hydrogen sulfide and less than about 10 parts per million by weight of free ammonia, ie, not ionically bound to an acid. In a process where there is sufficient gas-water contact and a vacuum of an absolute pressure of between about 10 to about 35 KPa is maintained, the resulting black water suspension can contain less than about 1 part per million by weight of hydrogen sulfide and less than 1 part per million by weight of free ammonia. Of course, it is possible to achieve a greater vacuum, that is, 5 KPa, but the cost of reaching such a vacuum normally can not be justified in the amount of extra gases removed. After evaporation, the black water suspension is cooled. Black water can not be easily cooled by conventional heat exchangers, since the surfaces would experience incrustations of solids and calcium salts. There is no cooling surface in the present invention, as would be found in a heat exchanger, for calcium solids and salts to be embedded. The cooled and degassed black water suspension can also be treated at atmospheric conditions to separate the solids from the water. The low level of gases, dangerous in other circumstances, particularly hydrogen sulfide, in black water does not pose a safety risk. Vacuum vapor is designed to handle this suspension, with its abrasive solids. The bottom of the vacuum drum and the connecting pipe are inclined at sufficient angles to prevent the solids from accumulating, that is, not less than about 10 degrees from the horizontal. The entrance of suspension of hot black water to vacuum steam is at level with the internal wall to prevent erosion of the nozzle. It is often advantageous to condense the steam to recycle it to the gas scrubbers. A condenser, or a heat exchanger and a liquid removal vessel, can be used to condense water vapor. It is preferred that the condensation be conducted under vacuum to minimize the amount of ammonia, carbon dioxide and hydrogen sulfide that dissolves in the condensed water. A short residence time in the condenser will also help to optimize the amount of gases that dissolve in the condensed water. The condenser preferably cools the gas to below about 40 degrees Centigrade. The cooled gas leaving the steam condenser contains ammonia, carbon dioxide, and hydrogen sulfide absorbed, as well as carbon monoxide and hydrogen and inerts such as nitrogen. It is advantageous to absorb the ammonia, carbon dioxide and hydrogen sulfide in cold basic water. The basic water may contain any of a base such as sodium hydroxide or be water rich in ammonia. It is preferred that the pH of the water be above 9, preferably above 11, more preferably above 12. Absorption of water and gas are carried out in intimate contact in a gas purification unit . The gas treatment unit can be of any type, including a jet scrubber, a trays or packed column, a venturi, or other gas scrubbers used in the industry. A counter-current gas purification tower with packing is preferred. Temperature control is important, and caustic soda or caustic or ammonia-rich water extracted from the bottom of the tower should be passed through a heat exchanger to cool the fluid before circulating the fluid back to the top of the tower. The preferred temperature of the caustic water is between about 0 and about 40 degrees Celsius, preferably between about 5 and about 30 degrees. Celsius Because the absorption of these gases depends on the temperature, it is preferred that the ammonia, carbon dioxide and hydrogen sulfide be absorbed in the caustic or ammonia rich water while still under vacuum. It is possible, especially if the vacuum pump is mechanical, to cause these gases to pass through the vacuum pump and absorb these gases at atmospheric pressure or even at high pressures. The method to generate a vacuum is important. Conventional means include jet and mechanical orifice pumps. The caustic or rich ammonia water is advantageously recycled to the gasification reactor. There, these gases help to moderate the pH of the gasification reactor by neutralizing some organic acids, particularly formic acid.

DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of the process. The raw hot black water enters from the gas scrubber vessels (not shown) via the conductive tube (30) between the vacuum evaporation drum (10). The black water evaporates in this vacuum, releasing the dissolved gases and vapors, and cooling the black water. The black water is extracted from vacuum evaporation (10) through the conductive tube 22 for further treatment (not shown). The gases and vapors leave the vacuum cleaning drum (10) and pass through a condenser, described here as a heat exchanger (12) and a liquid removal vessel (14). The condensed water vapor is passed through a pump (24) to the gas purification vessels (not shown). The gases leave the liquid elimination container (14) and enter the absorbent (16). The ammoniacal caustic water must be extracted from the bottom of the absorbent (16) through a circulation pump (20). A fraction of the water leaving this circulation (20) is sent to the gasifier through the high pressure pump (28). A fraction of the water leaving this circulation pump (20) is routed through a cooler (26) and back to the top of the absorbent. Fresh caustic water is added as necessary. The remaining gases, which include hydrogen, carbon monoxide and inert gases such as nitrogen, are routed via the conductive tube (18) through the steam generator (not shown) to a burner (not shown).

Claims

CHAPTER CLAIMEDICATORÍO Having described the invention, it is considered as a novelty and, therefore, the content is claimed in the following CLAIMS:

1. A process for degassing and cooling a suspension of black water obtained from gas scrubbers, characterized in that it comprises exposing the suspension of black water to a vacuum under conditions sufficient to separate the dissolved gases from the black water suspension, and remove the gases from the water. black water suspension.

2. The process according to claim 1, characterized in that it further comprises transporting the gases to a condenser and condensing the water vapor.

3. The process according to claim 2, characterized in that it further comprises transporting the condensed water to a gassin scrubber.

4. The process according to claim 1, characterized in that it further comprises exposing the gases to basic cold water under conditions sufficient to absorb ammonia, carbon dioxide and hydrogen sulfide.

5. The process according to claim 4, characterized in that the step of absorbing ammonia, carbon dioxide and hydrogen sulfide in caustic water or rich in cold ammonia occurs under vacuum.

6. The process according to claim 4, characterized in that the step of absorbing ammonia, carbon dioxide and hydrogen sulfide in caustic water or rich in cold ammonia occurs at a pressure greater than or equal to a pressure of 101 KPa. The process according to claim 3, characterized in that it also comprises the step of recycling the caustic or ammonia rich water to the gasification reactor. 8. The process according to claim 1, characterized in that it further comprises the step of further treating the black water at atmospheric conditions to separate the solids from the water. The process according to claim 1, characterized in that the pressure is an absolute pressure of about "10 to about 75 KPa 10. The process according to claim 1, characterized in that the pressure is an absolute pressure of about 10. to approximately 50 KPa 11. The process according to claim 1, characterized in that the pressure is an absolute pressure of about 35 to about 50 KPa 12. The process according to claim 1, characterized in that the pressure is a pressure absolute from about 10 to about 35 KPa 13. The process according to claim 1, characterized in that the resulting black water suspension contains less than about 10 parts per million by weight of hydrogen sulfide and less than about 10 parts per million. by weight of free ammonia 14. The process according to the claim 1, characterized in that the resulting black water suspension contains less than about 1 part per million by weight of hydrogen sulfide and less than about 1 part per million by weight of free ammonia. 15. An apparatus for evaporating black water under vacuum and recovering water and released gases, the apparatus is characterized in that it comprises a vacuum evaporation, a steam condenser, an absorbent containing caustic or ammoniacal water, a vacuum generator, where the Water enters the vacuum evaporation chamber, and where the evaporated steam passes sequentially from vacuum evaporation through the steam condenser, the absorbent, and the vacuum generator. 16. The apparatus according to claim 15, characterized in that the bottom of the vacuum drum and the connecting pipe are inclined at angles not less than 10 degrees from the horizontal to prevent the accumulation of solids. 1

7. The apparatus according to claim 15, characterized in that the black water entering the vacuum drum is evaporated with the internal wall to prevent erosion of the nozzle.