PLATE HEAT EXCHANGER AND METHOD OF USING

Abstract

A process for heating a cold stream with a hot stream is described. At least one of the cold inlet or outlet, the hot inlet or outlet, the cold inlet or outlet header, the hot inlet or outlet header, and the plurality of plates is made of one of two stainless steel alloys. One alloy has higher molybdenum content with copper for improved corrosion resistance and is resistant to chloride pitting, chloride SCC, and PTA SCC. The other alloy has significantly higher tensile and yield strength, which will reduce the susceptibility of the plate bundle to thermal stress damage.

Description

BACKGROUND OF THE INVENTION

Oil refineries typically incorporate one or more different processes for treating and/or converting hydrocarbons, such as, for example, those present in crude oil or another naturally occurring source, to produce specific hydrocarbon products with properties that are useful for particular applications.

To carry out the refining or processing operations to treat crude oil and other hydrocarbons to form usable products, oil refineries typically include one or more complexes or groups of equipment designed for carrying out one or more particular treating or conversion processes to prepare desired final products. In this regard, the complexes each may have a variety of interconnected equipment or vessels including, among others, tanks, furnaces, distillation towers, reactors, heat exchangers, pumps, pipes, fittings, and valves.

Many types of hydrocarbon processing operations are carried out under relatively harsh operating conditions, including high temperatures and/or pressures and within various harsh chemical environments. In addition, due to the large demands for hydrocarbon and petrochemical products, the volumetric flow rate of a hydrocarbon stream through various oil refinery complexes is substantial, and the amount of downtime of the processing equipment is preferably small to avoid losses in output.

High temperature hydrocarbon processing operations generally involve heating a hydrocarbon stream to a process temperature and flowing the hydrocarbon stream through one or more hydrocarbon processing vessels forming a refinery complex. Specific process techniques are utilized depending on the feed and the desired products, and may include flowing the hydrocarbon stream in the presence of other materials and/or reactants, including gases and liquids, adsorbents to remove particular components from the product stream, and/or catalysts to control reaction rates. In this manner, the hydrocarbon stream can be processed to, for example, modify one or more components within the hydrocarbon stream, react one or more components with other materials (e.g. gases) within a vessel, and remove components from the hydrocarbon stream either as potential products, sometimes upon further processing, or for disposal.

Traditionally, austenitic stainless steels have been used to fabricate the oil refinery equipment listed above, because these types of alloys are useful in a variety of harsh environments. The addition of 8% nickel to a stainless steel containing 18% chromium produces a remarkable change in microstructure and properties. The alloy solidifies and cools to form a face-centered cubic structure called austenite, which is non-magnetic. Austenitic stainless steels are highly ductile, even at cryogenic temperatures and have excellent weldability and other fabrication properties. The most widely used stainless steel is probably Type 304 (sometimes called T304 or simply 304) because of cost. Type 304 stainless steel is an austenitic steel containing 18 to 20% chromium and 8 to 10% nickel. This and other specialty austenitic stainless steels have been used in these applications but are susceptible to high temperature H₂S, sulfur, and chloride-SCC corrosion and high temperature hydrogen attack issues that are present in these processes.

Many metals, including austenitic stainless steels, can be subject to a highly localized form of corrosion known as stress-corrosion cracking (SCC). SCC often takes the form of branching cracks in apparently ductile material and can occur with little or no advance warning. In low pressure vessels, the first sign of stress corrosion cracking is usually a leak, but there have been instances of catastrophic failures of high pressure vessels due to stress corrosion cracking. Stress corrosion cracking occurs when the surface of the material exposed to a corroding medium is under tensile stress (applied or residual) and the corroding medium specifically causes stress corrosion cracking of the metal. Tensile stresses may be the result of applied loads, internal pressure in piping systems and pressure vessels, or residual stresses from prior welding or bending.

One particularly harsh environment in which austenitic stainless steels are typically observed to undergo stress corrosion cracking is an environment containing halides, usually in the form of inorganic chlorides. The presence of chlorides along with an aqueous phase and tensile stresses can result in chloride stress corrosion cracking (“chloride-SCC”) of austenitic stainless steels. This type of cracking is predominantly transgranular and is dependent on time, oxygen, and chloride concentration. Stress corrosion cracking due to chlorides is usually observed in areas of austenitic stainless steels subjected to tensile stresses in the presence of chlorides, and oxygen. In general, chloride-SCC will occur where high concentrations of chlorides are present, but it may also occur at lower concentrations and elevated temperatures. In addition, while high temperatures may reduce the amount of time required for a particular chloride concentration to result in chloride-SCC, lower temperatures can cause chlorides to condense on surfaces, thereby increasing the concentration of the chlorides on the surfaces. Thus, chloride-SCC can be problematic at many temperature ranges. For example, chloride-SCC can occur where chloride concentrations are able to build up, for example, by pitting or crevice corrosion of the material surface, on heated surfaces, or where chlorides present in the environment condense on a material surface. Chlorides are able to penetrate the passive film to allow corrosive attack of the material to occur. One particularly problematic area of chloride-SCC is in condensers where chloride condenses and concentrates on surfaces of the vessel.

Another type of harsh corrosive environment to which sensitized stainless steels are particularly susceptible is one that contains polythionic acid (PTA) formed from the decomposition of sulfide scale by moisture in air. Typically, chromium within the austenitic stainless steels reacts with oxygen to form a passive film of chromium oxide that protects the material from corrosion. The passivated metal is able to resist further oxidation or rusting. However, at high temperatures, usually somewhere in the range of between 370° C. and 815° C. depending on the stainless steel alloy, chromium-rich carbides precipitate out at the grain boundaries. The precipitation of chromium depletes the chromium content adjacent to the grain boundaries, forming chromium depleted zones and drastically reducing the corrosion and/or cracking resistance in corrosive environments in these zones. This phenomenon is known as sensitization. Due to the high temperature of operation and the presence of sulfur (S) and hydrogen sulfide (H₂S) in a reducing environment or in a cold stream in many oil refinery complexes and/or processes, an iron sulfide scale can form on the stainless steel surfaces. Upon shutdown of the equipment, if the sensitized stainless steel is exposed to moisture and oxygen from the surrounding environment, there is the potential that the sulfur and hydrogen sulfide will react with oxygen and moisture from the ambient environment to form PTA. The PTA can attack the chromium depleted zones formed by sensitization, causing corrosion and ultimately polythionic acid stress corrosion cracking (PTA-SCC) where the vessel is put under tensile stresses either by being pressurized or by having residual stresses from, for example, welding during fabrication. Unstabilized grades of austenitic stainless steels, such as Types 304 and 316, traditionally used in the fabrication of oil refinery complexes, have all exhibited sensitization and PTA-SCC due to polythionic acid. To minimize the formation of Cr-rich carbides or a sensitized microstructure, elements such as Nb or Ti are added to react with excess C and stabilize the microstructure. However, even these stabilized grades, such as Types 321 and 347, can exhibit sensitization and PTA-SCC if they are exposed to elevated temperatures (e.g., 400° C. to 800° C.) for a long enough time.

In some instances, protective coatings are applied to protect the outside of stainless steel vessels from exposure to chlorides in insulating jackets. In other applications, post welding heat treatment can be used to relieve residual stress in the steel alloys.

The risk of PTA-SCC and chloride-SCC in oil refinery equipment has primarily been addressed by known processes to either prevent the formation of PTA and/or the presence of chlorides, or to neutralize the PTA in the environment prior to exposure to air.

To reduce the affects of chloride-SCC, precautions are typically taken to minimize the amount of chloride in the process material or feed that will come into contact with austenitic stainless steel equipment. Precautions are also taken to limit the chloride content to low levels in any flushing, purging, or neutralizing agents used in the system. In addition, limiting exposure to oxygen containing gases and/or water also helps to avoid the occurrence of chloride-SCC.

Preventing PTA formation and subsequent cracking can be accomplished by either eliminating liquid phase water or eliminating oxygen, since these are the components responsible for reacting with the sulfide scale to form the PTA. One approach is to maintain the temperature of the austenitic stainless steel equipment above the dew point of water to avoid condensation of the moisture. Another approach is to purge the equipment with a dry nitrogen purge during any shutdown or startup procedure, when the system is depressurized and the equipment is opened and exposed to air, since this is generally the only time when significant amounts of oxygen might enter the system.

PTA that has or is likely to form within a complex or vessel may be neutralized by an ammoniated nitrogen purge or an aqueous solution of soda ash. With an ammoniated nitrogen purge, special procedures are utilized to form the ammoniated nitrogen, which is pressurized and blown into the system. A soda ash solution neutralization step involves completely filling the piping or piece of equipment involved with the solution and allowing the equipment to soak for a minimum of two hours prior to exposing the system to air. Each of these processes is time consuming and impractical during the operation of an oil refinery complex because it requires additional materials and additional downtime of the particular equipment to perform the purge or neutralization steps. In addition, special precautions must be taken to protect service workers working on the equipment when nitrogen, ammoniated nitrogen, or soda ash is present. Removal of these chemicals would reduce the need for special handling and waste disposal. Moreover, the catalyst in the reactor can be poisoned if trace levels of the chemicals remain, which is often the case.

In addition, chemically stabilized austenitic stainless steels like TP321 and TP347 have been used in reactors that process sulfur-containing streams because of their resistance to high temperature corrosion. However, such austenitic stainless steels are also susceptible to PTA-SCC as a result of exposure to polythionic acid, since the operating conditions of many hydrocarbon treatment processes fall within the time at temperature at which sensitization occurs. Similarly, these materials are still susceptible to chloride-SCC through exposure to chlorides, oxygen, water, and stress at sufficient times and temperatures. The need for special procedures during shutdown and startup of a refinery complex affects not only costs, but also production time since they take a certain amount of time to carry out.

Welded plate heat exchangers, which are used in a variety of processes to heat or cool stream in various processes, are often made from type 304 or type 316 or type 321 stainless steel and are subject to chloride-SCC and PTA-SCC.

Currently, corrosion in welded plate heat exchangers is avoided by preventing the presence of moisture and oxygen when chlorides or sulfur may be present. Chlorides are often present in the catalytic reforming process, and ammonium chloride salts have been found in the effluent side of the plate bundle. Small amounts of sulfur are injected into the catalytic reforming process to minimize carburization and metal catalyzed coke. During normal operation, liquid water and oxygen are not present. During shutdowns, the heat exchangers are kept under a nitrogen purge. If inspection or maintenance of the exchanger is required, then water washes and soda ash washes are performed before opening the exchanger to the atmosphere to remove the chlorides and neutralize the sulfides. However, even with these precautions, chloride corrosion of welded plate heat exchangers is often observed when liquid water is present in the feed leading to chloride pitting, chloride pitting due to under-deposit corrosion, and when a water dew-point occurs during regeneration of a fixed bed reforming unit leading to chloride pitting, and chloride-SCC due to the presence of chlorides, oxygen, and water. Any failure due to corrosion or cracking will reduce product quality by contaminating the product stream with the feed stream.

In addition, some welded plate heat exchangers are damaged by thermal stress which can cause mechanical damage to the heat exchanger. This type of mechanical damage accounts for the majority of all damage that causes bundle cross leakage. Thermal stresses can be caused by thermal transients that generate rapid changes in the bundle temperature or by flow mal-distribution which causes temperature differences between adjacent parts of the bundle. Thermal transients can occur during start-up, shutdown, and upset conditions. Flow mal-distributions can occur when there is a fouling of the bundle, when there is a sudden plugging of the bundle, when there is plugging of a distributor, or when there are low velocities resulting in poor liquid flow up in the bundle.

Thermal stress damage that causes bundle cross leaking is costly to the end user because it results in cross-leakage of the higher pressure stream into the lower pressure stream resulting in a reduction in heat transfer efficiency, as well as potentially contaminating a product stream with a feed stream. This damage can result in reduced throughput, reduced product quality, and eventually may require shutdown to repair or replace the heat exchanger. In corrosion and SCC cases, the bundles cannot be repaired because the damage typically occurs to most of or all the plate channels, requiring replacement of the bundle or the entire heat exchanger. In thermal stress damage cases, the bundle can be repaired during a shutdown by plugging channels to allow continued operation. However, when 10-20% or more of the channels are plugged, the resulting increase in pressure-drop can significantly reduce throughput. Furthermore, repaired bundles can be more susceptible to further damage by thermal stresses.

Currently the mechanical properties of type 304 and type 321 stainless steels limit the bundle design temperatures for welded plate heat exchangers to about 560° C. Some processes may require higher design temperatures, e.g., about 590° C. or more.

There is a continuing need, therefore, for improved processes for processing hydrocarbon streams while avoiding expensive, time consuming, and inconvenient additional steps for purging or neutralizing the internal environment to avoid forming polythionic acid and reducing the effects of chlorides within welded plate heat exchangers and causing PTA-SCC, chloride-SCC, or thermal stress.

SUMMARY OF THE INVENTION

One aspect of the invention is a process for heating a cold stream, such as a feed to a reaction zone, with a hot stream, such as a reactor effluent. In one embodiment, the process includes providing a plate heat exchanger. The plate heat exchanger comprises a plurality of corrugated plates forming cold flow channels and hot flow channels, a cold inlet in fluid communication with the cold flow channels, a cold outlet in fluid communication with the cold flow channels, a hot inlet in fluid communication with the hot flow channels, a hot outlet in fluid communication with the hot flow channels. The cold stream is introduced to the cold inlet and the hot stream is introduced to the hot inlet and heat is exchanged from the hot stream to the cold stream. The cold stream has a higher enthalpy at the outlet than at the inlet, and the hot stream has a lower enthalpy at the outlet than at the inlet. At least one of the cold inlet, the cold outlet, the hot inlet, the hot outlet, and the plurality of plates is made of: a first stainless steel alloy comprising 0.005 to 0.020 wt % carbon, 9.0 to 13.0 wt % nickel, 17.0 to 19.0 wt % chromium, 0.20 to 0.50 wt % niobium, and 0.06 to 0.10 wt % nitrogen; or a second stainless steel alloy comprising 0.0005 to 0.020 wt % carbon, 10 to 30 wt % nickel, 15-24 wt % chromium, 0.20 to 0.50 wt % niobium, 0.06 to 0.10 wt % nitrogen, up to 5 wt % copper, up to 1.00 wt % silicon, up to 2.00 wt % manganese, and 0.3 to 7 wt % molybdenum.

Another aspect of the invention is a welded plate heat exchanger. The welded plate heat exchanger comprises: a plurality of welded corrugated plates forming cold flow channels and hot flow channels, a cold inlet in fluid communication with the cold flow channels, a cold outlet in fluid communication with the cold flow channels, a hot inlet in fluid communication with the hot flow channels, a hot outlet in fluid communication with the hot flow channels. At least one of the cold inlet, the cold outlet, the hot inlet, the hot outlet, and the plurality of plates is made of: a first stainless steel alloy comprising 0.005 to 0.020 wt % carbon, 9.0 to 13.0 wt % nickel, 17.0 to 19.0 wt % chromium, 0.20 to 0.50 wt % niobium, and 0.06 to 0.10 wt % nitrogen; or a second stainless steel alloy comprising 0.0005 to 0.020 wt % carbon, 10 to 30 wt % nickel, 15-24 wt % chromium, 0.20 to 0.50 wt % niobium, 0.06 to 0.10 wt % nitrogen, up to 5 wt % copper, up to 1.00 wt % silicon, up to 2.00 wt % manganese, 0.3 to 7 wt % molybdenum, 0.01 to 2.0 wt % beryllium, and 0.1 to 0.5 wt % boron.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a fixed bed catalytic reforming process utilizing a combined feed-effluent heat exchanger.

FIG. 2 is an illustration of continuous catalyst regenerating reforming process utilizing a combined feed-effluent heat exchanger.

FIG. 3 is an illustration of liquid-liquid process utilizing a Lean-Rich Solvent heat exchanger.

FIG. 4 is an illustration of one embodiment of a plate heat exchanger.

FIG. 5 is an illustration of another embodiment of a plate heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION

Processes for treating a hydrocarbon cold stream including one or more different hydrocarbons and which may include other components and/or impurities typically include flowing the hydrocarbon stream through various pieces of equipment. The equipment may be included as part of a larger oil refinery complex capable of performing one or more particular types of hydrocarbon conversion or treatment processes for converting or treating one or more components of the hydrocarbon cold stream to form a desired product. Heat is applied to the hydrocarbon stream and/or the equipment during operation. Heat may be applied to the hydrocarbon stream while it is within or before entering the equipment to raise the temperature thereof to a processing temperature. Particular process parameters or operating conditions, such as temperature, pressure, and space velocity, are typically process specific and are selected to promote the particular reactions or treatment steps of the particular process. The process is maintained for a predetermined amount of time. The process may be shut down intermittently for servicing or replacement of equipment, inspection, or for other reasons.

Among the pieces of equipment used in some processes are welded plate heat exchangers. Welded plate heat exchangers include a bundle of corrugated plates. The corrugated plates are welded together to form flow channels for the cold and hot streams. In one embodiment using countercurrent flow, the cold stream enters the heat exchanger at one end and exits at the opposite end. The hot stream enters at the opposite end and exits at the end where the cold stream enters.

In some embodiments, cold inlet and outlet headers connect the cold inlet and outlet to the welded plate bundle, and hot inlet and outlet headers connect the hot inlet and outlet to the welded plate bundle. In other embodiments, headers are not provided.

In one approach, the process includes controlling the halide stress corrosion cracking, and more particularly, chloride-SCC, PTA-SCC, and/or thermal stress of at least a portion of the welded plate heat exchanger that is heated to a predetermined temperature, for example, a temperature at which sensitization will occur. The process may include controlling chloride-SCC, PTA-SCC, or thermal stress of at least a portion of the welded plate heat exchanger even though chloride, or polythionic acid is present in the welded plate heat exchanger during the operation thereof. In one form, controlling chloride-SCC, PTA-SCC, and/or thermal stress of at least a portion of the welded plate heat exchanger is achieved by employing one or both of the austenitic stainless steel compositions specified below in the welded plate heat exchanger.

In one approach, the process includes controlling sensitization of at least a portion of the welded plate heat exchanger that is heated to a predetermined temperature. Controlling sensitization includes restricting or reducing the amount of sensitization that occurs and may involve restricting or reducing the extent of precipitation of chromium carbides within the material of the welded plate heat exchanger. The precipitation of chromium carbides is controlled even though the welded plate heat exchanger is heated to a predetermined temperature for a predetermined amount of time, to a point where sensitization is typically observed within a sensitization envelope of a traditional austenitic stainless steel. Controlling precipitation of chromium carbides in the welded plate heat exchanger may be achieved when heating at least a portion of the welded plate heat exchanger because it is formed from one of the austenitic stainless steel compositions described below.

The welded plate heat exchanger may be heated to a predetermined temperature and maintained at the predetermined temperature for the predetermined amount of time. It has been found that heating the welded plate heat exchanger formed of one of the two austenitic stainless steel compositions to a predetermined time and temperature that falls within the normal operating conditions of welded plate heat exchanger, sensitization of the welded plate heat exchanger does not occur.

In one approach, the welded plate heat exchanger may be heated above a predetermined temperature by the flow of the hot hydrocarbon stream therethough, where the hydrocarbon stream is heated to the predetermined temperature before entering the heat exchanger, and heat is transferred from the hot hydrocarbon stream to the cold hydrocarbon stream through the plate bundle.

Surprisingly, sensitization has been reduced or restricted even where the predetermined temperature and the predetermined time for maintaining the process fall within or near the sensitization envelope of austenitic stainless steels traditionally used for the fabrication of welded plate heat exchangers. Not to be bound by theory, it is believed that the lower carbon content in the austenitic stainless steel compositions reduces or restricts the extent of precipitation of chromium carbides within the alloy along the grain boundaries. This in turn reduces or restricts the formation of chromium depleted zones and the resulting sensitization that typically is present in austenitic stainless steels used for oil refinery complex fabrication. It is further believed that the addition of niobium interacts with the carbon and nitrogen that is present in the material to restrict the formation and precipitation of chromium carbides. It is also believed that the addition of nitrogen in the austenitic stainless steel reduces any loss in strength of the welded plate heat exchanger that might otherwise occur due to the low carbon content.

Further, in addition to strengthening the steel, nitrogen is also believed to have a similar function as molybdenum in terms of resistance to pitting corrosion and chloride-SCC, because nitrogen restricts the formation of a chromium-molybdenum phase. In acidic environments, the corrosion of metals is generally comprised of concurrent metal dissolution reaction and hydrogen evolution reaction. Suppressing one or both of these reactions will reduce the corrosion. However, when higher levels of nitrogen are present, chromium nitrides form in the grain boundaries, resulting in sensitization. The molybdenum in the present novel austenitic stainless steel significantly suppresses hydrogen evolution in most of the reducing acids, such as most organic acids, and thus increases the metals' resistance to organic acids.

Chloride ions may be present in the welded plate heat exchanger at levels that would typically be sufficient to cause chloride-SCC. It is believed that the inclusion of molybdenum in the austenitic stainless steel compositions enhances the passivity of the material in chloride-containing environments by stabilizing the passive chromium oxide film on the material. It is believed that the molybdenum may even repair the passive film if it deteriorates. In this regard, the austenitic stainless steel compositions increase the pitting and crevice resistance of the welded plate heat exchanger. As a pit is normally the initiation site for chloride stress corrosion cracking, molybdenum also increases chloride stress corrosion cracking resistance.

Beryllium and boron can also be included. The beryllium and boron reduce the tendency of stainless steel to undergo chloride stress corrosion cracking.

In addition, polythionic acid when present in the welded plate heat exchanger will not cause polythionic stress corrosion cracking when these alloys are used. Not to be bound by theory, it is believed that because the occurrence of chromium depleted zones typically present as a result of sensitization does not occur with these alloys, and thus the polythionic acid is not able to attack the grain boundaries of the stainless steel. In this manner, the process includes exposing the welded plate heat exchanger to the external environment without taking steps to reduce or restrict the formation of polythionic acid and controlling corrosion of the welded plate heat exchanger by the polythionic acid.

The plate bundle is made from one of two alloys of stainless steel. In some embodiments, at least one of the cold inlet, the cold outlet, the hot inlet, and the hot outlet is also made from, or lined with, one of the two alloys of stainless steel. In some embodiments, all of these components are made from one of the two alloys. In some embodiments, the plate heat exchanger includes a cold inlet header in fluid communication with the cold inlet and the cold flow channels, a cold outlet header in fluid communication with the cold outlet and the cold flow channels, a hot inlet header in fluid communication with the hot inlet and the hot flow channels, or a hot outlet header in fluid communication with the hot outlet and the hot flow channels. If one or more of the headers are present, one or more of them can also be made from, or lined with, one of the two alloys, depending on how the welded plate heat exchanger is to be used.

The first stainless steel alloy has a composition comprising, or consisting essentially of, or consisting of 0.005 to 0.020 wt-% carbon, from 9.0 to 13.0 wt-% nickel, 17.0 to 19.0 wt-% chromium, 0.20 to 0.50 wt-% niobium, and 0.06 to 0.10 wt-% nitrogen. The remainder of the composition includes iron and may include one or more additional components. This alloy has significantly higher tensile and yield strength, which may allow plate bundles to be designed for higher pressures and/or temperatures. This alloy will also reduce the susceptibility of the plate bundle to thermal stress damage. Typically, when thin plates cool more rapidly or are colder than adjacent parts, they contract relative to the adjacent parts, resulting in mechanical failure in tension, known as thermal stress damage. Higher allowable tensile yield and yield stresses allow thicker parts, such as headers, spacer, and end plates, to be designed with smaller thicknesses, reducing the thermal inertia and reducing the thermal stresses that occur due to thermal transients. At the same time, the thin plates benefit from the higher tensile yield and yield strength, allowing them to handle larger thermal stresses. The thin plates are made from thin sheets (e.g., about 0.8 mm to about 1.2 mm) which are corrugated, stacked, and welded to form the welded plate heat exchanger bundle.

The second stainless steel alloy has a composition comprising, or consisting essentially of, or consisting of 0.0005 to 0.020 wt % carbon, 10 to 30 wt % nickel, 15-24 wt % chromium, 0.20 to 0.50 wt % niobium, 0.06 to 0.10 wt % nitrogen, up to 5 wt % copper, up to 1.00 wt % silicon, up to 2.00 wt % manganese, and 0.3 to 7 wt % molybdenum. The remainder of the composition includes iron and may include one or more additional components. The second stainless steel alloy, which has higher molybdenum content with copper for strengthening for improved corrosion resistance, is resistant to chloride pitting, chloride SCC, and PTA SCC. This would eliminate the concern of corrosion damage to the plate bundle, during both operation and downtime.

The first alloy, the second alloy, or both can optionally include 0.01 to 2 wt % beryllium, and 0.1 to 0.5 wt % boron. The beryllium and boron reduce the tendency of stainless steel to undergo chloride stress corrosion cracking.

In one embodiment, the welded plate heat exchanger can be used when the cold stream, known as a coolant or a cooling fluid, has a high chloride content. Cooling fluids include, but are not limited to, water.

Although the two alloys are more expensive than the alloys currently used for welded plate heat exchangers, this may be offset by the reduced risk of costly unplanned shutdowns or reduced throughput in the process since potential damage to the equipment is avoided.

In one approach, the process includes maintaining the operation of the welded plate heat exchanger for above 300 hours, with intermittent process shutdowns during the period of time, without sensitization of the welded plate heat exchanger occurring. In some embodiments, the process can be maintained for above 1,000 hours, or above 2,500 hours, or above 5,000 hours, or above 7,500 hours, or above 10,000 hours, or above 50,000 hours or above 100,000 hours or above 150,000 hours. It should be noted that as described herein, sensitization is not considered to occur where the amount of sensitization of the welded plate heat exchanger that occurs is insufficient to cause chloride-SCC, or PTA-SCC of the welded plate heat exchanger within the predetermined amount of time.

In one approach, the process includes a high temperature process for exchanging heat between a hot stream and a cold stream. In this approach, at least a portion of the welded plate heat exchanger is heated to a temperature by the hot stream flowing therethrough. In one approach, at least a portion of the welded plate heat exchanger is heated to a predetermined temperature. In another approach, some or all of the welded plate heat exchanger is heated to a temperature of above 400° C., or above 550° C., or above 565° C. In still another approach, a maximum temperature is below 700° C. As used herein, the term predetermined temperature does not necessarily refer to a constant or known temperature, and may include, for example, an average temperature, a median temperature, a temperature range and the like.

One example of a fixed bed catalytic reforming process 5 is illustrated in FIG. 1. The feed 10 is mixed with a gas 15 coming from compressor 20. The mixture of feed 10 and gas 15 is preheated in a combined feed-effluent (CFE) heat exchanger 25. The preheated mixture 30 then flows to reaction zone 35. As shown, reaction zone 35 includes a series of one or more heaters and reactors. The preheated mixture 30 is heated in heater 40 and the preheated mixture 45 then flows to reactor 50. The effluent 55 from reactor 50 is sent to heater 60. The heated effluent 65 from heater 60 is sent to reactor 70. The effluent 75 from reactor 70 is sent to heater 80. The heated effluent 85 from heater 80 is sent to reactor 90. The effluent 95 from reactor 90 is sent to CFE heat exchanger 25 where it is used to preheat the mixture of feed 10 and gas 15. The heat exchange in CFE heat exchanger 25 reduces the temperature of exiting stream 100. Stream 100 may be partially condensed. Stream 100 is further condensed in condenser 105. Partially condensed stream 110 is sent to separator 115 where it is separated into liquid product 120 and gas stream 125. Liquid product 120 can then be sent for further processing, such as fractionation (not shown). Gas stream 125 includes light end gases and hydrogen. A portion 130 of gas stream 125 is sent to compressor 20 to be mixed with feed 10. The remaining portion 135 is a net gas stream 135. The light end gases can be further separated in a recovery section (not shown).

FIG. 2 illustrates a continuous catalytic regenerating reforming process 300. The feed 310 is mixed with a gas 315 coming from compressor 320. The mixture of feed 310 and gas 315 is preheated in a combined feed-effluent (CFE) heat exchanger 325. The preheated mixture 330 then flows to the reaction zone. As shown, the reaction zone includes a series of four heaters and four stacked reactors. The preheated mixture 330 is heated in heater 340 and the preheated mixture 345 then flows to reactor 350. The effluent 355 from reactor 350 is sent to heater 360. The heated effluent 365 from heater 360 is sent to reactor 370. The effluent 375 from reactor 370 is sent to heater 380. The heated effluent 385 from heater 380 is sent to reactor 390. The effluent 395 from reactor 390 is sent to heater 400. The heated effluent 405 from heater 400 is sent to reactor 410. The effluent 415 from reactor 410 is sent to CFE heat exchanger 325 where it is used to preheat the mixture of feed 310 and gas 315. The heat exchange in CFE heat exchanger 325 reduces the temperature of exiting stream 420. Exiting stream 420 from CFE heat exchanger 325 may be partially condensed (condenser not shown). Exiting stream 420 is sent to separator 425 where it is separated into liquid product 430 and gas stream 435. Gas stream 435 is divided into a first portion 440 which is sent to the compressor 320 and a second portion 445 which is sent to a net gas compressor 450. The compressed net gas 455 is combined with liquid product 430 and sent to recovery section 480. The spent catalyst 460 is sent to catalyst regenerator 465 where the coke is oxidized. The regenerated catalyst 470 is sent to reactor 350.

FIG. 3 is an illustration of one embodiment of a liquid-liquid process 500 that uses physical solvent to remove acid process gases from synthetic or natural gas streams. The feed gas 505 is introduced into an absorber 510 where it is treated with a liquid solvent. The treated gas 515 is removed from the absorber 510. The cold solvent 520 is fed into the plate heater exchanger 525 where heat is exchanged with the hot solvent 530 coming from the stripper column 535. The warmed solvent 540 is sent to a concentrator 545, and the cooled solvent 550 is sent to the absorber 510. The overhead gas 555 from the concentrator 545 is sent to a compressor 560. The compressed gas 565 is sent to the absorber 510. The bottoms 570 from the concentrator 545 is sent to the stripper column 535 where it is separated into the solvent 530 and overhead 575. The overhead 575 is sent to a reflux accumulator 580 where it is separated into a acid gas stream 585 and a water stream 590. Water stream 590 can be combined with make-up water 595 to form water stream 600 and sent to stripper column 535.

One embodiment of the CFE heat exchanger 25 is shown in FIG. 4. In this embodiment, the welded plate bundle 150 is housed in a pressure vessel 155. The pressure vessel 155 contains a pressurizing fluid. The plate bundle is designed for differential pressure between the cold stream and the hot stream.

There is a hot inlet 160 which is connected to the welded plate bundle 150 by hot inlet header 165. At the opposite end of the CFE heat exchanger 25, there is a hot outlet 170 connected to the welded plate bundle 150 by hot outlet header 175. The cold inlet 180 is connected to the welded plate bundle 150 by cold inlet header 185, and the cold outlet 190 is connected to the welded plate bundle 150 by the cold outlet header 195, although this is not required. For the countercurrent arrangement shown, the hot inlet 160 and the cold outlet 190 are on one end of the CFE heat exchanger 25, while the hot outlet 170 and cold inlet 180 are at the opposite end. However, other arrangements are possible, as would be understood by those of skill in the art.

In some embodiments, the inlets and/or outlets are nozzles and may include connecting piping.

Not all welded plate heat exchangers have headers connecting the inlets and outlets with the flow channels. There can be isolated fluid communication between the cold inlet and outlet and the cold flow channels and between the hot inlet and outlet and the hot flow channels, whether or not there are headers present, depending on the type and design of the plate heat exchanger.

Another embodiment of the heat exchanger 525 is shown in FIG. 5. In this arrangement, there is no pressure vessel. A pair of end plates 297 is clamped on the welded plate bundle 250. The pair of end plates 297 can be clamped onto the welded plate bundle 250 using any known method, including but not limited to, tie rods 299. The end plates may also be made up of multiple pieces that are each clamped by multiple tie rods.

There is a hot inlet 260 which is connected to the welded plate bundle 250 by hot inlet header 265. At the opposite end of the CFE heat exchanger 525, there is a hot outlet 270 connected to the welded plate bundle 250 by hot outlet header 275. The cold inlet 280 is connected to the welded plate bundle 250 by cold inlet header 285, and the cold outlet 290 is connected to the welded plate bundle 250 by the cold outlet header 295, although this is not required. For the countercurrent arrangement shown, the hot inlet 260 and the cold outlet 290 are on one end of the CFE heat exchanger 525, while the hot outlet 270 and cold inlet 280 are at the opposite end. However, other arrangements are possible, as would be understood by those of skill in the art.

While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.

Claims

1. A process for heating a cold stream with a hot stream comprising:

providing a plate heat exchanger comprising: a plurality of corrugated plates forming cold flow channels and hot flow channels, a cold inlet in fluid communication with the cold flow channels, a cold outlet in fluid communication with the cold flow channels, a hot inlet in fluid communication with the hot flow channels, a hot outlet in fluid communication with the hot flow channels;

introducing the cold stream to the cold inlet and the hot stream to the hot inlet and exchanging heat from the hot stream to the cold stream, the cold stream at the cold outlet having an enthalpy higher than an enthalpy of the cold stream at the cold inlet, and the hot stream at the hot outlet having an enthalpy lower than an enthalpy of the hot stream at the hot inlet;

wherein the plurality of plates is made of: a first stainless steel alloy comprising 0.005 to 0.020 wt % carbon, 9.0 to 13.0 wt % nickel, 17.0 to 19.0 wt % chromium, 0.20 to 0.50 wt % niobium, and 0.06 to 0.10 wt % nitrogen; or a second stainless steel alloy comprising 0.0005 to 0.020 wt % carbon, 10 to 30 wt % nickel, 15-24 wt % chromium, 0.20 to 0.50 wt % niobium, 0.06 to 0.10 wt % nitrogen, up to 5 wt % copper, up to 1.00 wt % silicon, up to 2.00 wt % manganese, and 0.3 to 7 wt % molybdenum.

2. The process of claim 1 wherein the first stainless steel alloy, the second stainless steel alloy, or both further comprise 0.01 to 2.0 wt % beryllium, and 0.1 to 0.5 wt % boron.

3. The process of claim 1 wherein at least one of cold inlet, the cold outlet, the hot inlet, and the hot outlet is made of the first stainless steel alloy or the second stainless steel alloy.

4. The process of claim 1 further comprising at least one of a cold inlet header in fluid communication with the cold inlet and the cold flow channels, a cold outlet header in fluid communication with the cold outlet and the cold flow channels, a hot inlet header in fluid communication with the hot inlet and the hot flow channels, a hot outlet header in fluid communication with the hot outlet and the hot flow channels, and wherein at least one of cold inlet header, the cold outlet header, the hot inlet header, and the hot outlet header is made of the first stainless steel alloy or the second stainless steel alloy.

5. The process of claim 1 wherein the plurality of plates are welded.

6. The process of claim 5 wherein the plate heat exchanger further comprises a pressure vessel containing the plurality of plates.

7. The process of claim 5 wherein the plate heat exchanger further comprises a pair of end plates clamped on the plurality of plates.

8. The process of claim 1 wherein the hot stream comprises a vapor and wherein the hot stream at least partially condenses in the plate heat exchanger.

9. The process of claim 1 wherein the cold stream comprises a liquid and wherein the cold stream at least partially vaporizes in the plate heat exchanger.

10. The process of claim 1 wherein the cold stream comprises a cooling fluid.

11. The process of claim 1 wherein the plurality of plates are welded, the plurality of plates are contained in a pressure vessel, the cold stream is a liquid mixed with a recycle gas, the hot stream is a vapor, and wherein the cold stream at least partially vaporizes in the plate heat exchanger, and the hot stream at least partially condenses in the plate heat exchanger.

12. The process of claim 1 wherein the plurality of plates are welded, the plate heat exchanger further comprises a pair of end plates clamped on the plurality of plates, the cold stream is a liquid, and the hot stream is a liquid.

13. The process of claim 1 wherein the cold inlet and the hot outlet are at one end of the plate heat exchanger and the cold outlet and the hot inlet are at the other end of the plate heat exchanger.

14. The process of claim 1 wherein the plate heat exchanger operates at a temperature above about 590° C.

15. A welded plate heat exchanger comprising:

a plurality of welded corrugated plates forming cold flow channels and hot flow channels, a cold inlet in fluid communication with the cold flow channels, a cold outlet in fluid communication with the cold flow channels, a hot inlet in fluid communication with the hot flow channels, a hot outlet in fluid communication with the hot flow channels;

wherein the plurality of plates is made of: a first stainless steel alloy comprising 0.005 to 0.020 wt % carbon, 9.0 to 13.0 wt % nickel, 17.0 to 19.0 wt % chromium, 0.20 to 0.50 wt % niobium, and 0.06 to 0.10 wt % nitrogen; or a second stainless steel alloy comprising 0.0005 to 0.020 wt % carbon, 10 to 30 wt % nickel, 15-24 wt % chromium, 0.20 to 0.50 wt % niobium, 0.06 to 0.10 wt % nitrogen, up to 5 wt % copper, up to 1.00 wt % silicon, up to 2.00 wt % manganese, 0.3 to 7 wt % molybdenum, 0.01 to 2.0 wt % beryllium, and 0.1 to 0.5 wt % boron.

16. The welded plate heat exchanger of claim 15 wherein at least one of cold inlet, the cold outlet, the hot inlet, and the hot outlet is made of the first stainless steel alloy or the second stainless steel alloy.

17. The welded plate heat exchanger of claim 15 further comprising at least one of a cold inlet header in fluid communication with the cold inlet and the cold flow channels, a cold outlet header in fluid communication with the cold outlet and the cold flow channels, a hot inlet header in fluid communication with the hot inlet and the hot flow channels, a hot outlet header in fluid communication with the hot outlet and the hot flow channels, and wherein at least one of cold inlet header, the cold outlet header, the hot inlet header, and the hot outlet header is made of the first stainless steel alloy or the second stainless steel alloy.

18. The welded plate heat exchanger of claim 15 further comprising a pressure vessel containing the plurality of plates.

19. The welded plate heat exchanger of claim 15 further comprising a pair of end plates clamped on the plurality of plates.

20. The welded plate heat exchanger of claim 15 wherein the cold inlet and the hot outlet are at one end of the plate heat exchanger and the cold outlet and the hot inlet are at the other end of the plate heat exchanger.