WO2013192162A1 - Low temperature heat transfer fluid and system - Google Patents

Abstract

A heat transfer system employs an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide salt as a working fluid. In suitable concentrations, such a solution remains fluid down to -50°C, -75°C, or lower, permitting heat transfer at temperatures that are inaccessible with conventional aqueous salt solutions, such as NaCl or CaCl₂ brines.

Description

TITLE

LOW TEMPERATURE HEAT TRANSFER FLUID AND SYSTEM

Cross-Reference to Related Application

This application claims benefit of US Provisional Patent Application Serial No. 61/662042, filed June 20, 2012 and entitled "Low Temperature Heat Transfer Fluid And System," which is incorporated herein in its entirety by reference thereto.

Technical Field

This invention relates to a heat transfer fluid and heat transfer method and system and, more particularly, to a heat transfer fluid and system operable at sub-ambient temperatures.

Background

A need to heat or cool an object arises in a wide spectrum of industrial and manufacturing processes. For example, cutting, molding, and shaping operations used to fabricate objects having a desired configuration often result in the production of heat, which may need to be removed to limit the temperature of the object or tooling to an acceptable value. Chemical syntheses are often exothermic so that heat must be removed to avoid decomposition processes, control reaction pathways and/or kinetics, or prevent hazards such as fire and explosion. Human comfort and safety is promoted by regulating the temperature

environment, both during the normal activities of life, but especially during medical procedures of various kinds.

In many of the foregoing situations, heat must be transferred to or from an object or location. Such transfer is very frequently accomplished using a heat-transfer fluid. In the simplest situations, air acts as a heat- transfer fluid, whether driven merely by convection or impelled by blowers or fans. But in more demanding applications, greater amounts of heat energy must be transferred or a faster transfer rate is desired, so that either high pressure or high velocity gas or a liquid heat-transfer fluid are used. The fluid may be either discarded after use or recirculated through a closed system. Some heat-transfer systems rely on a thermodynamic cycle, wherein a heat transfer medium undergoes a reversible, first-order thermodynamic phase change during the heat transfer cycle. In a first-order change, a characteristic amount of latent heat is absorbed or released. For example, a hot workpiece may be quenched by immersing it in a vessel of liquid, such as oil or water. If the mass and temperature of the object are large enough, some amount of the liquid may be boiled, thereby absorbing a measure of heat energy carried in the workpiece. The amount of heat transferred is governed in general by both the specific heat of the medium and its latent heat of vaporization. These quantities are characteristic of a given material, the specific heat being the amount of heat per unit mass required to raise the temperature by one degree, and the latent heat of vaporization being the amount of heat required per unit mass to induce the phase transformation of converting the liquid to or from vapor at a particular phase-change temperature. Ideally, a heat transfer medium has high latent heat and high heat capacity.

Many simple heat transfer systems rely merely on changing the temperature of a transfer fluid without any phase change. In this situation, the amount of thermal or heat energy transferred is governed by Equation (1), which relates the amount of energy transferred ΔΕ to the temperature change ΔΤ of the material : AE = m - C - AT (1)

wherein C is the specific heat and m is the mass of the material. In SI units, heat capacity is measured in units of joules per kilogram per Kelvin (or °C).

Refrigeration systems frequently comprise a more sophisticated apparatus in which the heat-transfer fluid is circulated in a closed-loop system, wherein the fluid boils at the heat source (thus absorbing heat) and thereafter is circulated to the heat sink, where it condenses and gives up the latent heat. The source and sink are commonly termed the

"evaporator" and "condenser," respectively. The amount of heat absorbed on boiling and released on condensation is governed by the latent heat of vaporization. Refrigeration systems ordinarily include compression or pump means to facilitate the cycle and circulation of the fluid.

However, in many instances, heat transfer systems must operate at sub-ambient temperatures. Because water is environmentally benign and has a high heat capacity, it is commonly used as a heat transfer fluid, but is obviously limited to use above its freezing point of 0°C (32°F). Water- based salt brines have reduced melting points, e.g. -17°C (1°F) for 21 wt.% NaCI and -44°C (-47°F) for 30 wt.% CaCI₂, and so permit operating temperatures below 0°C, but they are also relatively corrosive.

Systems operable at lower temperatures also have been constructed using various silicone or organic fluids for heat transfer. Many of the silicone-based materials, however, are believed to present stability issues at elevated temperatures, such as potential polymerization and cross- linking, which adversely increase the fluid viscosity and decrease effective heat transfer. In addition, they frequently must be protected from exposure to air or water and may be flammable. Organic heat transfer fluids include certain hydrocarbons, glycols, and polymers, many of which present issues that may include

objectionable odor, toxicity, stability, volatility, and flammability. Some materials also do not have adequate stability. Certain chlorofluorocarbon- based materials formerly used for heat transfer have now been eliminated as presenting an unacceptable global warming potential. Other

hydrofluorocarbon materials present a reduced global warming potential and thus are presently allowable. Accordingly, there remains a need for heat transfer fluids that are operable at sub-ambient temperatures but reduce or eliminate operational, environmental, and safety issues presented by fluids now available.

Summary

In one aspect, the disclosure hereof provides a heat transfer system comprising:

(a) a heat source;

(b) a heat sink;

(c) a heat transfer fluid comprising an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide; and

(d) a fluidic connection configured to communicate flow of the heat transfer fluid between the heat source and the heat sink. In another aspect, there is provided herein a method for transferring thermal energy from a heat source to a heat sink. The method comprises the steps of:

(a) providing a heat transfer fluid comprising an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide;

(b) in sequence, thermally contacting the heat transfer fluid to one of the heat source and the heat sink and thereafter contacting the heat transfer fluid to the other of the heat source and the heat sink, whereby thermal energy is transferred from the heat source to the heat sink. Still further, there is provided a method for transferring thermal energy to or from a workpiece, comprising:

(a) providing a heat transfer fluid comprising an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide;

(b) thermally contacting the workpiece with the heat transfer fluid, whereby thermal energy is transferred between the workpiece and the heat transfer fluid.

Still another aspect of the disclosure hereof provides a heat transfer fluid comprising a near-eutectic aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide. In an embodiment, the heat transfer fluid comprises a a near-eutectic aqueous solution of lithium

bis(trifluoromethylsulfonyl)imide.

Brief Description of the Drawings

The disclosure hereof will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of certain preferred embodiments and the

accompanying drawing, in which:

FIG. 1 is a schematic diagram showing a closed-loop heat transfer system.

Detailed Description

The present disclosure arises from the discovery that aqueous solutions of certain organic salts have a freezing point that is well below room temperature, extending to below the temperature (T= -78.5°C) of a dry ice bath. Such fluids can therefore function as heat transfer fluids for many low-temperature heat transfer processes. Heat is thermal energy in the process of transfer. As used herein, the term "heat source" refers broadly to any object, substance, or medium having a thermal mass and from which energy in the form of heat can be extracted; the term "heat sink" refers broadly to any object, substance, or medium to which energy in the form of heat can be transferred. The term "heat transfer fluid" (also known as a heat transfer composition or a heat transfer fluid composition) refers to a working fluid used to carry heat from a heat source to a heat sink. In broadest usage, a heat transfer fluid can be either a liquid or a gas. The term "heat exchanger" refers to an apparatus by which heat energy in a first fluid substance can be transferred to a second fluid substance, with the first and second fluid substances being kept separate. Each of these fluid substances can be either a liquid or a gas. One simple form of heat exchanger is a radiator, wherein the ambient atmosphere functions as the second fluid substance. The first fluid substance circulates through internal passages in the radiator, and heat is transferred to or from the first substance to the ambient atmosphere. Other forms of heat exchangers permit both the fluid substances to circulate in closed-loop systems.

For example, an internal combustion-engine system used in an automobile ordinarily employs a heat transfer system by which waste heat generated in the engine that cannot be converted to useful shaft power is transferred to the ambient atmosphere. A pump urges an aqueous coolant fluid to circulate through a closed-loop, fluidic connection that joins internal cavities in the engine to a finned radiator. (The coolant fluid is commonly an aqueous ethylene glycol solution with functional additives, such as anticorrosive agents.) Unusable heat produced by combustion in the cylinders is transferred to the engine block and thence to the circulating coolant fluid, thus cooling the engine while heating the fluid. The fluid then passes through internal passages in the radiator and heats it.

Thereafter, the fluid, having been cooled by giving up heat to the radiator (and thus to the ambient air), returns to the engine block to close the loop. The radiator functions as a heat exchanger, transferring heat delivered to it by the coolant fluid to the ambient atmosphere, without any direct contact between the coolant fluid and the atmosphere. The radiator thereby serves as an intermediate heat sink, with the thermal energy ultimately being delivered into the ambient air. The effectiveness of this transfer is enhanced by providing the radiator with a number of fins that have a low impedance thermal pathway to its internal passages. The fins have a large surface area and at least the fins are typically constructed with a high thermal conductivity material, such as aluminum or copper. To improve the heat transfer to the atmosphere, the flow of air across the radiator fins is ordinarily enhanced by a fan. The heat-transfer cycle of the automobile engine described above is comparable to heat transfer methods used in a wide variety of

manufacturing and industrial processes.

In one embodiment of the subject matter hereof, the heat transfer fluid used in a heat transfer system and in a method for transferring heat from a heat source to a heat sink can comprise or consist essentially of an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide organic salt. This substance is often abbreviated as MTf2N, wherein M is any of the alkali metals Li, Na, K, Rb, Cs, or a mixture thereof. Dissolving this salt in water creates an ionic solution characterized by the presence of dissociated anions and cations. In further embodiments, the fluid employed in the system and method is an aqueous solution comprising 50% to 80%, or 50 to 70% by weight of lithium bis(trifluoromethylsulfonyl)imide. In other embodiments, the fluid employs an aqueous solution comprising 30% to 70% or 40% to 60% by weight of NaTf2N or KTf2N. The heat transfer fluid in any of these embodiments may further contain functional additives designed to inhibit corrosion, improve the flowability, improve the heat transfer at an interface between the fluid and a surface, provide lubrication, or accomplish other functions typically required for a heat transfer fluid, without substantially inhibiting the fluid's ability to accept thermal energy transferred to it or compromising its flowability or related rheological characteristics.

In another embodiment, a heat transfer fluid used in the present system and method can comprise or consist essentially of an aqueous solution which is a near-eutectic composition of water and an alkali metal bis(trifluoromethylsulfonyl)imide including, without limitation, a eutectic composition of water and lithium bis(trifluoromethylsulfonyl)imide. The term "near-eutectic composition" refers to a composition in a range encompassing a eutectic composition, the range being such that the melting point of any composition therewithin varies by no more than 2°C. Aqueous solutions of alkali metal-Tf2N beneficially provide low freezing point temperatures {e.g. -78.5°C or lower for suitable LiTf2N

compositions), a lack of flammability, and low toxicity and environmental impact. In various embodiments, heat transfer systems employing a heat transfer fluid comprising such an aqueous solution benefit from any one or more of these characteristics. In some implementations, the fluidic connection of the present heat transfer system provides a closed-loop arrangement defining a flow path for recirculation of the heat transfer fluid through the heat source and the heat sink. By "closed loop" is meant a fluidic connection in which the heat transfer fluid is recirculated through the system, with fluid not being added to, or removed from, the system in the course of ordinary operation. Of course, provision may be made for initially charging the system,

replenishing any fluid that is lost during operation {e.g. to a leak), and removing some or all the fluid during maintenance operations. The heat transfer fluid in some closed-loop systems is confined within a totally enclosed structure, such as a continuous piping loop. However, in other closed loop systems, part of the fluidic connection may be unenclosed. For example, the heat transfer fluid might be flowed or sprayed over a workpiece to be cooled, with the fluid thereafter collected in a catch basin, from which it is conveyed into piping to be recirculated to the heat sink, cooled, and thereafter conveyed to the workpiece to complete the circuit.

Fig. 1 depicts a simple form of a heat transfer system shown generally at 10. A heat transfer fluid comprising MTf2N in aqueous solution is conveyed through the system in a closed piping loop 16 by the action of pump 18, which may be of any type compatible with the system's operating conditions. The fluid circulates in the direction shown by the arrows, alternately bringing the heat transfer fluid into thermal contact with heat source 12 and heat sink 14. In an implementation, heat source 12 might be a chemical reactor or a workpiece being machined, treated, or tested. Heat produced in source 12 is transferred to the heat transfer fluid, causing its temperature to rise, commensurate with the amount of heat energy extracted from source 12. The fluid is then passed to heat sink 14, at which heat is extracted from the fluid, cooling its temperature. The loop is completed by passing the fluid through pump 18 and again to the heat source 12. In some implementations of heat transfer system 10, a workpiece might be cooled to a temperature well below ambient room temperature by holding heat sink 16 to a subambient temperature, so that sufficient heat is extracted to cool the heat transfer fluid to a sufficiently low temperature. Of course, the pump may be placed in other positions in within a

circulation loop.

In an alternative implementation, the heat transfer system may provide an open or closed loop configuration that functions as a thermosiphon. In such a configuration, natural convection, based on the temperature variation of the density of the heat transfer fluid, urges the flow of the heat transfer fluid through the system, in some cases avoiding the need for an external pump.

In an alternative embodiment, the present heat transfer system is operable over a range of temperatures that may be below 0°C, or as low as -50°C or -75°C, by use of a heat transfer fluid having a suitable composition, i.e. one that remains fluid to at least as low as the required minimum operating temperature experienced by the fluid during its recirculating cycle. In such a system and in the practice of the present method, the heat transfer fluid may be cooled to a temperature below 0°C, -50°C, or -75°C during its passage through at least a portion of its flow path. In some embodiments of the present system and method, the fluid may also experience an elevated temperature during at least a portion of its flow path. It has been found that the present aqueous solution of alkali metal bis(trifluoromethylsulfonyl)imide is stable at temperatures that in some compositions may be at least 200°C or more, facilitating operation of heat transfer systems operable well above room temperature as well as at subambient temperatures.

In other embodiments, the present heat transfer system

incorporates a heat exchanger as either the heat source or the heat sink, depending on whether heat is to be added or removed from a working object. Any suitable form of heat exchanger may be used including, without limitation, fin and tube heat exchangers, microchannel heat exchangers, and vertical or horizontal single pass tube or plate type heat exchangers.

Another aspect of the present disclosure provides a method for transferring heat to or from a workpiece of any type by thermally contacting it with a heat transfer fluid comprising an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide. By "thermal contact" is meant any physical contact that results in transfer of thermal energy between the heat transfer fluid and the appointed workpiece, whether the fluid and the workpiece are in direct physical contact or the contact is made through an intermediate structure. In representative

implementations of this aspect, the workpiece is an item being machined, a reactor vessel, or an operating electrical, electromechanical, or electronic device. Thermal contact may be effected by any suitable arrangement, including, without limitation, spraying, flowing, or otherwise impinging the heat transfer fluid directly onto the workpiece. Alternatively, the fluid may be delivered directly onto a support structure on which the workpiece is physically situated and optionally held or secured, or the fluid may flow through cooling passages within the support structure. By suitably selecting the temperature of the heat transfer fluid, the workpiece may be either heated or cooled. In an embodiment, a static amount of the fluid may be present and in thermal contact, the amount providing a thermal mass that is sufficiently high to maintain the workpiece at approximately a preselected temperature. Alternatively, the fluid may be supplied dynamically at a temperature and at a rate sufficient to transfer enough thermal energy to maintain the workpiece at approximately a preselected temperature. Examples

The operation and effects of certain embodiments of the present invention may be more fully appreciated from a series of examples

(Examples 1-4), as described below. The embodiments on which these examples are based are representative only, and the selection of those embodiments to illustrate aspects of the invention does not indicate that materials, components, reactants, conditions, techniques and/or configurations not described in the examples are not suitable for use herein, or that subject matter not described in the examples is excluded from the scope of the appended claims and equivalents thereof.

Solutions were prepared by dissolving lithium

bis(trifluoromethylsulfonyl)imide (LiTf2N) (Iolitec, Inc, Tuscaloosa,

Alabama, Lot 100420.4) in HPLC-grade water (Aldrich, Inc. Lot

1001159890), with nominal concentrations of 20.0, 40.4, 60.0 and 73.7 wt.% of the organic salt. Crystallization temperatures were measured using a Tamson silicone oil bath, which had a minimum operating temperature of about -40 °C. The samples were placed in glass vials and suspended in the oil bath. The bath temperature was monitored using a resistance temperature device (RTD). The bath temperature was lowered while manually agitating the samples until the appearance of solids was detected in the vials, at which point the onset of crystallization begins and the crystallization temperature recorded. The temperature values were confirmed by allowing the sample to freeze solid and thereafter slowly warming the sample until it became liquid.

It was found that the 60.0% and 79.2% samples remained liquid even at -35.1°C. Consequently, these samples were placed in a dry ice bath (T= -78.5°C). The 79.2% sample froze after about 4 min., whereas the 60.0% sample remained liquid, albeit with a high viscosity, and showed no evidence of crystallization, even after an hour of exposure. It was thus inferred that the freezing point of the 79.2% sample was between -35.1 and - 78.5°C. These findings are set forth in Table I below.

Table I

LiTf2N Heat Transfer Fluid Properties

The data of Table I indicate that even relatively dilute aqueous solutions of LiTf2N remain liquid at temperatures below the freezing point of water, and, in some cases, the liquid persists to below the sublimation point of dry ice (~ -78.5 °C). The data further indicate the existence of a water-LiTf2N eutectic composition between 40 and 80 wt.% LiTf2N, permitting a suitably chosen composition, such as an aqueous solution of 57.5 to 67.5 wt.%, or a near-eutectic aqueous solution, of LiTf2N, to be used as a heat transfer fluid operable at even lower temperatures.

Where a range of numerical values is recited or established herein, the range includes the endpoints thereof and all the individual integers and fractions within the range, and also includes each of the narrower ranges therein formed by all the various possible combinations of those endpoints and internal integers and fractions to form subgroups of the larger group of values within the stated range to the same extent as if each of those narrower ranges was explicitly recited. Where a range of numerical values is stated herein as being greater than a stated value, the range is nevertheless finite and is bounded on its upper end by a value that is operable within the context of the invention as described herein. Where a range of numerical values is stated herein as being less than a stated value, the range is nevertheless bounded on its lower end by a non-zero value.

In this specification, unless explicitly stated otherwise or indicated to the contrary by the context of usage, where an embodiment of the subject matter hereof is stated or described as comprising, including, containing, having, being composed of, or being constituted by or of certain features or elements, one or more features or elements in addition to those explicitly stated or described may be present in the embodiment. An alternative embodiment of the subject matter hereof, however, may be stated or described as consisting essentially of certain features or elements, in which embodiment features or elements that would materially alter the principle of operation or the distinguishing characteristics of the embodiment are not present therein. A further alternative embodiment of the subject matter hereof may be stated or described as consisting of certain features or elements, in which embodiment, or in insubstantial variations thereof, only the features or elements specifically stated or described are present.

In this specification, unless explicitly stated otherwise or indicated to the contrary by the context of usage, (a) amounts, sizes, ranges, formulations, parameters, and other quantities and characteristics recited herein, particularly when modified by the term "about", may but need not be exact, and may also be

approximate and/or larger or smaller (as desired) than stated, reflecting tolerances, conversion factors, rounding off, measurement error, and the like, as well as the inclusion within a stated value of those values outside it that have, within the context of this invention, functional and/or operable equivalence to the stated value; and

(b) all numerical quantities of parts, percentage, or ratio are given as parts, percentage, or ratio by weight; the stated parts, percentage, or ratio by weight may or may not add up to 100.

Claims

Claims What is claimed is:

1. A heat transfer system comprising:

(a) a heat source;

(b) a heat sink;

(c) a heat transfer fluid comprising an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide; and

(d) a fluidic connection configured to communicate flow of the heat transfer fluid between the heat source and the heat sink.

2. The heat transfer system of claim 1, wherein the aqueous solution comprises 50% to 80% by weight of lithium

bis(trifluoromethylsulfonyl)imide.

3. The heat transfer system of claim 1, wherein the aqueous solution comprises 50% to 70% by weight of lithium

bis(trifluoromethylsulfonyl)imide.

4. The heat transfer system of claim 1, wherein the aqueous solution comprises a near-eutectic composition of water and an alkali metal bis(trifluoromethylsulfonyl)imide.

5. The heat transfer system of claim 1, wherein the aqueous solution comprises a near-eutectic composition of water and lithium bis(trifluoromethylsulfonyl)imide.

6. The heat transfer system of claim 1, wherein the heat transfer fluid remains a liquid at a temperature at least as low as -50°C.

7. The heat transfer system of claim 1, wherein the fluidic connection provides a closed loop arrangement defining a flow path for recirculation of the heat transfer fluid through the heat source and the heat sink.

8. The heat transfer system of claim 7, further comprising a pump operable to circulate the heat transfer fluid between the heat source and the heat sink.

9. A heat transfer fluid comprising a near-eutectic aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide.

10. The heat transfer fluid of claim 9, comprising a near-eutectic aqueous solution of lithium bis(trifluoromethylsulfonyl)imide.

11. A method for transferring thermal energy from a heat source to a heat sink, the method comprising the steps of:

(a) providing a heat transfer fluid comprising an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide;

(b) in sequence, thermally contacting the heat transfer fluid to one of the heat source and the heat sink and thereafter contacting the heat transfer fluid to the other of the heat source and the heat sink, whereby thermal energy is transferred from the heat source to the heat sink.

12. The method of claim 11, wherein the heat transfer fluid is operable at -50°C.

13. The method of claim 11, wherein the aqueous solution comprises 50% to 70% by weight of lithium bis(trifluoromethylsulfonyl)imide.

14. The method of claim 11, wherein the heat source and heat sink are coupled by a fluidic connection defining a closed-loop recirculating flow path, and the method further comprises recirculating the heat transfer fluid through the heat source and the heat sink, such that thermal energy is imparted from the heat source to the heat transfer fluid and thermal is extracted from the heat transfer fluid to the heat sink.

15. The method of claim 14, wherein the heat transfer fluid is cooled to a temperature below 0°C during its passage through at least a portion of the flow path.

16. A method for transferring thermal energy to or from a workpiece, comprising:

(a) providing a heat transfer fluid comprising an aqueous solution of an alkali metal bis(trifluoromethylsulfonyl)imide; and

(b) thermally contacting the workpiece with the heat transfer fluid, whereby thermal energy is transferred between the workpiece and the heat transfer fluid.