EP0499897B1 - Process control in cracking furnaces used for the preparations of olefines - Google Patents

Abstract

In a process for controlling the transition temperature between the convection zone and radiant zone in cracking furnaces for olefin production from different hydrocarbon feedstocks, the feed stream is divided into two part streams before introduction into the convection zone, the first part stream is first preheated and then mixed with process steam, subsequently mixed with the second part stream untreated up to this point in time, and the resulting total process stream is passed after further heating and/or vaporisation into the radiant zone. The mixing of the first part stream with the process steam can take place before or after internal or external super-heating of the process steam. <IMAGE>

Description

The invention relates to a method for controlling the transition temperature between the convection and the radiation zone in cracking furnaces for olefin production from different hydrocarbon inserts.

In the thermal cracking of hydrocarbons for olefin production, it is necessary to heat the hydrocarbons in the cracking zone, that is to say the radiation zone, to high temperatures of the order of 550 ° C. to 900 ° C. in order to achieve the desired conversions when the hydrocarbons flow briefly through this zone to achieve. In order to achieve this, the hydrocarbons must be preheated to relatively high temperatures before they enter the radiation zone. Since the cleavage is usually carried out in the presence of water vapor as an inert diluent, it is also necessary to preheat the water vapor to the entry temperature of the radiation zone.

The required high temperatures in the cracking zone are usually achieved by passing the feed material through cracked tubes which are arranged in the radiation zone of a burner-heated cracking furnace. The hot flue gases generated during the firing form a large heat reservoir after exiting the radiation zone, which can be used to preheat the use of hydrocarbons and possibly other fluids and for this purpose is passed through a convection zone equipped with heat exchangers.

Under the idealized condition that a precisely defined, unchangeable hydrocarbon insert is processed with the operating conditions always unchanged, the design of the convection as well as the radiation zone could theoretically be determined very precisely with regard to the number of heat exchangers, length of the pipes, residence times etc. However, even when the use is almost constant in terms of time and composition, practice shows that due to the complex and not yet fully researched interdependency of the individual operating parameters, control interventions during operation are unavoidable. Operating practice also shows that in almost all cases the product composition of the hydrocarbon feedstocks fluctuates within wide limits, which gives a corresponding flexibility in the cracking conditions, i.e. especially the temperature profile, seems desirable.

DE-A-15 68 966 teaches what such a regulation can look like. It relates to a control method for the splitting of hydrocarbons, in which the transition temperature between a convection part and a radiation part of a cracking furnace is regulated by either part of the hot flue gases or part of the hydrocarbons to be preheated being circulated around the convection zone.

The known method is aimed at influencing the temperature profile within a can which penetrates a convection zone and a subsequent radiation zone. A greater steepness of the temperature profile in the radiation zone is achieved in that the preheating of the gap insert in the convection zone is reduced, so that the transition temperature from the convection zone to the radiation zone is reduced and preheating is initially continued in the radiation zone. An essential aspect of this method is to be seen in the fact that the regulation provided there works in such a way that the predetermined end temperature T _{E is} reached at the outlet end of the can. However, the cited German published application does not indicate any changes to the procedure to be undertaken in the event of a transition to a completely different gap insert, for example the transition from gasoline fractions to gas oil.

Since there is a constantly growing need for olefins, which can lead to a shortage and / or price increase of certain uses, efforts have been underway for some time to develop processes which allow the optimal utilization of different hydrocarbon uses.

In most cases, a change in the use of hydrocarbons necessitates changes in the process flow. Changes in steam dilution are also required. Such changes can cause large fluctuations in the flow rate through the heat exchangers arranged in the convection zone, which can cause operational problems.

From German patent specification DE-A-28 54 061 a method for preheating hydrocarbons prior to their thermal splitting in the radiation zone of a burner-heated cracking furnace is known, the hydrocarbons and other fluids being heated against flue gas by heat exchangers arranged in the convection zone of the cracking furnace. The design of the cracking furnace for the cracking of different hydrocarbon inserts results from the possibility of passing the hydrocarbons and other fluids through heat exchangers divided into a series of bundles, the flow sequence and / or the fluids flowing through the individual bundles using a switching device depending on the cracking insert can be changed. This is made possible by the fact that the heat exchanger bundles are at least partially connected to one another by lines provided with switching elements.

In order to be able to achieve the best possible yield from the respective hydrocarbon use with the lowest possible energy requirement, it is necessary to regulate the transition temperature, also called cross-over temperature, between the convection and radiation zone of the cracking, pyrolysis or cracking furnace.

The present invention is therefore based on the object of developing a method of the type mentioned above in such a way that it is possible to adapt the transition temperature between the convection and radiation zones in the most varied of hydrocarbon applications in a process-technically simple manner and with an improved economy of the process sequence.

This object is achieved in that the feed stream is divided into two sub-streams before being introduced into the convection zone, the first sub-stream initially preheated, then mixed with process steam, then mixed with the second, up to this point in time untreated partial stream and the total process stream thus formed is led into the radiation zone after further heating and / or evaporation.

One embodiment of the method according to the invention is to overheat the process steam before it is mixed with the first partial stream. This process steam superheating can take place on the one hand in a heat exchanger in the convection zone of the cracking furnace for hot flue gas coming from the radiation zone of the cracking furnace, and on the other hand in any manner outside the cracking furnace. The former embodiment is used above all in the case of liquid hydrocarbon inserts, since only partial evaporation of the first partial stream takes place here, and only when the superheated process steam and the first partial stream are mixed does an almost complete evaporation take place due to the heat released by the superheated process steam onto the hydrocarbons.

A further embodiment of the method according to the invention provides the possibility of initially mixing the process steam and the first partial stream and then feeding the gas mixture thus formed to a heat exchanger where it is heated to such an extent that the gas mixture overheats.

A further embodiment of the method according to the invention discloses the possibility of preheating the first partial stream in one or more stages before mixing with process steam.

Likewise, in a further embodiment of the invention, the heating of the overall process stream formed after the mixing of the two partial streams can take place in one or more steps.

A further embodiment of the method according to the invention consists in preheating the first partial stream and further heating the overall process stream formed after the two partial streams have been mixed by indirect heat exchange with the hot flue gas coming from the radiation zone.

The process according to the invention is applicable to hydrocarbon inserts as diverse as ethane, propane, butane, liquid gas, naphtha, kerosene, atmospheric gas oil, hydrogenated atmospheric gas oil and hydrogenated vacuum gas oil. For this purpose, it must be noted that in addition to the aforementioned, almost all other known hydrocarbon inserts can be used.

The method according to the invention now allows a relatively inexpensive adaptation of the splitting process to a wide variety of hydrocarbon uses. This enables an optimized and thus maximum utilization of the hydrocarbon feedstocks with the lowest possible energy consumption, specifically heating gas for operating the burners in the radiation zone and the resulting quantity and temperature of the flue gas. The constructional and operational expenditure compared to the cited prior art can be significantly reduced.

The method according to the invention thus reduces the investment and operating costs on the basis of the following improvements: Optimization of the utilization at different Hydrocarbon inserts, minimization of the heating media required and constructional simplification of the convection zone of a cracking furnace for olefin production from different hydrocarbon inserts.

Precise control of the transition temperature between the convection and radiation zones also enables the reduction of coke deposits in the heat exchanger through which the entire process gas flow last flowed. The rate of coking of the gas lines essentially depends both on the temperature of the gas mixture to be split and on the cracking severity. This means that the start of the actual fission processes in the last heat exchangers of the convection zone should be avoided. Rapid coking of the gas lines can drastically shorten the life of a cracking furnace.

The regulation of the transition temperature now makes it possible to set a temperature which is chosen for each hydrocarbon used in such a way that coking does not yet take place in the convection zone. The avoidance of operational interruptions due to decoking work in the convection zone leads to an improvement in the economy of the cracking furnace.

The invention will now be explained in more detail with reference to two schematically illustrated exemplary embodiments.

Figur 1:

Ein erfindungsgemäßes Verfahren, wobei der Prozeßdampf zunächst überhitzt und anschließend mit dem ersten Teilstrom vermischt wird.

Figur 2:

Ein erfindungsgemäßes Verfahren, wobei der Prozeßdampf und erste Teilstrom zunächst vermischt und anschließend soweit erwärmt werden, daß es zu einer Überhitzung der Prozeßdampfkomponente des gebildeten Gasgemisches kommt.

Show:

Figure 1:

A method according to the invention, the process steam first being overheated and then mixed with the first partial stream.

Figure 2:

A method according to the invention, wherein the process steam and first partial stream are first mixed and then heated to such an extent that the process steam component of the gas mixture formed overheats.

Figure 1 is the basic representation of the method according to the invention. It shows the convection zone of a cracking furnace for the thermal decomposition of different hydrocarbon inserts.

The hydrocarbon feed stream is led via line 1 upstream of the convection zone and divided into a first partial stream (line 2) and a second partial stream (line 3) in the desired ratio. The first partial flow is then fed via line 2 to a heat exchanger B, heated against cooling flue gas (represented by the dashed arrows) from the radiation zone of the cracking furnace and discharged via line 2a. The process steam required for diluting the hydrocarbon feed is fed to a heat exchanger C via line 4 and overheated in it against cooling flue gas. The superheated process steam is then drawn off via line 4a, mixed with the heated first partial stream from line 2a and brought together via line 2c with the second, previously untreated partial stream in line 3. The total process stream thus formed is then fed via line 5 to a heat exchanger D, where it is further heated or evaporated against cooling flue gas. After bypassing an intermediate heat exchanger E, which will be described below, the total process stream is fed via line 6 to a last heat exchanger F in the convection zone and heated in it against cooling hot flue gas to the desired transition temperature between the convection and radiation zones before being conducted via line 7 is directed into the radiation zone.

In the heat exchanger A, feed water supplied via line 10 is heated and then passed via line 11 into a high-pressure steam drum, not shown here. This feed water serves as a coolant for the quenching of the cracked gases, for which purpose it is fed to a quench cooler. When heat is exchanged with the hot cracked gases, steam is formed, which is collected in the upper area of the quench cooler and returned to the steam chamber of the high-pressure steam drum. From there it passes via line 12 into the heat exchanger E, where it is overheated against high-temperature flue gas. The superheated steam is drawn off via line 13 and can be used, for example, to operate a turbine for energy recovery.

The flue gas has a temperature of 1000 ° C to 1200 ° C at the transition from the radiation to the convection zone, leaves the convection zone shown here at the upper end with a temperature of about 130 ° C and is removed via a chimney, if necessary after cleaning .

Figure 2 shows a further embodiment of the method according to the invention. As already described in FIG. 1, the hydrocarbon feed is divided into a first partial stream (line 2) and a second partial stream (line 3) in the desired ratio. Here too, the first partial stream is fed via line 2 into a heat exchanger B, heated against cooling flue gas and drawn off via line 2a. Process steam required for dilution is introduced via line 4 and mixed with the first, already heated partial flow in line 2a, the gas mixture thus formed is fed via line 2b to a further heat exchanger C and heated in it to such an extent that the process steam component overheats. The gas mixture is then over Line 2c withdrawn and mixed with the second, up to this point in time untreated stream in line 3. The total process stream formed in this way is conducted analogously to FIG. 1 via lines 5, 6 and 7 and heat exchangers D and F into the radiation zone.

Points 21 and 22 each indicate a measuring point for flow rate control in the two partial streams in line 2 and line 3. Points 23 to 28 indicate six temperature measuring points.

Using six different ratios of the two partial flows, Table 1 shows the range in which the transition temperature, which corresponds to the temperature of the measuring point 28, can be varied. Ethane is chosen as the hydrocarbon feed.

The favorable mode of operation of the method according to the invention is shown below using three examples. In the first example, atmospheric gas oil with a partial flow ratio of 100/0 and in examples 2 and 3 ethane with a partial flow ratio of 30/70 or 60/40 are selected as the hydrocarbon feed. The three examples are described with reference to FIG. 1.

Example 1:

Gas oil at a pressure of 5.4 bar and a temperature of 82 ° C. is supplied in liquid form via line 1 and 2 to heat exchanger B. After heating to 357 ° C, half of the gas oil which has already evaporated is drawn off at a pressure of 5.4 bar via line 2a. Heat exchanger C water vapor is supplied via line 4 at a pressure of 5.6 bar and a temperature of 179.degree. After overheating, the water vapor is drawn off at a pressure of 5.4 bar and a temperature of 449 ° C. via line 4a and mixed with the gas oil from line 2a. The mixing results in a complete evaporation of the mixture formed, so that a mixture with a pressure of 5.0 bar and a temperature of 361 ° C. is supplied via line 2c and 5 heat exchangers D. The further heated mixture leaves heat exchanger D at a pressure of 4.9 bar and a temperature of 449 ° C and is then fed via line 6 to the last heat exchanger F of the convection zone. With a pressure of 4.8 bar and a temperature of 540 ° C, which corresponds to the transition temperature between the convection zone and the radiation zone, it is withdrawn from the heat exchanger F via line 7 and led into the radiation zone. In addition, feed water is preheated in heat exchanger A from 110 ° C to 140 ° C and superheated steam in heat exchanger E from 329 ° C to 527 ° C. The hot flue gas has a temperature of 1124 ° C when entering the convection zone and leaves the convection zone after overflowing the seven heat exchangers A to F at a temperature of 122 ° C.

Example 2:

Gaseous ethane with a pressure of 3.9 bar and a temperature of 68 ° C. is introduced via line 1 as the hydrocarbon feed. 30% of the feed are fed via line 2 to heat exchanger B, heated to 646 ° C. in it and discharged via line 2a at a pressure of 3.9 bar. Heat exchanger C, steam at a pressure of 3.9 bar and a temperature of 175 ° C., is fed in via line 4, overheated to 629 ° C., drawn off via line 4a at constant pressure and mixed with the heated ethane from line 2a. The gas mixture formed is brought together via line 2c with the remaining 70% of the hydrocarbon feed which has hitherto been untreated, and the mixture thus formed is fed via line 5 to heat exchanger D at a pressure of 3.5 bar and a temperature of 347 ° C. It is further heated to 496 ° C., whereupon the gas mixture is fed into the heat exchanger F via line 6 at a pressure of 3.4 bar. Here it heats up to a transition temperature of 649 ° C before the mixture reaches the radiation zone via line 7. In addition, feed water is preheated or evaporated in heat exchanger A from 110 ° C to 309 ° C and superheated steam in heat exchanger E from 330 ° C to 504 ° C. The hot flue gas has a temperature of 1182 ° C when entering the convection zone and leaves the convection zone after overflow of the seven heat exchangers A to F at a temperature of 183 ° C.

Example 3:

Gaseous ethane with a pressure of 4.0 bar and a temperature of 68 ° C. is introduced via line 1 as the hydrocarbon feed. 60% of the feed are fed via line 2 to heat exchanger B, heated to 643 ° C. in it and discharged via line 2a at a pressure of 3.9 bar. Heat exchanger C, steam at a pressure of 3.9 bar and a temperature of 175 ° C., is fed in via line 4, overheated to 636 ° C., drawn off via line 4a at constant pressure and mixed with the heated ethane from line 2a. The gas mixture formed is brought together via line 2c with the remaining 40% of the hydrocarbon feed which has hitherto been untreated, and the mixture thus formed is fed via line 5 to heat exchanger D at a pressure of 3.5 bar and a temperature of 482 ° C. It is heated further to 581 ° C., whereupon the gas mixture is fed into the heat exchanger F via line 6 at a pressure of 3.5 bar. Here it is heated to a transition temperature of 709 ° C before the mixture reaches the radiation zone via line 7. In addition, feed water is preheated or evaporated in heat exchanger A from 110 ° C to 256 ° C and superheated steam in heat exchanger E from 329 ° C to 527 ° C. The hot flue gas has a temperature of 1166 ° C when entering the convection zone and leaves the convection zone after overflow of the seven heat exchangers A to F at a temperature of 162 ° C.

Claims (9)

1. A process for regulating the transition temperature between the convection zone and the radiation zone in cracking ovens for the production of olefins from different hydrocarbon charges, characterised in that prior to entry into the convection zone the charge stream is divided into two sub-streams, the first sub-stream is initially preheated, then mixed with process steam, and then mixed with the second sub-stream, which up to this time is untreated, and following further heating and/or evaporation the total process stream thus formed is conveyed into the radiation zone.
2. A process as claimed in Claim 1, characterised in that the process steam is superheated and then mixed with the first sub-stream.
3. A process as claimed in Claim 2, characterised in that the process steam is superheated in a heat exchanger of the convection zone of the cracking oven in the presence of hot flue gas emanating form the radiation zone of the cracking oven.
4. A process as claimed in Claim 2, characterised in that superheated process steam is supplied outside of the cracking oven.
5. A process as claimed in Claim 1, characterised in that the first sub-stream and the process steam are initially mixed and then heated to the extent that superheating of the formed gas mixture occurs.
6. A process as claimed in one of Claims 1 to 5, characterised in that the preheating of the first sub-stream can take place prior to the mixing with the process steam in one stage or several stages.
7. A process as claimed in one of Claims 1 to 6, characterised in that the further heating of the total process stream, formed following the mixing of the two sub-streams, can take place in one stage or several stages.
8. A process as claimed in one of Claims 1 to 7, characterised in that the preheating of the first sub-stream and the further heating of the total process stream, formed following the mixing of the two sub-streams, takes place by indirect heat exchange with the hot flue gas emanating from the radiation zone.
9. A process as claimed in one of Claims 1 to 8, characterised in that ethane, propane, butane, liquid gas, naphtha, kerosine, atmospheric gas oil, hydrogenated atmospheric gas oil and hydrogenated vacuum gas oil can be used as hydrocarbon charges.

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