SE535209C2

SE535209C2 - Corrosion resistant plate heat exchanger with tantalum coating

Info

Publication number: SE535209C2
Application number: SE1050608A
Authority: SE
Inventors: Mats Nilsson; Maribel Nilsson
Original assignee: Alfa Laval Corp Ab
Priority date: 2010-06-15
Filing date: 2010-06-15
Publication date: 2012-05-22
Also published as: SI2583048T1; US20130068428A1; SE1050608A1; EP2583048A2; WO2011159238A3; DK2583048T3; CA2798155A1; EP2583048B1; CA2798155C; ES2673485T3; WO2011159238A2

Abstract

16 Abstract The present invention relates to a plate package for a plate heatexchanger and a plate heat exchanger made of materials with improved anti-corrosion properties for highly corrosive fluids such as hydrochloric acid.

Description

HEAT EXCHANGER WITH IMPROVED CORROSION RESISTANCE Field of the lnventionThe present invention relates to a plate package for a plate heatexchanger and a plate heat exchanger with improved corrosion resistance.

BackgroundPlate heat exchangers may be used for different types of fluids.

However, some fluids are considered very corrosive. When heat exchangingat least one corrosive fluid the demands on the heat exchanger increases.Today the choice is often between materials which may corrode giving a shortlife time of the plate heat exchanger with a risk of contaminating the fluid or aheat exchanger made of a more corrosion resistant material, the latter beingvery expensive in comparison. Unfortunately, several materials that areconsidered corrosion resistant are not able to be used for all parts ofpermanently assembled plate heat exchangers since the materials used areunable to give satisfying permanent joining. Brazed plate heat exchangersmay be made of a corrosion resistant plate material but the brazing material isa less corrosion resistant material thus constituting an obstacle for the heatexchangers to be used in connection with certain liquids or gases. Then thebrazing technique itself may mix plate material and brazing material duringassembly of the heat exchanger giving rise to more easily corroding areas.Also, corrosion resistant materials that can be applied to the plates of a heatexchanger before assembly can make it difficult or impossible for such a heatexchanger to achieve satisfying permanent joining with good anti-corrosionproperties.

Coating materials like plastics are considered not enough fatigue andcorrosion resistant for highly corrosive fluids. The stress put on a plasticcoating on a plate of a plate heat exchanger e.g. in the form of high pressuresand/or high temperatures also makes the coating degrade and/or lose itsadhesion to the plate. Also, high pressure differences and high temperaturedifferences during use of a plastic coated heat exchanger may cause thecoating to degrade and e.g. flake. Plastics also exhibit inferior thermal transmittance properties compared to metals which a plate heat exchanger ismade of.

Tantalum is a very corrosion resistant metal towards many fluids and itis known to make heat exchangers of this metal. However, tantalum is anexpensive metal and is mechanically considerably weaker than other knownmaterials for use in heat exchangers such as stainless steel. Thus, thickerplates must be used to withstand the mechanical stress put on a heatexchanger made of tantalum.

WO 92/16310 discloses a method of surface protecting heat transferplates in a heat exchanger using plastics as a surface protecting material.According to the method a gaseous medium containing the plastics isintroduced into the assembled plate heat exchanger which then forms a layeron the surfaces of the heat exchanger plates.

GB 1,112,265 discloses tubular heat exchangers in contact with highlycorrosive media. ln the document it is disclosed that mounting plates may becoated or lined with tantalum and the tubes may be made of tantalum.

WO 96/06705 discloses fully brazed heat exchangers which areresistant to corrosive media due to the brazing joints between the plates areprotected by a coating resisting the corrosive media. The plates are made ofstainless steel, the solder is copper solder and the protective coating intendedto cover the brazing joints is a metal such as tin or silver.

US 2010/0051246 discloses a high-temperature and high-pressurecorrosion resistant process heat exchanger, wherein the third system coolantchannel surfaces of the heat transmission fin and heat transmission plate,which come in contact with sulphuric acid and/or sulfite, are subjected to ionbeam coating and ion-beam mixing using a material having high corrosionresistance such as SiC, AlgOg, silicon steel and tantalum.

JP 4,334,205 discloses a plate heat exchanger with plates made oftitanium, stainless steel, copper, nickel or alloys thereof. ln order to suppresselution of electrode material from a plate a coating treatment is performed onat least 30% of the heat transfer plate electrode areas by the side of a coolingwater passage. The coating may comprise platinum metal oxide, manganese,tantalum, tin etc.

EP 110,311 discloses a flat heat-exchange plate comprising two plateswhich may be surface coated with tantalum or a tantalum a||oy and at leastone duct. The two plates are attached to each other to form the flat heat-exchange plate by use of an adhesive coat. lt would be desirable to find new ways to ensure more corrosionresistant heat exchangers in order to be able to process highly corrosivemedia and increase the life time of the heat exchangers. lt is also desirable tobe able to produce corrosion resistant heat exchangers from cheaper basematerials that have good mechanical properties and are easily and effectivelypermanently assembled. lt would also be desirable that all parts of a heatexchanger, e.g. both plates and joints, which are in contact with a highlycorrosive fluid are equally highly corrosion resistant. Further, it would bedesirable to achieve more fatigue and corrosion resistant internal parts ofheat exchangers in contact with highly corrosive fluids. lt would also bedesirable to find corrosion and fatigue resistant materials applied on theinside of a plate heat exchanger, which materials show good adhesion. Stillfurther, it would be desirable to achieve a good or improved heat transfer inthe plate heat exchanger.

Summary of the invention lt is an object of the present invention to solve the above mentionedproblems. Thus, it is an object of the present invention to provide goodmechanical properties and high corrosion resistance of all parts of a heatexchanger in contact with highly corrosive ﬂuids. lt is also an object of thepresent invention that good heat transfer is obtained.

This object is achieved by a permanently joined plate package for aplate heat exchanger being coated with a tantalum containing coatingeverywhere on the inside, such as both plates and joints, in at least one flowside of the plate package. By applying a coating comprising tantalum highlycorrosive media such as hydrochloric acid can be used in a plate heatexchanger without a rapid degradation of the heat exchanger.

The present invention relates to a permanently joined plate package fora plate heat exchanger made of stainless steel or carbon steel wherein all surfaces of at least one of the flow sides of the plate package have an alloybonded coating of a tantalum containing compound. The present inventionalso relates to a plate heat exchanger comprising said plate package.

One embodiment of a plate heat exchanger according to the presentinvention include the heat exchanger having frames and/or mounting platesthat are a part of at least one of the flow sides of the heat exchanger and saidframes and/or mounting plates are made of tantalum, or stainless steel orcarbon steel having an alloy bonded coating of a tantalum containingcompound, preferably stainless steel or carbon steel having an alloy bondedcoating of a tantalum containing compound, more preferably stainless steelhaving an alloy bonded coating of a tantalum containing compound.

Another embodiment of a plate heat exchanger according to presentinvention is when the plate heat exchanger is permanently joined and is madeof stainless steel or carbon steel and all surfaces of at least one of the flowsides of the plate heat exchanger have an alloy bonded coating of a tantalum containing compound.

Detailed description of the invention A conventional permanently joined plate package or plate heatexchanger may be made more corrosion resistant than it was from thebeginning with the present invention.

A plate heat exchanger is composed of multiple, thin metal plates thathave very large surface areas and fluid flow passages which may enable heattransfer. A heat exchanger is provided with at least two inlets and two outletsfor the fluids to be heat exchanged. Additional fluids may be used thenrequiring additional inlets and out lets of the heat exchanger. Plate heatexchangers comprise a series of heat transfer plates. These heat transferplates form what is called a plate package in the heat exchanger. The heattransfer plates are made of thin sheets of metal and are often provided withcorrugations or other protuberances in their heat transferring portions, whichin a heat exchanger abut against each other by a large force at a greatnumber of contact places distributed across the heat transferring portions.Then the heat transfer plates are assembled interspaces are formed between the plates. These plate interspaces are intended for at least one heatexchanging fluid flowing through. ln a plate heat exchanger the at least twofluids are flowing through the interspaces next to each other allowing the heattransfer to take place. These interspaces between the plates intended for flowof one of the fluids is in the present application considered as being part of aflow side. ln the present application the wording flow side is connected to theconstruction of a heat exchanger or plate package for the fluids, i.e. the fluidflow passages. Since at least two fluids are used in a plate heat exchanger, ithas has at least two flow sides, one flow side for a warm fluid and one flowside for a cold medium. For each flow side, all parts of a plate package or aheat exchanger being in contact with either the warm or cold flowing fluid areconsidered belonging to that flow side, e.g. plates, plate interspaces, joints,connections, inlet and/or outlet ports in frames or mounting plates. ln a platepackage or plate heat exchanger according to the present invention at leastone of the flow sides is designed for highly corrosive fluids when in use.

With the present invention simple rigid base materials for heatexchangers such as stainless steel and carbon steel can be used and with atantalum containing coating be made corrosion resistant to highly corrosivefluids. With the present invention also other parts of the plate package or heatexchanger like the joints which may be more sensitive parts of the plate heatexchanger due to e.g. welding during the assembly of the heat exchanger arecoated with a corrosion resistant material. The joints may also be sensitiveparts of the heat exchanger due to soldering, fusion bonding or brazing duringassembly of the heat exchanger. The term fusion bonding relates to the useof an iron based brazing material in accordance with the disclosures of e.g.EP 1 347 859 B1 and WO 02/098600. Assembly of a heat exchanger usingsoldering, fusion bonding or brazing the joints may be made of a differentmaterial than the plates. During the assembly process the soldering orbrazing material is applied to the plates, fully or partially covering the plates,and the soldering or brazing material may during the assembly be mixed withadditional coatings on the plate material or in some cases even the platematerial itself creating more corrosion sensitive parts of the heat exchanger.Since at least both plates and joints of a plate package or plate heat exchanger according to the present invention are coated the heat exchangeris made more corrosion resistant. Thus, in this way the joints or areas on theplates close to the joints can no more be a weak link for the heat exchanger.ln one embodiment of the present invention permanently assembled plate heat exchangers or plate packages for plate heat exchangers made ofstainless steel or carbon steel are coated with a tantalum containingcompound. The plate packages or heat exchangers may e.g. be permanentlyassembled by welding, soldering, fusion bonding or brazing. A tantalumcontaining compound is introduced into the heat exchanger in at least thoseplate interspaces being intended for through flow of one of the two heatexchanger fluids, i.e. at least one of the flow sides designated for being usedfor highly corrosive fluids when in use. lnside the heat exchanger or platepackage, the tantalum containing compound is deposited on all surfaces of atleast one of the flow sides of the heat exchanger or plate package, e.g.plates, joints and other parts intended to be in contact with heat exchangerfluids.

The use of a tantalum containing compound according to the presentinvention provides a plate package or plate heat exchanger with very goodproperties. Tantalum show better heat transfer properties then plastics whichare not considered thermally conductive materials. According to the presentinvention it is important to be able to present a coating which does not impairthe heat transfer. Tantalum show good heat transfer properties. Further, thetantalum containing coating according to the present invention is chemicallybonded to the materials of the plate package and plate heat exchanger. Thetantalum containing compound is bonded by alloying to said materials. Suchan alloying bonding give rise to more fatigue resistant plate packages andheat exchangers compared to e.g. heat exchangers coated with plasticmaterials. Since the tantalum is partially alloyed to the material the adhesionis superior. This makes it easy for the tantalum containing coating to followthe plate and joint materials movements due to thermal and pressurechanges within the plate heat exchanger when going form out of use to useand also during use. The tantalum containing coating has a gradual transitionof compounds within itself. When looking at the tantalum containing coating in a cross cut view, the intermediate phase closest to the heat exchangermaterial, e.g. a plate, show an a||oy of tantalum containing compound and theplate material, a gradual transition is thereafter made to only the tantalumcontaining compound, which thereafter is gradually transferred into tantalumoxide since the outer surface of the tantalum containing compound isoxidized. Thus, since not all of the tantalum containing coating applied toparts of a heat exchanger is an a||oy with said parts it is considered that thetantalum containing compound is partially alloyed to the heat exchangerparts.

The film thickness of the tantalum coating must not be too highbecause that would influence the heat transfer properties in a negative waysince an enlarged barrier, an increased plate thickness, between the heattransferring fluids decreases the heat transfer. lf the film thickness is to lowthe effect of the coating may not last as long as suspected when in contactwith a highly corrosive fluid.

According to the present invention a tantalum containing compound iscoated on the inside of a plate package or heat exchanger using a depositionprocess with chemical reactants in fluid form. The method of coating apermanently joined plate package or heat exchanger in accordance with thepresent invention, comprises the steps: 1) introducing gas or vapor phasechemical reactants into said plate package or heat exchanger in at least oneof the flow sides of the heat exchanger, wherein at least one of the reactantsis a reactant comprising tantalum, 2) formation of a solid film comprising atantalum containing compound on the surfaces of said plate package or heatexchanger from the reaction of the gas or vapor phase chemical reactants.

The application process relates to formation of a non-volatile solid filmon a substrate, in the present case parts of a plate package or heatexchanger, from the reaction of gas or vapor phase chemical reactants,wherein at least one reactant is a reactant comprising tantalum. A reactionchamber is used for the process, into which the reactant gases or vapors areintroduced to decompose and react with the substrate or in the case ofmultiple applications the previously applied layer to form the film. lnside thereaction chamber the reactants are forced into the plate package or heat exchanger. ln one embodiment the reactant comprising tantalum in fluid formis tantalum pentachloride.

The application process disclosed above could also be used for partsof a heat exchanger such as frames or mounting plates not part of apermanently joined heat exchanger. Such coated frames or plates may thenbe used together with a permanently joined plate package coated inaccordance with the present invention.

The application process of the tantalum containing composition ispreferably done by Chemical Vapor Deposition (CVD) or Atomic LayerDeposition (ALD), preferably by CVD.

A basic CVD process consists of the following steps: 1) a predefinedmix of reactant gases and diluent inert gases are introduced at a specifiedflow rate into the reaction chamber; 2) the gas species move to the substrate;3) the reactants get adsorbed on the surface of the substrate; 4) the reactantsundergo chemical reactions with the substrate to form the film; and 5) thegaseous by-products of the reactions are desorbed and evacuated from thereaction chamber.

The growth of material layers by ALD consists of repeating thefollowing characteristic four steps: 1) Exposure of the first precursor. 2) Purgeor evacuation of the reaction chamber to remove the non-reacted precursorsand the gaseous reaction by-products. 3) Exposure of the second precursor -or another treatment to activate the surface again for the reaction of the firstprecursor. 4) Purge or evacuation of the reaction chamber. Each reactioncycle adds a given amount of material to the surface, referred to as thegrowth per cycle. To grow a material layer, reaction cycles are repeated asmany as required for the desired film thickness. ln one embodiment the method of coating a permanently joined heatexchanger made of stainless steel or carbon steel comprises the steps: 1)introducing gas or vapor phase chemical reactants into said heat exchangerin at least one of the flow sides of the heat exchanger, wherein at least one ofthe reactants is a compound comprising tantalum, 2) formation of a solid filmcomprising tantalum on the surfaces of said heat exchanger from the reactionof the gas or vapor phase chemical reactants, is for the steps 1) and 2) preferably carried out at a temperature of 600-1000°C, more preferably 700-900°C. ln another embodiment of the present invention steps 1) and 2) arecarried out at atmospheric pressure, subatmospheric pressure or at very lowpressure.

According to the present invention it is important that the heatexchange plates of a plate package or plate heat exchanger not only arepermanently joined to each other along their peripheral portions, it is alsoimportant that at a variety of areas of contact in their heat exchange portionsare permanently joined. lf plates are only joined along their peripheralportions other areas of contact may move/be dislocated during use. lf onlycontact surfaces along their peripheral portions are permanently joined theplates may separate at some areas of contact which are not permanentlyjoined during use when the plate heat exchanger is e.g. pressurized on one ofthe ﬂuid flow sides. ln the case of areas of contact shifting due to e.g.pressurizing, a coated heat exchanger which is notjoined at all areas ofcontact within the fluid flow would then have areas not coated exposed to thefluid in the heat exchanger and thus resulting in corroding areas if the fluidused is corrosive. Thus, it is important that all areas of contact betweenplates, where the areas of contact are in contact with or surrounded bycorrosive fluid, are permanently joined by welding, soldering, fusion bondingor brazing.

A permanently joined plate package for a plate heat exchanger asdisclosed herein is to be interpreted as a non-accessible plate packagewherein at least all areas of contact between plates in contact with corrosivefluid are permanently joined. Thus, since the plate package is non-accessibleit is to be interpreted that the complete plate package may not bedisassembled in any way.

Such a plate package according to the present invention can be used in aplate heat exchanger having e.g. frames and/or mounting plates of anymaterial, as long as they are not in contact with the corrosive fluid in at leastone of the flow sides. lf e.g. frames or mounting plates are a part of at leastone of the flow sides of the heat exchanger and is in contact with a highly corrosive fluid said frames and/or mounting plates preferably are made oftantalum, or stainless steel or carbon steel having an alloy bonded coating ofa tantalum containing compound on at least the parts of the at least one ofthe flow sides of the heat exchanger. For such frames and/or mounting platespreferably stainless steel or carbon steel having an alloy bonded coating of atantalum containing compound are used, more preferably stainless steelhaving an alloy bonded coating of a tantalum containing compound.

A permanently joined plate heat exchanger as disclosed herein is to beinterpreted as a non-accessible heat exchanger comprising a permanentlyjoined plate package wherein at least all areas of contact between plates incontact with corrosive fluid are permanently joined. Thus, since the plate heatexchanger is non-accessible it is to be interpreted that the plate heatexchanger may not be disassembled. For a plate heat exchanger this meansthat not even any frames or mounting plates that are located around a platepackage and are to be in contact with at least one corrosive heat exchangefluid can be removed. The permanently joined plate heat exchangeraccording to the present invention is for the parts in contact with at least onefluid, e.g. a corrosive fluid, impossible to disassemble in any way. Thewordings permanently joined and permanently assembled in view of platepackages and plate heat exchangers are regarded as being interchangeablein the present application.

The present invention relates to application of a solid film of a tantalumcontaining coating onto surfaces within a permanently joined plate package orplate heat exchanger. The tantalum containing compound used as coating,preferably metal tantalum, tantalum oxide and/or tantalum nitride, applied onthe surfaces of the heat exchangers to be in contact with highly corrosivefluid. ln a preferred embodiment the tantalum containing compound is metaltantalum and/or tantalum oxide, preferably metal tantalum. lf the tantalumcoating is made of metal tantalum naturally the uppermost part of the coatingis oxidized and thus is tantalum oxide, and the nethermost part of the coatingis then alloyed with the materials of a permanently joined plate package orplate heat exchanger. 11 The permanently joined plate package and permanently joined heatexchanger coated in accordance with the present invention is made ofstainless steel or carbon steel. Stainless steel and carbon steel areconsidered materials with good mechanical properties. The permanentlyjoined plate package or permanently joined heat exchanger is assembledusing welding, soldering, fusion bonding or brazing, preferably using weldingfusion bonding or brazing. Brazing is preferably done by use of copper asbrazing material. Preferably the heat exchanger is made of stainless steel andwas assembled using welding, fusion bonding or brazing, preferably fusionbonding or copper brazing.

According to the present invention the coating comprising tantalumapplied onto the surfaces in at least one of the flow sides designated for beingused for highly corrosive fluids has preferably a film thickness of about 1-125um, preferably 1-50 um, preferably 10-40 um and even more preferably 15-25um.

ExamplesTwo copper brazed stainless steel units, CB14, and two Alfa Fusion stainless steel units, AN14, from Alfa Laval have been processed with theCVD process to coat with tantalum. Conventional Alfa Fusion units, AN14,were used as reference. All units contained plates of stainless steel but inCB14 they were copper brazed and in AN14 they were fusion bondedtogether.

Process: Tantalum reacts with chlorine gas to form TaCl5. The gas is led into anvacuum oven at 850 °C were the TaCl5 will react with available surfaces(stainless steel, copper, carbon steel, graphite etc) to form a CVD coating oftantalum. The pressure of the gas is about 25 mB, and the process is runningfor about 8 hours.

The chorine released during the process will react with hydrogen toform hydrochloric gas which is led out of the process and neutralized withsodium hydroxide. 12 TaCl5 gas is led from the centre pipe to the units. The small, hangingspacers attached to the inlet and outlet are used for evaluation of thethickness of the tantalum layer. According to weight measuring of the spacersbefore and after process the average thickness of the tantalum layer is about45 um in the inlet and 38 um in the outlet.

Analysis: The tantalum CVD processed units (CB 14 and AN 14 units) werecorrosion tested with 75 °C hydrochloric acid during 48 h. The hydrochloricacid used for the test showed almost no change in color after recirculation inthe tantalum treated units. The tantalum coated CB 14 and AN 14 unitsshowed no without leaking or other signs of corrosion damages during or afterthe corrosion test. After the corrosion test the units were pressure tested withcompressed air at 8 bar. No external or internal leaks were found in the units.

A conventional AN14 unit was corrosion tested in hydrochloric acid aswell. For the conventional AN14 unit the hydrochloric acid reacted stronglywith the stainless steel surfaces under emission of hydrogen gas, the acidhad to be replaced a couple oftimes because of depletion. A strong greencolorization from iron chloride was found in the acid from the standard unit.The conventional AN14 unit showed no leakage after 90 minutes, but after 6hours numerous large leaks were detected.

After the corrosion tests the units were cut up and cross cuts of thesurfaces were metallograhipcally prepared and examined with microscope.The tantalum treated units were cut up and four cross cuts were examinedfrom each unit. The CB 14 unit showed very good adhesion between thecopper and tantalum in all investigated locations. The CB 14 unit showedslightly better adhesion between the copper and tantalum than the stainlesssteel and tantalum in the AN 14 unit. A reason for this might be that thesurface of the AN 14 unit may have been contaminated or, to a lower extent,be because of the higher surface roughness in the AN 14 unit.

The thickness of the tantalum layer varies from about 105-125 um inthe areas around the inlet to just over 10 um at the diagonal maximumdistance from inlet on the AN 14 unit. 13 The thickness of the tantalum layer varies from about 150 pm in theinlet to thin, most probably less than 5 pm at the diagonal maximum distancefrom inlet on the CB 14 unit.

Claims

1. A permanently joined plate package for a plate heat exchanger made ofstainless steel or carbon steel c h a r a c t e r i s e d in that all surfaces of atleast one of the flow sides of the plate package have an alloy bonded coatingof a tantalum containing compound.

2. A permanently joined plate package according to claim 1, wherein thetantalum containing compound is metal tantalum, tantalum oxide and/ortantalum nitride, preferably metal tantalum and/or tantalum oxide, morepreferably metal tantalum.

3. A permanently joined plate package according to claims 1 or 2, wherein theplate package was assembled using welding, fusion bonding or brazing,preferably fusion bonding or copper brazing.

4. A permanently joined plate according to any one of claims 1-3, wherein theplate package is made of stainless steel and was assembled using welding,fusion bonding or brazing, preferably fusion bonding or copper brazing.

5. A permanently joined plate package according to any one of claims 1-4,wherein the coating of tantalum containing compound has a thickness ofabout 1-125 um, preferably 1-50 um, more preferably 10-40 um and evenmore preferably 15-25 um.

6. A plate heat exchanger c h a r a c t e r i s e d in that it comprises a platepackage of the kind as defined in any one of claims 1-5.

7. A plate heat exchanger according to claim 6, wherein said heat exchangerhas frames and/or mounting plates that are a part of at least one of the flowsides of the heat exchanger and said frames and/or mounting plates aremade of tantalum, or stainless steel or carbon steel having an alloy bondedcoating of a tantalum containing compound, preferably stainless steel orcarbon steel having an alloy bonded coating of a tantalum containing compound, more preferably Stainless steel having an alloy bonded coating ofa tantalum containing compound.

8. A plate heat exchanger according to claim 7, wherein the plate heatexchanger is permanently joined and is made of stainless steel or carbonsteel and all surfaces of at least one of the flow sides of the plate heatexchanger have an alloy bonded coating of a tantalum containing compound.