TRANSFER LINE EXCHANGER WITH THERMAL SPRAY COATING

Abstract

The present disclosure refers to transfer line exchangers with improved cooling capacity, equipped with a thermal spray coating in crackers and a method for the maintenance of transfer line exchangers with the help of a thermal spray coating. In some embodiments, the thermal spray coating is obtained from a spray material formed of Cr.sub.3C.sub.2 and a NiCr alloy. Preferably, the spray coating material comprises at least 60 wt.-%, preferably at least 70 wt.-% of Cr.sub.3C.sub.2, based on the total weight of the spray material.

Claims

1. 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 obtained from a spray coating material comprising Cr.sub.3C.sub.2 and a NiCr alloy.

2. The transfer line exchanger of claim 1, wherein the spray coating material comprises at least 60 wt.-% of Cr.sub.3C.sub.2, based on the total weight of the spray coating material.

3. The transfer line exchanger of claim 1, wherein the gas inlet side of each cooling tube, which is impacted by gas as the gas enters the cooling tube, is equipped with the thermal spray coating.

4. The transfer line exchanger of claim 1, wherein the thermal spray coating extends at least 100 mm into the cooling tube from the gas inlet side.

5. The transfer line exchanger of claim 1, wherein the thermal spray coating has a thickness of 0.1 to 0.5 mm.

6. The transfer line exchanger of claim 1, wherein the thermal spray coating is directly applied on-site to the surface of the cooling tube.

7. The transfer line exchanger of claim 1, wherein the thermal spray coating is a weldable coating.

8. The transfer line exchanger of claim 1, 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.

9. The transfer line exchanger of claim 1, wherein each cooling tube is 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.

10. The transfer line exchanger of claim 1, wherein at least one cooling tube comprises at least two tube segments welded to each other at a welding area and at least one segment is provided with the thermal spray coating adjoining to the welding area.

11. An apparatus for the production of olefins comprising the transfer line exchanger of claim 1.

12. A process for cracking hydrocarbons, comprising the steps of: (i) feeding a feedstock of hydrocarbons into a cracking coil located inside a thermal reactor; (ii) heating the feedstock, thereby obtaining 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 claim 1.

13. The process for cracking hydrocarbons of claim 12, further comprising a maintenance phase comprising the steps of: (iv) detecting faults on the inner surface of the cooling tubes; and (v) applying the thermal spray coating to the faults, wherein the cooling tube has a process side and a water side.

14. The process of claim 13, wherein the step of detecting of faults is conducted by measuring the wall thickness of the cooling tube on the process side.

15. The process of claim 13, wherein the water side surface of the cooling tubes of the transfer line heat exchanger is monitored by measuring the wall thickness of the cooling tubes on the process side of the tubes.

Description

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] FIG. 1 provides a schematic overview of a common maintenance process for transfer line exchangers, according to an embodiment of the disclosure;

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

[0046] FIG. 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 Cr.sub.3C.sub.2 and a NiCr alloy. Preferably, the spray coating material comprises at least 50 wt.-%, at least 60 wt.-%, preferably at least 70 wt.-% of Cr.sub.3C.sub.2, based on the total weight of the spray material. The spray coating material may in one embodiment comprise about 75 wt.-% of Cr.sub.3C.sub.2. In some embodiments, the spray coating material may comprise up to 95 wt.-%, up to 90 wt-%, preferably up to 80 wt.-% of Cr.sub.3C.sub.2. 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 Cr.sub.3C.sub.2 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 dis-cussed 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 addi-tional 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 Cr.sub.3C.sub.2/NiCr alloy.

[0059] Within the meaning of this application, a single layer coating is defined as a layer obtained from a single feed comprising Cr.sub.3C.sub.2/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 Cr.sub.3C.sub.2 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 Cr.sub.3C.sub.2 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 Cr.sub.3C.sub.2 and a NiCr alloy. Preferably, the spray coating material comprises at least 60 wt.-%, preferably at least 70 wt.-% of Cr.sub.3C.sub.2, 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 com-bined 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] FIG. 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] FIG. 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] FIG. 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 U.S. Pat. No. 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 Cr.sub.3C.sub.2 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 Cr.sub.3C.sub.2 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-US-00001 TABLE 1 Comparative Comparative Comparative Present Example 1 Example 2 Example 3 Disclosure Protection No Sleeve 3-layer 1-layer Technique protection protection coating coating Coating No No Yes Yes Reduction of Process 0.45 0 0 0 wall thickness side of cooling tubes Water 0.08 0.08 0.08 0.08 [mm/year] side Coke Deposit Yes Yes Yes No Wall Thickness Yes No, due to Difficult due Yes Measurement gap between to different Possible? sleeve and layers and wall inhomogeneous layer transitions Weldable? Yes Yes, but No, upper Yes sleeves layer contains have to be non-weldable removed first ceramic elements Duration of Replacement of Replacement Extensive Application/ Repair affected tube of affected replacement restoration section: ~13 tube section and of coated of wear days installation of parts: ~40 protection: ~3 new sleeves: ~18 days days days Potential Risks Furnace failure; Furnace failure; Furnace failure; None, as wall furnace outage furnace outage; furnace outage thickness risk due to reductions are flying protective measurable sleeves and the bundle tubes no longer need to be partially replaced.

[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-US-00002 TABLE 2 Average Temperature Average Outlet Temperature during production run after start-up 20 days 30 days 40 days TLE TLE TLE Temp. TLE Temp. TLE Temp. inlet, outlet, outlet, increase, outlet, increase, outlet, increase, ? C. ? C. ? C. ? C. ? C. ? C. ? C. ? C. Without 803 330 363 33 372 42 376 46 coating With 803 330 357 27 363 33 364 34 coating

[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 fea-tures 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 numeri-cal 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 encom-passed 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.