EP0021167B1 - Process and apparatus for the thermal decoking of an apparatus for the thermal cracking of hydrocarbons such apparatus comprising a cracking zone followed by a cooler for the product gas - Google Patents

Description

The invention relates to a method for thermal decoking of a device for thermal cracking of hydrocarbons, the cracking tubes arranged in a cracking zone and a subsequent cracking gas cooler for cooling the cracked products by indirect heat exchange with a cooling medium, wherein a water stream and oxygen-containing gas flow through the cracking tubes and the Cracked gas cooler passed and the cooling medium is also passed through the cracked gas cooler during decoking. The invention also relates to a device suitable for carrying out the method.

The thermal cracking of hydrocarbons, especially the thermal cracking aimed at the formation of ethylene and other lower olefins, is of great technical importance. When such processes are carried out, however, a number of side reactions occur which lead to economically uninteresting or even disruptive products. Such a disruptive side reaction can be seen in the coking of the can and the gas cooler, since it leads to a deterioration in the heat transfer both in the externally heated gap zone and in the subsequent gas cooler and, in extreme cases, can even lead to the blockage of individual line elements. As coking progresses, it is therefore necessary to heat the can more strongly in order to ensure the heating of the hydrocarbons required for the reaction. This increased energy expenditure is offset by a reduced high-pressure steam production in the cracked gas cooler, in which as large a portion of the cracked gas heat as possible is to be recovered for generating high pressure steam. In addition, the worsened cooling is, of course, unsatisfactory from a process engineering point of view, because rapid cooling and thus an interruption of the cleavage reactions with a view to a desired product yield is sought.

It is therefore necessary to interrupt the thermal splitting from time to time and to decoke the system. This is usually done by passing a mixture of air and water vapor through the canned tubes, which are still heated from the outside, and the canned gas cooler, which is still cooled. At the high temperatures in the can, which can be between 750 and 850 ° C, for example, the deposits burn off. However, cleaning of the cracked gas cooler is not possible in this way, since the temperatures prevailing here no longer burn up. At most, in the entrance area of the cracked gas cooler, in which the entering gases still have the high temperature of the cracking zone, a limited burn-up can occur, which, however, is quickly ended due to the cooling. A common decoking process for the cracked gas cooler therefore consists in cooling the system and then separating the cracked gas cooler from the cracking zone and cleaning it mechanically. This cleaning can be carried out by a water jet which emerges from a nozzle under very high pressure, for example 700-1000 bar, and causes the deposits to spring off. This process, which usually takes about three days, is not only time-consuming, but also leads to a thermal load on the system due to the periodically repeated heating and cooling cycles, which limits the service life of the can.

The invention is therefore based on the object of designing a method of the type mentioned at the outset in such a way that the cost and time required for decoking is reduced.

According to the invention, this object is achieved in that, in a first process stage, the gas stream is passed through the device in such an amount that the temperature of the deposits on the heat-exchanging surfaces of the cracked gas cooler is in the range of the operating temperature prevailing during thermal cracking, that the first process stage as long as it continues until the can is largely decoked and that the gas flow is then increased in a second process stage to such an extent that the temperature of the deposits on the heat-exchanging surfaces of the cracked gas cooler increases to such an extent that the temperature of the gas stream at the outlet from the cracked gas cooler is at least 400 ° C.

According to the invention, a decoking process is thus proposed in which it is no longer necessary to separate the cracked gas cooler from the cracking zone and to cool the plant.

A cracked gas cooler has already become known, which is also decoked by thermal means (Bulletin of the Japan Petroleum Institute, Vol. 13, No. 2, November 1971, pages 279 to 284), but deviates in essential points from the method according to the invention becomes. In this known cracked gas cooler, the cracked gases are cooled in tubes arranged in a spiral. The decoking process is practically the same as that in a cracking zone, because the cooling water is removed from the cracking gas cooler during the decoking phase, causing the cooling pipes to heat up to over 700 ° C and the contaminants burn off.

A major disadvantage of this known cracked gas cooler is, however, that the temperature of the tubes in the cracked gas cooler is subject to large fluctuations. This is particularly important because the pipes are arranged in a high-pressure container, which has an operating pressure of the order of 100 bar, for example. If such a container is heated to over 700 ° C at regular intervals compared to the operating temperature of the order of 300 ° C, special measures must be taken for the operational safety of such a cracked gas cooler. In addition, this well-known fission gas cooler deviates from the most commonly used type. In contrast to conventional cracked gas coolers, it does not use straight heat exchange tubes, but rather spirally arranged coils. A transfer of this known concept to straight tube constructions would not be possible because of the thermal expansion or shrinkage when the temperature rises or when cooling back to operating temperature. Straight pipe constructions are preferred, among other things, because a corrosion-inhibiting magnetite layer forms on the outside of the pipes during operation, which is retained in the event of larger temperature fluctuations, while pipe coils cause this layer to flake off and thus increase susceptibility to corrosion.

Furthermore, from FR-PS 153'2127 a method is known in which the thermal decoking takes place by the action of water vapor and air on the inner surfaces of the tubes both in the cracking zone and in the cracking gas cooler. It is stated that the heat exchanger for cracked gas cooling can be left in operation during decoking, but if this is actually done, this has two consequences for decoking: either there is insufficient decoking of the cracked gas cooler, as effects occur as they do have already been mentioned at the beginning, or the decoking gas must be passed through the entire system with such a high throughput that damage to the system occurs after a relatively short time. FR-PS 1 532127, on the other hand, does not provide any references to a two-stage process, as is provided according to the invention. Finally, a method is known from US Pat. No. 3,365,387 _' which describes the decoking of individual, parallel-connected canned tubes in a split zone. However, references to a two-stage decoking cannot be found in this document either.

According to the invention, on the other hand, a two-stage process is proposed in which continuous steam production is made possible and in which there is only a comparatively small change in the temperature of the tubes in the cracked gas cooler. It is therefore also possible to carry out this process in conventional cracked gas coolers with straight tubes.

In the first stage of the process according to the invention, a gas mixture containing water vapor and oxygen, usually a mixture of water vapor and air, is passed through the cracking plant, the deposits in the cracking tubes being burnt off in the manner customary hitherto. During this gentle decoking phase, most of the coke is removed from the cracking tubes, while the cracking gas cooler is only cleaned slightly, since the temperature here is too low to burn up.

After completion of this first process stage, which can last for a few hours, for example 4-8 hours, the second process stage follows, in which a substantially larger amount of the gas mixture is passed through the plant. In this stage of the process, the canned tubes are further cleaned and, moreover, the coke in the cracked gas cooler is largely broken down. This is due to the fact that the gas mixture is passed through the cracked gas cooler in such an amount that the temperature of the coke deposits on the inner wall of the tubes increases to such an extent that a noticeable water gas reaction sets in. This temperature increase is possible despite the cooling of the pipes because the thermal conductivity of the coke layer is very low. However, there is no erosion, as occurs in the gap tubes of the gap zone, since if the gap gas cooler is continuously cooled, a temperature increase to the values required for this, which are above 600 ° C., cannot be achieved. In contrast, the decoking process is probably favored by flaking of deposits due to the increased mass throughput.

The two-step procedure is necessary with regard to the service life of the can. If, at the beginning of the decoking process, the gas flow was selected to be so strong that the water gas reaction starts in the cracked gas cooler, there would be a risk that coke parts flaking off in the cracking zone would have an erosive effect on the cracked tubes and damage them.

It is important for the implementation of the process according to the invention that a gas mixture containing water vapor and oxygen is also used in the second process stage, although actually only water vapor is required for the water gas reaction. However, the presence of oxygen is advantageous for the rate at which the coke is broken down in the cracked gas cooler. This is related to the fact that the water gas reaction is catalyzed by trace constituents from the pipe materials, in particular chromium and nickel, which are contained in the coke by diffusion from the pipe materials. However, this catalytic effect only occurs when the sulfur components that are always contained in the coke have been broken down. The presence of oxygen in the gas stream now leads to the sulfur traces predominantly being converted into SO ₂ delt so that they can no longer act as a catalyst poison.

When carrying out the process according to the invention, it has proven advantageous to increase the gas flow in the second process stage to such an extent that the temperature of the decoking gases at the outlet from the cracked gas cooler is at least 400 ° C. It has been shown that the rate of coke removal in the cracked gas cooler is too low at lower temperatures to ensure an effective decoking treatment. If the amount of decoking gas is kept constant during the second stage of the process, it is expedient to choose the outlet temperature at the beginning of the second stage of the process considerably above the minimum temperature of about 400 ° C, since the outlet temperature decreases with decoking and the minimum temperature should not be fallen below.

During the decoking of the cracked gas cooler, the deposition layer in the tubes is continuously thinning, which improves the heat exchange with the coolant, so that the outlet temperature drops as the decoking progresses. A termination of the decoking process can therefore be easily determined by checking the outlet temperature, since in this case it remains practically constant. The method according to the invention is illustrated below using an example. The high-severity cracking of a heavy atmospheric gas oil resulted in a cracked gas cooler outlet temperature of 634 ° C after 60 days of operation at an oven outlet temperature of 800 ° C, which indicated strong coking. In a first phase of decoking, a steam-air mixture with a mass velocity of 25 kg / sm ^{2 was used} in the cracked gas cooler for 8 hours, the furnace outlet temperature being 750 ° C. The mass velocity in the cracked gas cooler was then increased to 45 kg / sm ² and the furnace outlet temperature to 800 ° C. After a two-hour induction period in which the coke breakdown rate was slow, there was a marked increase in the coke breakdown rate. (The rate of coke extraction is the lowering of the cracked gas cooler outlet temperature during decoking under completely constant conditions.) The coke extraction rate during the induction period was 2 K / h, while it subsequently reached a maximum value of 15 K / h. This second phase of decoking was ended after 16 hours.

After a total decoking time of 24 hours, the exit temperature of the decoking gases from the cracked gas cooler was about 400 ° C. The decoking process was ended here.

After restarting the cracking furnace with heavy gas oil, the cracking gas cooler outlet temperature reached about 470 ° C. This means that the cracked gas cooler has been almost completely cleaned. In the subsequent runtime, 60 days could also be reached again, which indicates that the coking rate of the cracked gas cooler cleaned according to the invention is not greater than that of a mechanically cleaned cracked gas cooler.

The method according to the invention in its form described so far has proven to be advantageous in decoking a plant for splitting heavy hydrocarbons such as gas oil or vacuum gas oil. However, it has been shown that effective decoking for cracked gas coolers in a plant for splitting lighter uses, such as naphtha or ethane, cannot be achieved in this way. This is primarily due to the fact that the cracked gas coolers when splitting such inserts are designed so that the cracked gas temperature is below the temperature at which a noticeable water gas reaction still occurs. This increased cooling, which can be achieved, for example, by using longer cooling tubes, is possible when lighter hydrocarbons are split, since these contain fewer constituents in the cracked gas that tend to precocculate. For example, while cooling a cracked gas obtained from gas oil leads to rapid coking of the cracked gas cooler when cooling down to about 470 ° C, cooling down to temperatures of about 350 to 370 ° C can be carried out with a cracked gas obtained from naphtha, without any more Fear of coking tendencies. The gas emerging from the cracked gas cooler is then usually further cooled by direct heat exchange with a quench oil.

In order to enable the decoking process according to the invention to be extended to cracked gas coolers with a low outlet temperature, such as are used in the cracking of hydrocarbons boiling below approximately 200 ° C., it is proposed in a further embodiment of the invention that the cracked gas cooler is decoked step by step each step the gas flow is only passed through part of the heat-exchanging surface. It is essential here that the heat-exchanging surface is reduced during the decoking of the cracked gas cooler, in order in this way to achieve a temperature increase in the sections through which flow occurs. When using a cracked gas cooler with a shell-and-tube heat exchanger, this can be done, for example, by passing the entire stream of decoking gas through only part of the cooling tubes while other tubes are being shut down. The heat supply to the individual sections of the coked cracked gas cooler can hereby be increased to such an extent that the temperatures required for a sufficiently strong water gas reaction are reached. In contrast, this tempe is due to the exclusive increase in mass throughput temperature increase is not possible because the gas flow is then no longer heated to the required high temperature when it passes through the gap zone. The complete decoking of the cracked gas cooler takes place in this embodiment of the invention in that after decoking a first part of the heat-exchanging surface, the latter is shut off and the gas flow is then passed through a further part in which the process is repeated. This process is continued until the entire cracked gas cooler is decoked.

It has proven to be advantageous to decoke the cracked gas cooler in two steps. It has been shown that by halving the heat-exchanging surface of the cracked gas cooler, the temperature required for a sufficient water gas reaction is already reached. Limiting to as few decoking steps as possible is of course desirable in order to keep the decoking time as short as possible, but it must be taken into account that the temperature for a sufficiently strong water gas reaction is reached in each individual step.

It is also expedient to only gradually decoke the cracked gas cooler during the second process stage, ie. H. with increased mass flow rate of the decoking gas. In some cases, however, it may also be expedient to pass the gas flow through a small area of the cracked gas cooler during the first stage of the process, in which the cracking tubes are decoked. The process according to the invention can be carried out in the cracking of heavy hydrocarbons using the cracked gas coolers usually used for this purpose. In contrast, when hydrocarbons boiling below 200 ° C are split, a modified cracked gas cooler is used, which, in addition to the usual features such as a gas inlet hood, a gas outlet hood, and cooling pipes surrounded by a coolant, also has shut-off devices that allow part of the cooling pipes to be shut down becomes. It has proven to be advantageous to arrange the shut-off elements in the area of the gas outlet hood of the cracked gas cooler. Since the shut-off elements are arranged in the colder part of the cracked gas cooler in this way, a structurally simpler design is possible. While shut-off devices arranged in the area of the gas inlet hood must remain functional at temperatures of, for example, 850 ° C., it is sufficient to provide valves in the area of the outlet hood that are functional at temperatures of up to, for example, 550 ° C. A particularly simple way of dividing the heat-exchanging surface of the cracked gas cooler has been found to be to split the gas outlet hood into several separate areas. Each area is connected to a number of cooling pipes and each has a gas outlet that can be shut off. The only structural change compared to conventional cracked gas coolers is the subdivision of the gas outlet hood and can therefore be carried out at low cost even with existing systems.

Claims (9)

1. A process for the thermal decoking of apparatus for the thermal cracking of hydrocarbons, which apparatus comprises cracking tubes arranged in a cracking zone, and a subsequent cracking gas cooler for cooling the cracking products by indirect heat exchange with a cooling medium, in which process a gas stream containing steam and oxygen is passed through the cracking tubes and the cracking gas cooler, and the cooling medium is also passed through the cracking gas cooler during the decoking, characterised in that, in a first process step, the gas stream is led through the apparatus in an amount such that the temperature of the deposits on the heat exchange surfaces of the cracking gas cooler lies in the region of the operating temperature which prevails during the thermal cracking; that the first process stage is continued until the cracking tubes are substantially decoked; and that in a second process stage, the gas stream is thereafter increased to an extent such that the temperature of the deposits on the heat exchange surfaces of the cracking gas cooler increases to an extent such that the temperature of the gas stream at the outlet from the cracking gas cooler is at least 400°C.

2. A process as claimed in claim 1, characterised in that, in the second process stage, a gas stream of constant amount is passed through the apparatus, and that this second process stage is terminated when the outlet temperature of the gas stream from the cracking gas cooler has attained an approximately constant value.

3. A process as claimed in claim 1 or claim 2, characterised in that the cracking gas cooler is decoked in stepwise mariner, in each step, the gas stream being led only over a part of the heat exchange surface.

4. A process as claimed in claim 3, characterised in that the cracking gas cooler in decoked in two steps.

5. A process as claimed in claim 3 or claim 4, characterised in that the stepwise decoking of the cracking gas cooler takes place only during the second process stage.

6. A process as claimed in one of claims 3 to 5, characterised in that, in the second process stage, a gas stream of constant amount is passed through the apparatus, and that one step of the second process stage is terminated when the outlet temperature of the gas stream from the cracking gas cooler has attained an approximately constant value.

7. Apparatus for carrying out the process claimed in one of claims 3 to 6, characterised by a cracking gas cooler having a gas inlet cap, a gas outlet cap, cooling tubes running between these, and shut-off devices for shutting off a part of the cooling tubes.

8. Apparatus as claimed in claim 7, characterised in that the shut-off devices are arranged in the region of the gas outlet cap of the cracking gas cooler.

9. Apparatus as claimed in claim 8, characterised in that the gas outlet cap is divided into a plurality of separate regions each of which is connected to a number of cooling tubes; and that each region is provided with a shut-off device.