TECHNICAL FIELD

The present disclosure is drawn to polymerization autoclaves which utilize exterior heating coil alloys that substantially avoid low cycle fatigue damage from repeated cyclic temperature differentials.

BACKGROUND Polymer use has dramatically increased over the past several decades due, at least in part, to the variety of material properties which are achievable and relatively low costs associated with formation of complex shapes. Both

continuous and batch polymerization processes are utilized for a variety of polymer production facilities. Each process has advantages and drawbacks depending on a variety of variables such as capital costs, throughput, polymer type, polymerization kinetics, and other priorities. Batch polymerization processes typically utilize a polymerization reactor or autoclave which is heated to an appropriate process temperature. Polymerization autoclaves can most often be designed to tolerate high temperatures and pressures during processing.

However, specific polymerization conditions can vary widely depending on the specific polymerization reactions, choice of additives, throughput, and a variety of other variables.

Heating of such polymerization reactors typically involves the use of closed loop heating systems which transfer heat from a heating fluid into the reactor. Heating systems can include exterior heating coils, internal heating loops, jacketed systems, or other similar heat transfer systems. Such systems can have inherent limitations in terms of heat distribution, heating rates, reliability, and operational constraints. As such, improvements to such heating systems on polymerization reactors continue to be sought and developed.

SUMMARY

A polymerization autoclave having an exterior heating assembly can include an autoclave vessel body, an autoclave inlet, a pressure release valve, and a heating conduit. The autoclave vessel body defines an interior reaction chamber within which a polymerization reaction can occur. The vessel body has an outer surface and an inner surface having a vessel wall thickness. The vessel body is also formed of a steel alloy. The autoclave inlet is oriented through the autoclave vessel body and is capable of directing polymerization reactants into the interior reaction chamber. The pressure release valve is in fluid

communication with the interior reaction chamber. More specifically, the pressure release valve is also capable of selectively discharging vapor from the interior reaction chamber. The heating conduit is wrapped around and attached to a heat transfer portion of the outer surface. The heating conduit has multiple wraps around the autoclave vessel body such that heating fluid can be circulated through the heating conduit to transfer heat into the interior reaction chamber across the heat transfer portion. The heating conduit also has a heating conduit wall thickness and is formed of a chromium-molybdenum steel alloy. The heating conduit is also attached to the heat transfer portion so as to substantially avoid low cycle fatigue damage from repeated cyclic temperature differentials greater than 80 °C.

In another example, a method of assembling a polymerization autoclave having an exterior heating assembly can comprise providing an autoclave vessel body, wrapping a heating conduit around the heat transfer portion of the out surface multiple times, and welding the heating conduit to the heat transfer portion. The autoclave vessel can define an interior reaction chamber and can have an outer surface with a heat transfer portion and an inner surface for containing reactants. The autoclave vessel body can also be formed of a steel alloy and having a vessel wall thickness. Furthermore, the assembly can be such that the heating fluid can be circulated through the heating conduit to transfer heat across the heat transfer portion into the interior reaction chamber. Likewise, the heating conduit can have a heating conduit wall thickness and can be formed of a chromium-molybdenum steel alloy so as to substantially avoid low cycle fatigue damage from repeated cyclic temperature differentials greater than 80 °C.

Additional features and advantages of the invention will be apparent from the detailed description that follows, which illustrates, by way of example, features of the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a polymerization autoclave in

accordance with one embodiment of the present disclosure.

It should be noted that the figure is merely exemplary of several embodiments of the present invention and no limitations on the scope of the present invention are intended thereby.

DETAILED DESCRIPTION

Although the following detailed description contains many specifics for the purpose of illustration, a person of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the herein disclosed embodiments.

Accordingly, the following embodiments are set forth without any loss of generality to, and without imposing limitations upon any claimed invention. Before the present disclosure is described in greater detail, it is to be understood that this disclosure is not limited to particular embodiments described, as this may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. As used in this specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a heating fluid" includes a plurality of such fluids and "an inlet" refers to one or more of such features.

In this disclosure, "comprises," "comprising," "containing" and "having" and the like can have the meaning ascribed to them in U.S. Patent law and can mean "includes," "including," and the like, and are generally interpreted to be open ended terms. The term "consisting of" is a closed term, and includes only the devices, systems, methods, components, structures, steps, or the like specifically listed, and that which is in accordance with U.S. Patent law. "Consisting essentially of or "consists essentially" or the like, when applied to devices, systems, methods, components, structures, steps, or the like encompassed by the present disclosure refers to compositions like those disclosed herein, but which may contain additional structural groups, composition components, or method steps, etc. Such additional structural groups, composition components or method steps, etc., however, do not materially affect the basic and novel characteristic(s) of the compositions or methods, compared to those of the corresponding compositions or methods disclosed herein. In further detail, "consisting essentially of or "consists essentially" or the like, when applied to devices, systems, methods, components, structures, steps, or the like

encompassed by the present disclosure have the meaning ascribed in U.S. Patent law and the term is open-ended, allowing for the presence of more than that which is recited so long as basic or novel characteristics of that which is recited is not changed by the presence of more than that which is recited, but excludes prior art embodiments. When using an open ended term, like

"comprising" or "including," it is understood that direct support should be afforded also to "consisting essentially of language as well as "consisting of language as if stated explicitly.

Phrases such as "suitable to provide," "sufficient to cause," or "sufficient to yield," or the like, in the context of methods of synthesis, refers to reaction conditions related to time, temperature, solvent, reactant concentrations, and the like, that are within ordinary skill for an experimenter to vary to provide a useful quantity or yield of a reaction product. It is not necessary that the desired reaction product be the only reaction product or that the starting materials be entirely consumed, provided the desired reaction product can be isolated or otherwise further used.

It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range includes "about 'x' to about 'y'". To illustrate, a concentration range of "about 0.1 % to about 5%" should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also include individual concentrations (e.g., 1 %, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1 %, 2.2%, 3.3%, and 4.4%) within the indicated range. In an

embodiment, the term "about" can include traditional rounding according to significant figures of the numerical value. In addition, the phrase "about 'x' to 'y " includes "about 'x' to about 'y'".

The term "about" as used herein, when referring to a numerical value or range, allows for a degree of variability in the value or range, for example, within 10%, or, in one aspect within 5%, of a stated value or of a stated limit of a range.

In addition, where features or aspects of the disclosure are described in terms of a list or a Markush group, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group. For example, if X is described as selected from the group consisting of bromine, chlorine, and iodine, claims for X being bromine and claims for X being bromine and chlorine are fully described as if listed individually. For example, where features or aspects of the disclosure are described in terms of such lists, those skilled in the art will recognize that the disclosure is also thereby described in terms of any combination of individual members or subgroups of members of list or Markush group. Thus, if X is described as selected from the group consisting of bromine, chlorine, and iodine, and Y is described as selected from the group consisting of methyl, ethyl, and propyl, claims for X being bromine and Y being methyl are fully described and supported.

As used herein, all percent compositions are given as weight-percentages, unless otherwise stated. When solutions of components are referred to, percentages refer to weight-percentages of the composition including solvent (e.g., water) unless otherwise indicated.

As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features that may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure. Any recited step can be carried out in the order of events recited or in any other order that is logically possible.

Furthermore, embodiments of the present disclosure employ, unless otherwise indicated, techniques of chemistry, metallurgy, welding, and the like, which are within the skill of the art.

Batch polymerization reactions frequently involve repeated cycling of temperatures during various stages of processing. Typically, polymerization reactants are charged into an autoclave and the materials are heated sufficient to initiate polymerization and drive the reaction during the process. Once

completed, the materials are extruded or otherwise removed from the autoclave. Often intermediate cleaning and washing stages between polymerization batches can also be used to provide more consistent product quality. Unfortunately, such processes introduce significant cyclic variations in temperatures within the autoclave, along the autoclave walls, and along adjacent heating systems such as external heating coils. Furthermore, differentials in temperatures across each portion of the autoclave and heating system can create mechanical stresses at weld joints and adjacent materials.

As an illustrative example, formation of dyadic polyamides such as nylon 6,6 can involve converting/reacting nylon salt to form nylon polymer at elevated temperatures around 189-250 °C for several hours. Autoclave wall temperatures are initially below typical heating fluid temperatures. Temperature and pressure conditions in a nylon 6,6 batch process cycle include variations during initial startup, charging of nylon salt, reaction stages, extrusion stage, and clean out stages. Among these various stages, temperatures changes create temperature differentials between the autoclave walls and heating coils.

Table 1 provides one example of cyclic polymerization operating conditions.

Table 1 : Temperature Conditions

Notably, the temperature differential between the heating coil and the autoclave wall can vary dramatically. External heating coils are most often welded to outer surfaces of the autoclave. During start-up of the reactor, expansion of the heating coil is restrained by the relatively cooler autoclave walls. This results in a compression load which may exceed elastic deformation limits of the welds. Such permanent deformation leads to stress in weld joints as autoclave walls subsequently heat up to normal operating temperatures. Similar deformation results during variations in temperature conditions such as boil out, steam cleaning, washing, shutdown and startup of the reactor. In each case, changes in temperature results in a variable temperature gradient across the weld as a function of time. Furthermore, as a vessel is used over and over again for multiple batch cycles during normal use, temperatures are cycled from low to high within the various batch cycles.

In the case of nylon 6,6 production, batch cycle times can often range about 100-120 minutes for standard production volumes. Ultimately, such repeated temperature differentials can lead to low cycle fatigue damage at weld joints between the external heating coils and the autoclave walls. The degree of thermally induced compression and deformation can be varied based on choice of materials for the autoclave heating vessel, the heating conduits and the weld alloys used to attach heating conduits. Furthermore, relative size and dimensions of each component can also affect degree of deformation and consequently whether low cycle fatigue problems arise over time.

Accordingly, as generally illustrated in FIG. 1 , a polymerization autoclave 100 can include an autoclave vessel body 104 and an exterior heating assembly 102 which includes a heating conduit 1 10. Also included is a plurality of ports at a top end of the vessel for transferring reactant and/or additives into the vessel, venting gases, etc. Any of these inlet/outlet ports (or others that can be optionally included) can be used for these purposes. For example, the vessel might be designed to have an autoclave inlet 106, a pressure release valve 108, and an optional secondary inlet, though other arrangements can be used as may be practical for a given polymerization process. The autoclave vessel body defines an interior reaction chamber 1 12 within which a polymerization reaction can occur. Of particular interest for the present invention, the polymerization reaction can be adapted for the formation of polyamides such as nylon 6,6, although other polymerization reactions can also be suitable.

The autoclave vessel body 104 can be any vessel in which a

polymerization reaction can be performed. Generally suitable vessel bodies can include an interior reaction chamber which is enclosed by an autoclave wall and capable of pressurization. Although operating conditions can vary, the autoclave vessel body can be adapted to retain pressures of at least 300 psia, and in some cases at least 600 psia. The autoclave vessel body can have an outer surface 1 14 and an inner surface 1 16. The autoclave vessel body has a cylindrical central portion 1 18, with a domed top portion 120, and a conical bottom portion 122. Other shapes can likewise be used. The autoclave vessel body can be formed as a single unitary vessel. Alternatively, the autoclave vessel body can be formed of multiple segments. Such segmentation of the autoclave vessel body can facilitate manufacture, assembly, cleaning, and repair of the device. For example, an internal heating assembly 124 can be engaged through the conical bottom portion of autoclave vessel body. Although discussed in more detail below, the polymerization autoclave can be assembled and disassembled by removing fasteners along flange 126 to allow removal of the internal heating assembly as a single unit. Additional features can be provided as part of the polymerization autoclave for convenience and improved performance. For example, retainer 127 can be connected to a fixed structural member to provide mechanical stability to the autoclave.

The autoclave vessel body can be formed of materials which are structurally sufficient to withstand expected operating conditions. However, the autoclave vessel body is typically formed of a steel alloy. Non-limiting examples of suitable steel alloys can include carbon steel alloys (e.g. HII carbon steel, St35.8, P235GH, P265GH, P295GH, P355GH, or the like), refractory metal alloys, composites thereof, and combinations thereof.

Table 2: Carbon Steel Alloys (balance Fe)

"P235GH also requires Cr+Cu+Mo+Ni < 0.70%

**P295GH also requires Cr+Cu+Mo < 0.70% In one example, the vessel walls can be formed of a chromium- molybdenum steel alloy. Suitable chromium-molybdenum steel alloys can include, but are not limited to, 16Mo3, 13CrMo4-5, P235GH, P265GH, and combinations thereof. In a particular example, the vessel walls can be formed of 16Mo3 which exhibits desirable properties over a wide range of operating conditions. Table 2 outlines chemical compositions for these chromium- molybdenum steel alloys. Table 2: Chromium-Molybdenum Steel Alloys (balance Fe)

*P235GH also has Cr+Cu+Mo+Ni < 0.7

Depending on materials chosen to form the autoclave vessel body 104, polymerization process conditions can present corrosive and damaging conditions. Thus, in some alternative embodiments, the autoclave vessel body can be a cladded vessel having a primary vessel wall and at least one interior cladding layer coated on the inner surface. Such cladding layers can provide increased resistance to corrosion beyond materials used for the primary vessel wall. The cladded walls can often represent a compromise to achieve both mechanical integrity which is characteristic of the primary vessel wall and corrosion resistance which is characteristic of the cladding layers. Thus, the primary vessel wall can be coated on interior surfaces with at least one interior cladding layer. Cladding can be coated on the primary vessel wall using known techniques such as explosion welding, although other deposition techniques may be suitable. Cladding layer materials can also provide corrosion protection to inner surfaces which are in contact with polymerization reactants and products at high temperatures and pressures.

Accordingly, the primary vessel wall can be formed of steel alloys as outlined above, and in one optional aspect are formed of a carbon steel alloy such as HII carbon steel or St35.8. Alternatively, the primary vessel wall can be formed of a chromium-molybdenum steel alloy such as those described above. However, the interior coating layer can be stainless steel (e.g. SS321 , 314 and 306), or the like. Non-limiting examples of stainless steel 321 alloys can include X6CrNiTi18-10, X12CrNiTi8-9, or the like. Table 3 outlines nominal chemical compositions for several suitable stainless steels. Table 3: Stainless Steel Chemical Composition

*X6CrNiTi 18-10 also requires Ti%≥ 5x%C but < 0.8.

Thicknesses of the autoclave vessel body 104 can vary. Vessel thickness can substantially affect rates of heat transfer during temperature transitions and can also vary degree of thermal expansion and contraction during such temperature transitions as well. Accordingly, vessel wall thicknesses can vary but are generally from about 15 mm to about 50 mm, and in many cases from about 20 mm to 40 mm. In the case of cladded vessels, the primary vessel wall can often have a thickness from 20 mm to 28 mm and the cladding layer can include a single cladding layer having a thickness of 1 .5 mm to 5 mm. In one example, the vessel wall thickness can be 24 mm carbon steel (HII) with a 3 mm stainless steel 321 interior cladding.

In one optional aspect, the conical bottom portion 122 can have a wall thickness which is larger than a wall thickness of the cylindrical central portion 1 18. For example, the conical bottom portion thickness can be about 10% to 40% greater than the cylindrical central portion wall thickness.

Various ports, inlets, and outlets can be formed in the autoclave vessel body 104. Such features can provide for control of feedstock, product removal, pressure control, venting, introduction of additives, introduction of staged polymerization reactants, temperature probes, pressure probes, cleaning, video feed, and the like. Although inlets can be configured for any of these purposes, the autoclave inlet 106 is oriented through the autoclave vessel body and is capable of directing polymerization reactants into the interior reaction chamber 1 12. Typically the autoclave inlet can be sufficiently wide to allow for introduction of reactants quickly to reduce charging time. Although any suitable inlet diameter can be used, the autoclave inlet often has a diameter from 1 inch to 4 inches, and in many cases about 3 inches.

During processing, it can frequently be desirable to control pressures within the interior reaction chamber 1 12. One convenient approach is to charge materials and adjust heating rates while selectively releasing material to reduce pressure. For example, charging of materials can result in pressure increases. Similarly, heating of polymerization reactants and materials during processing can also increase pressure within vapor space which is located above liquid reactants, solids, and polymerized flowable materials. As pressures rise, a suitable mechanism can be used to maintain pressure within a desired target range. For example, during processing, pressures may vary from about 0 psi up to about 300 psi. High pressure cycles can often have an upper bound around 280 psi. Similarly, extrusion stages usually involve elevated pressures from about 100 psi to 200 psi, and often about 125 psi.

Accordingly, the pressure release valve 108 can be in fluid communication with the interior reaction chamber 1 12. More specifically, the pressure release valve is also capable of selectively discharging vapor from the vapor space of the interior reaction chamber and is most often oriented within the domed top portion 120 of the autoclave vessel body 104. The pressure release valve is illustrated as a flanged conduit; however any suitable pressure release valve can be operatively connected to the flange. The pressure release valve can often be adjustable in order to provide for variable pressure within the interior reaction chamber throughout each polymerization process cycle. Such adjustable pressure relief valves can be operatively connected to a process controller module for remote control of pressure conditions, along with other monitored and controlled conditions.

As another option, the secondary inlet 128 can be oriented in the domed top portion. This secondary inlet can be used for addition of additives during processing, staged polymerization reactants or other components. Non-limiting examples of additives can include colorants, ultraviolet stabilizers, plasticizers, cross-linking agents, anti-microbials, fillers, process aids, flame retardants, biodegradability enhancers, and the like. Optional spray nozzles can be oriented on internal ends of inlets described herein. Typically, inlets which introduce material into the interior reaction chamber 1 12 can terminate a distance away from the interior wall surface 1 16. Accordingly, in order to more uniformly distribute such materials into the interior reaction chamber, nozzles can be used to increase and control dispersion patterns.

Polymerization processing can often involve careful control of reaction temperatures within the interior reaction chamber 1 12. Such control of

temperatures can be provided by exterior heating and optional interior heating. As illustrated in FIG. 1 , the exterior heating assembly 102 can include a heating conduit 1 10 which is wrapped around the autoclave vessel body 104. The outer surface 1 14 can include an exposed portion and a heat transfer portion.

Specifically, the heating conduit can be directly attached to the heat transfer portion of the outer surface. Conversely, the exposed portion of the outer surface is free of heating mechanisms and is exposed to surrounding environment. The heating conduit has multiple wraps around the autoclave vessel body such that heating fluid can be circulated through the heating conduit to transfer heat into the interior reaction chamber across the heat transfer portion. The number of wraps can vary considerably, but is often more than ten wraps, and in some cases up to or more than twenty. Depending on the specific configuration, the exterior heating assembly can cover the heat transfer portion of the outer surface. Although degree of coverage can vary, the exterior heating assembly can cover from 30% to 80% of the outer surface, in some cases 5% to 70%, and often from 40% to 70% of the outer surface. With respect to surface coverage, gaps between windings in the heating conduit are included in these percentages as covered by the exterior heating assembly.

Each wrap typically has a small gap such that the outer surface 1 14 is exposed in between successive wraps of the heating conduit 1 10. Such gaps are generally limited in order to improve heat transfer into the autoclave, but are most often limited to no more than 50% (and in many cases less than 20%) of the width of surrounding heating conduit. The heating conduit can also be formed having a variety of cross-sectional shapes such as, but not limited to, half-pipe, full-pipe, U-channel, V-channel, and the like. In one aspect, the heating conduit can have a half-pipe cross-sectional shape. Alternatively, the heating conduit can have an overlapping cross-section where one edge is welded to the outer surface of the autoclave vessel body, while an opposing edge is welded to a suspended portion (i.e. an outer surface) of an adjacent wrap of the heating conduit. In this manner only the one edge of the heating conduit is welded directly to the outer surface. As a result, the opposing edge is suspended remote from the outer surface of the autoclave vessel body such that temperature differentials are reduced during transitions in operating conditions throughout the polymerization process. Regardless of the specific configuration, the heating conduit can typically have a uniform cross-sectional shape along an entire length of the heating conduit which is attached to the outer surface.

Dimensions of the heating conduit 1 10 can also be a factor in long term reliability and performance of the exterior heating assembly 102. For example conduit wall thickness can be varied and is generally less than the vessel wall thickness. Corresponding wall thicknesses of the heating conduit can also affect the degree of stress transferred to the heating coils during transitions in temperatures. Heating conduit wall thicknesses are generally less than about 25 mm. In one example, the heating conduit wall thickness can be from about 3 mm to about 6 mm and in a specific example 4 mm. Similarly, a ratio of vessel wall thickness to the conduit wall thickness can influence performance. As a general guideline, the ratio of vessel wall thickness to conduit wall thickness can be 2: 1 to 15:1 , and in some cases 5:1 to 9:1 .

Heating conduit thickness can affect stress induced in welds, as well as heat losses to surrounding environment. However, outer width of the heating conduit 1 10 can also determine percentage coverage of the outer surface 1 14 for purposes of heat transfer. Furthermore, cross-sectional heating conduit profiles can also determine patterns of heat flow within the conduit. Although heating conduit height (i.e. perpendicular distance from the outer surface to a distal portion of the heating conduit) can allow for greater heating fluid volumes, excessive heights can result in undesirable heat currents and increased heating fluid volume requirements. Similarly, increased outer width (i.e. width of heating fluid contact area with the outer surface) can increase available effective heat transfer surface. In addition, a reduction in the number of contact points with the outer surface results in fewer welds and potential failure points. Conversely, excessively wide conduits can also result in non-uniform heat flow patterns. Regardless, the outer width of heating conduits can vary considerably, such as from about 50 mm to about 100 mm.

In one optional aspect, the heating conduit 1 10 can be formed in whole or in part as a jacketed chamber which circumvents at least a portion of the heat transfer portion of the outer surface 1 14 of the autoclave vessel body 104. Such a jacketed chamber can optionally include internal baffles or walls which direct the heating fluid across portions of the outer surface. A variety of baffle arrangements can be used, although complex and or numerous baffle configurations can cause excessive pressure head. Such increased pressure within the heating assembly can increase chances of causing a failure within the heating assembly, need for high pressure heating fluid pumps, or increased operational costs.

The heating conduit 1 10 can be formed of a material which is operable at temperatures of at least 350 °C with repeated heating and cooling cycles. Of particular interest are heating conduits which are formed of a chromium- molybdenum steel alloy. Non-limiting examples of suitable chromium- molybdenum steel alloys include 16Mo3, 13CrMo4-5, P235GH, P265GH, and combinations thereof. Chemical compositions of these chromium-molybdenum steel alloys are provided in Table 4.

Table 4: Chromium-Molybdenum Steel Alloys (balance Fe)

*P235GH also has Cr+Cu+Mo+Ni < 0.7 In one aspect, the heating conduits 1 1 0 can be formed of a dissimilar steel alloy from the autoclave vessel body 104 and in particular the outer surface 1 14. However, in one particular aspect, the heating conduits can be formed of the same steel alloy as used in formation of the autoclave vessel body, especially the outer surface.

Typically the exterior heating assembly 102 can be welded to the outer surface 1 14 of the autoclave vessel body 104 using a high temperature weld alloy. Non-limiting examples of suitable weld alloys can include, Mn-Mo alloys, W2 Mo, G 46 AM G4MO, E Mo B32 H5, AG42, W 42 5 W2Si, or combinations thereof. In one aspect, the weld alloy can be AG42. In another aspect, the weld alloy can be W2 Mo. Commercial weld alloys which can be suitable include filler metals such as, but not limited to, Nertalic 86®, Union® l-Mo, or SL® 12G.

Welding of the exterior heating assembly 102 to the outer surface 1 14 can create a closed heating loop which encloses the heating fluid in a recirculating heating system. For example, the exterior heating assembly can include an external heating inlet 130 where heating fluid is introduced. As heating fluid circulates around the autoclave vessel body 104, heat is transferred into the interior reaction chamber 1 12 such that the heating fluid cools in upper portions of the heating conduit. The heating conduit can thus have an exterior heating outlet 132 which allows the heating fluid to be recirculated and reheated.

The exterior heating assembly 102 can be welded to the outer surface 1 14 using any number of welding techniques. Non-limiting examples of welding techniques include gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and the like. Overlapping offset weld deposits in building up weld joints can be used to increase weld joint strength. Furthermore, weld joints can be full penetration welds (i.e. as opposed to tack welds or partial penetration welds). As a guideline, root spacing of weld joints can be 1 mm to 5 mm, although other spacing may be suitable.

Through a combination of choice of materials, heating conduit thicknesses, vessel wall thicknesses, and weld alloys materials, the heating conduit can be attached to the heat transfer portion of the outer surface 1 14 so as to

substantially avoid low cycle fatigue damage from repeated cyclic temperature differentials greater than 80 °C, and in some cases greater than 100 °C. Although a variety of heating fluids can be used, non-limiting examples include Thermanol 66® (a proprietary mixture of hydrogenated terphenyl, partially hydrogenated quaterphenyls and higher polyphenyls, and terphenyl), Dowtherm® (mixture of biphenyl and diphenyl oxide), and mixtures thereof. Suitable heating fluids can operate at temperatures from 330 °C to 340 °C and have a vapor pressure less than 100 psi.

Referring again to FIG. 1 , the polymerization autoclave 100 can include the interior heating assembly 124 which includes internal heating manifolds. As an example, an internal heating manifold can include a conical reservoir 134 having an inlet 136. The conical reservoir can be housed within the conical bottom portion 122 of the autoclave which is attached to the vessel body at flange 126. The internal heating manifold can further include one or more heating tubes 138 which are each fluidly connected to the conical reservoir. The heating tubes can be connected to a common outlet 140 for recycling and/or reheating of cooled heating fluid. In addition, the heating tubes can be vertically oriented and substantially parallel to one another. Further, the heating tubes can be oriented within a lower region of the interior reaction chamber 1 12 to form an axially oriented annular heating manifold. An optional upper housing 142 can be provided to distributed heat more uniformly within the interior reaction chamber. The upper housing illustrated has a ring shape and a square cross-section with a peaked ceiling.

The internal heating tubes can be formed of any material which is operable up to at least 350 °C and in some cases up to about 400 °C. Non-limiting examples of suitable materials can include those previously listed for use with the autoclave vessel walls, exterior heating conduit, stainless steel, and the like. In one alternative the internal heating tubes can be formed of a stainless steel. Non- limiting examples of stainless steels include stainless steel 304, 316 and 321 .

The interior heating assembly 124 can be integrally connected to a portion of the conical bottom 122 which can be removed as a single unit. Such segmentation of the autoclave vessel body can facilitate manufacture, assembly, cleaning, and repair of the device. Furthermore, the conical bottom portion 122 includes an autoclave outlet 144 which allows for removal of product from the interior reaction chamber 1 12 for further processing (e.g. extrusion, drawing into fiber, molding, etc.)- Extrusion can be performed by increasing pressure within the interior reaction chamber 1 12 and optionally increasing temperatures in order to decrease viscosity of polymerization products.

In yet another alternative, the polymerization autoclave 100 can be an agitated autoclave which includes an internal mixer (not shown). The internal mixer can typically be oriented vertically along a centerline of the interior reaction chamber 1 12. Internal mixers can allow for increased uniformity of polymerization conditions and decreases in polymerization reaction times. In such cases, the internal heating assembly 124 can be omitted or reconfigured to provide clearance for operation of the internal mixer.

The polymerization autoclaves and configurations described herein can provide effective heat transfer, while also minimizing or eliminating chances of low cycle fatigue. As a result, polymerization autoclaves incorporating such features can provide increased service life, higher reliability, and more uniform heat distribution.

Although the subject matter has been described in language specific to structural features and/or operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features and operations described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the described technology.