WO1991004308A1

WO1991004308A1 - A method for the thermal cracking of hydrocarbon oils and other liquid/gas reactions

Info

Publication number: WO1991004308A1
Application number: PCT/FI1990/000211
Authority: WO
Inventors: Antero Ollila
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-09-13
Filing date: 1990-09-11
Publication date: 1991-04-04
Anticipated expiration: 1992-03-13
Also published as: FI85598C; FI894313A7; AU6298790A; FI894313A0; FI85598B

Abstract

A method for the thermal cracking of hydrocarbons and other liquid/vapour processes, in which method the liquid feed is heated, and led to a reaction vessel (14) that is in an essentially horizontal position, in which, due to the large flow cross-sectional area, the flow velocity decreases and creates a delay to achieve the desired additional reactions, and in which method vapour is removed from the beginning of the reaction vessel (14). The intention of the invention is to create a more effective cracking process. In the system in accordance with the invention a vapour phase layer (20) is arranged on top of the liquid phase layer (19) over the greatest length of the reaction vessel (14), and that the liquid level is kept essentially constant by removing the vapour as required. The invention also concerns a corresponding soaking-cracker.

Description

A METHOD FOR THE THERMAL CRACKING OF HYDROCARBON OILS AND OTHER LIQUID/GAS REACTIONS

The object of the invention is a method for the thermal cracking of hydrocarbons and other liquid/gas processes, in which method the liquid feed is heated to the cracking temperature and is led to a soaking-cracker in an essentially horizontal position, in which, due to the large cross-sectional area of the flow, the velocity of the flow decreases and a delay is created that achieves the desired additional reactions, and in which method the gas is removed from the initial section of the reaction vessel. The invention is also concerned with a soaking-cracker to implement the method. The method is also suitable for other liquid/gas processes, in which the presence of a vapour phase is not necessary in terms of the reaction technique, but which appears in the conditions of the reaction vessel. The intention of the thermal cracking of hydrocarbons is to split the heavy oil fractions to lighten them, and to increase the amount of the more valuable light fractions. Two principal methods are in use. In the simpler method, the oil feed is led through a cracking oven to a still. In the oven, the temperature of the oil rises to 450 - 480°C, when part of the heavy fractions split into smaller ones. The retention time at the process temperature is 1 - 4 minutes.

In the other method, a lower temperature (410 -450ºC) is used with a longer delay or retention time. This takes place by feeding the hot oil after the oven to a reaction vessel, in which the retention time is 10 - 30 minutes.

The reaction vessel of the latter method is formed by a normal cylindrical pressure vessel, to one end of which the feed is led from the cracking oven, and from the other end the liquid/vapour mixture is removed to the following stage of the process. The direction of flow in the reaction vessel has been either from upper to lower, or from lower to upper, which is more common nowadays. Two main type of reaction take place in the thermal cracking of hydrocarbons. One is the desired cracking reaction, in which the long-chain hydrocarbon molecules split into smaller ones, causing a reduction in viscosity. The other type of reaction is undesirable polycondensation, in which the molecules combine to form pitch, coke, and hydrogen as well as causing the increase of asphaltenes, which reduce the quality of the cracked heavy fuel oil. The condensation reactions increase as the temperature rises, so that attempts have been made to use lower reaction temperatures and correspondingly longer retention times. This is the essential advantage compared with simple oven cracking/ Other advantages of the method are reduced energy consumption and the slower blocking of the pipe of the cracking oven with coke, which makes longer periods of operation possible.

The retention time in the reaction vessel is, from the point of view thermal cracking, an important quantity. The desired reactions are not able to take place, if the retention time is too short. If the retention time is too long, the undesirable reactions form detrimental compounds, which cause difficulties is the later use of heavy fuel oil. The aim is therefore the most controlled and even retention time possible, because a homogenous final product is then obtained The main problem with the latter reaction vessel method lies in the presence of the vapour phase. Light components arise in the cracking reactions, which are in a vapourous form in the reaction vessel. In one investigation, it has been shown by both retention time and density measurements that the volume of vapour phase in the reaction vessel is about 60 per cent (Lauri Rantalainen, 'Lämpökrakkausyksikkö 2:n virtausdynamiikan kartoitus radioaktiivisella merkkiaineella', [The charting of the secondary flow dynamics of a thermal cracking unit using a radioactive indicator], 1979, TKK Espoo). The proportion of light fractions in the amount of the final products is generally in the region of 3 - 7 per cent by weight, which leads to a similar estimate of the proportion of vapour phase. A high proportion of vapour phase cause two problems. Firstly, the effective volume of the reaction vessel is less than half the theoretical value, so that the average retention time of the liquid phase is correspondingly half the theoretical value. Secondly, the velocity of the vapour phase is greater than that of the liquid phase. This causes a strong mixing phenomenon. Due to this, the retention time of the liquid phase is not, in accordance with the plug flow nearly even, instead the retention time distribution is quite wide. This is due to the fact that part of the liquid travels more rapidly through the reaction vessel along with the moving gas bubbles, and part of the liquid mixes and stays for a longer time in the reaction vessel. This has certain effects, from the point of view of both desirable and especially undesirable reactions. Because the actual retention time is less than half the theoretical value, the process temperature must be correspondingly raised to achieve the same conversion value. Among other things, this weakens the thermal efficiency of the process and accelerates coking.

By adjusting the reaction vessel pressure, the amount of the vapour phase can be affected to some degree, but this in no way removes the problem. An attempt has been made to solve the problems caused by the vapour phase to the retention time of the liquid phase by various arrangements in the reaction vessel, in which the flow takes place from the foot upwards. In US Patent 4.247.387, a perforated false bottoms, which form numerous mixing points in the reactor, is placed in the reaction vessel, and thus an even retention time is intended to be achieved. This solution does not eliminate the loss of reaction volume caused by the vapour phase. It causes maintenance work in the cleaning of the false bottoms, and a certain risk of the entire reactor coking-up in the event of a mistake in operation.

In Finnish Patent 65275, the aim has been to create the same flow velocity for the vapour phase and the liquid phase by taking into consideration the increase in the bubble size caused by a reduction in pressure and giving the reaction vessel a form that opens conically upwards. The flow velocity of the vapour phase is, however, greater than that of the liquid phase due to the nature of the mixing phenomenon, so that the situation is not decisively improved.

In Finnish Patent 65274, three different methods for evening-out the retention time are presented. One form of application is to locate a spiral coil that fills the reaction vessel in the reactor. In the two other methods, the intention is to create a tangential revolving method in the liquid/vapour mixture by means of nozzles, or of a feed that is led tangentially to the reactor. The vapour phase tries to separate from the liquid phase and causes a mixing phenomenon throughout the entire reaction vessel, despite these arrangements.

None of the known solutions described above tries to separate the vapour phase from the liquid phase.

Finnish Patent Application 843423 describes three different reactor solutions, in all of which the main goal is to separate the vapour phase from the liquid phase. This is achieved by means of flow guides located inside the reaction vessel. Thus dead areas are formed, the gas collecting inside them being led out through several gas connections. The weakness of all of the solutions is that the even flow, which would be important precisely from the point of view of this process, is intentionally disturbed. It is also necessary to locate complicated constructions, which increase the risk of coking-up and maintenance expenses, inside the reactor. Numerous gas removal connections demand the construction of as many surface controls, which increases costs. The aim of the invention is to create an improvement in the present known methods. The characteristic features of the method in accordance with the invention are described in the accompanying Patent Claim 1, and the characteristic features of a soaking-cracker in accordance with the invention in Patent Claim 5. The method in accordance with the invention makes it possible to rapidly separate the vapour phase from the liquid phase, while disturbing the even flow inside the reactor as little as possible. Thus a great effective volume and a longer average retention time for the liquid flow are achieved, as well as a more even retention time distribution due to a smaller mixing. Also no extra constructions need be placed inside the reactor vessel.

Other special features and advantages of the invention are described in greater detail in. connection with the form of application described later. The basic principles of the invention, and a soaking-cracker in accordance with the invention are illustrated with the aid of the following figures.

Figure 1 and 2 show a cracking method in accordance with the state of the art

Figure 3 shows the results of a retention time measurement Figure 4 shows the velocity of a gas bubble in a horizontal vapour/liquid phase flow

Figure 5 shows the method in accordance with the invention as a schematic process diagram of one advantageous form of application

Figures 6a and 6b show the reaction vessel shown in Figure 5 in greater detail

In the simpler known method, Figure 1, cracking takes place by leading the oil feed along pipe 1 to the pipework 3 of the cracking oven 2, in which most of the cracking takes place, and the reaction continues in the following pipework 4, along which the hydrocarbons are led to the next stage 5 of the process. This is normally distillation, in which the products are separated for example as light and heavy petrol fractions 6, as light fuel oil 7, and as heavy fuel oil 8. The retention times are, depending on the process temperature, in the region of 1 - 4 minutes, and the temperature in the region of 450 - 480°C. In the second, more developed, cracking method in accordance with the state of the art, Figure 2, the oil feed in led along pipe 1 to the pipework 3 of the cracking oven 2, in which its temperature is raised to the cracking temperature (410 - 450°C) and it is then led along pipe run 4 to the reaction vessel 9, in which the retention time is in the order of 10 - 30 minutes. The hydrocarbons can be led from the reaction vessel to a distillation process similar to that in Figure 1. Heat is not led to the reaction vessel, and because the reactions are principally endothermic, the temperature drops by 10 - 20 degrees. At a lower temperature the process equipment becomes dirty more slowly, so that periods of operation are longer.

The reaction vessel may also be set at a slight slant. Most advantageously the vapour phase layer extends for the entire length of the reaction vessel.

Figure 3 shows the results of the previously-mentioned indicator experiment in a vertical reaction chamber. Curve 11 shows the theoretical retention time distribution and duration. Curve 10 shows that in practice the average retention time is about 40 per cent of the theoretical value and that it is quite wide.

By using a horizontal reaction vessel in accordance with the invention, it is possible to separate the vapour phase and liquid phase from one another and thus essentially prevent the mixing phenomenon in the entire length of the reactor. When the hydrocarbon feed is led to the reaction vessel, it already contains vapour phase due to the reactions in the cracking oven. On account of their lightness, the vapour bubbles tend to rise immediately to the upper section of the reaction vessel, from which they can be led away in a controlled manner, by means of surface measurement and adjustment As has been shown previously on the basis of research results, the flow velocity of the vapour phase is greater than that of the liquid phase. Figure 4 demonstrates diagrammatically how the horizontal component 24 of the velocity of the liquid flow, and the vertical component 25 of the velocity of a vapour bubble lead to the magnitude and direction of the final velocity vector 26. This being the case, even in the most unfavourable case, if a vapour bubble separates from the liquid phase at the foot of the reaction vessel, it rises to the vapour space of the reaction vessel along a horizontal distance that is less than the diameter of the reaction vessel. The distance, along which vapour bubbles disturb the liquid flow, is clearly less than the distance that is possible in vertically placed reactors. From the point of view of the dimensioning of the reaction vessel, it is advantageous to use a large length/diameter ratio, the optimal values of which would be in the order of 15:1 - 25:1.

The control of the flow to and from the reaction vessel is most advantageously arranged through either conically or elliptically shaped end funnels. An additional advantage, which is achieved by separating the vapour phase, is normally achieved in the distillation that forms the next stage of the process. This potential problem of distillation is foaming, which is mainly due to the fact that the feed contains 50 - 60 per cent vapour. In the distillation column the vapour phase tends to rise rapidly to the upper part of the column, pulling with it heavy fractions, and thus seriously disturbing the operation of the distillation column.

When the vapour phase has been mainly separated already in the reaction vessel, it can be led to an optimal point at a level above the liquid feed to the distillation column, and thus substantially reduce the foaming problem. From the point of view of the cracking reaction, it has been demonstrated that at an industrial scale a suitable temperature is between 410 - 470°C and a suitable temperature 2 - 20 bar. The presence of vapour phase in the reaction conditions after the cracking oven it not necessary from the point of view of desirable reactions, but rather creates only problems.

In Figure 5 the oil feed is led through pipe 1 to the pipework 3 of the cracking oven 2, in which its temperature is raised to between 420 - 470ºC. From oven 2 the oil is led through pipe 4 to the reaction vessel 14, which is principally horizontal, but which can also be set at a slant, with the extreme positions varying between -5° - +5º. The liquid phase is led from the reaction vessel along pipe 23 and the vapour phase along pipe 22. In the next stage of the process it is, for example, possible to separate in the distillation column 5 vapour+petrol 6, light fuel oil, and heavy fuel oil 8. The average retention time in the reaction zone is 5 - 100 minutes, depending on dimehsioning.

Figure 6a shows a reaction vessel 14 in a horizontal position without thermal insulation, in which vessel there is a conical entry section 12 and a reaction zone proper, in which it is possible to separate the liquid phase 19 from the vapour phase 20. The liquid phase is removed through the conical exit section 15, and the vapour phase through the conical or tube-like exit section 16. The location of the vapour phase exit section is not critical in a horizontal construction, but the optimal location is in the beginning of the reaction vessel, in which the greatest part of the vapour separates from the liquid phase.

Operationally certain surface measurement under reaction vessel conditions can be constructed using radioactive measurement. By exploiting the volume of the reaction vessel the construction of separate structures inside the reaction vessel can be avoided. Figure 6b shows a cross-section of the reaction vessel 4 at the point of the exit section 16.

Claims

Patent Claims

1. A method for the thermal cracking of hydrocarbons and other liquid/vapour processes, in which method the liquid feed is heated, and led to a reaction vessel (14) that is in an essentially horizontal position, in which, due to the large flow cross-sectional area, the flow velocity decreases and creates a delay to achieve the desired additional reactions, and in which method vapour is removed from the beginning of the reaction vessel (14), characterized in that a vapour phase layer (20) is arranged on top of the liquid phase layer (19) over the requisite length of the reaction vessel (14), and that the liquid level is kept essentially constant by removing the vapour as required.

2. A method in accordance with Patent Claim 1, characterized in that the flow to the reaction vessel (14) is directed to expand evenly to the entire flow cross-sectional area and that the flow is removed as an evenly decreasing flow.

3. A method in accordance with Patent Claims 1 or 2, characterized in that the vapour removal is controlled on the basis of surface measurement and adjustment.

4. A method in accordance with Patent Claims 1, 2, or 3, characterized in that the liquid is heated to 410 - 470ºC, the pressure is between 2 - 20 bar, and the average retention time is 5 - 100 minutes.

5. A method in accordance with one of Patent Claims 1 - 4, characterized in that the vapour phase layer extends essentially over the entire length of the reaction vessel (14).

6. A soaking-cracker, intended to implement the method in accordance with Patent Claim 1, which consists of an essentially horizontal tube-like reaction vessel (14) to create the retention of the liquid, and vapour removal devices, characterized in that the reaction vessel (14) includes feed and removal funnel ends (12, 15), and that the vapour removal equipment includes a surface measurement sensor and transmitter (21), an adjustment device (18), and a valve (17) connected to the vapour removal (6), which are so adjusted that they keep the surface of the liquid at a constant height in the reaction vessel (14) and the longitudinal vapour phase layer (20) in the upper section of the reaction vessel (14).

7. A soaking-cracker in accordance with Patent Claim 6, characterized in that the angle of development of the feed funnel (12) is less than 15 A