GB2115538A

GB2115538A - Heating furnace for hydrocarbon treatment

Info

Publication number: GB2115538A
Application number: GB08304503A
Authority: GB
Inventors: Keizo Konoki; Takanobu Shinohara; Yutaka Kita
Original assignee: Toyo Engineering Corp
Current assignee: Toyo Engineering Corp
Priority date: 1982-02-22
Filing date: 1983-02-18
Publication date: 1983-09-07
Also published as: GB8304503D0; DE3305905A1; JPS58142978A; FR2522126A1

Abstract

The furnace comprises a combustion chamber containing heating tubes for the passage of hydrocarbon to be treated, the heating tubes having a wall thickness which varies in the longitudinal direction in correspondence with different temperatures at various points of the longitudinal temperature distribution on the outer surface of the tubes in order to save on tube material. For the steam reforming of naphtha, the surface temperature distribution is indicated by the characteristic (3) in the diagram. The tube wall thickness distribution is indicated by characteristic (2). (1) indicates the uniform thickness tubes of the prior art. <IMAGE>

Description

SPECIFICATION Heating furnace for hydrocarbon treatment The present invention comprises an improvement in heating furnaces used for hydrocarbon treatment such as heating, reforming and thermal cracking.

Known heating furnaces for reforming and thermally cracking hydrocarbons are provided with a plurality of elongated heating tubes. These heating tubes comprise 3 to 5 tubes, hereinafter referred to as unworked tubes, which are manufactured by centrifugal casting and are welded together to form heating tubes which are 2 to 3 metres in length. The temperature of heating tubes is normally distributed in the longitudinal direction of the tubes and creep damage becomes greatest where the temperature of the tubes becomes the highest. When the unworked tubes are welded in the vicinity of a position where the highest temperature is reached, creep damage, particularly at the welded part, presents a problem.

Conventionally, heating tubes are given a wall thickness which is calculated on the basis of the creep strength requirement of the tube material at the highest temperature region regardless of the longitudinal temperature distribution. In other words, the wall thickness of heating tubes at portions exposed to lower temperatures is made excessively thick. Because of the very high temperatures encountered during reforming and thermal cracking of hydrocarbons, heating tubes are usually made of expensive materials such as 25 Cr-20 Ni steel and 25 Cr-35 Ni steel and the excessive wall thickness inevitably results in unnecessarily expensive heating tubes.

The present invention provides a heating furnace for treating hydrocarbons comprising a combustion chamber, a fuel burner attached to the wall thereof, an exhaust gas outlet port and at least one heating tube provided in said combustion chamber, said heating tube having a wall thickness which varies in the longitudinal direction in correspondence with different temperatures at various points of longitudinal temperature distribution on the outer surface thereof.

Preferably, the heating tube comprises a plurality of unworked tubes welded together, the length of said unworked tubes being so selected that the welds do not coincide with the high temperature region on the outer surface of the heating tube.

A specific embodiment of the present invention will now be described by way of example with reference to the accompanying drawing which illustrates an example of the wall thickness distribution in the longitudinal direction of heating tubes in a heating furnace according to the prior art and a heating furnace according to the present invention as well as their outer surface temperature distribution in the longitudinal direction for the steam reforming of naphtha.

With reference to the accompanying drawing, the longitudinal wall thicknesses of the heating tubes to be used in the heating furnace according to the present invention is determined by the following procedure: First, the longitudinal temperature distribution of the heating tubes is calculated on the basis of such factors as the type of hydrocarbons passing through the tubes, the type of diluent employed as needed, the flow rate, the pressure, etc., depending on the kind of hydrocarbon treatment, for example, reforming or thermal cracking. Second, the allowable stress is calculated on the basis of the creep strength of the tube material which is determined by the design life and temperature distribution. Third, the wall thickness required at various portions of the heating tubes is calculated according to applicable regulations or standards.Fourth, the profile of the heating tube with the required wall thickness at respective portions is defined.

It is preferred that the thickness of each unworked tube be varied in a continuous manner faithfully following the above-mentioned profile, but the inner diameter thereof should be uniform.

It is also possible to reasonably ignore minor irregularities in the profile for easy manufacture of the unworked tubes or to have the thickness vary, not continuously, but stepwise.

Centrifugal casting is a preferred method for manufacturing the unworked tubes. Heating tubes can be manufactured by making a male profile to correspond to the thickness profile which is used as a female profile. With the male and female profiles used for the mold, tubes are cast with a core having a diameter equivalent to the tube's inner diameter. As mentioned above, the length of the mold for unworked tubes can be adjusted so that the welds do not coincide with the highest temperature region.

The thickness profile of the heating tube differs depending on the type of hydrocarbon treatment to be carried out in the tube and thus is difficult to define in general terms. The temperature may rise from relatively low to extremely high values within the region extending from the inlet port to 20 to 40% of the total length of the tube. The thickness of the tube in this region may therefore be increased from the minimum to the maximum to correspond with this rise in temperature.

In the case of reforming or thermal cracking of hydrocarbons which raises the outer surface temperature of the heating tubes, 25 Cr-20 Ni steel, 25 Cr-35 Ni and the like are used as the material for the heating tube. When the outer surface temperaure is relatively low, heating tubes can be made of 1 to 9 Cr-0.5 to 3 Mo steel, carbon steel or the like.

The heating furnace according to the present invention can be used for different treatments of various hydrocarbons such as gaseous hydrocarbon (e.g. methane), naphtha, iight oil, heavy oil and the like. Different treatments include: heating hydrocarbons to a relatively low temperature; reforming hydrocarbons with the use of steam; partially oxidizing hydrocarbons; and, thermally cracking hydrocarbons. In other words, the heating furnace according to the present invention can be widely used for treatments with which the outer surface temperature of the heating tubes falls within the range of from 300 to 1,1 000C. The pressure is from 0 to 100 kg/cm2 of the gauge pressure. The heating tubes may be packed with a suitable catalyst depending on the purpose.

As an example, application of the furnace according to the present invention to steam reforming of naphtha will now be described with reference to the accompanying drawing. In the drawing, the curve 3 shows the calculated longitudinal temperature distribution on the outer surface of the heating tubes in which the steam reforming of naphtha is carried out. From this, the longitudinal profile shows as the curve 2 of the wall thickness of the heating tubes can be obtained. The stratight line 1 shows the longitudinal profile of the wall thickness of the prior art heating tubes which is uniform. The maximum thickness in the profile shown as the curve 2 corresponds to the thickness of the prior art heating tubes.As is evident from the drawing, the heating tubes employed in the present invention requires less material, the difference in amount corresponding to the area defined by the straight line 1 and the curve 2. Calculations indicated that the material required to manufacture the heating tubes in this particular case may be reduced to 80% of the amount required in the prior art.

As the heating tubes of the present invention heating furnace for hydrocarbon treatment is varied in its longitudinal thickness in correspondence with the temperature distribution on the outer surface, it requires a smaller amount of material as compared with the prior art heating tube and thus is less expensive. Considering that these heating tubes must be replaced after a certain period of time and that they are used in plural for each unit of a heating furnace, it is quite evident that savings in the amount of material is extremely advantageous. Further, by welding the unworked tubes at portions away from the high temperature region, creep damage at the welded part can be reduced. Reliability and stability of the heating tubes can be greatly improved which is advantageous in terms of operation and maintenance.

Claims

1. A heating furnace for treating hydrocarbons comprising a combustion chamber, a fuel burner attached to the wall thereof, an exhaust gas outlet port and at least one heating tube provided in said combustion chamber, said heating tube having a wall thickness which varies in the longitudinal direction in correspondence with different temperatures at various points of longitudinal temperature distribution on the outer surface thereof.

2. A heating fumace as claimed in claim 1 wherein said heating tube comprises a plurality of unworked tubes welded together, the length of said unworked tubes being so selected that the welds do not coincide with the high temperature region on the outer surface of the heating tube.

3. A heating furnace as claimed in claim 1 wherein the wall thickness at various points on the outer surface of the heating tube in the longitudinal direction is determined on the basis of an allowable stress at the temperature at each of said points.

4. A heating furnace as claimed in claim 1 wherein the wall thickness of said heating tube is gradually varied from the minimum to the maximum within a region extending from the inlet port to 20 to 40% of the total length of the heating tube.

5. A heating fumace substantially as hereinbefore described with reference to the accompanying drawing.