WO1996021836A1

WO1996021836A1 - Heat exchanger having enhanced heat transfer capability

Info

Publication number: WO1996021836A1
Application number: PCT/US1995/015306
Authority: WO
Inventors: Kyung W. Suh; Creston D. Shmidt; John M. Kennedy
Original assignee: The Dow Chemical Company
Priority date: 1995-01-12
Filing date: 1995-11-27
Publication date: 1996-07-18
Also published as: TW290632B

Abstract

The surfaces are further adapted to continuously remove or provide heat to the fluid. The surfaces comprise a metal/fluorinated polymer coating structure. Further disclosed is a process for cooling or heating a fluid.

Description

HEAT EXCHANGER HAVING ENHANCED

HEAT TRANSFER CAPABILITY

This invention relates to a heat exchanger having enhanced heat transfer efficiency and reduced pressure drop.

Conventional heat exchangers have typically been constructed of common metals and alloys such as carbon steel, stainless steel, galvanized steel, and aluminum. Such metals are relatively inexpensive, and are readily fabricable. Contact surfaces across which heat is transferred are typically constructed of the same metal as the remainder of the heat exchanger.

A problem with heat transfer contact surfaces comprised of common metals is fouling or deposition of impurities or degradation products on those surfaces. Fouling or deposition diminishes the heat transfer capability of the contact surfaces, and, in turn, the heat exchanger as a whole.

Fouling of heat transfer contact surfaces is particularly troublesome when heating or cooling polymer melts or polymer-containing fluids. Polymers are subject to degradation and oxidation, and also are relatively large and heavy in particulate form, which makes deposition onto contact surfaces likely.

One means of preventing fouling is to coat or laminate heat transfer contact surfaces with a fluorinated polymer such as polytetrafluoroethylene (PTFE). Surfaces of PTFE are lubricious and non-porous, and substantially prevent fouling. Heat exchangers with contact surfaces of PTFE are well known in the art. Representative patents include U S Patent Nos 4,705,101 ; 4,776,391 ; 4,557,202; 4,479,359; and 4,461,347.

A problem with employing heat transfer contact surfaces of PTFE is diminution ot heat transfer capability. PTFE has a significantly lower thermal conductivity than common metals on which is it typically coated.

It would be desirable to have a heat exchanger with heat transfer contact surfaces which resist fouling. It would further be desirable to have a heat exchanger with contact surfaces which have a thermal conductivity comparable or similar to those of metals commonly employed in heat exchangers.

According to the present invention, there is a heat exchanger comprising a plurality of heat transfer contact surfaces and a means for continuously removing heat from or providing heat to those surfaces. The surfaces are fashioned to contact and are adapted to continuously convey a fluid to be heated or cooled along or through them . The surfaces are further adapted to continuously remove heat from or provide heat to the fluid. The surfaces comprise a metal/fluorinated polymer coating structure of a porous metal matrix layer infused and coated with a fluorinated polymer (metal/fluorinated polymer coating structure). The exchanger is particularly useful in heating or cooling a flowable polymer melt. A preferred fluorinated polymer is PTFE.

Further according to the present invention, there is a process for cooling or heating a fluid The process comprises providing the fluid; contacting the fluid with a heat transfer contact surface; and removing or providing heat to the liquid. The surface is maintained at a temperature different than the temperature of the fluid The surface is adapted to continuously remove or provide heat. The surface comprises a metal/fluorinated polymer coating structure of a porous metal matrix layer infused and coated with a fluorinated polymer.

The present heat exchangers afford enhanced heat transfer efficiency over those of the prior art. Heat exchange is more effective and efficient because fouling, deposition, or buildup of solids on heat transfer contact surfaces is substantially prevented or reduced. Heat exchange is further effective and efficient because the heat transfer contact surfaces have relatively high thermal conductivity.

The present heat exchanger provides greater long-term heat transfer efficiency than prior art heat exchangers having heat transfer contact surfaces of common metals because the metal/fluorinated polymer coating structure of the present exchanger is substantially not susceptible to fouling, deposition, or buildup of solids. The metal/fluorinated polymer coating structure is not susceptible because it has a significantly lower coefficient of friction than contact surfaces comprised solely of common metals under most conditions due to the presence of the fluorinated polymer. Though virgin contact surfaces of solely common metals will afford more efficient heat transfer than a surface comprised of a metal and a fluorinated polymer, the long-term heat transfer efficiency of the metal/fluorinated polymer coating structure will be greater than those surfaces of solely common metals because of substantially lower fouling or buildup.

The present heat exchanger further provides greater short-term and long-term heat transfer efficiency than prior art heat exchangers having heat transfer contact surfaces with surface coatings entirely of fluorinated polymers. The metal/fluorinated polymer coating structure of the present exchanger provides greater short-term and long-term heat transfer efficiency because it has a higher thermal conductivity under most conditions and, thus, is more heat conductive. The metal/fluorinated polymer coating structure has a higher thermal conductivity than surface coatings entirely of PTFE under most conditions because of the presence of metal matrix in the coating structure.

The enhanced heat transfer efficiency of the present heat exchanger further promotes more uniform heat distribution or lower temperature gradient in fluid contacting us heat transfer contact surfaces than corresponding prior art heat exchangers. The lower temperature gradient is particularly apparent when processing fluids of relatively low heat transfer capability such as flowable polymer melts Temperature gradient is the difference in temperature of the fluid contiguous to the heat transfer contact surfaces and a region of the fluid stream further from the contact surf aces.

Another advantage of the present heat exchanger is reduced pressure drop compared to prior art heat exchangers with heat transfer contact surfaces entirely of common metals. The metal/fluorinated polymer coating structure of the present heat exchanger has a much lower coefficient of friction than contact surfaces entirely of common metals in most useful heat exchange temperature ranges. The lower coefficient of friction manifests itself in the form of lower pressure drop across the heat exchanger. The lower pressure drop is particularly apparent when processing viscous fluids such as polymer melts. The lower pressure drop affords energy cost savings, lower process and equipment pressure duties, higher throughput potential, and broader processing conditions.

In summary, the present heat exchanger is advantageous over prior art heat exchangers having heat transfer contact surfaces solely or entirely of common metals and those having contact surfaces entirely of fluorinated polymers because it provides the advantages of each without their disadvantages. The present heat exchanger has the advantage of high heat transfer efficiency similar to that of heat exchangers with virgin or un-fouled contact surfaces of solely common metals without the disadvantage of diminution in heat transfer efficiency encountered with those contact surfaces within a short time due to fouling and buildup. The present heat exchanger also has the advantages of not being susceptible to fouling and buildup and having a low pressure drop similar to heat exchangers having heat transfer contact surfaces entirely of fluorinated polymers without their disadvantage of low heat transfer efficiency. The present heat exchanger also provides a more uniform temperature gradient in the fluid to be heated or cooled than prior art heat exchangers.

The performance of the present heat exchanger is maximized or optimized according to operating conditions and the fluid to be heated or cooled. Performance attributes such as heat transfer efficiency, coefficient of friction, and pressure drop of the coating structure may vary according to coating structure composition, coating structure operating temperature, fluid (to be heated or cooled) composition and physical properties, and fluid temperature.

The metal/fluorinated polymer coating structures are positioned or configured within the heat exchanger such as to receive, contact, and convey the fluid being heated or cooled. The coating structures are preferably applied to or formed upon metal support surfaces which are part of the basic structure of the heat exchanger. The metal support surfaces function, at least in part, to provide physical support for the metal/fluorinated polymer coating structures. The metal upon which the coating structure is situated is typically, but not necessarily, the primary metal ot construction of the heat exchanger. Typically, the coating structure is applied to or formed by treatment of heat transfer contact surfaces of an existing conventional heat exchanger. Heat is transferred across the coating structures to the fluid to be heated or cooled .

The metal/fluorinated polymer coating structures of the present heat exchanger may be formed by any of several methods known in the art In the broadest aspect, a metal surface is rendered porous or microporous, and a fluorinated polymer such as PTFE is infused in a particulate or microparticulate form into the interstices of the porous metal surface, and then over the top or upper surface of the porous metal surface to substantially cover it with a thin layer of the PTFE. In the another aspect, a porous or microporous layer of metal is applied to an existing metal support surface, and a fluorinated polymer such as PTFE is infused in a particulate or microparticulate form into the interstices of the porous metal layer, and then over the top or upper surface of the porous metal layer to substantially cover it with a thin layer of the PTFE. Generally, PTFE particles are infused in porous metal matrixes to form a hard, lubricious, and corrosion-resistant metal/fluorinated polymer surface . Useful metals in the porous metal matrixes include cobalt, nickel, chromium, tungsten, aluminum, phosphorous, and alloys of any of the foregoing.

The metal/fluorinated polymer coating structures are available commercially in a variety of compositions and configurations The General Magnaplate Company applies a variety of such coating structures under the tradenames NEDOX, TUFRAN, MAGNADIZE, CANADIZE, LECTROFLUOR, HI-T-LUBE, and MAGNAPLATE HCR Poly-Plating, Inc applies a variety of such coatings under the POLY-OND tradename.

By way of example, the commercial process for making NEDOX coating structures is generally as follows a hard, microporous layer of nickel is electroplated onto a clean, existing metal support surface; the micropores of nickel are enlarged; PTFE microparticles are infused into the microporous layer of nickel and ultimately over the top or upper surface of it to substantially cover it with a thin layer of the PTFE; and the resulting coating structure is heat treated to ensure thorough infusion and curing.

Further by way of example, the commercial process for making TUFRAN is generally as follows an existing metal aluminum surface is cleaned; the surface of aluminum crystals are then expanded; PTFE microparticles are infused into the aluminum and, ultimately, over the top or upper surface of it to substantially cover it with a thin layer of the PTFE; and the resulting coating structure is then cured to ensure interlocking of the PTFE microparticles.

Metal/fluorinated polymer coating structures have been employed in a variety of end uses in the prior art. U.S. Patent Nos 4,309, 115 and 4,623,546 relate the use of such coating structures on various internal surfaces of extruders to enhance extruder throughput U.S. Patent No 5,008,056 relates use of such coating structures in extrusion dies U.S. Patent No 4,204,406 relates the use of such coating structures to resist corrosion. U.S. Patent No 4,970,560 relates the use of such coating structures on blades to reduce wear Other known characteristics and properties of metal/fluorinated polymer coating structures include enhanced mold release, chemical resistance, moisture resistance, and high dielectric strength.

Fluorinated polymers useful in the metal/fluorinated polymer coating structures may be any of those known in the art for reducing friction and/or enhancing lubricity.

Preferred fluorinated polymers are the tetrafluoroethylene polymers. The tetrafluoroethylene polymers help provide heat exchange contact surfaces with a low coefficient of friction and low degree of stickiness or adhesion with the fluid being cooled. Tetrafluoroethylene

homopolymers (polytetrafluoroethylene) (PTFE) are especially preferred. Excellent teachings to tetrafluoroethylene polymers are seen in Tetrafluoroethylene Polymers, "Encyclopedia of Polymer Science and Technology" , Volume 16, S.V.Gangal, Pages 577-642.

The present invention is particularly useful for heating or cooling viscous liquids such as polymer melts Polymer melts present particular problems with respect to heat transfer because of their tendency to foul heat exchange contact surfaces with polymer degradation and oxidation by-products. Melts of polymers of useful thermoplastics include polyolefins, polyvinylchlonde, alkenyl aromatic polymers, polycarbonates, polyamides, polyesters, polyvinylidene chloride, polyvinyl chloride, polymethylmethacrylate, copolymers, terpolymers, polymer blends, and rubber modified polymers Suitable polyolefins include polyethylene and polypropylene. Suitable alkenyl aromatic polymers include polystyrene and copolymers of styrene and other monomers, such as acrylic acid, butyl acrylate, acrylonitrile, and 1 ,3-butadiene. Suitable polyethylenes include those of high, medium, low, linear low, and ultra low density types The present heat exchanger is particularly efficacious in heating or cooling viscous, flowable polymer melts at or just above or below their melting point or glass transition temperatures.

Polymer-containing fluids such as polymer/solvent compositions and aqueous polymer compositions may also be heated or cooled in the present heat exchangers. Useful polymer/solvent compositions include polymer/gas or polymer/vapor mixtures and

polymer/organic solvent mixtures. Useful aqueous polymer compositions include aqueous polymer latexes, emulsions, and suspensions.

Nonpolymeric fluids may also be heated or cooled in the present heat exchangers As when processing polymer melts or polymer-containing fluids, fouling or deposition may occur on heat transfer surfaces from non-polymeric fluids. The present invention enhances heat exchange by minimizing such fouling and deposition.

The present heat exchanger may take the form of any heat exchanger known i n the art Useful types include concentric tube exchangers, shell and tube exchangers, and plate exchangers. The heat exchanger may be of either counter-current or co-current flow configuration.

Heat is typically provided to or removed from heat transfer contact surfaces by any of those conventional means known in the art such as heat transfer fluids and gases. Useful gases include superheated steam, air, and other atmospheric gases . Useful fluids include water and various conventional petroleum and synthetic heat transfer oils.

In the following examples, polymer/solvent mixtures were cooled by heat exchangers having metal/fluorinated polymer coating structures and heat exchangers without such coating structures. The pressure drops and heat transfer coefficients exhibited by the heat exchangers with and without the coating structures were compared.

The two apparatuses employed each comprised an extruder and two coolers (heat exchangers) in series. The extruder was a 1.125 inch (2 86 centimeters) screw extruder, and had a mixing head attached to the end of the extruder screw. The first heat exchanger (# 1) was oil-cooled, and the second heat exchanger (#2) was water-cooled. In one apparatus, the heat exchange contact surfaces of both heat exchangers were coated with metal/fluorinated polymer coating structures In the other apparatus, the heat exchange contact surfaces of the exchangers were left uncoated. The coating structure was NEDOX SF-2 PTFE/nickel/cobalt alloy (General Magnaplate).

The polymer was gravity fed to the extruder. The polymer was melted, and a solvent was introduced into the polymer melt. The polymer and solvent were homogeneously mixed in the mixing head of the extruder to form the polymer/solvent mixture, which was conveyed through the heat exchangers. The extruder throughput was 15 pounds of polymer per hour (6 75 kilograms of polymer per hour).

Pressure drops for the exchangers with and without the coating structures were measured by subtracting the pressure of the polymer mixture at the inlets of each of the exchangers from the pressure of such mixture at the outlets of each of the exchangers.

Heat transfer coefficients for each of the exchangers were calculated by the following expression.

U = (m•Cp•ΔT_mix) / A•ΔT_{ℓ m}

where

exchanger and the cooling medium (oil or water) entering the heat exchanger Example 1 and Comparative Example 1

A polymer/solvent mixture was processed in the apparatuses described above to determine the impact of the coating structure on heat exchanger performance and efficiency The data are set forth in Table 1.

The polymer employed was a polystyrene of a weight average molecular weight of about 200,000 according to size exclusion chromotography. The solvent was a 50/50 weight mixture of methyl chloride and dichlorodifluoromethane, and was employed at 11 parts per hundred (pph) based upon the weight of the polymer.

The coating structure generally significantly increased heat transfer coefficient across the first exchanger compared to the corresponding first exchanger without the coating structure.

The coating structure generally significantly reduced pressure drop and significantly increased heat transfer coefficient in the second exchanger compared to the corresponding second exchanger without the coating structure.

Example 2 and Comparative Example 2

A polymer/solvent mixture was processed in the apparatuses described above to determine the impact of the coating structure on heat exchanger performance and efficiency The results are set forth in Table 2.

The polymer employed was a polyethylene resin with a melt index of 2. The solvent employed was 17 pph dichlorodifluoromethane based upon the weight of the polymer.

The coating structure generally significantly reduced pressure drop across the first exchanger compared to the corresponding first exchanger without the coating structure.

While embodiments of the heat exchanger and the process for heating and cooling a fluid of the present invention have been shown with regard to specific details, it will be appreciated that depending upon the manufacturing process and the manufacturer's desires, the present invention may be modified by various changes while still being fairly within the scope of the novel teachings and principles herein set forth

Claims

1 A heat exchanger, comprising:

a) a plurality of heat transfer contact surfaces fashioned to contact and continuously convey a fluid along or through them; the surfaces being adapted to

continuously remove or provide heat to the liquid; and

b) a means for continuously removing heat from or providing heat to the surfaces, the heat exchanger being characterized in that surfaces have metal/fluorinated coating structures of a porous metal matrix infused and coated with a fluorinated polymer

2. The heat exchanger of Claim 1 , wherein the metal is selected from the group consisting of cobalt, nickel, molybdenum, chromium, tungsten, phosphorous, and alloys of any of the foregoing.

3. The heat exchanger of any of the preceeding claims, wherein the fluorinated polymer is polytetrafluoroethylene.

4. The heat exchanger of any of the preceeding claims, wherein the heat exchanger is a shell and tube heat exchanger.

5. The heat exchanger of any of the preceeding claims, wherein the heat exchanger is a plate heat exchanger.

6. The heat exchanger of any of the preceeding claims, wherein the heat exchanger is of counter-current flow configuration.

7. A process for cooling or heating a fluid, comprising

a) providing the fluid ;

b) contacting the fluid with a heat transfer contact surface maintained at a temperature different than the temperature of the fluid; the surface being adapted to continuously remove heat from or provide heat to the fluid;

c) removing heat from or providing heat to the fluid; the process being characterized in that the fluid is contacted with a surface having a metal/fluorinated polymer coating structure of a porous metal matrix layer infused and coated with a fluorinated polymer.

8. The process of Claim 7, wherein the fluid is a flowable polymer melt.

9. The process of Claim 7 or 8, wherein the metal is selected from the group consisting of cobalt, nickel, molybdenum, chromium, tungsten, phosphorous, and alloys of any of the foregoing.

10. The process of any of Claims 7-9, wherein the fluorinated polymer is polytetrafluoroethylene.

11. The process of any of Claims 7-10, wherein the heat is removed from the fluid. Disclosed is a heat exchanger comprising a plurality of heat transfer contact surfaces and a means for continuously removing heat from or providing heat to those surfaces. The surfaces are fashioned to contact and continuously convey a fluid along or through them.