NESTED-FLOW HEAT EXCHANGERS AND CHEMICAL REACTORS
Disclosed is a technology based upon the nesting of tubes to provide chemical reactors or chemical reactors with built in heat exchanger. As a chemical reactor, the technology provides the ability to manage the temperature within a process flow for improved performance, control the location of reactions for corrosion control, or implement multiple process steps within the same piece of equipment. As a chemical reactor with built in heat exchanger, the technology can provide large surface areas per unit volume and large heat transfer coefficients. The technology can recover the thermal energy from the product flow to heat the reactant flow to the reactant temperature, significantly reducing the energy needs for accomplishment of a process.
- 1. (canceled)
- 2. A chemical reactor comprising:
a first tube; a second tube positioned in the first tube and defining an annular space between the first tube and the second tube; and a manifold assembly coupled to a first end of the first tube and a first end of the second tube, wherein the manifold assembly defines a first opening in direct fluid communication with the annular space between the first tube and the second tube and a second opening in direct fluid communication with a channel along the length of the second tube, wherein a second end of the second tube is open proximate a second end of the first tube.
- View Dependent Claims (3, 4, 5, 6, 7, 8, 9)
- 10. A method comprising:
flowing a gas along a bayonet flow path of a chemical reactor, the bayonet flow path being defined by; an annular space defined between a first tube and a second tube positioned in the first tube; a channel along the length of the second tube; a first opening defined in a manifold assembly coupled to a first end of the first tube and a first end of the second tube, wherein the first opening is in direct fluid communication with the annular space between the first tube and the second tube; and a second opening defined in the manifold assembly, wherein the second opening is in direct fluid communication with the channel along the length of the second tube.
- View Dependent Claims (11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22)
This application is a continuation of U.S. patent application Ser. No. 16/229,474, filed Dec. 21, 2018, entitled “NESTED-FLOW HEAT EXCHANGERS AND CHEMICAL REACTORS,” which is a continuation of U.S. patent application Ser. No. 15/934,763, filed Mar. 23, 2018, (now U.S. Pat. No. 10,401,092, issued Sep. 3, 2019), entitled “NESTED-FLOW HEAT EXCHANGERS AND CHEMICAL REACTORS,” which is a divisional of U.S. patent application Ser. No. 14/645,490, filed Mar. 12, 2015, (now U.S. Pat. No. 9,958,211, issued May 1, 2018), entitled “NESTED-FLOW HEAT EXCHANGERS AND CHEMICAL REACTORS,” of which the entire contents of each is hereby incorporated by reference herein in its entirety.
The present invention relates to equipment for processing chemicals and other fluid cycles bases on temperature change, including power cycles. The equipment includes heat exchangers and chemical reactors. The present invention significantly reduces the capital investment required for this type of equipment through use of standard construction techniques and materials. The present invention also significant improved heat transfer performance, where needed, which significantly reduces energy consumption for product production.
Heat exchangers and chemical reactors are commonly used equipment in chemical processing and recuperated power cycles. This equipment is used in a broad spectrum of industries and in all stages of processing.
Heat exchangers are used to transfer heat (thermal energy) from one process flow to another. Heat exchangers come in a variety of designs, including: tube-in-shell, flat plate, tube-in-tube, spiral-flow, and recently developed bonded-etched-plates usually referred to as printed circuit heat exchangers. Heat exchanger technologies are characterized by their respective flow path hydraulic diameters and the heat transfer area per unit size. Both the hydraulic diameter and heat transfer area per size scale inversely with the hydraulic diameter, thus the total energy transferred per unit time for a fixed temperature difference scales inversely with the square of the hydraulic diameter. This means that for a fixed sized heat exchanger, if one is constructed using tubes that are one-half the tube size in the other, the one with the smaller tubes can transfer four times the energy per unit time for the same temperature difference. This has led to smaller and smaller hydraulic diameter heat exchanger until now we are constructing heat exchangers using printed circuit etching technologies. These heat exchangers use hydraulic diameters flow paths down to 0.5 mm and resulting areas per volumes of up to 1300 m2/m3 (http://www.heatric.com/typical_characteristics.html). But the construction technique for making printed circuit heat exchanger is quite complex and only a limited production capability exists, resulting in both high cost and long delivery times for these heat exchangers. Heat exchanger costs per performance determine the overall heat exchanger efficiency that one can afford to implement into a process. A higher cost heat exchanger technology limits the total heat exchanger efficiency due to the tradeoff between heat exchanger cost and replacement energy cost. A lower cost heat exchanger technology achieves a higher efficiency as a result of that same tradeoff.
Chemical reactors are used to change a fluid from one chemical species to another. Chemical reactors operate at a variety of pressures and temperatures, where these parameters are adjusted for best performance within the chemical reactors capabilities. Some chemical reactors include catalyst while others have no need for catalyst, depending upon the kinetics of the chemical reaction. Four examples of chemical reactors are presented here for prior art description: hydrogen production using the Steam Methane Reforming (SMR) process; hydrogen production using the Water Shift Reaction (WSR) process; ammonia production using the Haber-Bosch process; and urea production from carbon-dioxide and ammonia. These processes are used in commercial fertilizer production, which is one of the targets for application of this invention.
Today'"'"'s SMR process involves the combustion of methane within a combustion chamber which is lined with pressurized and catalyst loaded reaction tubes in which the endothermic reaction of turning a mixture of methane and water to carbon-monoxide and hydrogen is accomplished. Although this process can be operated at low pressures, the tendency is to operate this process at 10-20 atmospheres to minimize the number of reaction tubes and subsequent pumping operations. Because this process is an endothermic reaction, it is favored at higher temperatures and usually operated at temperatures between 850° C. and 1000° C. The catalyst is used to improve the reaction kinetics (speed of conversion) also in an effort to reduce the number of reaction tubes. The energy conversion in these systems is reported to be in the low 70% levels due to several factors, these include: an additional 30% energy requirement to heat the SMR reactants from room temperature to reaction temperature as compared to the energy needs to accomplish the process at temperature; and consumption of 35% of the combustion energy to raise the combustion reactants from room temperature to SMR reaction temperature. In addition to the low energy efficiency of today'"'"'s SMR process, the capital cost of the system is high due to the strength of material required for the SMR reaction tubes. These tubes are operated at high temperatures, while coupled to a low pressure source of heat; forcing their construction from expensive materials, adding significantly to the cost of the SMR system. Because the cost of natural gas is significantly higher in Europe than it is in the US, several European fertilizer production facilities have incorporated a heat exchanger to recover the SMR product flow energy and utilize it for heating up the SMR reactant flow. This energy recuperation improves one of the energy demands in this system.
Today'"'"'s Water Shift Reaction (WSR) usually combines the carbon-monoxide from the SMR process and an outside source of steam to produce carbon-dioxide and hydrogen. Because of the characteristics of the WSR, the process is favored at lower temperatures, although a lower temperature bond exists due to catalyst performance. The process is slightly exothermic, and in today'"'"'s systems the WSR result in a temperature rise in the process flow temperature. This temperature rise limits the WSR conversion fraction, to offset that limitation the WSR is usually conducted in two stages with cooling provided between these stages for higher conversion.
Today'"'"'s Haber-Bosch process systems produce ammonia from hydrogen and nitrogen. The process is mildly exothermic, with a significant reduction in the number of molecules as a result of the chemical reaction. These characteristics result in more conversion at lower temperature as well as higher pressure. The process is usually accomplished in two stages to avoid the temperature rise from a single stage and cooling is provided between the two stages. Even with these steps, the once through conversion rate for the Haber-Bosch process is usually quite low, usually less than 20%. To obtain better utilization of the feed stock (hydrogen and nitrogen) the product flow is cooled to condense out the ammonia, then reheated and returned to the inlet of the process to achieve large total conversion efficiencies. Today'"'"'s system use significant energy to reheat the process flow and significant capital investment to provide the pressure capability of the reactor and heat removal capability of the increased flow through this system. At 18% once through conversion, the reactor size, the heat rejection system and the system required to reheat the process flow to the desired operating temperature are twice the size of a system that can achieve 36% once through conversion.
Today'"'"'s urea production systems produce urea through the Bosch-Meiser process from a feedstock of ammonia and carbon-dioxide. The production is usually described as a two-step process, with the ammonia and carbon-dioxide first forming an ammonium-carbamate which includes one ionic bond, that ionic bond is replaced over time with a covalent bond and release of a water molecule. The ammonium-carbamate formation is mildly exothermic with a reduction in the number of molecules from three to one. As a result, very high pressures and low operating temperatures are desired to promote the product formation, with pressures in excess of 2000 psi commonly used. The ammonium-carbamate slowly converts to urea as an endothermic process with a 15-30 minute hold time usually used to achieve the urea conversion of approximately 80%. The energy needs for the ammonium-carbamate to urea conversion is usually obtained from the exothermic ammonium-carbamate formation. Although the ammonium-carbamate can be removed from the urea through the simple reduction of the pressure over the mixture, today we accomplish this removal through the reduction of the partial pressure of either the carbon-dioxide or ammonia over the mixture. This stripping process permits the released reactants to be captured and returned as input feed to the process. The urea production reactor and the ammonium-carbamate to urea hold up column are very high pressure system. In addition, the ammonium-carbamate is highly corrosive to metal because of its ionic nature. Because of this very high pressure and corrosive nature, the chemical reactor and transforming column are costly.
U.S. Pat. No. 7,645,437 describes an Integrated boiler, superheater, and decomposer for sulfuric acid decomposition for the thermo-chemical production of hydrogen. In that application we (Robert Moore, Paul Pickard, Ed Pama, Fred Gelbard, Roger Lenard and Milton Vernon) described the nesting of ceramic tubes to accomplish the chemical processing of sulfuric acid with a built in heat recovery design. That invention permitted the highly concentrated sulfuric acid to enter the unit at approximately room temperature; be heated to nearly the decomposition temperature through recovery of the sensible heat in the product discharge; be decomposed through the use of an external heat source; and then be cooled by transferring the sensible heat of the product back to the sulfuric acid reactant feed. This permitted the connections of the ceramic decomposer to be made with moderately low temperature materials, of which we had many choices. That patent described only an endothermic process utilizing one flow path for reactants and one flow path for products.
U.S. Pat. No. 5,275,632 describes a bayonet reformer in which the outside of a closed tube is heated with an externally fired source to heat the reforming reactant to produce hydrogen. This patent introduces the concept of improved combustion thermal energy recovery through the use of small gap annular combustion flow. This patent limits the improved energy performance to the improved efficiency of thermal energy recovery from combustion down to the operating temperature of the reformer, offers no thermal energy recovery of the combustion energy below the reformer temperature and provides no recovery of the reforming product sensible heat.
U.S. Pat. No. 5,429,809 describes a bayonet chemical reactor in which the outside of a closed tube is heated with a thermal fluid that raises the process reactants to the desired temperature then that fluid limits the product temperature by acting as a heat source or sink as is necessary. Although this invention limits the temperature swings within a process, it does nothing for improvement of efficiency.
U.S. Pat. No. 5,639,431 describes a bayonet reformer in which the emphasis of the patent is on removing hydrogen through the use of hydrogen permeating tubes or cylinders, in an effort to lower the operating temperature of the reformer and thus achieve some energy efficiency. This patent introduces a complicated membrane in an attempt to achieve added efficiency, but fails to address major efficiency losses such as SMR product temperature and combustion exhaust temperature.
U.S. Pat. No. 5,876,469 describes a bayonet reformer in which the emphasis of the patent is on controlling the uniformity of the heating to the pressure tubes of the reformer, which are in the bayonet design, and the recovery of the flue gas sensible heat down to the operating temperature of the reforming process. This invention fails to recover flue gas sensible heat at temperatures less than the reforming process temperature or recover any of the reforming sensible heat.
Against this background, the present invention was developed.
Nested-flow technology is a system of flow channels developed through the nesting of circular tubes. These tubes provide a flow path bounded by the inside of an outer tube and the outside of an inner tube. The flow path is maintained open through the use of spacers within the flow path that do not substantially restrict flow. The spacing between the outer and inner tubes provides a characteristic hydraulic diameter and flow area; while the length of the tube provides a characteristic flow length.
A single nested-flow unit refers to the aggregate of all tubes nested within one another which essentially share the same axial centerline. The minimum number of nested tubes within a single nested-flow unit is determined from the number of process flows required to meet the desired result, while the maximum number of nested tubes is usually determined from other considerations such as internal pressure and material stress limits. The hydraulic diameter of flow channels can be the same or drastically different depending upon the needs of the system.
A manifold nested-flow unit refers to the assembly of multiple single nested-flow units into a common unit through the use of flow distribution manifolds. Each of the single nested-flow units performs the same process and it is the sum of these parallel operations that determine the quantitative production capability of the manifold nested-flow unit.
The nested-flow technology can be assembled into a heat exchanger by the repeated nesting of tubes one within the other, with each tube attached to a separate manifold plate at each end of the tube. A simple trade-off can be made between adding more tubes per single nested-flow unit and adding more single nested-flow units per production unit to minimize the cost of the system. The centermost tube of the single nested-flow unit can provide a flow path on the inside of the tube or not; if it provides a flow path then a final manifold plate is included to provide a corresponding flow path in the manifold assembly; if the centermost tube does not provide a flow path, the tube could be replaced by a rod and the rod or tube connected to the last manifold plate at both ends of the nested-flow tubes. For heat transfer between two fluid flows, with one fluid being heated while the other is being cooled, one flow path option is to flow the fluid being heated in one direction within all odd numbered channels, when channels are numbered from the centermost channel, while flowing the fluid being cooled in the opposite direction within all even numbered channels. For heat transfer between three fluid flows, for example say two fluids being heated and one fluid being cooled, one option is for the central most channel to be used for the first fluids being heated, the next channel out would be of the fluid being cooled and the third channel out would be of the remaining fluid being heated. This pattern would continue until all the flow channels in the single nested-flow are consumed.
The Nested-flow Technology can be assembled into a chemical reactor by the incorporation of chemical reactions sites within one or more of these nested-flow channels. Those chemical reaction sites are defined by the presence of catalyst or by the mixing of two or more fluid flows into a common volume providing an initial start point for the chemical reaction. In a single nested-flow unit the flow paths can be from one end to the other or operated as a bayonet flow with the flow returning back to the end from which it originated. These flow paths can be utilized to transfer heat, provide isolation flows for corrosion compatibility or provide resonance time for fluid flow to achieve the desired chemical conversion. A phase change jacket can be incorporated into the chemical reactor as a method of adding or removing energy at a constant temperature. Fluid can be added to the jacket at the desired temperature and state (vapor or liquid), the fluid can then change state (condense or boil) to either add energy to or remove energy from a process at a fixed temperature, then the fluid removed from the jacket in the new state. The fluid may be added or removed through a temperature gradient if the combined sensible and latent heat management is desired. A simple boiler is one example of the use of a phase change jacket which could incorporate a temperature gradient heat-up flow.
It is a primary objective of the present invention to provide a heat exchanger which is more cost effective per unit of energy transferred at the same driving temperature than we have today.
It is another objective of the present invention to provide a chemical reactor that incorporates heat transfer between the reactant and the products at a sufficiently low cost that the reactor will provide a significantly improved energy efficiency for the process.
It is another objective of the present invention to provide a chemical reactor that provides heat transfer during chemical reactions to better manage the process for improved production per product pass and reduction in non-desirable product production.
It is another objective of the present invention to provide a chemical reactor that utilizes the flow paths to limit the corrosion impact of product produced.
These and other objectives of the present invention will become apparent to those skilled in this art upon reading the accompanying description, drawings and claims.
Nested-flow technology as used herein shall mean a system of nested tubes with associated flow channels, created by those tubes, which are maintained as open flow channels through the use of spacers within those flow channels.
Single nested-flow unit as used herein shall mean a group of nested tubes with a common axial centerline, more or less, providing as many flow channels as desired.
Manifold nested-flow unit as used herein shall mean more than one single nested-flow unit assembled onto a common manifold for achieving desired product flow rates.
Wire wrap as used herein shall mean the installation of a small rod or wire spiraling from one end of a tube to the other end. The wire wrap can spiral fairly quickly from one end of the tube to the other, meaning that several inches of tube length would be incorporated for each wrap around the tube. The wire in this wire wrap need not be metal, but shall be compatible to the tube onto which it is wrapped and the fluid flowing within the channel in which it resides.
Phase change jacket is a larger volume enclosed by a single tube which contains a number of single nested-flow units in which the principle heat transfer mechanism is either boiling or condensation.
Endothermic process as used herein shall mean a process in which energy must be added else the reaction temperature would decrease.
Exothermic process as used herein shall mean a process in which energy must be removed else the reaction temperature would increase.
Best mode of the invention as contemplated by the inventor is to maximize the energy savings within the design until the incremental energy savings from a size increase does not recover the added capital and pumping cost. As contemplated by the inventor, any tube placed within another should first have a spacer installed on the outside of the tube, and in the inventors opinion that spacer should be a “wire” spiral wrap from, essentially, one end of the tube to the other. That wire wrap should be the gap size minus any uncertainty in that gap size as a result of size uncertainties in the tubes.
Although a single nested-flow unit can be operated, most production units will consist of a multitude of single nested-flow units connected to a common manifold system consisting of several plates, depending upon the number of flow channels in the single nested-flow unit. A plate is machined to accept the number of same size tubes needed to meet the users desired flow as well as to provide a flow path for the fluid. All nested-flow tubes of the same size are installed on the same manifold plate, and every tube size within a single nested-flow unit has an independent plate. Thus if seven size tubes are incorporated into the single nested-flow unit, then at least seven manifold plates are provide, one for each tube size. If the inner-most tube is used for a flow channel, then an eighth plate is added to provide a corresponding channel in the manifold. All tubes which go inside another are first wire wrapped to provide a physical spacer to maintain the resulting flow path open, then the tubes are attached to the manifold plates; then nested together with inclusion of catalyst, when needed, to produce a manifold nested-flow unit. The manifold can be welded, brazed, or incorporate gasket and seals to prevent loss of process fluid.
The nested-flow technology can be used as a heat exchanger or as a chemical reactor or as a combined unit accomplishing both chemical reaction and heat exchange.
Operation of the ammonia manifold nested-flow unit is accomplished by first heating the water in region 45 to the operation temperature for the water. The reactant mixture of approximately one molar part nitrogen and three molar parts hydrogen is introduced at the common port 42. The reactants are heated by the tube surfaces as they flow toward the warm end of the nested-flow, where they form ammonia when in contact with the catalyst 44. Most of the heat of formation is deposited in the common boiling water volume 45, where the steam produced is removed for use elsewhere. The ammonia and un-reacted hydrogen and nitrogen are cooled as they move towards the cooler end of the nested-flow, providing the heat to the tubes which is used to heat both the reactants and water to their desired temperature. The flow from all the nested-flows is gathered inside the manifold where it exits at the common port 41. The ammonia will condense as it is cooled in flow channel 49. If operated vertically, then the ammonia, nitrogen and hydrogen are separated shortly after exiting the nested-flow manifold. If operated horizontally, then a lower and upper port can be incorporated for port 41, permitting separation of ammonia from un-reacted reactants within the manifold. Significantly higher once through production rates will occur because the temperature is not permitted to rise in the production of ammonia. As a result, significantly less recycling will occur and the total ammonia production size will be smaller.
The methane flow for combustion is through channel 62. This methane flow is heated as it moves toward the warm end of the nested-flow nested tubes. The air flow for combustion is through channel 63. This air flow is heated as it moves toward the warm end of the nested-flow nested tubes. The methane and air are mixed at the warm end of the nested-flow unit, where the temperatures are greater than 800° C. Since the auto-ignition temperature for methane air is approximately 550° C., the methane and air will auto-ignite. The reactant flow for the SMR process is a mixture of steam and methane introduced through channel 65. The SMR reactant flow is heated as it travels toward the warm end of the nested-flow nested tubes, but will not substantially react until it enters the SMR catalyst 70 region of the channel 71. The SMR process has a significant entropy gain, going from three molecules of reactants to four molecules of products and proceeds nearly to completion provided sufficient energy is provided through the combustion process occurring in region 72. By having the combustion process so tightly coupled with the SMR process, the combustion process is only a few degrees hotter than the SMR process. As a result, the methane combustion process of the nested-flow system will have an equilibrium level significantly higher than 85% combustion level of the standard 2000° C. methane combustion. In addition, a combustion catalyst 69 is included in the system to take the methane combustion to even higher values as the combustion gases cool from loss of energy to the SMR process. Assembled into a manifold nested-flow unit, the SMR production unit will be twenty feet long and approximately twenty inches in diameter to meet the hydrogen production needs for twenty tons of ammonia per day. The efficiency improvement provided by the nested-flow technology will reduce the cost of hydrogen production from $140/ton-of-ammonia to $110/ton-of-ammonia based on natural gas cost of $4/million-BTUs.
These chemical reactors permit the modularization of a fertilizer plant. One modularization option would be to house these chemical reactors into standard forty feet long eight feet wide and eight feet tall shipping containers. To meet production goals of thirty-five tons per day of urea, only two shipping containers would be required to house all the chemical reactors, purification equipment, pumps and compressors. Such an arrangement would permit the equipment to be deployed rapidly to both regions that are short of fertilizer production capability and locations where natural gas is being wasted through flaring operations. For locations that are not supported or under-supported with electrical power, hydrogen production can be increased to provide additional hydrogen for use in hydrogen based fuel cells for production of needed electrical power. To meet 20 tons/day ammonia or 35 tons/day urea production, 1½ million cubic feet per day of hydrogen measured at STP will be required. If the hydrogen production is doubled, a 35 ton/day urea production plant could provide approximately 3.5 MW of electrical power using hydrogen fuel cell technology.