Process and apparatus for indirect cooling of a hot gas

Abstract

 Process and apparatus for indirectly cooling a hot gas that contains impurities which, on cooling, deposit in the form of water-soluble solids, the hot gas being passed through a heat exchanger provided with tubes. A water film flows downwards along each of the tubes, and the water flowing away from beneath the tubes is discharged from the heat exchanger. Preferably use is made of a substantially vertical heat exchanger in which the gas is passed through the tubes and the water for the water film is fed onto a substantially horizontal tube plate in the top of the heat exchanger, to which tube plate the tubes are connected such that their ends project somewhat above the tube plate.

Description

[0001] The invention relates to a process for the indirect cooling of a hot gas, the hot gas being passed through a heat exchanger provided with tubes.

[0002] A process of this kind is well known. The gas is cooled by the transfer of heat to a coolant. If the gas is passed through the tubes, the coolant surrounds the tubes. If, on the other hand, the gas is passed around the tubes, the coolant is routed through the tubes.

[0003] Problems can arise if the hot gas contains impurities which, on cooling, deposit as solids on the walls of the tubes. This can seriously impair the heat transfer between the gas and the coolant. Moreover, the deposited solids can accumulate to such an extent that an excessive pressure drop is created across the heat exchanger, and it is even possible for blockages to occur in or between the tubes. This calls for regular cleaning of the tubes, which usually necessitates interruption of the gas flow. If the impurities are water-soluble, these problems can be overcome by the process according to the invention.

[0004] The aim of the invention is therefore to clean the heat exchanger without interrupting the gas flow. The invention thus relates to a process for indirectly cooling a hot gas that contains impurities which, on cooling, deposit in the form of water-soluble solids, the hot gas being passed through a heat exchanger provided with tubes, characterized in that a water film flows downwards along each of the tubes, and that the water flowing away from beneath the tubes is discharged from the heat exchanger.

[0005] The impurities, being cooled and solidifying, are dissolved in the water film, transported in this film to the bottom of the heat exchanger, and from there discharged. Already deposited impurities, if any, are dissolved and discharged, too. The heat exchanger tubes are therefore cleaned without interruption of the gas flow.

[0006] The heat exchanger can be positioned in many ways, for example horizontally or vertically. If it is positioned substantially horizontally and the tubes are therefore also substantially horizontal, the coolant is preferably passed through the tubes and the gas passed around them. Water is fed into the heat exchanger from above, the water forming a film on the tubes and dissolving any deposited solids. It drips from one tube to another and finally ends up on the bottom of the heat exchanger, from where it is discharged.

[0007] The heat exchanger is preferably positioned vertically. Either the gas or the coolant can be passed through the tubes. In either case the process according to the invention is simpler to carry out in this position than in a horizontal one.

[0008] The water film can be formed by spraying water in the top of the heat exchanger. The water droplets thus formed collide with the tubes and thus form a water film. Some droplets, however, will fall to the bottom without touching the cooling surface and forming a film. This cannot be entirely prevented even in a vertical heat exchanger in which a distribution plate is provided above the tubes. Droplets will still be able to fall past or through the tubes without becoming incorporated in the film. If the gas is passed through the tubes it is easy to ensure that substantially all the added water becomes incorporated in the films. A substantially vertical heat exchanger is therefore advantageously used in which the gas is passed through the tubes. In such an embodiment the water for the water film is preferably fed onto a substantially horizontal tube plate in the top of the heat exchanger, to which tube plate the tubes are connected. In this way the water flows evenly over the tube plate into the tubes and forms a water film. More preferably, the ends of the tubes project somewhat above the tube plate. Advantageously, they project above the tube plate such that their openings lie in an imaginary horizontal plane. The water fed onto the tube plate has the same level over the entire tube plate and thus flows over the edges of the tubes in equal amounts per tube (assuming that the tubes have equal diameters). For this purpose the ends of the tubes suitably project 0.1 to 10 cm above the tube plate.

[0009] To obtain a more even distribution of water over the tube plate, the tube plate is preferably divided into compartments which can be connected individually to the water supply. To this end the tube plate may be divided into segments, but it can also be divided by concentric circular partitions or by parallel straight partitions. Combinations of these systems are also possible. In the present patent application, water is understood to include all aqueous solvents. It may also contain other substances, e.g. salts or alcohols which raise or lower the boiling point of the aqueous mixture.

[0010] The supply rate of the water is determined in accordance with the total area of the walls of all tubes. The rate is not critical. Preferably, the water film is present over the whole area of the tubes. The rate should not be so high that the tubes became entirely filled with water for the case of the gas being passed through the tubes, nor that the space between the tubes becomes almost filled with water for the case of the coolant being passed through the tubes.

[0011] The process according to the invention can be carried out either intermittently or continuously, depending on, amongst other things, the amount of impurities in the gas and the solubility of the deposit in water. If the amount of impurities is small and the deposit dissolves readily in water, an intermittent process is preferable.

[0012] The water falling down from the tubes and discharged from the heat exchanger is preferably recirculated for at least a part to the top of the heat exchanger. The recirculated water is somewhat heated by the gas. The solid dissolves more readily in this relatively warm water. This is also an advantage if the process takes place under pressure. Recirculation eliminates the need to pressurize a large quantity of water.

[0013] Advantageously, a part of the water discharged from the heat exchanger is not recirculated but is removed from the system and replaced by fresh water. This prevents excessive accumulation of the impurities in the water for the water film. It is therefore desirable to add fresh water, either periodically or continuously, to the recirculated water.

[0014] In the case of a substantially vertical heat exchanger, the gas can be passed through the heat exchanger either from the bottom to the top, or from the top to the bottom, i.e. either counter to the flow of the water film or in the same direction. Since the water film is more likely to be disturbed by the gas flowing counter to it, the gas is preferably passed through the heat exchanger from top to bottom.

[0015] The process according to the invention is advantageously applied if the gas consists of an ammonium chloride-containing synthesis gas. Synthesis gas is formed by the reaction of oxygen with a fuel such as gaseous or liquid hydrocarbons, coal, wood, etc. Synthesis gas consists mainly of carbon monoxide and hydrogen. Depending on the fuel, it can also contain small amounts of ammonia, ammonium chloride, chlorine and other halogens, carbon monoxide, hydrogen sulphide, methane, carbon oxysulphide and hydrogen cyanide. When this gas is cooled, solid ammonium chloride, amongst others, is deposited. Also other ammnonium halide compounds and ammonium bicarbonate may be deposited. The deleterious effect of this deposit on the heat exchanger being used can be prevented by cooling according to the invention.

[0016] The synthesis gas, that usually has a temperature of 350 to 100°C and a pressure of 50 to 1 bar before being introduced into the heat exchanger, is preferably cooled to a temperature of 200 to 50°C. The water constituting the water film remains predominantly liquid under these conditions.

[0017] The invention also relates to an apparatus for carrying out the described process, comprising a pressure vessel to which are connected an inlet and an outlet for the gas, -an inlet and outlet for a coolant, and which contains tubes through which the gas is passed and which are connected to two tube plates, said pressure vessel being substantially vertical, characterized in that at least one water inlet is provided above the above tube plate and at least one water outlet is provided at the bottom of the pressure vessel.

[0018] Preferably, the ends of the tubes project somewhat above the top tube plate. Preferably, they project 0.1 to 10 cm above the tube plate, more preferably 0.5 to 5 cm.

[0019] The invention will now be explained in more detail with reference to the following figures, which, however, in no way restrict the scope of the invention. The figures do not show auxiliary equipment such as pumps, compressors, valves, monitoring instruments, etc.

[0020] 

Figure 1 shows schematically a vertical sectional view of the apparatus according to the invention.

Figure 2 shows a cross section of the apparatus along the line II-II.

In Figure 3 the application of an apparatus of this type is shown for the preparation of synthesis gas.

[0021] In Figure 1 a pressure vessel 1 has an inlet 2 for the hot gas. The gas entering through the inlet 2 into the heat exchanger passes downwards through tubes 10. The tubes 10 are connected to tube plates 8 and 9. Around the tubes 10 is passed a coolant which is introduced into the heat exchanger through an inlet 6 and discharged from the heat exchanger through an outlet 7. As can be seen in Figure 2, the tube plate 8 is divided into 12 segments. The segments are separated from one another by partitions 11. Each segment comprises a water inlet 4 so that water can be fed to each segment individually. Water is fed to the tube plate 8 via the inlets 4. Due to the tubes 10 projecting somewhat above the tube plate 8, a layer of water is formed on it. As more water is added it flows over the ends of the tubes 10 and frcms a water film on the inner wall of each of the tubes. The film flows downwards and deposited impurities dissolve in it. The water flowing out of the pipes 10 is caught in a space at the bottom of the pressure vessel 1. The water is discharged periodically or continuously from the pressure vessel via an outlet 5 in the bottom. The cooled gas leaves the pressure vessel via a gas outlet 3.

[0022] . Figure 3 shows the present invention applied in the field of coal gasification.

[0023] In a reactor 20 coal, supplied via a pipe 21, is gasified at high pressure and temperature with steam and oxygen supplied via pipes 22 and 23 respectively. A large part of the slag formed is discharged from the reactor 20 via a pipe 25. The synthesis gas thus formed and containing fine slag droplets, is led via a pipe 24 to a cooler 26 where the gas is somewhat cooled and the slag droplets solidify to particles. The synthesis gas contains, in addition to carbon monoxide and oxygen, some ammonium chloride. The cooler 26 is here shown as an indirect heat exchanger in which a coolant enters and leaves via pipes 27 and 28 respectively. The cooling of the gas can, however, also be performed directly, for example by injection of water and/or a cool gas into the hot gas. The mixture of synthesis gas and solid slag particles is passed via a pipe 29 to a separating device 30. Various systems are possible for the separating device, which operates at high pressure and relatively high temperature. For example, electrostatic precipitators can be used, as well as all types of granular bed filters, such as stationary, moving, magnetically stabilized or electrostatic granular bed filters. Acoustic agglomeration, ceramic membrane filters or ceramic fibre filters can also be used. Further possibilities are dry plate scrubbers or electrocyclones. The figure shows schematically a ceramic bag filter. The slag particles separated in the bag filter 30 are discharged from the system via a pipe 31. The hot gas is fed via a pipe 32 to a cooling device 33. From there the gas mixture passes via a pipe 34 to the top of a heat exchanger 35 where it is cooled in the manner according to the invention. The gas mixture flows downwards through tubes in the heat exchanger from where it is discharged via a pipe 36. Around the tubes is passed a coolant supplied via a pipe 41 and discharged from the heat exchanger via a pipe 42. Any coolant can be used, e.g. (boiling) water or oil. Water is fed via a pipe 37 into the heat exchanger 35 and flows as a film down along the inner wall of the tubes. The water is discharged from the heat exchanger via a pipe 38. This water stream is split into a stream 39 which is recirculated and added to the water in the pipe 37, and a stream 40 which is discharged from the system. Amnonium chloride entrained in the gas and solidified during cooling is dissolved in the water film and finally removed from the system by stream 40.

[0024] The cooled gas mixture is discharged via pipe 36 for further processing.

EXAMPLE

[0025] In a heat exchanger, substantially as described with reference to Figure 1, having 1020 tubes with an external diameter of 1.9 cm and a length of 10.8 m, 44.2 kg/s of synthesis gas at 250°C and having the following composition is fed through the gas inlet 2:

[0026] The pressure in the heat exchanger is 24 bar. 7.8 kg/s water at 80°C is introduced for two minutes via one of the water inlets 4 into one of the twelve segments of tube plate 8. After two minutes the water supply is stopped for three minutes, whereupon it is resumed for a further two minutes into the following segment of the tube plate 8. In this way each segment receives a two minutes supply of water once per hour. The total supply time is 24 minutes per hour. The water contains 0.24 kg/s ammonium chloride. The ends of the tubes project 2 cm above the tube plate. As coolant, 16.6 kg/s water at 50°C is fed via the inlet 6 into the heat exchanger. The temperature of the cooling water discharged from the outlet 7 is 200°C. At the water outlet 5 water at 80°C is tapped off. The amount of ammonium chloride that it carries in an hour is 547.2 kg.

[0027] The synthesis gas is discharged through the gas outlet 3. It has a temperature of 80°C and contains no ammonium chloride.

Claims

1. Process for indirectly cooling a hot gas that contains impurities which, on cooling, deposit in the form of water-soluble solids, the hot gas being passed through a heat exchanger provided with tubes, characterized in that a water film flows downwards along each of the tubes, and that the water flowing away from beneath the tubes is discharged from the heat exchanger.

2. Process as claimed in claim 1, characterized in that use is made of a substantially vertical heat exchanger in which the gas is passed through the tubes.

3. Process as claimed in claim 2, characterized in that the water for the water film is fed onto a substantially horizontal tube plate in the top of the heat exchanger, to which tube plate the tubes are connected.

4. Process as claimed in claim 3, characterized in that the water for the water film is fed onto a tube plate to which the tubes are connected such that their ends project somewhat above the tube plate.

5. Process as claimed in any one of claims 1-4, characterized in that the water for the water film is supplied intermittently to the heat exchanger.

6. Apparatus for carrying out a process as claimed in any one of claims 1-5, comprising a pressure vessel to which are connected an inlet and an outlet for the gas, an inlet and an outlet for a coolant, and which contains tubes through which the gas is passed and which are connected to two tube plates, said pressure vessel being substantially vertical, characterized in that at least one water inlet is provided above the above tube plate and at least one water outlet is provided at the bottom of the pressure vessel.

7. Apparatus as claimed in claim 6, characterized in that the ends of the tubes project somewhat above the top tube plate.

8. Apparatus as claimed in claim 6 or 7, characterized in that the tube plate is divided into compartments.

9. Apparatus as claimed in claim 6, as described hereinbefore with special reference to the Figure.

10. Process as claimed in claim 1, as described with special reference to the Example.

Drawing