WO1998017961A1

WO1998017961A1 - Heat exchanger

Info

Publication number: WO1998017961A1
Application number: PCT/SE1997/001752
Authority: WO
Inventors: Jan Olov Olsson
Original assignee: Kanthal Ab
Priority date: 1996-10-22
Filing date: 1997-10-20
Publication date: 1998-04-30
Also published as: SE9603890L; AU4797797A; SE9603890D0

Abstract

The present invention is for a heat exchanger for cooling of gases having high temperature and high carbon potential as is the case by production of carbon black. Natural gas and/or oil and possible additives, such as aromatic oils and steam, are supplied to a burner (1). After the combustion the gases are brought to a heat exchanger (2). This may schematically be split up in two sections, a lower section (4) with high gas temperatures and an upper section (5) where the gas temperatures are lower. In order to make the heat exchanger tubes in the lower part of the heat exchanger closest to the inlet of the combustion gases resistant to corrosion and heat, tubes made from an alloy comprising 2-10 weight % aluminium, 10-40 weight % chromium, balance mainly iron are used.

Description

Heat exchanger.

The present invention is for a heat exchanger for cooling of gases of high temperatures and high carbon potential as is the case by the production of carbon black

Carbon black is produced by combustion of natural gas which may be mixed with other oil products Combustion takes place at a great deficiency of oxygen and a corresponding surplus of fuel Normally there is supplied about six times more of combustible products than what corresponds to the combustion which takes place in the plant After the combustion some the heat which is developed is recuperated in a heat exchanger, whereupon the product is further cooled and recovered in the form of particles of a few millimeters diameter

Several process parameters are influenced by the temperature of the combustion gases at the inflow to the heat exchanger An increase of this temperature will bring with it a considerable improvement of the yield of the process and important energy savings In the production there is also a considerable erosion of the material of the heat exchanger, primarily on the inside of the tubes through which the hot combustion gases flow immediately following the retort in which combustion takes place In these tubes there also occur thick deposits of solid particles from the combustion gases and deposits of products from reaction between the material of the tubes and the gases These reaction products comprise an essential content of chromium carbide The formation of these deposits bring with them a reoccuring need for cleaning with consequent shutdown of the production and costs associated with that The object of the present invention is to considerably reduce the risks for formation of these deposits by improvement of the tubes By this is achieved a longer life for the tubes of the heat exchanger and a lower production cost due to reduced need for interruptions of the production for cleaning purposes Another object of the invention is to make possible an increase of the production temperature and to achieve the advantages which follow with that.

The invention will below be described more in detail with reference to the embodiment which is shown in the enclosed figures.

Figure 1 schematically shows a plant for production of carbon black.

Figure 2 is a cross-section of a part of the heat exchanger.

Figure 3 is a cross-section of a heat exchanger tube according to a preferred embodiment of the invention.

Natural gas and/or oil and possibible additives, such as aromatic oils or steam, are supplied to a burner 1. After the combustion the gases are supplied to a heat exchanger 2. This may be divided into two sections, a lower section 4 with high gas temperatures and an upper section 5 with lower gas temperatures, so that the combustion gases which exit from the heat exchanger have a temperature of about 625 °C. After that the gases flow through a cooler 6 after which the final product in the form of small particles of carbon black, preferrably having a diameter of 2-3 mm, are separated in a filter 7. Combustion takes place at a temperature of 1500- 1600 °C. According to the process of prior art the temperature of the combustion gases is then reduced so that at it the inflow to the heat exchanger is about 850 °C. During the passage through the heat exchanger the temperature of the combustion gases drops to about 625 °C, and the heat is transferred to the air which is used for the combustion in the burner. The heat exchanger tubes 8 which are located in the lower part 4 of the heat exchanger are in prior art made from steel alloys having a nickel content of 20-40%. The ability of these materials to resist high temperatures decides the highest allowable temperature of the combustion gases at the inlet of the heat exchanger. Moreover there takes place a reaction between carbon in the combustion gases, which have a high carbon potential, and the mateπal of the tubes leading to a build up of carbon depositions on the tubes This means that at regular intervals the production has to be shutdown and the plant cleaned

According to the invention an alloy containing 2-10 weιght-% aluminium, 10-40 weιght-% chromium, balance mainly iron, is used for the tubes of the heat exchanger in the lower part thereof closest to the inlet for the combustion gases Such alloys are commonly named FeCrAI alloys This alloy may be used for the tubes of the whole of the lower part of the heat exchanger and as an alternative only the lower part of these tubes or an insert into the tubes closest to the inlet of the combustion gases may be made from said alloy That surface of the tubes which will be exposed to the combustion gases is preoxidized at a temperature of at least 1050 °C in order to ensure that a dense layer of aluminium oxide is formed on the surface

Conventionally used materials for the heat exchanger tubes 8 have a comparatively high chromium content At the combustion a carbon rich, low oxygen atmosphere is formed and chromium carbides are formed by contact with chromium containing alloys This brings with it changes of the composition of the alloy which means less good properties of the material The chromium carbides may be eroded away by the gas which flows by and which contains large quantities of particles of carbon black, which causes damages of the tube walls If the carbides remain on the surface the formation of carbides causes a volume increase and a rough surface is formed both for this reason and because of the erosion of the chromium carbide Deposits of carbon black and chromium carbide cause plugging of the tubes why the plant has to be shutdown for cleaning comparatively often

In a preferred embodiment of the invention the tubes have an outer layer 9 made from conventional material and a closely adjacent inner tube 10 made from FeCrAI alloy Also in this case it is suitable that the inner FeCrAI tube 10 is shorter than the outer tube 9 and extends to about half of the length of the outer tube Preferably the inner tube is produced separately and is inserted into the surrounding outer tube but it is also possible to produce the inner tube directly on to the inside of the outer tube. Preoxidation of the inside of the inner tube, i. e. that surface which is in contact with the combustion gases and the carbon black, may be performed either before or after that the inner tube has been inserted into the outer tube.

The invention makes possible an increase of the temperature of the combustion gases by at least 100 °C. This brings with it important advantages by up to about 10 % reduction of the fuel comsumption and an increased efficiency of the heat recuperation. These factors bring about an improvement of the efficiency of the process as a whole with 10-20 % and means an considerably saving of energy. This is mainly achieved thereby that the air which is used as cooling air and is then heated is directed to the burner and used for the combustion. Beside the improved efficiency the increased inlet temperature of the combustion air makes it possible to use oil fractions which otherwise can not be used.

Another advantage which is achieved by the invention is that shutdown for cleaning of the plant is normally not required, as no deposits of carbon build up on the walls of tubes made from the material according to the invention. This is due to that at high temperatures a thin layer of aluminium oxide is formed on the outside of these tubes different from most other high temperature resistant alloys on which layers of chromium carbide are formed. Carbon is not soluble in aluminium oxide and thus does not in any way react with the material of the tubes, and consequently there is no build up of deposits on the tubes. Together these technical advantages bring considerable econmical advantages.

Within the frame of the inventive idea the invention may, by various embodiments, be adapted to an individual application and its special process conditions and the design of the plant. Thus the length of that part of the tubes which is made from FeCrAI alloy may vary from just a small part closest to the combustion chamber 1 up to the full length of the tubes. When inserts made from FeCrAI are used these may be of very different lengths and also protrude out from the encircling outer tubes.

Claims

1. Heat exchanger for cooling of combustion gases of high temperature and high carbon potential by production of carbon black, where the combustion gas flows through tubes which are surrounded by a cooling medium characterized in that on the inside of the tubes, which are in contact with the combustion gases, at least the cooling surfaces closest to the gas inlet of the heat exchanger, are made from an alloy comprising 2- 10 weight-% aluminium, 10-40 weight % chromium and balance mainly iron (FeCrAI).

2. Device according to claim 1 characterized in that each tube along a part of its length has an insert of FeCrAI alloy which is surrounded by a longer outer tube.