FIELD OF THE DISCLOSURE

[0001] The present disclosure refers to transfer line exchangers with improved cooling capacity, equipped with a thermal spray coating for increasing the erosion resistance and a method for the maintenance of transfer line exchangers with the help of a thermal spray coating.

BACKGROUND OF THE DISCLOSURE

[0002] Ethylene is the basic feedstock in the production of plastics. Ethylene and other olefins such as propylene are produced by thermal cracking of hydrocarbons in pyrolysis furnaces in the presence of steam, with ethane, naphtha and other mineral oil fractions serving as the main feedstocks.

[0003] The gas generated in the furnace usually has a temperature of around 850 to 1000 °C and is rapidly cooled after leaving the reaction zone of the furnace to prevent secondary reactions and to stabilize the gas composition. In some modern ethylene plants, this rapid cooling of the cracked gas is achieved by transfer line exchangers (TLE). The high-temperature gas is cooled down by passing the gas through cooling pipes which are surrounded by a cooling medium, thus employing the principle of heat transfer.

[0004] Transfer line exchangers operate under very harsh conditions. Apart from the extreme temperatures, the gas entering the transfer line exchangers may contain coke particles which cause two major problems in traditional heat exchangers: erosion and fouling. A third potential problem is corrosion on the water side of the exchanger. In light of these problems, careful monitoring of the equipment, in particular the cooling tubes of the transfer line exchanger, is mandatory as faulty tubes bear the risk of furnace failure. The state of the transfer line exchanger may be controlled by regularly measuring the wall thickness of the tubes. Affected tube sections are replaced by de-assembling of the heat exchanger, replacing the affected tube and re-assembling the exchanger, a process which is not only time consuming but due to the size of the equipment also a safety hazard. Further, any repairs also require extended downtimes of the cracking furnace which in turn can affect the production performance of the entire cracker.

[0005] Commonly used transfer line exchangers do not have adequate protection against erosion by coke particles resulting in a limited lifetime of the transfer line exchangers and the necessity of repair or regular replacement of faulty sectors or even whole tubes.

[0006] Several measure of erosion protection of transfer line exchangers are described in various references. [0007] EP 1 331 465 relates to transfer line exchangers used for cooling gaseous output from furnaces of ethylene production where high pressure water vapor is produced outside of tubes, with a shell under pressure, and with an inlet tube plate separating the inside of the shell from an inlet manifold of the fluid to be cooled, with the tube plate having, on the manifold side, an antierosion layer and the tube plate having passages in it for communication with the interior of the tubes of the tube nest and around each passage, on the opposite side of the tube plate relative to the manifold, there being a projecting neck on which is welded a corresponding tube of the nest, the tube plate being made of steel lightly alloyed with molybdenum or chromo-molybdenum and the anti-erosion layer being made of a weld deposit of Nickel-Chrome Alloy 625, harder than the tube plate, and the tubes being welded on an "internal bore welding" at a distance from the plate surface which is approximately equal to at least the thickness of the plate. It is believed that the combination of the anti-erosion layer and the high necks provides erosion protection against coke particles in the gaseous output.

[0008] US 7,237,601 describes a heat exchanger for cooling hot gas that contains solid particles, comprising a casing; respective tube plates disposed at ends of the casing, heat exchanger tubes through which said hot gas flows, wherein said heat exchanger tubes are surrounded by said casing, and wherein ends of said heat exchanger tubes are welded into bores of said tube plates via weld seams; and a protective layer that coats an end face of that one of said tube plates disposed on a gas inlet side, an inner wall of said bores, said weld seams, and an inlet region of said heat exchanger tubes, and wherein said protective layer comprises a metallic adhesive layer, a high temperature and erosion resistant ceramic layer, and a high temperature and erosion resistant metal layer disposed between said adhesive layer and said ceramic layer. However, the ceramic top layer does not allow repair by welding so that extensive replacement of the coated parts is required.

[0009] WO 2007/006446 refers to a shell-and-tube heat exchanger equipped with a tube plate lining which is resistant to wear for use in thermal cracking equipment comprising cooling tubes through which the gas to be cooled is circulated, each tube being secured by a tube plate at both ends of the tube and enclosed in a casing through which a coolant material is circulated; the surface of the tube plate on the gas inlet side which is impacted by gas as it enters the shell-and-tube heat exchanger is faced, at least partially, by a protective layer; the protective layer comprising sleeves with faces; the sleeve faces being aligned side-to-side and end-to-end at the outer edges; the sleeves being at least partially inserted into the cooling tube ends; and the sleeves being made from a heat resistant metallic material. The proposed solution increases rather than shortens the time required for repairs as the sleeves have to be removed before any affected tube sections can be replaced. Furthermore, with the sleeves it is not possible to measure regularly the tube wall thicknesses. This means that corrosion from the boiler feedwater side cannot be detected at an early stage. [0010] US 2011/277888 A1 discloses a method of providing sulfidation corrosion resistance and corrosion induced fouling resistance for a heat transfer component. The heat transfer component includes a heat exchange surface formed from a chromium-enriched oxide containing material formed from the composition 8, e and is a metal containing at least 5 to about 40 wt.-% chromium, e is a chromium enriched oxide formed on the surface of the steel 5, and 8 is a top layer formed on the surface of the chromium-enriched oxide e comprising sulfide, oxide, oxysulfide and mixtures thereof. A chromium enriched oxide is formed on the surface of the steel by exposing the steel to a low oxygen partial pressure environment at a temperature of from about 300°C to 1000°C for a time sufficient to effect the formation of the chromium enriched oxide.

[0011] US 2008/073063 A1 relates to a method for reducing the formation of deposits on the inner walls of a tubular heat exchanger through which a petroleum-based liquid is flowing. The method comprises applying one of fluid pressure pulsations to the liquid flowing through the tubes of the exchanger and vibration to the heat exchange surfacer to effect a reduction of the viscous boundary layer adjacent the inner walls of the tubular heat exchange surfaces.

[0012] US 6 074 713 A concerns a method of lessening the tendency of carbon to deposit on a hot metal surface, particularly a component in a furnace for thermally cracking hydrocarbons that comprises a coating, a chromium containing metal surface with a layer of porous, dry, pulverized glass and heating the coated metal to form an adherent, vitreous coating on the metal surface. The coating may be a barium aluminosilicate or strontium-nickel aluminosilicate glass.

[0013] US 2013/220523 A1 concerns a method for forming protective coatings on equipment. The coating is formed from a single-component iron based alloy composition comprising at least two refractory elements selected from Cr, V, Nb, Mo and W.

[0014] EP 2 772 563 A1 discloses a method that involves arranging a heating element on one side of a carrier, which is formed as heating side. The other side of the carrier is formed as medium side for heating of water. A functional coating layer with non-adhesive effect is formed on the surface of the carrier. The medium side of the carrier is treated before applying the functional coating layer. The coating layer is made of diamond like carbon, nano-coating, metal oxide coating, ceramic, enamel coating and silicon oxide layer.

[0015] Apart from the problem, that some of the suggested measures are not compatible with established safety measures, the suggested solutions suffer from the drawback the tubes are still prone to thickness losses and time- and cost-consuming repairs at regular intervals.

[0016] The above-described measures also disregard the need for continuous monitoring of the wall thickness of the cooling tubes as a means to ensure safe operation. Rather, due to the proposed measures, determination of the wall thickness becomes more complicated or even impossible due to gaps between the protection means and the tube wall or due to inhomogeneous layer transitions.

[0017] In light of the above, there still exists the need to prolong the lifetime of transfer line exchangers without effecting the established safety inspection procedures of the equipment and allowing for fast and uncomplicated repairs. This need is addressed by the present disclosure which proposes a new transfer line exchanger as well as an adapted maintenance process.

SUMMARY OF THE DISCLOSURE

[0018] In general, the present disclosure provides a transfer line exchanger with cooling tubes through which the gas to be cooled is processed, wherein the inner surface of each cooling tube is at least partially equipped with a thermal spray coating.

[0019] The thermal spray coating is obtained from a spray material formed of Cr3C2 and a NiCr alloy. Preferably, the spray coating material comprises at least

50 wt.-%, at least 60 wt.-%, preferably at least 70 wt.-% of Cr3C2, based on the total weight of the spray material. The spray coating material may in one embodiment comprise about 75 wt.-% of CrsC2. In some embodiments, the spray coating material may comprise up to 95 wt.-%, up to 90 wt-%, preferably up to 80 wt.-% of CrsC2.

[0020] In some embodiments, the content of Ni in the spray coating material may be around 15 to 25 wt.-%, preferably, the content of Ni in the spray coating material may comprise about 20 wt.- %. Additionally or alternatively, the content of C in the spray coating material may be less than 15 wt.-%. In some embodiments, the C content in the spray coating material may be 10 wt.-%.

[0021] In some embodiments, the spray coating material may be agglomerated and sintered.

[0022] In some embodiments, the gas inlet side of each cooling tube which is impacted by gas as it enters the cooling tube is equipped with the thermal spray coating.

[0023] In some embodiments, the thermal spray coating extends at least 100 mm, preferably at least 150 mm, more preferably at least 200 mm, especially at least 500 mm into the cooling tube from the gas inlet side.

[0024] In some embodiments, the thermal spray coating covers at least 0,5% of the inner surface of the cooling tube from the gas inlet side, preferably at least 2%, more preferably at least 5%, based on the complete inner surface of the cooling tube. [0025] In some embodiments, the thermal spray coating has a thickness of 0.1 to 0.5 mm, more preferably 0.15 to 0.3 mm.

[0026] In some embodiments, the thermal spray coating is directly applied on-site to the surface of the cooling tube.

[0027] In some embodiments, the thermal spray coating is weldable coating, preferably a single layer coating.

[0028] In some embodiments, at least one cooling tube comprises at least two segments welded to each other, wherein at least one segment is provided with the thermal spray coating adjoining to the welding area.

[0029] In some embodiments, the cooling tube comprises a weld seam, wherein a martensitic structure is formed at the weld seam, when welding is performed on the Cr3C2/NiCr coating for the purpose of repair or replacement of the tube sections.

[0030] In some embodiments, the thermal spray coating does not contain any layers formed of a different material than the thermal spray material, preferably no different material than Cr3C2/NiCr alloy.

[0031] In some embodiments, the thermal spray coating is applied by HVOF (High Velocity Oxy-Fuel Spraying), Combustion Flame Spraying, Plasma Spraying, Vacuum Plasma Spraying or Two-Wire Electric Arc Spraying, preferably HVOF.

[0032] In some embodiments, the cooling tube is made of heat-resistant carbon steel.

[0033] In some embodiments, the cooling tubes are secured by a tube plate at both ends of the tubes and the tube plate at the gas inlet side is equipped with a thermal spray coating, preferably a thermal spray coating obtained from a thermal spray material formed of CrsC2 and a NiCr alloy.

[0034] A further object of the present disclosure is an apparatus for the production of olefins, in particular ethylene and propylene, comprising the transfer line exchanger of the present disclosure.

[0035] A still further object of the present disclosure is a process for the cracking of hydrocarbons, in particular for the production of olefins, comprising: (i) feeding a feedstock of hydrocarbons into a cracking coil located inside a thermal reactor; (ii) heating the feedstock to obtain a cracking gas; and (iii) introducing the cracking gas into a transfer line exchanger for cooling, wherein the transfer line exchanger is a transfer line exchanger of the present disclosure.

[0036] In a further object, the present disclosure refers to a process for the maintenance of a transfer line exchanger comprising the steps of: (i) detecting faults on the inner surface of the cooling tubes; and (ii) applying a thermal spray coating to the faults.

[0037] In some embodiments, detection of faults is conducted by measuring the wall thickness of the cooling tube on the process side, preferably via non-destructive methods, in particular via ultrasound.

[0038] In some embodiments of the process, the thermal spray coating is applied by HVOF (High Velocity Oxy-Fuel Spraying), Combustion Flame Spraying, Plasma Spraying, Vacuum Plasma Spraying or Two-Wire Electric Arc Spraying, preferably by HVOF.

[0039] The thermal spray coating is obtained from a spray material formed of CrsC2 and a NiCr alloy.

[0040] In some embodiments of the process, the spray coating material comprises at least 50 wt.-%, at least 60 wt.-%, preferably at least 70 wt.-% of Cr3C2, based on the total weight of the spray material. The spray coating material may in one embodiment comprise about 75 wt.-% of CrsC2. In some embodiments, the spray coating material may comprise up to 95 wt.-%, up to 90 wt-%, preferably up to 80 wt.-% of CrsC2.

[0041] Another object of the present disclosure is a method for monitoring the water side surface of cooling tubes of transfer line exchangers by measuring the wall thickness of the cooling tubes from the process side of the tubes.

[0042] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. However, many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.

BRIEF DESCRIPTION OF THE FIGURES

[0043] Certain embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate the various implementations described herein and are not meant to limit the scope of various technologies described herein, and: [0044] Figure 1 provides a schematic overview of a common maintenance process for transfer line exchangers, according to an embodiment of the disclosure;

[0045] Figure 2 provides a flow chart of the maintenance process for transfer line exchangers of the present disclosure; and

[0046] Figure 3 is a cross-sectional illustration of an example transfer line exchanger cooling tube, according to an embodiment of the disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0047] In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments are possible.

This description is not to be taken in a limiting sense, but rather made merely for the purpose of describing general principles of the implementations. The scope of the described implementations should be ascertained with reference to the issued claims.

[0048] In a first embodiment, a transfer line exchanger for application in thermal cracking equipment is provided which addresses wear protection of the cooling tubes while at the same providing a solution that does not interfere with established safety measures or negatively impacts the cooling capacity of the transfer line exchanger.

[0049] It was surprisingly found that the thermal spray coating of the transfer line exchanger of the present disclosure is compatible with certified measurement procedures for determining the wall thickness of the cooling tubes, while at the same time limiting coke deposition in the tubes, thus reducing the risk of fouling and the need for decoking of the equipment.

[0050] Additionally, it was surprisingly found that the thermal spray coating also resulted in an improved cooling efficiency, resulting in a lower temperature increase of the cracking gas passing through the transfer line exchanger during olefin production.

[0051] The thermal spray coating is in particular chosen with regard to its wear protection properties. Accordingly, preference is given to materials with a high wear resistance, in particular Nickel-Chromcarbides. The thermal spray coating is obtained from a spray material formed of CrsC2 and a NiCr alloy. Preferably, the spray coating material comprises at least 50 wt.-%, at least 60 wt.-%, preferably at least 70 wt.-% of Cr3C2, based on the total weight of the spray material. The spray coating material may in one embodiment comprise about 75 wt.-% of Cr3C2. In some embodiments, the spray coating material may comprise up to 95 wt.-%, up to 90 wt-%, preferably up to 80 wt.-% of Cr3C2. In the resulting thermal spray coating the chromium carbide particles are embedded in the NiCr metallic matrix. On one hand, the NiCr matrix allows for an easier and faster repassivation when the coating is subjected to wear, thereby enhancing the corrosion resistance. On the other hand, the fine grain structure with a homogenous distribution of the skeleton network of the hard carbide phases provide excellent erosion resistance and is therefore a better alternative to hard oxide coatings.

[0052] The content of Ni in the spray coating material may be around 15 to 25 wt.-%, preferably, the content of Ni in the spray coating material may comprise about 20 wt.-%. Additionally or alternatively, the content of C in the spray coating material may be less than 15 wt.-%. In some embodiments, the C content in the spray coating material may be 10 wt.-%.

[0053] In particular, the spray coating material may consist of CrsC2 and a NiCr alloy. The spray coating material may for example be an agglomerated and sintered powder. Such powders show a good melting behavior due to the large surface to volume ration and, at the same time, good flow behavior due to the mostly spherical shape.

[0054] The thermal spray coating is in particular intended to protect the gas inlet site of the cooling tubes against erosion caused by coke particles present in the gas stream. Therefore, an embodiment is preferred, wherein the gas inlet side of each cooling tube, which is impacted by gas as it enters the cooling tube, is equipped with the thermal spray coating.

[0055] Although the main site of impact of any solid particles in the gas stream is at the gas inlet of the cooling tubes, maximum protection could be achieved if the thermal spray coating was further extended into the cooling tube. In a preferred embodiment, the thermal spray coating extends at least 100 mm, preferably at least 150 mm, more preferably at least 200 mm, especially at least 500 mm into the cooling tube from the gas inlet side. In an especially preferred embodiment, the entire inner surface of the cooling tube is covered with a thermal spray coating.

[0056] In a further preferred embodiment, the thermal spray coating covers at least 0,5% of the inner surface of the cooling tube from the gas inlet side, preferably at least 2%, more preferably at least 5%, based on the complete inner surface of the cooling tube.

[0057] The thermal spray coating should provide sufficient protection against erosion without affecting the cooling of the gas stream. In consideration thereof, a thickness of the thermal spray coating of no more than 0.7 mm and no less than 0.05 mm was found to be advantageous. In a particular preferred embodiment of the present disclosure, the thermal spray coating thus has a thickness of 0.1 to 0.5 mm, more preferably 0.15 to 0.3 mm. [0058] Common wear protection coatings are usually made of several different layers to provide sufficient adhesion as well as sufficient mechanical and thermal protection. However, as discussed above, multilayer coatings render it impossible to reliably determine the wall thickness of the cooling tubes with standard measurement equipment. It was surprisingly found that no additional layers are needed in the course of the present disclosure. Rather, the thermal spray coating can be directly applied to the surface of the cooling tube. Therefore, in a preferred embodiment, the thermal spray coating is directly applied to the surface of the cooling tube, preferably on-site. In a further preferred embodiment, the thermal spray coating is a weldable coating, preferably a single layer coating. In a particular preferred embodiment, the thermal spray coating does not contain any layers formed of a different material than the thermal spray material, preferably no different material than Cr3C2/NiCr alloy.

[0059] Within the meaning of this application, a single layer coating is defined as a layer obtained from a single feed comprising Cr3C2/NiCr.

[0060] The protection systems for transfer line exchangers against erosion which are described in the state of the art suffer from the drawback that they cannot be incorporated into existing transfer line exchangers and that their installation and repair is a rather tedious process. The disadvantages of the prior art are overcome by the present disclosure which offers an easy way to improve the wear resistance of existing equipment by application of a thermal spray coating. In this regard, an embodiment of the present disclosure is preferred wherein the thermal spray coating is applied by HVOF (High Velocity Oxy-Fuel Spraying), Combustion Flame Spraying, Plasma Spraying, Vacuum Plasma Spraying or Two-Wire Electric Arc Spraying, preferably HVOF, a technique which does not require stationary equipment and can also be easily used in tight spaces.

[0061] The transfer line exchanger of the present disclosure is particularly designed for use in cracker applications where rapid cooling of the cracked gas is necessary. Accordingly, the material of the cooling tubes is preferably selected for its efficient heat transfer while at the same being able to withstand the harsh process conditions. Therefore, in a preferred embodiment, the cooling tube is made of heat-resistant carbon steel.

[0062] The cooling tubes of the transfer line exchanger are normally secured by a tube plate at both ends of the tube. In particular the plate at the gas inlet side is thus also in contact with the hot gas entering the cooling tubes and also prone to erosion and fouling. Within the course of the present disclosure, it was surprisingly found that the concept of erosion protection by thermal spray coating could also be applied to the tube plate. Therefore, in a preferred embodiment, each cooling tube is secured by a tube plate at both ends of the tube and the tube plate at the gas inlet side is equipped with a thermal spray coating, preferably a thermal spray coating obtained from a thermal spray material formed of CrsC2 and a NiCr alloy. [0063] The transfer line exchanger of the present disclosure is in particular designed for application in the production of olefins by cracking. Therefore, a further object of the present disclosure is an apparatus for the production of olefins, in particular ethylene and propylene, comprising the transfer line exchanger of the present disclosure.

[0064] A further object of the present disclosure is a process for the cracking of hydrocarbons, in particular for the production of olefins, comprising: (i) feeding a feedstock of hydrocarbons into a cracking coil located inside a thermal reactor; (ii) heating the feedstock to obtain a cracking gas; and (iii) introducing the cracking gas into a transfer line exchanger for cooling, wherein the transfer line exchanger is a transfer line exchanger of the present disclosure.

[0065] Cracking of hydrocarbons is normally carried out at temperatures of up to 1000 °C. In a preferred embodiment of the present process, the gas entering the transfer line exchanger has a temperature of 750 to 1000 °C, preferably 800 to 950 °C.

[0066] Although the wear protection properties of thermal spray coatings are reported in other references, their applicability in cracking equipment and in particular with regard to transfer line exchangers came as a surprise within the course of the present disclosure. A further object of the present disclosure is therefore the use of a thermal spray coating in the erosion protection of transfer line exchangers, in particular the use of a thermal spray coating obtained from a coating material formed of CrsC2 and NiCr alloy, especially applied to the gas inlet area of the cooling tubes.

[0067] The present disclosure also takes into the account the need for an easy and cost and time saving process for the maintenance of transfer line exchangers. Accordingly, a further object of the present disclosure is a process for the maintenance of a transfer line exchanger comprising the steps of: (i) detecting faults on the inner surface of the cooling tubes; and (ii) applying a thermal spray coating to the faults.

[0068] Detection of the faults is preferably conducted by measuring the wall thickness of the cooling tube, preferably via non-destructive methods, in particular via ultrasound.

[0069] The process of the present disclosure can in particular be applied for repairing any faults due to erosion on the inside of the cooling tubes. It was surprisingly found that the process of maintenance of the present disclosure could be applied to transfer line exchangers which are already equipped with a thermal spray coating as well as to common transfer line exchangers without any wear protection. Further, the process was found to offer a fast and cost-effective way of ensuring safe operation of the transfer line exchanger. In contrast to measures suggested by others, the coating did not interfere with the detection of any faults on the water side of the tubes, which under normal circumstances are not accessible and are thus monitored by way of measuring the wall thickness of the cooling tubes in the process side. Rather, the faults could still be detected even after the coating was put in place inside the tube. Also, tedious disassembly of the transfer line exchanger and partial or complete exchange of tubes could be reduced.

[0070] The maintenance process of the present disclosure is in particular intended for on-site operation. Thus, suitable means had to be found to also apply the thermal spray coating directly on-site, as needed. The coating is preferably applied by HVOF (High Velocity Oxy-Fuel Spraying), Combustion Flame Spraying, Plasma Spraying, Vacuum Plasma Spraying or Two-Wire Electric Arc Spraying. In this regard HVOF was found to be the most suitable technique which could also be adapted to the limited space inside the cooling tube. Therefore, in a preferred embodiment, the thermal spray coating is applied by HVOF.

[0071] The thermal spray coating is obtained from a spray material formed of Cr3C2 and a NiCr alloy. Preferably, the spray coating material comprises at least 60 wt.-%, preferably at least 70 wt.-% of Cr3C2, based on the total weight of the spray material. It was surprisingly found that the material not only provided sufficient protection against erosion but also limits the deposition of coke. Further, the material was found to be easily reparable by either welding or applying a new coat of the thermal spray material. Consequently, it is possible to weld the tube plate to the cooling tubes comprising the thermal spray coating and thus replace an entire cooling tube, or even replacing a segment of the cooling tube by removing a damaged part of the cooling tube and welding a replacement segment to the remainder of the cooling tube.

[0072] The cooling tubes of a transfer line exchanger are normally surrounded by a cooling media, commonly water. Therefore, the water side of the tubes is not accessible under normal operation conditions but bears a high risk of damage due to corrosion. It is therefore important to not only regularly inspect the process side of the tubes but also the water side to ensure safe operation. Therefore, a further object of the present disclosure is a method for monitoring the water side surface of cooling tubes of transfer line exchangers by measuring the wall thickness from the cooling tubes on the process side of the tubes. Within the course of the present disclosure, the process side depicts the inside of the tubes which comes into contact with the cracked gas while the water side refers to the outside of the tubes which in contact with the cooling medium, usually water. Any deviation of the wall thickness, especially a decrease, compared to a pre-set reference value is defined as a fault or defect within the course of the present disclosure. Any defects on the water side of the cooling tube will impact the wall thickness of the cooling tubes and thus be easily detected by the method of the present disclosure. In a particular preferred embodiment, the method of the present disclosure for monitoring the water side of transfer line exchangers is combined with the process of the present disclosure for the maintenance of a transfer line exchanger [0073] The process of the present disclosure provides the means to monitor any impact of corrosion which is made impossible by some of the wear protection means usually taken such as multilayer coatings or the use of protection sleeves. Although the present disclosure allows for convenient maintenance of cooling tubes which suffer due to erosion on the inside of the tubes, any defects on the outside of the tubes, namely the water side, may need to be repaired by replacing the affected section of the tube. However, it was surprisingly found that the concept of the present disclosure to apply a thermal spray coating to the inner surface of the tubes does not interfere with the replacement process. Even if the tube is supplied with a thermal spray coating, the tube may still be cut and the replacement section welded to the remaining tube which was found to be impossible, e.g. if multilayer coatings were used as wear protection, in particular those comprising ceramic layers. The process and the transfer line exchanger of the present disclosure may therefore be conveniently incorporated into existing crackers.

[0074] The present disclosure will be explained in more detail with reference to the following figures and examples which by no means are to be understood as limiting the scope and spirit of the disclosure.

[0075] Figure 1 shows a flow chart of a common maintenance process of transfer line exchangers in crackers. Within the normal course of decoking and cleaning the transfer line exchanger, the inlet cone from the transfer line exchanger is removed and the wall thickness on the inside of the cooling tube is determined. In cases were the determined value is found to be below a pre-set threshold value, the tubes are removed, the broken sections replaced and the transfer line exchanger re-assembled. In particular removal of the tubes is difficult due to the size and weight of the tubes and accidents regularly occur.

[0076] Figure 2 shows a flow chart of the maintenance process of the present disclosure. Within the course of the decoking and cleaning the transfer line exchanger, the wall thickness of the tubes is measured. If the determined value is found to be below a pre-set minimum threshold value, a thermal spray coating is applied either to the affected area if the transfer line exchanger is already equipped with a thermal spray coating, or a larger section of the tube is equipped with the coating should no coating have been present. Apart from increasing the safety of the maintenance process of transfer line exchangers, the time for repair is also considerably shortened, resulting in a more effective operation of the cracker.

[0077] Figure 3 schematically shows the concept of the present disclosure for monitoring and maintaining cooling tubes of transfer line exchanger. A measuring device (3) is placed inside a cooling tube of transfer line exchanger and the wall thickness (1) of the tube is measured. Here, the measuring device is inserted from the gas inlet side (4) of the tube. If any defects, i.e. a decrease in the wall thickness, are detected on the inside of the tube, the thermal spray coating (2) can easily be renewed. At the same time, also defects on the outside of the tube can be detected this way which can then be repaired by replacing the affected section of the tube.

[0078] Table 1 summarizes some of the advantages of the present disclosure in comparison with commonly used wear protection techniques. The present disclosure is compared to concepts described in other references such the use of heat sleeves as described in WO 2007/006446 and a 3-layer coating as described in US 7,237,601 . As can be seen from Table 1 , the time needed for repairs may be significantly shortened by the present disclosure and the risk of furnace failure reduced. Further, transfer line exchangers equipped in accordance with the present disclosure showed a reduced tendency for coke deposition.

[0079] In particular, it has been surprisingly found that the thermal spray coating does not negatively affect the weldability of the cooling tubes. Consequently, a cooling tube may comprise at least two tube segments, which are welded together, wherein at least one of the at least two tube segments is provided with the thermal spray coating adjoining to the welding zone. Alternatively or in addition, at least one cooling tube may be welded to the tube plate, wherein the cooling tube is provided with the thermal spray coating adjoining to the welding zone. In contrast to the thermal spray coating obtained by the thermal spray material formed of CrsC2 and a NiCr alloy, the presence of other coatings, such as ceramic coatings, in close proximity to the welding zone has proven to negatively affect the weld seam, such that these pipes will likely fail a quality inspection or a stress test.

[0080] In particular, it has been found that a martensitic structure is formed at least partially at the weld seam, when welding in the presence of the thermal spray coating obtained by the thermal spray material formed of CrsC2 and a NiCr alloy. The martensitic structure may further increase the hardness of the weld further improving the erosion resistance of the cooling tube. The martensitic structure may be formed of the weld. Thus, the weld seam may be harder than a base material of the cooling tube, the hardness being measured according to DIN EN ISO 6507- 1 :2018-07. The weld seam may particularly be formed on the outward facing side of the weld.

Table 1 :

[0081] In addition, to the improved process of maintaining TLEs, it was further surprisingly found that the thermal spray coating also leads to an improvement of the cooling efficiency of the TLE. As shown in Table 2, the temperature increase of the cracking gas in the coated TLEs was lower compared to common TLEs without coating. Table 2:

[0082] The particular embodiments disclosed above are merely illustrative, as the present disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and such variations are considered within the scope and spirit of the present disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. While compositions and methods are described in broader terms of "having”, “comprising," "containing," or "including" various components or steps, the compositions and methods can also "consist essentially of’ or "consist of’ the various components and steps. Use of the term “optionally” with respect to any element of a claim means that the element is present, or alternatively, the element is not present, both alternatives being within the scope of the claim.

[0083] Numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, each range of values (of the form, "from about a to about b," or, equivalently, "from approximately a to b," or, equivalently, "from approximately a-b") disclosed herein is to be understood to set forth each number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and unambiguously defined by the patentee. Moreover, the indefinite articles "a" or "an", as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents, the definitions that are consistent with this specification should be adopted. [0084] Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, such scope including equivalents of the subject matter of the claims.