ES2673485T3 - Heat exchanger with greater corrosion resistance - Google Patents

Abstract

A permanently attached plate set for a stainless steel, copper or carbon steel plate heat exchanger, characterized in that at least all media contacting surfaces of at least one of the flow sides of the plate set have an alloy-bonded coating of a tantalum-containing compound, wherein the coating is about 1-300 µm thick, and because the coating has been applied after the plate assembly has been permanently bonded.

Description

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DESCRIPTION

Heat exchanger with greater corrosion resistance Field of the invention

The present invention relates to a set of plates for a plate heat exchanger and a plate heat exchanger with greater corrosion resistance.

Background

WO 96/06705 discloses a set of plates having the characteristics in the preamble of claim 1. Documents DE 10 2008 013358 A1 and GB 933295 A disclose plate heat exchangers having coatings. Plate heat exchangers can be used for different types of fluids.

However, some fluids are considered very corrosive. When heat from at least one corrosive fluid is exchanged, the demands on the heat exchanger increase.

Nowadays, the choice is usually between materials that can corrode which results in a short service life of the plate heat exchanger with risk of contamination of the fluid or a heat exchanger made of a material more resistant to corrosion, being the latter in comparison very expensive. Unfortunately, various materials that are considered corrosion resistant cannot be used for all parts of permanently mounted plate heat exchangers since the materials used cannot provide a satisfactory permanent bond. Heat exchangers of brazing plates can be made of a corrosion resistant plate material, but brazing material is a less corrosion resistant material, thus constituting an obstacle for heat exchangers to be used in connection with certain liquids or gases. In addition, the brazing technique itself can mix plate material and brazing material during heat exchanger assembly resulting in more easily corroded areas.

In addition, the corrosion-resistant materials that can be applied to the plates of a heat exchanger before assembly can make it difficult or impossible for said heat exchanger to achieve a satisfactory permanent bond with good anti-corrosion properties.

Coating materials, such as plastics, are considered insufficient in terms of resistance to fatigue and corrosion for highly corrosive fluids. The stress exerted on a plastic coating on a plate of a plate heat exchanger, for example, in the form of high pressures and / or high temperatures also causes the coating to degrade and / or lose its adhesion to the plate. In addition, large pressure differences and large temperature differences during the use of a plastic coated heat exchanger may cause the coating to degrade and scale. Plastics also exhibit lower thermal transmission properties compared to the metals from which a plate heat exchanger is made.

Tantalum is a very corrosion resistant metal for many fluids and is known to produce heat exchangers for this metal. However, tantalum is an expensive and mechanically considerably weaker metal than other materials known for use in heat exchangers such as stainless steel. Therefore, thicker plates should be used to withstand the mechanical stress exerted on a heat exchanger made of tantalum.

WO 92/16310 discloses a method of surface protection heat transfer plates in a heat exchanger that uses plastics as a surface protection material. According to the method, a gaseous medium containing the plastics is introduced into the mounted plate heat exchanger which then forms a layer on the surfaces of the heat exchanger plates.

GB 1,112,265 discloses tubular heat exchangers in contact with highly corrosive media. In the document it is disclosed that the mounting plates may be coated or coated with tantalum and the tubes may be made of tantalum.

WO 96/06705 discloses heat exchangers entirely made of brazing that are resistant to corrosive media because the brazing joints between the plates are protected by a coating that resists corrosive media. The plates are made of stainless steel, the solder is made of copper and the protective layer intended to cover the solder joints is a metal such as tin or silver.

US 2010/0051246 discloses a high temperature and high pressure corrosion resistant process heat exchanger, in which the surfaces of the third cooling channel of the fin system

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Heat transmission and the heat transmission plate, which come into contact with sulfuric acid and / or sulphite, are subjected to an ionic beam coating and ionic beam mixture using a material that has a high corrosion resistance, such as SiC, Al2Ü3, silicon steel and tantalum.

JP 4,334,205 discloses a plate heat exchanger with plates made of titanium, stainless steel, copper, nickel or alloys thereof. In order to suppress the elution of the electrode material of a plate, a coating treatment is carried out on at least 30% of the electrode areas of the heat transfer plate by the side of a cooling water passage. The coating may comprise platinum metal oxide, manganese, tantalum, tin, etc.

EP 110,311 discloses a flat heat exchange plate comprising two plates that may be surface coated with tantalum or a tantalum alloy and at least one conduit. The two plates are joined together to form the flat heat exchange plate by using an adhesive layer.

It would be desirable to find new ways to guarantee more corrosion resistant heat exchangers in order to process highly corrosive media and increase the life of heat exchangers. It is also desirable to be able to produce corrosion-resistant heat exchangers from cheaper base materials that have good mechanical properties and that are easily and efficiently assembled permanently. It would also be desirable that all parts of a heat exchanger, for example, both the plates and the joints that are in contact with a highly corrosive fluid are equally highly resistant to corrosion. In addition, it would be desirable to achieve more internal parts of heat exchangers resistant to fatigue and corrosion in contact with highly corrosive fluids. It would also be desirable to find corrosion and fatigue resistant materials applied inside a plate heat exchanger, whose materials show good adhesion. Even more, it would be desirable to achieve a good or improved heat transfer in the plate heat exchanger.

Summary of the invention

It is an objective of the present invention to solve the problems mentioned above. Therefore, it is an object of the present invention to provide good mechanical properties and high corrosion resistance of all parts of a heat exchanger in contact with highly corrosive fluids. It is also an objective of the present invention that a good heat transfer is obtained.

This objective is achieved by a set of plates permanently attached to a plate exchanger that is coated with a coating containing tantalum inside, such as plates and joints, on at least one side of the flow of the plate assembly. By applying a coating comprising tantalum, highly corrosive means, such as hydrochloric acid, can be used in a plate heat exchanger without rapid degradation of the heat exchanger.

The present invention relates to a permanently attached plate assembly for a stainless steel or carbon steel plate exchanger in which at least all surfaces in contact with means of at least one of the flow sides of the plate assembly have an alloy bonded coating of a tantalum-containing compound. The present invention also relates to a plate heat exchanger comprising said set of plates.

An embodiment of a plate heat exchanger according to the present invention includes the heat exchanger having racks and / or mounting plates that are part of at least one of the flow sides of the heat exchanger and said racks and / or plates Mounting are made of tantalum, or stainless steel or carbon steel having an alloy bonded coating of a tantalum containing compound, preferably stainless steel or carbon steel having an alloy bonded coating of a tantalum containing compound, more preferably stainless steel having an alloy bonded coating of a tantalum-containing compound.

Another embodiment of a plate heat exchanger according to the present invention is when the plate heat exchanger is permanently attached and made of stainless steel or carbon steel and all surfaces of at least one side of the flow of the heat exchanger. Plates have an alloy bonded coating of a compound containing tantalum.

Detailed description of the invention

A set of permanently attached plates or conventional plate heat exchanger can be made more resistant to corrosion than it was at the beginning with the present invention.

A plate heat exchanger consists of multiple thin metal plates that have very large surface areas and fluid flow passages that can allow heat transfer. A heat exchanger is provided with at least two inlets and two outlets for fluid heat exchange. Be

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they can use additional fluids that require additional inputs and outputs from the heat exchanger. Plate heat exchangers comprise a series of heat transfer plates. These heat transfer plates form what is called a set of plates in the heat exchanger. Heat transfer plates are made of thin sheets of metal and are often provided with corrugations or other protrusions in their heat transfer parts, which in a heat exchanger collide with each other by a large force in a large number of places of contact distributed throughout all heat transfer portions. Next, the heat transfer plates are assembled, and intermediate spaces are formed between the plates. These intermediate plate spaces are intended for at least one heat exchange fluid to flow. In a plate heat exchanger, at least two fluids are flowing through the intermediate spaces, next to each other, allowing heat transfer to take place. These intermediate spaces between the plates intended for the flow of one of the fluids are considered in the present application as part of one side of the flow. In the present application, the flow side is connected to the construction of a heat exchanger or set of fluid plates, that is, the fluid flow passages. Since at least two fluids are used in a plate heat exchanger, it has at least two flow sides, a flow side for a hot fluid and a flow side for a cold medium. For each side of the flow, all parts of a plate assembly or a heat exchanger that is in contact with the fluid flowing hot or cold are considered to belong to that flow side, for example, plates, spaces between plates, joints , connections, input and / or output ports in frames or mounting plates. In a plate or plate heat exchanger assembly according to the present invention, at least one of the flow sides is designed for highly corrosive fluids when in use.

With the present invention, simple rigid base materials can be used for heat exchangers such as stainless steel, copper and carbon steel and with a coating containing tantalum that has become resistant to corrosion to highly corrosive fluids. With the present invention, other parts of the plate or heat exchanger assembly are also coated with a corrosion resistant material such as gaskets that may be more sensitive parts of the plate heat exchanger due to, for example, welding during assembly of the heat exchanger. The joints can also be sensitive parts of the heat exchanger due to welding, fusion bonding or brazing during heat exchanger assembly. The term "fusion bonding" refers to the use of an iron-based solder in accordance with the disclosures of, for example, EP 1 347 859 B1 and WO 02/098600. The assembly of a heat exchanger by welding, fusion bonding or strong welding of the joints can be made of a different material than the plates. During the assembly process, the solder or solder material is applied to the plates, covering all or part of the plates, and the solder or solder material can be mixed during assembly with additional coatings on the plate material or in some cases even the plate material itself creating more corrosion sensitive parts of the heat exchanger. Since at least both plates and gaskets of a plate assembly or plate exchanger according to the present invention are coated, the heat exchanger becomes more resistant to corrosion. Therefore, in this way, the joints or areas on the plates near the joints can no longer be a weak link for the heat exchanger.

In one embodiment of the present invention, the permanently attached mounted plate heat exchangers or the plate heat exchanger plate assemblies made of stainless steel or carbon steel are coated with a tantalum-containing compound. The sets of plates or heat exchangers can, for example, be permanently assembled by autogenous welding, soft welding, fusion bonding or brazing. A tantalum-containing compound is introduced into the heat exchanger in at least the intermediate plate spaces intended for the flow of one of the two heat exchanger fluids, that is, at least one of the flow sides designated to be used for highly corrosive fluids when in use. Within the heat exchanger or set of plates, the tantalum-containing compound is deposited on all surfaces of at least one of the flow sides of the heat exchanger or set of plates, for example, plates, seals and other parts intended for be in contact with fluid heat exchangers.

The use of a tantalum-containing compound according to the present invention provides a plate assembly or plate heat exchanger with very good properties. Tantalum shows better heat transfer properties than plastics that are not considered thermally conductive materials. In accordance with the present invention, it is important to be able to present a coating or layer that does not impair heat transfer. Tantalum shows good heat transfer properties. In addition, the tantalum-containing coating according to the present invention is chemically bonded to the plate assembly materials and the plate heat exchanger. The tantalum-containing compound is bound by alloy to said materials. In this context, an alloy joint is a solid metallic solution composed of two or more elements of two or more different metal bodies composed of different materials, in the present invention tantalize it and the plate material, for example stainless steel, copper or steel to carbon, in an interface layer between the bodies. Such alloy bonding results in more sets of fatigue-resistant plates and heat exchangers compared to, for example, heat exchangers coated with plastic materials. Since the tantalum is partially alloyed to the material, the adhesion is superior. This makes it easier for the coating containing tantalum to follow the movements of the plate and the joint materials due to thermal and pressure changes within the

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plate heat exchanger when out of use and during use. The tantalum-containing coating has a gradual transition of compounds within itself. When looking at the coating containing tantalum in a cross-sectional view, the intermediate phase closest to the heat exchanger material, for example, a plate, shows an alloy of compound containing tantalum and the plate material, then a gradual transition is made only to the tantalum-containing compound, which is then gradually transferred to the tantalum oxide since the outer surface of the tantalum-containing compound oxidizes. Therefore, since not all of the tantalum-containing coating applied to the parts of a heat exchanger is an alloy with said parts, it is considered that the tantalum-containing compound is partially alloyed to the parts of the heat exchanger.

The thickness of the tantalum lining film should not be too high because that would negatively influence the heat transfer properties since an increased barrier, an increased plate thickness, between heat transfer fluids decreases heat transfer. If the film thickness is low, the effect of the coating may not last as long as it is suspected when it comes into contact with a highly corrosive fluid.

In accordance with the present invention, it is coated with a compound containing tantalum inside a set of plates or heat exchanger using a deposition process with chemical reactants in fluid form. The method of coating a set of permanently attached plates or heat exchanger in accordance with the present invention comprises the steps: 1) introducing chemical reactants in the gas or steam phase into said set of plates or heat exchanger in at least one of the flow sides of the heat exchanger, wherein at least one of the reactants is a reactant comprising tantalum, 2) formation of a solid film comprising a compound containing tantalum on the surfaces of said set of plates or exchanger of heat of reaction of chemical reagents in the gas or vapor phase.

The application process refers to the formation of a non-volatile solid film on a substrate, in this case parts of a set of plates or heat exchanger, of the reaction of chemical reactants in the gas or vapor phase, in which At least one reactant is a reactant comprising tantalum. A reaction chamber is used for the process, in which the gases or vapors of the reactants are introduced to decompose and react with the substrate or, in the case of multiple applications, the previously applied layer to form the film. Inside the reaction chamber, the reactants are introduced into the plate or heat exchanger assembly. In one embodiment, the reactant comprising tantalum in fluid form is tantalum pentachloride.

The application process disclosed above could also be used for parts of a heat exchanger such as racks or mounting plates that are not part of a permanently attached heat exchanger. Said coated racks or plates can be subsequently used together with a set of permanently bonded plates, coated in accordance with the present invention.

The application process of the tantalum-containing composition is preferably performed by chemical vapor deposition (CVD) or atomic layer deposition (ALD), preferably by CVD.

A basic CVD process consists of the following steps: 1) a predefined mixture of reactant gases and diluent inert gases are introduced at a specified flow rate in the reaction chamber; 2) the kind of gas moves towards the substrate; 3) reagents are adsorbed on the surface of the substrate; 4) reagents undergo chemical reactions with the substrate to form the film; and 5) the gaseous byproducts of the reactions are desorbed and evacuated from the reaction chamber.

The growth of layers of material by ALD consists in repeating the following four characteristic stages: 1) Exposure of the first precursor. 2) Purging or evacuating the reaction chamber to eliminate unreacted precursors and by-products of the gas reaction. 3) Exposure of the second precursor or other treatment to reactivate the surface for the reaction of the first precursor. 4) Purging or evacuating the reaction chamber. Each reaction cycle adds a certain amount of material to the surface, called growth per cycle. To grow a layer of material, the reaction cycles are repeated as many times as required for the desired film thickness.

In one embodiment, the method of coating a permanently bonded heat exchanger made of stainless steel, copper or carbon steel comprises the steps: 1) introducing chemical reactants in the gas or steam phase into said heat exchanger in at least one of the flow sides of the heat exchanger, wherein at least one of the reactants is a compound comprising tantalum, 2) forming a solid film comprising tantalum on the surfaces of said heat exchanger from the reaction of the reactants Chemicals in the gas or vapor phase, for stages 1) and 2), and preferably carried out at a temperature of 600-1000 ° C, more preferably 700-900 ° C.

In another embodiment of the present invention, steps 1) and 2) are carried out at atmospheric pressure, subatmospheric pressure or at very low pressure.

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In accordance with the present invention, it is important that the heat exchange plates of a plate assembly or plate heat exchanger not only be permanently joined together along their peripheral parts, it is also important that in various areas of Contact in your heat exchange portions are permanently attached. If the plates only join along their peripheral portions, other contact areas may move / dislocate during use. If only the contact surfaces along their peripheral parts are permanently joined, the plates may separate into some contact areas that are not permanently joined during use when the plate heat exchanger is, for example, pressurized in one of the sides of the fluid flow. In the case of contact change areas due to, for example, pressurization, a coated heat exchanger that is not attached in all contact areas within the fluid flow would then have uncoated areas exposed to the fluid in the heat exchanger. , resulting in corroded areas if the fluid used is corrosive. Therefore, it is important that all contact areas between the plates, where the contact areas are in contact or surrounded by corrosive fluid, are permanently bonded by autogenous welding, soft welding, fusion bonding or brazing.

A set of plates permanently attached to a plate heat exchanger as disclosed herein should be construed as a set of non-accessible plates in which at least all areas of contact between plates in contact with corrosive fluid are permanently attached. Therefore, since the plate assembly is not accessible, it should be interpreted that the complete plate assembly cannot be disassembled in any way. Such a plate assembly according to the present invention can be used in a plate heat exchanger having, for example, racks and / or mounting plates of any material, provided they are not in contact with the corrosive fluid in at least One side of flow. If, for example, the racks or mounting plates are part of at least one of the flow sides of the heat exchanger and are in contact with a highly corrosive fluid, said racks and / or mounting plates are preferably made of tantalum, stainless steel or carbon steel having an alloy bonded coating of a compound containing tantalum in at least the parts of the at least one of the flow sides of the heat exchanger. For such racks and / or mounting plates, preferably stainless steel or carbon steel having an alloy bonded coating of a tantalum containing compound, more preferably stainless steel having an alloy bonded coating of a tantalum containing compound is used.

A permanently bonded plate heat exchanger as disclosed herein should be construed as a non-accessible heat exchanger comprising a set of permanently bonded plates in which at least all contact areas between plates in contact with corrosive fluid are permanently united. Therefore, since the plate heat exchanger is not accessible, it should be interpreted that the plate heat exchanger cannot be disassembled. For a plate heat exchanger, this means that not even the racks or mounting plates that are around a set of plates and must be in contact with at least one corrosive heat exchange fluid can be separated. The permanently attached plate heat exchanger according to the present invention is impossible to disassemble in any way in the parts in contact with at least one fluid, for example, a corrosive fluid. Expressions permanently attached and permanently assembled with respect to plate assemblies and plate heat exchangers are considered interchangeable in the present application.

The present invention relates to the application of a solid film of a coating containing tantalum on surfaces within a plate assembly or plate heat exchanger permanently attached. The compound containing tantalum used as a coating, preferably metallic tantalum, tantalum oxide and / or tantalum nitride, applied on the surfaces of the heat exchangers to be in contact with a highly corrosive fluid. In a preferred embodiment, the tantalum-containing compound is metallic tantalum and / or tantalum oxide, preferably metallic tantalum. If the tantalum lining is made of metallic tantalum, naturally, the upper part of the lining is oxidized and, therefore, is tantalum oxide, and the lower part of the lining is alloyed with the materials of a plate assembly or permanently exchanged plate exchanger.

The set of permanently bonded plates and the permanently bonded heat exchanger coated in accordance with the present invention is made of stainless steel or carbon steel. Stainless steel and carbon steel are considered materials with good mechanical properties. The set of permanently bonded plates or the permanently bonded heat exchanger is assembled by autogenous welding, soft welding, fusion bonding or brazing, preferably by autogenous welding, fusion bonding or brazing. Brazing is preferably performed by using copper as brazing material. Preferably, the heat exchanger is made of stainless steel and is assembled using autogenous welding, fusion bonding or brazing, preferably fusion bonding or copper brazing.

According to the present invention, the coating comprising tantalum applied on the surfaces on at least one of the flow sides designated to be used for highly corrosive fluids preferably has a film thickness of about 1-300 pm, preferably 1-125 pm, more preferably 1-50 pm, even more preferably 10-40 pm and more preferably 15-25 pm.

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Examples

Two stainless steel units welded with copper, CB14, and two stainless steel units Alfa Fusión, AN14, from Alfa Laval have been processed with the CVD process to coat with tantalum. The conventional Alfa Fusion units, AN14, were used as reference. All units contained stainless steel plates, but in CB14 they were welded copper and in AN14 they were joined by fusion.

Process:

Tantalum reacts with gaseous chlorine to form TaCl5. The gas is conducted to a vacuum oven at 850 ° C where the TaCl5 will react with the available surfaces (stainless steel, copper, carbon steel, graphite, etc.) to form a tantalum coating with CVD. The gas pressure is approximately 25 mB, and the process runs for approximately 8 hours.

The chlorine released during the process will react with hydrogen to form hydrochloric gas that is removed from the process and neutralized with sodium hydroxide.

TaCl5 gas is conducted from the central tube to the units. The small hanging spacers attached to the entrance and exit are used to assess the thickness of the tantalum layer. According to the measurement of the weight of the spacers before and after the process, the average thickness of the tantalum layer is approximately 45 gm at the entrance and 38 gm at the exit.

Analysis:

The tantalum processed units by CVD (units CB 14 and AN 14) were tested for corrosion with hydrochloric acid at 75 ° C for 48 h. The hydrochloric acid used for the test showed almost no change in color after recirculation in the units treated with tantalum. CB 14 and AN 14 units coated with tantalum showed no leakage or other signs of corrosion damage during or after the corrosion test. After the corrosion test, the units were tested under pressure with compressed air at 8 bar. No external or internal leaks were observed in the units.

Corrosion was also tested in a conventional AN14 unit in hydrochloric acid. For the conventional AN14 unit, hydrochloric acid reacted strongly with stainless steel surfaces under hydrogen gas emission, the acid had to be replaced a couple of times due to depletion. A strong green coloration of iron chloride was observed in the acid of the standard unit. The conventional AN14 unit showed no leaks after 90 minutes, but after 6 hours numerous large leaks were detected.

After corrosion tests, the units were cut and the cross sections of the surfaces were prepared metallographically and examined with a microscope. The units treated with tantalum were cut and four cross sections of each unit were examined. The CB 14 unit showed very good adhesion between copper and tantalum in all the locations investigated. The CB 14 unit showed slightly better adhesion between copper and tantalum than stainless steel and tantalum in unit AN 14. One reason for this could be that the surface of unit AN 14 may have been contaminated or, to a lesser extent , due to the greater surface roughness in the AN 14 unit.

The thickness of the tantalum layer varies from about 105-125 gm in the areas around the entrance just over 10 gm at the maximum diagonal distance from the entrance in the AN 14 unit.

The thickness of the tantalum layer varies from about 150 gm at the entrance to the finest thickness, most likely less than 5 gm at the maximum diagonal distance from the entry into the CB 14 unit.

Claims (8)

5 10 fifteen twenty 25 30 35 1. A set of plates permanently attached for a heat exchanger of stainless steel, copper or carbon steel plates, characterized in that at least all surfaces in contact with the means of at least one of the flow sides of the assembly of Plates have an alloy-bonded coating of a tantalum-containing compound, wherein the coating has a thickness of about 1-300 pm, and why the coating has been applied after the plate assembly has been permanently attached. 2. A set of permanently bonded plates according to claim 1, wherein the tantalum-containing compound is metallic tantalum, tantalum oxide and / or tantalum nitride, preferably metallic tantalum and / or tantalum oxide, more preferably metallic tantalum . 3. A set of permanently bonded plates according to claims 1 or 2, wherein the set of plates has been assembled using solder, fusion bonding or brazing, preferably fusion bonding or brazing copper. 4. A set of permanently bonded plates according to any one of claims 1-3, wherein the plate set is made of stainless steel and has been assembled using autogenous welding, fusion bonding or brazing, preferably fusion bonding or copper solder. 5. A set of permanently bonded plates according to any one of claims 1-4, wherein the coating of a tantalum-containing compound has a thickness of about 1-125 pm, preferably 1-50 pm, more preferably 10- 40 pm and most preferably 15-25 pm. 6. A plate heat exchanger characterized in that it comprises a set of plates of the type defined in any one of claims 1-5. 7. A plate heat exchanger according to claim 6, wherein said heat exchanger has racks and / or mounting plates that are part of at least one of the flow sides of the heat exchanger and said racks and / or mounting plates are made of tantalum, or stainless steel or carbon steel having an alloy bonded coating of a tantalum containing compound, preferably stainless steel or carbon steel having an alloy bonded coating of a tantalum containing compound , more preferably stainless steel having an alloy bonded coating of a tantalum-containing compound. 8. A plate heat exchanger according to claim 7, wherein the plate heat exchanger is permanently attached and made of stainless steel or carbon steel and all surfaces of at least one of the flow sides of the plate heat exchanger have an alloy bonded coating of a tantalum-containing compound.