GB1602995A - Method of cooling a hot cracked gas - Google Patents

Description

(54) METHOD OF COOLING A HOT CRACKED GAS (71) We, SCHMIDT'SCHE HEISSDAMPF GmbH, a Company organised under the laws of the Federal Republic of Germany, of Ellenbacher Strasse 10, 3500 Kassel-Bettenhausen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a heatexchange process for cooling hot cracked gases, in which a hot cracked gas constituting a first flowing heat-exchange medium transfers its heat to another flowing heat-exchange medium via an intermediate heat transfer vehicle which is physically separated from both flowing media.

A heat exchange process of this kind has long been used in heat exchangers for nuclear power stations (see Liquid-Metals Handbook, Sodium-Nak Supplement of the Atomic Energy Commission-Department of the Navy. Washington, D.C., 1.7.1955). In this case the heat exchange is between liquid Na and NaK and between NaK and water via an intermediate liquid metal heat transfer vehicle, mercury being used as the intermediate heat transfer vehicle. In addition to its function as a heat-exchange medium, the latter is intended to protect the operating personnel from the danger that would arise in the event of leakage in the heat-exchange tubes if the two media between which heat was being exchanged were to come into direct contact with one another.To this end, in the known heat-exchangers the intermediate heat-transfer vehicle is always kept at the maximum level so as to completely fill the space between the areas where the flowing media could make contact in order to prevent any chance of direct contact between the heat exchange media at any time.

This process known from nuclear engineering does not allow one to vary the heatexchange capacity -of the apparatus. It is therefore unsuitable for cooling hot cracked gases.

Cracked gases are obtained by the thermal cracking of hydrocarbons in the presence of an inert gas (e.g. steam, or CO2) by hydropyrolysis of hydrocarbons preferably heavy hydrocarbons, in the presence of hydrogen participating in the reaction, or by catalytic cracking. The cracked gases must then be cooled. In these conditions, petroleum coke forms to a varying degree on the heating surfaces of the heat exchanger and necessitates decoking of the apparatus from time to time. The tendency of the cracked gas to coke and hence the frequency of the decoking processes depends on the feedstock used (e.g. naphtha, or gas oil).Since the known heat-exchange process does not permit control of the heat-exchange capacity, the operating time of the apparatus between the decoking operations cannot be optimized, to suit the material being cracked when it is wished to use the apparatus to cool the cracked gases.

In addition, in such cases it would not be possible to decoke the heat exchanger by an on-line decoking process without discharging the cooling water before-hand. On-line decoking is carried out by burning or gasifying the coke with a mixture of water vapour and air or else just with water vapour at high temperature, gasification or combustion of the petroleum coke deposited on the heating surfaces occurring in these conditions. The on-line decoking process does away with the otherwise very time-consuming mechanical cleaning of the heat exchanger, which would in principle necessitate opening the heat exchanger by removing the gas hoods.In a heat exchanger operating by the known process, the parts of the apparatus exposed in use to water and high pressure would have to be exposed to a very high temperature during the on-line decoking process because of the good thermal conductivity of the intermediate heat transfer vehicle, and this would result in the apparatus being damaged by destruction of the protective layer of magnetite (Fe,O4) on the water contacting surfaces of the apparatus.

The object of the invention is to provide a process for cooling hot cracked gas which allows the heat-exchange rate to be varied as desired and which is also such as to permit decoking of the heat-exchange surfaces by the on-line decoking method without it being necessary previously to discharge the cooling water when the apparatus is used for cooling hot cracked gases.

According to the present invention there is provided a method of cooling a hot cracked gas in which the hot cracked gas constituting a first flowing medium is caused to transfer heat to a second flowing medium via an intermediate heat transfer material from which both flowing media are physically separated by members affording first and second heat exchange surface areas respectively in contact with the first and second media, the amount of intermediate heat transfer material located between the said members being adjusted to vary the proportion of the first and second surface areas in direct conductive contact with the said intermediate heat transfer material whereby the exchange of heat between the first and second media occurring in the process is varied in dependence on the composition of the cracked gas so as to minimise the tendency for hydrocarbon deposits to form on the first heat exchange surface.

The intermediate heat transfer vehicle used may be any suitable heat-exchange medium, more particularly a liquid metal.

Low-melting metals, preferably heavy-metal alloys, are of advantage for this purpose.

If the intermediate heat transfer vehicle is discharged in part, the effective heating or cooling surface governing the heat-exchange capacity of the apparatus is reduced. If, on the other hand, more intermediate heat transfer vehicle is fed to the apparatus, then the effective area of the heating or cooling surface increases, with a resultant increase in the heat-exchange capacity of the apparatus.

In this way the heat-exchange capacity can be varied to suit the nature of the feedstock being used to produce the cracked gases at any time.

The temperature on the gas exit side can be varied to minimize the tendency of the cracked gases to coking in order always to achieve the longest possible operating time before it is necessary for decoking to be carried out. When it is wished to decoke the apparatus, the intermediate heat transfer vehicle is completely discharged so that there is then practically no heat transfer to the tubes through which the cooling water flows.

The heating surfaces are then heated up by water vapour or a mixture of water vapour and air, gasification or combustion of the deposited petroleum coke taking place. The water-contacting parts of the apparatus are under these conditions not subject to high temperatures, thus avoiding damage to the apparatus by destruction of the protective iron oxide layer on the surfaces which contact the cooling water. In this way decoking can be carried out without the need to discharge the cooling water, so that it is possible to switch over very rapidly from use of the apparatus for cooling to decoking for maintenance purposes and then back to production again. The amount of intermediate heat transfer vehicle can be rapidly and easily changed by discharging the intermediate heat transfer vehicle by pressure and supplying the heat transfer vehicle by natural gravity.This enables one to vary the heat exchange capacity of the apparatus rapidly.

The invention may be put into practice in various ways and a number of specific embodiments will be described to illustrate the invention with reference to the accompanying drawings in which: Figure 1 is a tube heat exchanger for cooling cracked gases with a reservoir for intermediate heat transfer material at a higher level and a nitrogen battery for supplying and discharging the intermediate heat transfer material, in diagrammatic form; Figure 2 is a longitudinal section through the heat exchanger on the line II--II in Figure 1; Figure 3 is a cross-section through the heat exchanger on the line III--III in Figure 2; Figure 4 is a longitudinal section of an intermediate heat transfer vehicle element of the heat exchanger shown in Figures 1 to 3;; Figure 5 is a section through the intermediate heat transfer vehicle element on the line V-V in Figure 4; Figure 6 shows another embodiment of the intermediate heat transfer vehicle element in a double tube construction in longitudinal section; Figure 7 is a section through the intermediate heat transfer vehicle element on the line VIl-VIl in Figure 6; Figure 8 is a longitudinal section of another embodiment of the intermediate heat transfer vehicle element for a heat exchanger of the field tube type; and Figure 9 is a section through the intermediate heat transfer vehicle element on the line VIlI-VIlI in Figure 8.

The plant shown in Figure 1 comprises a cracked gas cooler 1, which is connected via lines 2 and 3 to a vapour drum 4, and via lines 5 and 6 to a liquid metal reservoir 7.

The latter has a vapour or electrically operated heater 8 which keeps the metal iri the reservoir liquid. Lines 5 and 6 respectively contain an intake valve 9 and an outlet valve 10. A branch line 11 containing a shut-off valve 12 is provided between the cracked gas cooler 1 and the intake valve 9. The branch line 11 leads to a battery 13 of compressed nitrogen gas which serves as a pressure source.

The cracked gas cooler 1 comprises a housing 14, a gas inlet head 15 and a gas outlet head 16 (see Figures 2 and 3). Several rows of heat exchange tubes 17 are provided in the interior of the housing 14, each row of tubes being surrounded by an intermediate vehicle element 18 in the form of a plate.

As will be seen from Figures 4 and 5, the ends of the heat-exchange tubes 17 are welded into headers l9a and l9b at one end into distributors 20a and 20b at the other end. The headers and distributors extend laterally out of the intermediate vehicle elements 18 and at these places they have welded-on flanges 21 connected to the boxshaped part of the intermediate vehicle elements 18 by way of expansion compensators 22. Each of the elements 18 has an inlet spigot 23 and an outlet spigot 24, the spigots 23 being connected to the line 5 via distributors 25 while the outlet spigots 24 are connected to the line 6 via headers 26.

Headers l9a and 19b and distributors 20a and 20b are in turn connected to lines 2 and 3 respectively via main headers 27a and 27b; and main distributors 28a and 28b respectively.

The cracked gas for cooling enters the interior of the heat exchanger via the gas inlet head 15, flows between the intermediate vehicle plate elements 18 and finally through the gas outlet head 16. At the same time, cooling water flows out of the vapour drum 4 through the line 2, the main distributors 28a and 28b and the distributors 20a and 20b, into the cooling tubes 17, through the latter and then in the form of a mixture of vapour and water, through the headers l9a and 19b, the main headers 27a and 27b, and the line 3 back to the vapour drum 4.

The intermediate vehicle elements 18 are provided with a liquid metal filling which fills the spaces between the inner walls of each element 18 and the outer surfaces of the water-carrying cooling tubes 17 located in each element 18, and establishes a very satisfactory thermally conductive pathway between water flowing through the cooling tubes and the outer surfaces of the intermediate vehicle elements 18, which are in contact with the hot gas in use.

The heat exchange between the hot cracked gases and the cooling water is therefore by way of the liquid metal filling, a low-melting metal, preferably a heavy metal alloy, for example being used for the purpose.

The heat-exchange capacity of the apparatus is controlled by varying the level to which the intermediate vehicle elements 18 are filled. For example, if the heat-exchange capacity is to be reduced, some of the intermediate heat transfer filling material is discharged via the discharge spigot 24, the outlet valve 10 being opened while inlet valve 9 is closed and the filling being subjected to the pressure of the nitrogen battery 13 by opening of the shut-off valve 12. The active heating or cooling surface of the intermediate vehicle elements 18 and thus of the cooling tubes 17 is reduced, with the result that the heat-excbange capacity of the apparatus is also reduced. If, conversely, the heat-exchange capacity is to be increased, liquid metal is fed to the elements 18 from the reservoir 7.This is done by opening the valve 9 with the valves 10 and 12 closed, or opening both valves 9 and 10 so that liquid metal flows by gravity from the reservoir 7 to the elements 18 which are at a lower level than the reservoir 7. The supply of liquid metal could also be effected under pressure, using a suitable pressure source (not shown) connected to the reservoir.

When it is wished to decoke the apparatus by the on-line decoking process, the liquid metal is withdrawn from the elements 18, so that there is then practically no heat transport from the external surface of each plate 18 to the cooling tubes 17 located within each element 18. The elements 18 are then heated up to a high temperature by means of a mixture of vapour or water vapour and air flowing round them, so that the petroleum coke deposited on the heating surfaces of the elements 18 is gasified or burnt off. Since the cooling water is still present in the cooling tubes, these are kept at low temperatures.

The plate bodies of the intermediate vehicle elements 18 which in use are exposed to high temperatures are preferably made from heatresistant alloys. The cooling water tubes 17, on the other hand, can be made from carbon steel or low-alloy materials.

In the embodiment shown in Figures 6 and 7, the elements 18' consist oftubes 30 ofheatresistant material which surround the cooling water tubes 17 and are welded at both ends to half-shell header chambers 31 and 32. Here the liquid metal fills the annular gap between each tube 17 and the surrounding tube 30 as well as the cavities in the header chambers.

In the embodiment shown in Figures 8 and 9, the cooling tubes 17 are constructed as Field tubes (Field rohre) consisting in each case of a jacket tube 34 and an immersed tube 35 open at its bottom end. Each field tube consists of an inner tube connected at one end to a first header or manifold and open at its other end and surrounded by a wider outer tube connected at the same end as is the narrower tube to a second header or manifold which surrounds the first header, each outer tube being closed at its other end whereby flow can be in through the first header down the narrow tube out of its open end and back through the space between the inner and outer tube to the second header and thence out through it or flow can be in the opposite sense.The elements 18" again consist of tubes 30 as in the embodiment shown in Figures 6 and 7, tubes 30 surrounding the Field tubes 17 and being welded to a header chamber 31 at one end. At the other end they communicate with the header 26 and the liquid metal discharge line 6, via connecting tubes 39 and a header line 40.

The cooling water flows to the Field tubes 17 via a distributor tube 37, to which the immersed tubes 35 are connected. The mixture of water and vapour then flows through the annular gap between the tubes 34 and 35 into the header 36 and is discharged via the outlet spigot 38. With this construction the problems of differential expansion can be controlled without using compensators or similar aids since the tubes 35 are free to expand and contract within the tubes 34.

The method proposed may be used with any combination of the second flowing medium and the intermediate heat transfer vehicle and any direction of heat flow.

Solenoid valves or similar control means can be used to control the level of filling of the intermediate heat transfer vehicle containing elements. The gas can flow in the reverse direction, i.e. upwards, without modifying the process.

WHAT WE CLAIM IS: 1. A method of cooling a hot cracked gas in which the hot cracked gas constituting a first flowing medium is caused to transfer heat to a second flowing medium via an intermediate heat transfer material from which both flowing media are physically separated by members affording first and second heat exchange surface areas respectively in contact with the first and second media, the amount of intermediate heat transfer material located between the said members being adjusted to vary the proportion of the first and second surface areas in direct conductive contact with the said intermediate heat transfer material whereby the exchange of heat between the first and second media occurring in the process is varied in dependence on the composition of the cracked gas so as to minimise the tendency for hydrocarbon deposits to form on the first heat exchange surface.

2. A heat-exchange process as claimed in Claim I in which a liquid metal is used as the intermediate heat-transfer material.

3. A heat-transfer process as claimed in Claim 1 or Claim 2 in which the intermediate heat-transfer material is discharged by the application of pressure and is supplied by natural gravity flow.

4. A heat exchange process as claimed in Claim 1 substantially as specifically described herein with reference to Figures 1, 2, 3, 4 and 5 or Figures 1, 2, 3, 6 and 7, or Figures 1, 2, 3, 8 and 9.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. header chamber 31 at one end. At the other end they communicate with the header 26 and the liquid metal discharge line 6, via connecting tubes 39 and a header line 40. The cooling water flows to the Field tubes 17 via a distributor tube 37, to which the immersed tubes 35 are connected. The mixture of water and vapour then flows through the annular gap between the tubes 34 and 35 into the header 36 and is discharged via the outlet spigot 38. With this construction the problems of differential expansion can be controlled without using compensators or similar aids since the tubes 35 are free to expand and contract within the tubes 34. The method proposed may be used with any combination of the second flowing medium and the intermediate heat transfer vehicle and any direction of heat flow. Solenoid valves or similar control means can be used to control the level of filling of the intermediate heat transfer vehicle containing elements. The gas can flow in the reverse direction, i.e. upwards, without modifying the process. WHAT WE CLAIM IS:

1. A method of cooling a hot cracked gas in which the hot cracked gas constituting a first flowing medium is caused to transfer heat to a second flowing medium via an intermediate heat transfer material from which both flowing media are physically separated by members affording first and second heat exchange surface areas respectively in contact with the first and second media, the amount of intermediate heat transfer material located between the said members being adjusted to vary the proportion of the first and second surface areas in direct conductive contact with the said intermediate heat transfer material whereby the exchange of heat between the first and second media occurring in the process is varied in dependence on the composition of the cracked gas so as to minimise the tendency for hydrocarbon deposits to form on the first heat exchange surface.

2. A heat-exchange process as claimed in Claim I in which a liquid metal is used as the intermediate heat-transfer material.

3. A heat-transfer process as claimed in Claim 1 or Claim 2 in which the intermediate heat-transfer material is discharged by the application of pressure and is supplied by natural gravity flow.

4. A heat exchange process as claimed in Claim 1 substantially as specifically described herein with reference to Figures 1, 2, 3, 4 and 5 or Figures 1, 2, 3, 6 and 7, or Figures 1, 2, 3, 8 and 9.