FI65275C - FOERFARANDE FOER TERMISK KRACKNING AV KOLVAETEOLJA - Google Patents

Description

65275

Method for the thermal cracking of hydrocarbon oils Förfarande för termisk krackning av kolväteolja

The invention relates to a process for the thermal cracking of hydrocarbons, in which process the hydrocarbon feed is heated to the cracking temperature and passed to a separate reaction zone where the cracking is allowed to continue as the flow occurs from the bottom upwards.

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In the thermal cracking of hydrocarbon oils, heavy oil fractions are cracked into lighter fractions, thus increasing the yield of the latter. In cracking, the feed oil is heated in the heating tubes of the cracking furnace to the cracking temperature. There are usually two alternative methods. In the second, the cracking takes place in the heating pipes of the cracking furnace and partly in the pipelines leading to the process steps following the cracking. In this cracking method, the residence times el are not known exactly, but they are relatively short, i.e. in the order of minutes. The pressure varies greatly from the furnace inlet to the furnace outlet. In the second cracking process, the hydrocarbon feed is first heated in a cracking furnace to a suitable reaction temperature and the actual cracking reaction takes place in a separate reaction zone with a significantly longer residence time than in the previous process, i.e. of the order of 10-30 minutes. No heat is introduced into the reaction zone.

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In the latter molding process 8a, the reaction zone is usually formed by a vertical cylindrical pressure vessel, at one end of which an oil feed heated in a cracking furnace is introduced and at the other end a mixture of liquid and gas is removed for further processing steps, e.g. distillation. 25 The flow direction in the reaction zone has been either top-down or bottom-up.

In thermal cracking of hydrocarbon oils, there are mainly two types of reactions. The second is the cracking reaction itself, in which long-chain molecules are broken down into smaller molecules, causing a decrease in viscosity. Another type of reaction is polycondensation, in which the molecules coalesce to form pitch and coke as hydrogen is released. The latter reaction is undesirable because it causes an increase in the amount of 2,65275 asphaltenes. As condensation reactions become relevant at higher temperatures, lower reaction temperatures and correspondingly longer residence times are sought.

5 The residence time is very important for thermal cracking. Cracking does not have time to occur if the residence time is too short. If the residence time is too long, the cracking products will start to react further, forming undesired reaction products. The result is an unstable product that causes difficulties in further fuel use. The aim is thus to ensure the most even cracking possible. If the flows in the pressure vessel acting as the reaction zone are uneven, varying residence times are created.

The cracking reaction produces light components which evaporate at the temperature and pressure of the reaction zone. For this reason, the density of the liquid / gas mixture decreases as the mixture flows upward in the pressure vessel. Due to the hydrostatic pressure difference in the pressure vessel, the density of the gas portion also decreases as the mixture flows upwards. The density of the liquid fractions generated in the cracking reactor is lower than the feed density, which also lowers the density of the liquid / gas-20 mixture. For this reason, the flow rate in a commonly used uniformly thick cylindrical reactor is not constant, but accelerates as the mixture flows upward.

The thermal cracking process disclosed in U.S. Patent 4,247,387 has a cylindrical vertical reaction zone pressure vessel in which torn intermediate bases are installed to prevent backflows inside the reactor, forming a plurality of mixing points in the reactor. The aim is to achieve as uniform a residence time as possible for the fraction fed into the zone. There are disadvantages to using spacers. As a result of the fault operations of the reactor 30, it may happen that the whole reactor is coking. The midsoles make coke removal and reactor cleaning cumbersome and expensive to perform.

It is an object of the invention to provide an improvement over the currently known methods. It is a more detailed object of the invention to provide a method in which it is possible to achieve a uniform residence time without torn intermediate bases or other similar devices to be placed in the reaction vessel.

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The objects of the invention are achieved by a method which is mainly characterized in that the reaction zone is formed from a pressure vessel whose cross-sectional area increases from the bottom upwards.

Other features of the method according to the invention are set out in claims 2-8.

By using a bottom-up cross-sectional area in the reaction zone according to the invention, the generation of large velocity gradients is reduced and a plug-like flow model is promoted which is optimal for the cracking result. Although the density of the liquid / gas mixture flowing from the bottom upwards decreases during the reaction, the conical, upwardly expanding shape of the reaction zone acts on the contrary, tending to slow down the flow in the reaction zone.

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The angle of the walls of the reaction zone with respect to the central axis is preferably dimensioned so that the velocity of the liquid / gas mixture remains approximately unchanged under normal conditions. This is usually achieved when said angle is between 2 and 15 degrees. At angles greater than this, harmful backflows begin to occur and, correspondingly, at an angle of less than 2 degrees, its effect begins to be insignificant.

Since flow rate gradients are easily generated at the inlet of the reaction zone, it is advantageous according to the invention also to make the inlet part of the reaction zone conical. The cone angle is selected based on the flow rate in the supply pipe. The higher the speed in the supply pipe, the lower the angle selected. In practice, suitable angles range from 2 to 30 degrees.

The outlet from the reaction zone can be formed in the shape of a cone. In this case, the angle is selected on the basis of the flow rate normally prevailing in the outlet pipe. The higher the output speed, the smaller the angle selected. In practice, suitable angles are between 2 and 30 degrees. The outlet part may also be rounded according to the elliptical or spherical surface 35 or it may be provided with flow guides or similar means to prevent backflows in this area.

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From the point of view of the cracking reaction, it has been found that a suitable temperature is between 410-470 degrees and a pressure of 2-20 bar. The ratio of the average diameter to the length of the reaction zone preferably ranges from 1: 1 to 1:20.

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The invention will be explained in detail with reference to a preferred embodiment of the invention shown in the figures of the accompanying drawing, to which, however, the invention is not intended to be exclusively limited.

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Figure 1 shows a schematic process diagram of a preferred embodiment of the method according to the invention.

Figure 2 shows the reaction vessel shown in Figure 1 on a larger scale.

In Fig. 1, the feed oil is led through a pipe 11 to a furnace 12, where its temperature is raised to between 410 and 470 degrees. From the furnace 12, the oil is led via a pipe 13 to a reactor 14, where it flows from the bottom upwards and exits the top of the reactor via a pipe 15 to a separate unit (not shown) where, for example, gas, petrol, light and heavy fuel oil can be separated. The average residence time in the reaction zone is between 5 and 100 minutes.

Fig. 2 shows the reaction vessel 14 of Fig. 1 with an inlet section 16, an actual reaction zone 17 and a conical discharge section 18. In the respective sections, the angle between the walls and the central axes is denoted by the letters a, 8 and γ. In the reaction zone according to Fig. 2, the angle α is larger than the angle 8 · It is quite possible that the angles α and 8 are chosen to be equal, whereby a separate inlet part 16 is not detectable. It is likewise possible and even desirable for any points of change in the cone angles to be rounded so that no sharp turning points occur.

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Claims (8)

65275 Pa tent t ivat suction door t A process for the thermal cracking of hydrocarbons, wherein the hydrocarbon feed is heated to a cracking temperature and passed to a separate reaction zone where the cracking is allowed to continue with a bottom-up flow, characterized in that the reaction zone (17) is formed by a pressure vessel (14) up. Process according to Claim 1, characterized in that the walls and the central axis of the reaction zone (17) form an angle 10 (g) of 2 to 15 degrees. Process according to Claim 1 or 2, characterized in that the rate of advance of the hydrocarbons in the reaction zone (17) is kept substantially constant. 15 Method according to one of the preceding claims, characterized in that a conical inlet part is used as the inlet part (16) of the reaction zone (17). Method according to Claim 4, characterized in that the angle (α) between the wall of the inlet part (16) and the central axis is 2 to 30 degrees. Process according to one of the preceding claims, characterized in that the temperature in the reaction zone (17) is between 410 and 470 degrees, the pressure between 2 and 20 bar and the average residence time between 5 and 100 min. Method according to one of the preceding claims, characterized in that a cone is used as the outlet part (18) of the reaction zone (17), in which the angle (γ) between the wall and the central axis is between 2 and 30 degrees. Process according to one of the preceding claims, characterized in that a ratio of 1: 1 to 1:20 is used as the average diameter to height ratio of the reaction zone (17). 65275