JPS6327042B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of operating a distillation column. More specifically, the present invention relates to a method of operating a distillation column equipped with a natural circulation reboiler. Distillation columns with natural circulation reboilers using the thermosiphon principle are used in various processes in the fuel industry or the organic chemical industry. For example, hydrodesulfurization equipment for petroleum fractions, hydrocracking equipment,
In hydrotreating equipment, etc., a distillation column equipped with a natural circulation reboiler is used to isolate reaction products, separate low-boiling point components and high-boiling point components, improve the flash point, or remove water. Used a lot. To give a specific example, in a hydrodesulfurization equipment for kerosene and gas oil, the effluent from the hydrodesulfurization reaction tower is separated into gas and liquid, and the separated liquid is separated from the hydrogen sulfide-containing gas by a hydrogen sulfide stripper, and then released into the natural environment. A distillation column equipped with a circulating reboiler adjusts the flash point of the product kerosene and gas oil, and separates water and low-boiling components from the water condensed from the steam blown in at the bottom of the hydrogen sulfide stripper column as an overhead liquid. In this type of distillation column, the amount of refluxed top liquid is reduced to the extent that quality standards such as the flash point and moisture content of the bottom liquid (product kerosene, etc.) are not compromised, thereby saving energy. things are being done. However, if the amount of refluxed liquid at the top of the column is reduced and the amount of heat supplied to the natural circulation reboiler is below any value, the amount of distillation column bottom oil boiling in the reboiler will decrease, and the circulation in the natural circulation reboiler will become steady. Since the condition cannot be maintained, the amount of heat supplied to the distillation column changes and the distillation operation becomes disordered. In this case, the flash point, water content, etc. of the product tower bottom liquid vary, making it impossible to maintain predetermined product properties. In other words, in the natural circulation reboiler of this type of distillation column, there is a minimum amount of heat supply required to maintain normal natural circulation, and the amount of heat supply below this minimum amount of heat is stable. It was impossible to drive. In the past, several methods have been proposed for stably maintaining natural circulation in natural circulation reboilers, including in such cases. For example, JV Smith,
Chem.Eng.Progr., vol 70, No. 7, (1974) p. 70 states that one way to increase the stability of natural circulation reboilers is to inject nitrogen gas, such as nitrogen gas, into the liquid inlet of the reboiler. A method of injecting gas has been proposed. In addition, as a method to facilitate the generation of natural circulation when starting up a natural circulation reboiler, a method has been proposed in which a small amount of components with lower boiling points than the bottom components are supplied to the outlet and inlet of the reboiler at the time of startup. (1976-83880). However, in the method by JV Smith mentioned above, it is necessary to introduce a valuable gas such as nitrogen gas from outside the distillation system equipped with a natural circulation reboiler, and it is also necessary to introduce a valuable gas such as nitrogen gas from the outside into the distillation column top gas. There is a disadvantage that the gas mixed in with the top of the column deteriorates the original purity of the top gas. For this reason, continuous introduction of such external gas during steady operation of the distillation column is problematic in terms of operating costs and maintaining the purity of the top gas components, and it is difficult to actually operate the distillation column during steady operation. It was difficult to apply this method continuously. In addition, the method of JP-A-51-83880 aims to facilitate the start-up of the reboiler, and since the liquid to be injected must be procured from outside, it can be applied in series to the steady operation of the natural circulation reboiler. There was nothing I could do. The present inventor has variously modified the operating characteristics of this type of distillation column in order to avoid these problems of the conventional technology and to achieve stable operation of a distillation column equipped with a natural circulation reboiler and to save energy by reducing the reboiler heat source. As a result of analysis and repeated experiments, we found that when a part of the top liquid and/or side distillate of the distillation column is supplied to the circulating liquid of the reboiler, the vapor pressure of the circulating liquid at the reboiler liquid outlet increases, and the circulating liquid At the same time, the gas-liquid multiphase density at the reboiler liquid outlet decreases, and the density difference between the circulating liquid near the reboiler liquid inlet and the circulating liquid near the liquid outlet increases, resulting in natural circulation. found that the driving force of
We have arrived at the present invention. That is, an object of the present invention is to provide a method for stably operating a distillation column equipped with a natural circulation reboiler with a small amount of heat supply; or/and a part of the side distillate is transferred to the liquid inlet of the reboiler or/
This can be easily achieved by supplying the liquid to the liquid outlet. The present invention will be explained in detail below. A distillation column to which the present invention can be applied is a distillation column equipped with a natural circulation reboiler in the fuel industry and organic chemical industry, especially the petroleum and petrochemical industry, and is a distillation column equipped with a natural circulation reboiler in the fuel industry and organic chemical industry, particularly in the petroleum and petrochemical industry. Those whose purpose is to separate components and/or moisture are suitable. Examples of such distillation columns include matching columns, strippers, regeneration columns, etc.
In particular, the following distillation columns are used for the purpose of separating or removing low-boiling components present in the distillation column raw material liquid in petroleum refining plants or petrochemical plants. Distillation column for adjusting the flash point of product oil in a hydrodesulfurization unit for kerosene and gas oil A matching column for adjusting the vapor pressure of naphtha or gasoline fractions Dissipating gas from absorbed oil in a gas separation unit or gas recovery unit Regeneration tower or stripper for the absorption liquid of harmful gases such as hydrogen sulfide and carbon dioxide Dehydration tower used to remove water from raw material components Solvent from the mixture of extract and solvent in solvent extraction equipment for aromatic hydrocarbons etc. The present invention is particularly preferably applied when the low-boiling component to be separated by distillation is water, hydrogen sulfide, non-hydrocarbon gas such as carbon dioxide gas, or light hydrocarbon gas. In the present invention, natural circulation in the reboiler is promoted by supplying a part of the top liquid and/or side distillate of the distillation column to the liquid inlet and/or outlet of the reboiler. However, it is desirable that the difference between the average boiling point of the top liquid and/or side distillate liquid and the average boiling point of the distillation column bottom liquid is large, preferably 50°C or higher, more preferably 100°C or higher. In addition, when supplying the top liquid and/or side distillate of the distillation column to the reboiler in this way, the properties of the distillation column bottom liquid and the column top liquid or/and side distillate are relatively close, As a result, the flash point that the bottom liquid should have,
vapor pressure, light gas content, content of low-boiling components,
There may be cases where the target property values such as moisture content cannot be achieved. In order to avoid such inconveniences, when applying the present invention, the liquid inlet of the reboiler of the distillation column top liquid or/and side distillate liquid or/and
The amount supplied to the liquid outlet must be kept within a range that does not impair the target properties of the tower bottom liquid, tower top liquid, and side distillate liquid. The appropriate supply amount of the top liquid and/or side distillate liquid depends on the type of distillation column and operating conditions (especially the amount of heat supplied to the reboiler and the amount of reflux of the top liquid), and the types of the top liquid and bottom liquid. Although it depends on factors such as (particularly the boiling point difference and density difference between the two), a small amount of 1/100th of the circulation amount of the bottom liquid in the reboiler, for example, 1/100 to 1/1000, is sufficient for the effect. is recognized. The optimal value of the feed rate can be easily determined experimentally by conducting distillation operations in a specific distillation system by increasing or decreasing the feed rate, while paying attention to the effect on the properties of the bottom liquid as described above. can do. The top liquid and/or side distillate may be supplied to either the liquid inlet or the liquid outlet of the reboiler,
Alternatively, it may be supplied to both of these. Next, the present invention will be explained in more detail with reference to FIG. 1, which shows an example of a specific embodiment of the method of operating a distillation column according to the present invention. FIG. 1 is a process diagram showing a distillation column for adjusting the flash point of product kerosene and gas oil in a kerosene and gas oil hydrodesulfurization apparatus. In FIG. 1, A represents a distillation column, B represents a natural circulation reboiler, C represents a condenser, and D represents a receiving tank. In the kerosene and gas oil hydrodesulfurization equipment, the effluent from the hydrodesulfurization reaction tower is separated into gas and liquid, and the liquid is sent to a hydrogen sulfide stripper, where the hydrogen sulfide-containing gas is separated, and then sent to distillation column A through conduit 1. The water and low boiling point components (mainly light hydrocarbons) contained therein are extracted from the conduit 2 as overhead oil. On the other hand, water and low boiling point components are removed from the bottom of the column, and kerosene whose water content and flash point are adjusted to within standard values is obtained as product kerosene through the conduit 10. A natural circulation reboiler B is provided at the bottom of the distillation column A to supply heat to the bottom liquid, and the bottom liquid extracted from the bottom of the distillation column through the conduit 8 is heated by the high temperature fluid flowing in the tube 11. and is refluxed via conduit 9 to the distillation column. The reboiler B is designed so that the internal circulating liquid flows upward by heating by the tube 11 and circulates naturally. Now, the extracted tower top oil is partially condensed in a condenser C, and then introduced into a receiving tank D, where it is separated into gas and liquid. The separated gas consisting mainly of light hydrocarbons is removed via gas line 4, while the liquid is further separated into water and light hydrocarbons, and the water is drawn out of the system via drain line 5. On the other hand, part of the light hydrocarbons is sent to the distillation column A as a reflux liquid through the reflux line 3, further part is sent to the liquid inlet of the reboiler B via the supply line 7, and the remaining part is sent to the liquid inlet of the reboiler B via the withdrawal line 6. Extracted from the system. According to the present invention, the amount of heat supplied to the reboiler is
Compared to conventional operation methods, the natural circulation rate of the natural circulation reboiler is maintained at a high level, even if the amount is small, and the heat transfer is stabilized, while at the same time the reflux rate of the distillation column overhead liquid can be significantly reduced. It becomes possible. As a result, energy savings can be achieved while the operation of the distillation column is stable. That is, by reducing the amount of heat supplied from the high-temperature fluid to the reboiler according to the present invention, it is possible to save thermal energy in the distillation column system. Since the surplus heat can be used for other equipment, it is possible to save energy in the entire equipment group, which is of great industrial benefit. EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to the following Examples unless the gist of the present invention is exceeded. Example 1 In a kerosene and gas oil hydrodesulfurization unit, the liquid obtained by separating the effluent from the hydrodesulfurization reaction tower into gas and liquid is sent to a hydrogen sulfide stripper, where the hydrogen sulfide-containing gas is separated, and the obtained crude gas oil is Distillation treatment was carried out according to the steps shown in FIG. That is, crude light oil was introduced into distillation column A through conduit 1, water and low-boiling components were distilled out as a top liquid from the top of the column, and product light oil was obtained from the bottom of the column. A portion of the light hydrocarbons separated from the top liquid was supplied to the liquid inlet of reboiler B through the supply line 7. The best condition is when the light hydrocarbon reboiler supply rate is 0.5 m ³ /h.
We were able to stably slow down the distillation column. For example, when the injection rate is set to 0.8 m ³ /h, the average boiling point of the distillate oil at the top of the column rises and it becomes heavier, the separation of the top oil and the bottom oil becomes worse, and the circulation at the reboiler liquid outlet increases. The increase in oil vapor pressure was reduced, and evaporation of the circulating fluid in the reboiler was suppressed, making it difficult to maintain natural circulation. Conversely, the injection volume
When it was reduced to 0.4 m ³ /h or less, the effect of maintaining natural circulation was insufficient.

【table】

[Table] From the above results, it is clear that according to the present invention, the amount of heat supplied to the reboiler and the amount of top liquid reflux can be reduced without impairing the quality properties such as the flash point and water content of the product light oil. However, it is clear that the natural circulation amount of the circulating liquid in the reboiler can be maintained at a high level, and therefore the distillation column can be operated stably.

[Brief explanation of the drawing]

FIG. 1 is a process diagram showing an example of a specific embodiment of the method for operating a distillation column according to the present invention. In the figure, A...distillation column, B...natural circulation reboiler, C...condenser, D...receiving tank.

Claims (1)

[Claims] 1. A distillation column equipped with a natural circulation reboiler, characterized in that a part of the top liquid and/or side distillate is supplied to the liquid inlet and/or liquid outlet of the reboiler. How to operate a distillation column. 2. The method for operating a distillation column according to claim 1, characterized in that the difference between the average boiling point of the column top liquid and/or side distillate liquid and the average boiling point of the column bottom liquid is 50°C or more. how to.