WO2010051013A1 - Steam heated polyester production process - Google Patents

Abstract

Processes for making polyesters in a polyester production facility are disclosed, that include the steps of forming a reaction medium comprising at least one monomer that includes terephthalic acid (TPA) and/or an ester derivative of TPA; subjecting at least a portion of the reaction medium to one or more chemical reactions in the polymer production facility to thereby produce the polyester; and heating the reaction medium at one or more locations in the polyester production facility, wherein at least 50 percent of the total energy input employed for the heating of the reaction medium is provided by indirect heat exchange between the reaction medium and steam.

Description

Steam Heated Polyester Production Process

[001] This application claims priority to U.S. Provisional Application Serial No. 61/109,983, filed October 31, 2008 the disclosure of which is incorporated herein by reference in its entirety.

Background

[002] It is known to produce polyester polymers through condensation polymerization in the melt phase starting from diacids and/or diesters as one type of monomer and diols as the other type of monomer. In particular, it is known to form polyester polymers based on aromatic dicarboxylic acids such as terephthalic acid (TPA) as the principal diacid and ethylene glycol (EG) as the principal diol with the resulting polymer being called polyester terephthalate (PET) herein. In addition to TPA and EG, PET polymers often contain lesser amounts of other multi-carboxylic acids, e.g. aromatic acids such as isophthalic acid (IPA) and trimellitic acid (TMA), and other multi- alcohols, e.g. 1 ,4-cyclohexane dimethanol (CHDM) and diethylene glycol (DEG). DEG monomer may be formed in situ from dimerization of EG. Throughout this disclosure and in claims, TPA may be replaced optionally in whole or part by any of its ester derivatives, including but not limited to dimethyl terephthalate (DMT) and bis(2-hydroxyethyl)terephthalate (BHET). Collectively and individually, these various multi-carboxylic acids, multi-esters, and multi-alcohols are termed monomers, as used herein. As used herein, process materials are monomers, catalysts, polymer additives, reaction medium, PET melt product, and various byproducts of the PET formation reactions. Typical byproducts from the formation of PET comprise water, methanol and EG, and various degradation byproducts. Typical degradation byproducts from the formation of PET comprise acetaldehyde, DEG, various dioxanes, and various colored conjugated aromatic molecules. As used herein, reaction medium is a mixture comprising at least a portion of EG mixed with at least a portion of TPA and/or an ester of TPA. Optionally, reaction medium may comprise various catalysts, polymer additives, and byproducts of the PET formation reactions.

[003] The melt-phase synthesis of PET is typically executed at temperatures in excess of 25O⁰C. Such high temperatures are needed both to maintain the reaction medium in a flowing molten state and to promote chemical reaction rates. Generally, the importance of higher temperature increases as the degree of polymerization increases.

[004] It is known that the heat of reaction for formation of the ester bond linkages in PET is only mildly exothermic in comparison to the thermal energy input needed to elevate the monomers to the temperature of the reaction medium and to remove as vapors the byproducts of the condensation reaction, e.g. water, methanol. In a commercial-scale PET melt product synthesis facility, very large amounts of thermal energy are added to process materials, especially to the reaction medium. Very large heat-transfer areas are needed to exchange this thermal energy through conductive, isolating, heat-exchange boundary surfaces, which typically comprise various metals and metal alloys.

[005] The large input of thermal energy during synthesis of PET melt product is typically provided by one or more types of heat-transfer fluid through conductive, isolating, heat-exchange boundary surfaces. The conventional choices for heat-transfer fluids are various relatively high molecular weight organic materials that have relatively low vapor pressures even at temperatures in excess of 300⁰C. This relatively low vapor pressure, as compared to the vapor pressure of water or of low molecular weight organic materials, ameliorates the mechanical design and cost of the conductive, solid boundary surface needed to isolate the reaction mass from the heat-transfer mass while enabling the transfer of thermal energy. These low-vapor-pressure organic heat-transfer fluids are used in various combinations of liquid and vaporized forms, yielding both sensible heat and latent heat of vaporization to the process materials. Suitable low-vapor- pressure organic heat-transfer fluids are available in the Dowtherm, Therminol, and other commercial product lines; and Dowtherm A and Therminol 66 are preferred embodiments.

[006] Unfortunately, such low-vapor-pressure organic heat-transfer fluids have several drawbacks. The low-vapor-pressure organic heat-transfer fluids have relatively low sensible heat capacity and latent heat of vaporization, especially as compared to water, and thus require relatively large mass flow rates to transfer the required amount of thermal energy into the process materials. Conduit diameters for conveying low-vapor-pressure organic heat-transfer fluids in a large PET melt product synthesis facility may exceed 0.5 m. The low-vapόr-pressure organic heat-transfer fluids are flammable and require considerable safety precautions. For example, a "river of fire" can occur when an exchanger tube ruptures inside a fuel-fired furnace where the low-vapor-pressure organic heat-transfer fluid is typically heated. Despite careful selection of the organic molecules and rigorous minimization of dissolved oxygen therein, the low-vapor-pressure organic heat-transfer fluids are challenged on thermal stability in the exchanger tubes of a fuel-fired furnace where they are typically heated. The low-vapor-pressure organic heat-transfer fluids are expensive, with the filling inventory for a world-scale PET melt product synthesis facility costing in excess of one million US dollars. Thus, the combined costs for using low-vapor-pressure organic heat-transfer fluids add considerably to the cost of manufacturing PET melt product: capital cost for fuel-fired furnace; capital cost for large diameter piping, valves, insulation, controls, and pumps; capital cost for fire protection; circulation pump energy consumption; fluid degradation losses; thermal energy losses on large pipe sizes, even with thick insulation; and working capital for fluid inventory.

Summary

[007] In one aspect, the invention relates to processes for making polyesters in a polyester production facility, which include the steps of: forming a reaction medium comprising at least one monomer that includes terephthalic acid (TPA) and/or an ester derivative of TPA; subjecting at least a portion of the reaction medium to one or more chemical reactions in the polymer production facility to thereby produce the polyester; and (c) heating the reaction medium at one or more locations in the polyester production facility, wherein at least 50 percent of the total energy input employed for the heating of the reaction medium is provided by indirect heat exchange between the reaction medium and steam.

[008] The inventors have discovered an economical way to use high pressure water vapor (steam) as the principal heat-transfer fluid in a polyester production facility, such as a PET melt product synthesis facility, at commercial scale, overcoming the previous economic barriers that prevented use of steam heating for this application. In certain embodiments, the polyester production facility employs a small diameter, elongated heat- exchange conduits rather than jacketed vessels, jacketed conduits, and larger diameter elongated heat-exchange conduits inside vessels. The small diameter, elongated heat-exchange conduits allow economical mechanical construction despite the very much higher pressure of steam as compared to low-vapor-pressure organic heat-transfer fluids at comparable temperature. In certain embodiments, the polyester production facility uses relatively high superficial flow velocities for process materials near conductive, isolating, heat-exchange boundary surfaces so that the heat transfer rate per unit boundary surface area is increased and the required boundary surface area is thus diminished.

[009] In certain embodiments, the polyester production facility comprises a shell-and-tube exchanger using steam at pressures above 8 megapascals on the shell-side and with a three-phase reaction medium comprising liquid, solid, and vapor flowing with a superficial velocity above 1.4 meters per second on the tube-side in order to provide the majority of all thermal energy input to a polyester production facility.

Brief Description of the Drawings

[010] FIG. 1 is a schematic depiction of a polyester production facility showing the primary reactants introduced into the facility, the primary products produced from the facility, and two separate furnaces heating two thermal energy mediums ("steam" and "other HTM") that provide heat to the facility.

[011] FIG. 2 is a more detail depiction of a polyester production facility showing a series of reactors (Rx#1-RX#5), with high-pressure steam providing thermal energy to the initial reactors (Rx#1-Rx#3).

[012] FIG. 3 is a schematic depiction of a polyester production facility similar to the facility of FIG. 1 , but employing only a single furnace to heat steam that is subsequently used to provide thermal energy to the facility, both directly and through heat exchange with another heat transfer medium (HTM).

[013] FIG. 4 is a schematic depiction of a monomer production facility and a polyester production facility with heat-integration between the two facilities.

[014] FIG. 5 is a more detailed depiction of the monomer production facility of FIG. 4 showing the individual steps of monomer production, as well as locations where heating can be provided by low-pressure steam.

[015] FIG. 6 is a schematic depiction of a system for heat-integrating a monomer production facility and a polyester production facility using flashed steam and steam-heating of another HTM.

Detailed Description of the Preferred Embodiments

[016] The inventors have discovered and disclose herein a process for forming a polyester melt product, such as PET, from polycarboxylic acid and glycol monomers, such as TPA and EG, wherein the thermal energy input sourced from steam heat-transfer fluid is at least about 30, 50, 70, 80 percent of the total thermal energy input to the polyester production facility from all heat-transfer fluids. A schematic depiction of an exemplary polyester production facility is provided in FIG. 1. In one embodiment the polyester production facility is a PET melt product synthesis facility. FIG. 2 shows a more detailed depiction of a polyester production facility that includes a plurality of serially connected reactors (Rx#1 , Rx#2, Rx,#3, Rx#4, and Rx#5). FIG. 2 also shows that each of the first three reactors has a shell-and-tube heat exchanger associated therewith, and that the shell-and-tube heat exchangers heat the reaction medium by indirect heat exchange with high- pressure steam.

[017] The inventors have discovered that it is preferable that at least about 30, 50, 70, 80 percent of the thermal energy input from steam heat- transfer fluid into a polyester production facility is provided by direct transfer of thermal energy from steam through conductive, isolating, heat-exchange boundary surfaces into reaction medium. This is as distinguished from transferring thermal energy from steam heat-transfer fluid into separated monomers not combined into reaction medium.

[018] The inventors have discovered a process and preferred apparatus for forming polyester melt product, such as PET, from monomers, such as TPA and EG, wherein the total thermal energy input sourced from steam heat-transfer fluid is at least about 400, 800, 1200, 1400 joules per gram of polyester melt product. More preferably, at least about 350, 700, 900, 1200 joules per gram of polyester melt product is exchanged directly from steam heat-transfer fluid into reaction medium, as distinguished from going into separated monomers not combined into reaction medium.

[019] Preferably the supply pressure of at least about 30, 50, 70, 90 weight percent of the steam heat-transfer fluid is at least about 2, 5, 6, 7, 9 megapascal at the locations where thermal energy is transferred into process materials. In contrast, the supply pressures of low-vapor-pressure organic heat-transfer fluids used in a polyester production facility are typically less than about 2 megapascals, requiring much less metal thickness for containment.

[020] More preferably, the condensing temperature matching the pressure of the steam heat-transfer fluid used to heat reaction medium is at least about 260, 275, 290, 305 degrees Celsius. Higher condensing temperatures provide a larger temperature gradient for exchanging thermal energy into reaction medium, thus relatively reducing the areas required for conductive, isolating, heat-exchange boundary surfaces. [021] However, higher condensing temperatures also require greater supply and containment pressures for steam heat-transfer fluid. Thus, it is preferred that the supply and condensing pressures of the steam heat- transfer fluid are limited to less than about 20, 16, 14, 12 megapascal for reasons of economy in mechanical design for supply conduits and vessels and for conductive, isolating, heat-exchange boundary surfaces.

[022] In contrast to the steam pressure, the pressure of the process materials is much less, preferably being less than about 1 , 0.5, 0.2, 0.1 megapascal for the monomers, reaction medium and byproducts. As the degree of polymerization and molecular weight of the reaction medium are increased, it is preferred that the pressure of the reaction medium is lowered to less than about 0.05, 0.01 , 0.001 , 0.0001 megapascal.

[023] Also, it is preferred that the temperature of the steam supplied to the polyester production facility is less than about 600, 500, 400, 350 degrees Celsius. At any given pressure, the steam can be far hotter than its equilibrium condensing temperature. In some applications, such as power generation, it is highly desirable that the steam supply contain a great amount of superheat. However, in a polyester production facility, excessive superheat increases costs for metal used for mechanical containment of the steam. Also, the temperature of superheated steam is potentially damaging to the process materials, especially in films of process materials contacting conductive, isolating, heat-exchange boundary surfaces, especially during operating excursions with less than design flow rates of process materials.

[024] In certain embodiments, the polyester production facility employs conductive, isolating, heat-exchange boundary surfaces that have a small diameter. This lowers the cost for mechanical containment of high pressure steam, and it also proves important in maximizing the heat-transfer coefficient on the process side, as is disclosed farther below. For at least about 30, 50, 70, 90 percent of the thermal energy transferred from steam to the polyester production, it is preferred that the outside diameter of the conductive, isolating, heat-exchange boundary surfaces is less than about 0.3, 0.15, 0.06, 0.03, or 0.003 meters. [025] Elongated conduits comprising small diameter, isolating, heat- exchange boundary surfaces are referred to herein as tubes. The inside of the tubes is referred to herein as tube-side. The outside of the tubes is referred to herein as shell-side.

[026] In certain embodiments, the polyester production facility provides relatively high superficial flowing velocities for the reaction medium in order to promote the amount of thermal energy transferred compared to the size and cost of the heat-exchange means. In a conventional process for forming polyester melt product using low-vapor-pressure organic heat-transfer fluids, both sides of the heat-exchange surfaces are contacted by organic liquids. The thermal conductivities of monomers and reaction medium are roughly comparable to those of typical low-vapor-pressure organic heat- transfer fluids. Especially when the degree of polymerization of the reaction medium and its viscosity are relatively low, the heat-transfer film coefficients are similar for reaction medium and for typical low-vapor-pressure organic heat-transfer fluids. In contrast, the heat-exchange properties of condensing steam are far greater. For example, a film coefficient for heat-exchange from condensing steam is typically greater than about 6,000 watts per square meter per degree Celsius, which contrasts with a film coefficient for heat- exchange from liquid low-vapor-pressure organic heat-transfer fluid that is typically less than about 2,000 watts per square meter per degree Celsius. Such a large difference in film coefficient for heat-exchange indicates that the process-side film coefficient, rather than the steam-side film coefficient, will largely control the overall heat-exchange rate across the conductive, isolating, heat-exchange boundary surfaces. Increasing the superficial velocity of the process flows usefully increases the heat-transfer film coefficient on the process side of the heat-exchange boundary surface. Increasing the heat- transfer film coefficient on the process side is particularly valuable with steam heat-transfer fluid because of the greater thickness and cost of metal for pressure containment, as compared to a low-vapor-pressure organic heat- transfer fluid. [027] In one embodiment of the invention, it is preferred to provide a superficial velocity of process materials, especially reaction medium, that is at least about 0.7, 1.4, 3, 5 meters per second near conductive, isolating, heat- exchange boundary surfaces. It is preferable that the above superficial velocities are provided within tubes. Superficial velocity has the usual meaning of time-averaged volumetric flow divided by the cross sectional area surrounded by the containing and isolating boundary surface and measured in a plane orthogonal to the direction of the time-averaged volumetric flow.

[028] In another embodiment of the invention, it is preferred to provide a liquid and vapor multiphase flow of reaction medium within a heat-exchange means. The vapor phase preferably comprises both EG monomer and reaction byproducts. In this embodiment, it is preferable that the volume fraction of vaporized reaction medium is at least about 25, 50, 75, 95 volume percent of the total volume of the reaction medium in the heat-exchange means. It is preferable that the above volume fractions are provided in a portion of reaction medium within or near the outlet of reaction medium from a tube in a heat-exchange means.

[029] In another embodiment of the invention, it is preferred to provide a liquid and solid multiphase flow of reaction medium within a heat-exchange means. More preferably, the solid phase comprises a polycarboxylic acid, such as TPA. In this embodiment, it is preferable that the mass fraction of polycarboxylic acid solid particles is at least about 2, 4, 6, 8 weight percent of the reaction medium. In this embodiment, it is also preferable that the mass fraction of polycarboxylic acid solid particles is less than about 60, 40, 25, 15 weight percent of the reaction medium in the heat-exchange means. It is preferable that the above mass fractions are provided in a portion of reaction medium within or near the inlet of reaction medium into a tube in a heat- exchange means.

[030] In still yet another embodiment of the invention, it is preferred to provide a three-phase reaction medium comprising solid TPA particles, vapor components, and liquid components within the heat-exchange means. The above preferred volume fractions of vapor and the above preferred mass fractions of solid apply in all physically realizable combinations for this three- phase mixture.

[031] The inventors have discovered that a preferred embodiment of the invention places the steam heat-transfer fluid on the shell side and the process materials on the tube side. Indeed, this is a preferred embodiment when the process material is specifically reaction medium, more specifically when the reaction medium comprises TPA solid, and most specifically when the reaction medium is a three-phase mixture comprising solid TPA, vapor components, and liquid components.

[032] This arrangement with steam heat-transfer fluid on the shell-side requires the mechanical design of both the tube-side and shell-side to withstand the pressure of the steam heat-transfer fluid. This causes greater capital cost than if the lower pressure process material were on the shell-side. However, the inventors have discovered that placing the process material on the tube-side provides very great advantages. First, this disposition provides greater control of the velocity of the process material near the conductive, isolating, heat-exchange boundary surfaces, thus helping to maximize film coefficients for heat transfer in order to reduce overall capital cost. Second, this disposition provides greater control of the maximum film temperature of the process material near the conductive, isolating, heat-exchange boundary surfaces. This can be important when the process material is reaction medium in order to optimize the balance between desired polymerization and undesirable side reactions such as dimerization of EG to diethylene glycol (DEG) and such as formation of colored impurities in the polyester melt product. Third, this disposition provides greater control of the residence time and residence time distribution of the process materials. Again, this is especially important for reaction medium in order to optimize the balance between desired polymerization and undesirable side reactions.

[033] It is preferred that the overall heat-exchange coefficient between steam heat-transfer fluid and process materials, especially reaction medium, is at least about 5, 100, 250, 500 watts per square meter per degree Celsius. This is achieved by selection of steam pressures and temperatures, tube diameters, process material compositions, process material superficial velocities, and tube metallurgies according to the disclosures herein. The overall heat-exchange coefficient is based on the outside diameter of the tubes and is calculated according to methods and standards of the Heat Transfer Research Institute.

[034] The inventors disclose that certain embodiments of the present invention can be particularly useful for the emerging scale of polyester production facilities with capacities from a single facility and or a single polyester melt product reactor of at least about 100, 200, 400, 600 million kilograms per year. Existing steam boilers are already extremely large, with combustion power inputs even greater than one gigawatt. Fired furnaces for heating low-vapor-pressure organic heat-transfer fluids can also be made quite large. However, design of a fired furnace for low-vapor-pressure organic heat-transfer fluids must pay strict attention to film temperatures in order to avoid thermal degradation of the fluids. This imposes more restrictive constraints on the firing temperature and on the size and layout of the heater tubes, as compared to a steam boiler. Mass flow rates per watt of thermal energy content are far less for condensing steam than for low-vapor-pressure organic heat-transfer fluids. Pipe sizes for the condensate-water return to a boiler are much smaller than for low-vapor-pressure organic heat-transfer fluids. Pipe sizes for steam supply are also smaller, albeit at a higher pressure rating.

[035] The inventors have discovered that certain embodiments of the present invention can be particularly useful for providing thermal energy to a reaction medium of a single polyester synthesis process facility wherein the thermal energy thermal input from steam heat-transfer fluid is at least about 10, 20, 30, 40 megawatts from steam heat-transfer fluid.

[036] The inventors have discovered that certain embodiments of the present invention can be particularly useful for providing thermal energy to a reaction medium of a single reaction step within a polyester synthesis process facility wherein the thermal energy input from steam heat-transfer fluid is at least about 5, 10, 15, 25 megawatts. [037] The inventors have discovered that certain embodiments of the present invention can be particularly useful with shell and tube heat-exchange means having tube areas in a single means of at least about 300, 1000, 2500, 4000 square meters, based on the outside diameter of the tubes.

[038] The inventors have discovered that certain embodiments of the present invention can be useful for exchangers wherein the tubes are made from any of the many stainless steel alloys, with 304L especially preferred, and the shell is made from any steel alloy, including any stainless steel alloy.

[039] As yet another aspect of the invention, the inventors have discovered that a preferred method for heating a low-vapor-pressure organic heat-transfer fluid for use in a polyester production facility comprises using steam heat-transfer fluid to provide thermal energy to a low-vapor-pressure organic heat-transfer fluid. This is especially preferred in a polyester production facility wherein at least a portion of thermal energy is provided to process materials by steam heat-transfer fluid directly through conductive, isolating, heat-exchange boundary surfaces according to at least one disclosure herein. In a polyester production facility, a considerable number of relatively low duty heating systems, such as piping and equipment tracer circuits, are needed to provide small thermal energy duties to process materials and to offset ambient losses of thermal energy, such as losses through insulation. These low duty heating systems are particularly required for preheating conduits and equipment before admitting the flow of process materials, for when the flows of process materials are temporarily idled, and for when the flows of process materials are reduced to very low rates. Although steam heat-transfer fluid as disclosed herein is sufficiently hot for this dispersed, low duty service, the very high pressures required cause capital and maintenance costs to be economically unfavorable for using the steam in the service. Thus, even when larger thermal energy duties for process materials are converted to steam heat-transfer fluid, there remains a need in a polyester production facility for a thermal energy supply system using a low-vapor pressure organic heat-transfer fluid. For such relatively low duty heating systems using a low-vapor-pressure organic heat-transfer fluid, the inventors have discovered that it is more economical to increase the size of the fuel-fired steam boiler and to use part of this steam to heat the low- vapor-pressure organic heat-transfer fluid, as compared to providing a separate fuel-fired furnace for the low-vapor-pressure organic heat-transfer fluid.

[040] The advantage of using steam heat-transfer fluid to provide thermal energy to a low-vapor-pressure organic heat-transfer fluid is especially pronounced when reliability is considered. The additional complex failure modes intrinsic with providing at least one fuel-fired furnace for low- vapor-pressure organic heat-transfer fluid are efficaciously replaced by a simple heat-exchange means, which means typically comprises no moving mechanical parts and which does not require controls and interlocks for combustion safety and environmental emissions.

[041] Thus, the inventors disclose a polyester production facility using steam heat-transfer fluid, according to according to at least one disclosure herein, as the only heat-transfer fluid that receives thermal energy directly from a fuel-fired furnace. More preferably, the polyester production facility also receives at least a portion of thermal energy from another heat-transfer fluid that receives at least a portion of thermal energy from the fuel-fired steam. Such a configuration is schematically illustrated in FIG. 3. Most preferably, the polyester production facility receives at least a portion of thermal energy from a low-vapor-pressure organic heat-transfer fluid that is heated by the steam heat-transfer fluid in a heat exchange means comprising tubes. It is preferred that the low-vapor-pressure organic heat-transfer fluid is heated by steam to a supply bulk temperature of at least about 250, 270, 290, 310 degrees Celsius. It is preferred that the low-vapor-pressure organic heat- transfer fluid is heated by steam to a supply bulk temperature of less than about 400, 360, 340, 330 degrees Celsius. It is preferred that the maximum film temperature of the low-vapor-pressure organic heat-transfer fluid is less than about 400, 360, 340, 330 degrees Celsius at the conductive, isolating, heat-exchange boundary surfaces contacted by steam heat-transfer fluid. [042] The inventors also disclose preferred ranges for the amount thermal energy provided by a low-vapor-pressure organic heat-transfer fluid that provides thermal energy to a polyester production facility. These energy values are much lower than in a conventional polyester production facility because the major duties for thermal energy are transferred to steam heat- transfer fluid according to certain embodiments of the invention disclosed herein. It is preferred that the thermal energy supplied to a polyester production facility by low-vapor-pressure organic heat-transfer fluid is less than about 800, 600, 400, 300 joules per gram of polyester melt product. More preferably, the organic heat-transfer fluid is heated by steam heat- transfer fluid according to the disclosures herein. It is preferred that the thermal energy supplied to a polyester production facility by low-vapor- pressure organic heat-transfer fluid is at least about 40, 80, 120, 160 joules per gram of polyester melt product. More preferably, the organic heat-transfer fluid is heated by steam heat-transfer fluid according to the disclosures herein.

[043] In yet another embodiment of the invention, it is preferred that condensate-water is taken from at least one heat-exchange means that is using steam heat-transfer fluid operating according to at least one embodiment herein and that the condensate-water is reduced in pressure to form lower pressure flashed-steam heat-transfer fluid. The means for converting the condensate-water to the flashed-steam can be located either inside or outside the polyester production facility. It is preferred that at least a portion of the flashed-steam heat-transfer fluid is formed having a pressure that is less than about 8, 7, 5, 2 megapascals. It is preferred that at least a portion of the flashed-steam heat-transfer fluid is formed having a pressure that is at least about 0.2, 0.6, 1.0, 1.5 megapascals. It is preferred that at least a portion of the flashed-steam heat-transfer fluid is produced from condensate-water that is formed from steam heat-transfer fluid used to heat reaction medium. It is preferred that at least a portion of the flashed-steam heat-transfer fluid is produced from condensate-water that is formed from steam heat-transfer fluid used to heat a low-vapor-pressure organic heat- transfer fluid.

[044] It is preferred that at least a portion of the flashed-steam heat- transfer fluid is used to provide energy to a polyester production facility and/or other adjacent chemical synthesis facilities. It is preferred that at least a portion of the flashed-steam heat-transfer fluid is used within a polyester production facility to provide thermal energy to process materials. More preferably, at least a portion of the flashed-steam heat-transfer fluid is used within a polyester production facility to form at least a portion of vaporized process material. Most preferably, at least a portion of the flashed-steam heat-transfer fluid is used within a polyester production facility to heat liquid EG to form vapor EG. Such a configuration is schematically illustrated in FIG. 4.

[045] Additionally, it is preferred that at least a portion of the flashed- steam heat-transfer fluid is used in an adjacent terephthalic acid synthesis facility. An exemplary monomer production facility, such as a terephthalic acid (TPA) synthesis facility, is schematically depicted in FIG. 5. The temperature of many major heat duties in a monomer production facility are usefully less than in a polyester production facility, and this enables a favorable, efficient, energy integration when steam heat-transfer fluid is used. The temperature of the reaction medium in a polyester production facility is preferably at least about 250 degrees Celsius; and, as disclosed above, it is preferred that the condensing temperature of the steam heat-transfer fluid therein is at least about 260, 275, 290, 305 degrees Celsius.

[046] FIG. 6 depicts one embodiment of a system for integrated steam heating of a polymer production facility, such as a PET melt product synthesis facility, and a monomer production facility, such as a TPA synthesis facility. In particular, FIG. 6 shows how the heating duty of high-pressure and lower- pressure steam can be divided up among the polymer and monomer facilities.

[047] In the monomer production facility, it is preferred that at least about 30, 50, 70, 80 percent of the total thermal energy input from heat transfer fluids is to a process temperature of less than about 295, 280, 265, 250 degrees Celsius. It is preferred that the terephthalic acid synthesis facility is located such that the minimum horizontal distance from the polyester production facility is less than about 1800, 900, 300, 100 meters. It is preferred that the terephthalic acid synthesis facility forms at least a portion of terephthalic acid product by partial oxidation of para-xylene, more preferably using molecular oxygen. It is preferred that at least a portion of the terephthalic acid product is fed into a reaction medium of the adjacent polyester production facility within less than about 72, 24, 12, 4 hours after being formed from para-xylene.

[048] The invention may include various separate aspects, as set forth herein. Thus, in one aspect, the invention relates to a process for making a polyester in a polyester production facility, the process including the steps of: forming a reaction medium comprising at least one monomer, wherein the at least one monomer comprises terephthalic acid (TPA) and/or an ester derivative of TPA; subjecting at least a portion of the reaction medium to one or more chemical reactions in the polymer production facility to thereby produce the polyester; and heating the reaction medium at one or more locations in the polyester production facility, wherein at least 30 percent of the total energy input employed for the heating of the reaction medium is provided by indirect heat exchange between the reaction medium and steam. In other aspects, at least 50, 70, or 80 percent of the total energy input employed for the heating of the reaction medium may be provided by indirect heat exchange with steam.

[049] In another aspect, the amount of the heating of the reaction medium provided by indirect heat exchange with steam may be at least 350, or at least 700, or at least 900, or at least 1 ,200 joules per gram of the polyester produced from the polyester production facility.

[050] In yet another aspect, the polyester may be produced from the polyester production facility at a rate of at least 100, or at least 200, or at least 400, or at least 600 million kilograms per year.

[051] In a further aspect, at least 30, or at least 50, or at least 70, or at least 90 percent of the heating of the reaction medium provided steam is accomplished by indirect heat exchange with steam having a pressure of at least 2, or at least 5, or at least 6, or at least 7, or at least 9 megapascals. In yet another aspect, at least 30, or at least 50, or at least 70, or at least 80 percent of the total energy input employed for heating any stream, conduit, and/or equipment in the polyester production facility is provided by indirect heat exchange with steam.

[052] In one aspect, the process further includes a step of generating pressurized steam in one or more boilers, wherein the pressurized steam directly or indirectly provides at least 30, or at least 50, or at least 70, or at least 80 percent of the total heat energy input required by the polyester production facility. The thermal energy input provided by the pressurized stream, in turn, may be at least 200, or at least 500, or at least 1 ,000, or at least 1 ,500 joules per gram of the polyester produced from the polyester production facility.

[053] In yet another aspect, the heating of the reaction medium by indirect heat exchange with steam is carried out in one or more shell-and-tube heat exchangers. The reaction medium may, in turn, flow on the tube-side of the shell-and-tube heat exchangers and the steam may flow on the shell-side of the shell-and-tube heat exchangers. The reaction medium may, in turn, flow through the tube-side of the shell-and-tube heat exchangers at a superficial velocity of at least 0.7, or at least 1.4, or at least 3, or at least 5 meters per second, measured on a time-averaged and cross-sectional-area- averaged basis. The tubes of the shell-and-tube heat exchanger may, in turn, have an outside diameter of less than 0.1 , or less than 0.05, or less than 0.01, or less than 0.005, or less than 0.003 meters.

[054] In further aspects, the at least one monomer may further comprise an alcohol, and the alcohol may comprise a diol, for example ethylene glycol.

[055] In yet another aspect, the reaction medium may be maintained at a temperature greater than 25O⁰C during at least one of the chemical reactions. [056] In a further aspect, the steam employed for the heating of the reaction medium may have an initial temperature of at least 25O⁰C.

[057] In another aspect, the chemical reactions include esterification and polycondensation.

[058] In yet another aspect, at least 75 percent of the heating of the reaction medium by indirect heat exchange with steam may be used to heat the reaction medium for the esterification reaction.

[059] In a further aspect, the one or more chemical reactions may be carried out in a plurality of serially connected reactors, wherein the serially connected reactors include a first reactor, a second reactor, and a third reactor. In yet another aspect, each of the first, second, and third reactors may have a respective first, second, and third heat exchanger associated therewith. In yet another aspect, each of the first, second, and third reactors may have a respective first, second, and third vapor-liquid separation zone associated therewith, and the the heat exchangers and vapor-liquid separation zones associated with each of the first, second, and third reactors may be located in separate vessels.

[060] In yet another aspect, the first, second, and third heat exchangers may be shell-and-tube heat exchangers with steam on the shell- side and the reaction medium on the tube-side.

[061] In yet another aspect, the first vapor-liquid separation zone may be located downstream of the first heat exchanger, the second vapor-liquid separation zone may be located downstream of the second heat exchanger, and the third vapor-liquid separation zone may be located downstream of the third heat exchanger. In a further aspect, esterification may be carried out in the first reactor, and the reaction medium may have a conversion of at least 50, or at least 70, or at least 80, or at least 90 percent upon exiting the first reactor. In yet another aspect, esterification is carried out in the second reactor, and the reaction medium may have a conversion of at least 80, or at least 90, or least 95, or at least 97 percent upon exiting the second reactor.

[062] In a further aspect, the first reactor has a first heat exchanger associated therewith, wherein at least 20, or at least 40, or at least 50, or at least 60 percent and/or up to 100, or up to 90, or up to 80, or up to 70 percent of the total energy input employed for the heating of the reaction medium is provided by indirect heat exchange with steam in the first heat exchanger.

[063] In yet another aspect, the amount of the heating of the reaction medium provided by indirect heat exchange with steam in the first heat exchanger may be at least 200, or at least 500, or at least 800, or at least 1 ,000 joules per gram of the polyester produced from the polyester production facility and/or up to 10,000, or up to 5,000, or up to 3,000, or up to 2,000 joules per gram of the polyester produced from the polyester production facility.

[064] In a further aspect, the amount of the heating of the reaction medium provided by indirect heat exchange with steam in the first heat exchanger may be at least 5, or at least 10, or at least 15, or at least 25 megawatts.

[065] In yet another aspect, the second reactor has a second heat exchanger associated therewith, wherein at least 0.5, or at least 1 , or at least 2, or at least 4 percent and/or up to 50, or up to 30, or up to 15, or up to 10 percent of the total energy input employed for the heating of the reaction medium may be provided by indirect heat exchange with steam in the second heat exchanger. In another aspect, the amount of the heating of the reaction medium provided by indirect heat exchange with steam in the second heat exchanger may be at least 20, or at least 50, or at least 80, or at least 100 joules per gram of the polyester produced from the polyester production facility and/or up to 1 ,000, or up to 500, or up to 300, or up to 200 joules per gram of the polyester produced from the polyester production facility.

[066] In yet another aspect, the third reactor has a third heat exchanger associated therewith, wherein at least 0.05, or at least 0.1, or at least 0.5, or at least 1 percent and/or up to 30, or up to 20, or up to 10, or up to 5 percent of the total energy input employed for the heating of the reaction medium may be provided by indirect heat exchange with steam in the third heat exchanger. In a further aspect, the amount of the heating of the reaction medium provided by indirect heat exchange with steam in the third heat exchanger may be at least 1 , or at least 5, or at least 10, or at least 20 joules per gram of the polyester produced from the polyester production facility and/or up to 500, or up to 200, or up to 100, or up to 50 joules per gram of the polyester produced from the polyester production facility.

[067] In a further aspect, polycondensation may be carried out downstream of the first, second, and/or third reactor.

[068] In yet another aspect, the serially connected reactors may include a fourth reactor and. a fifth reactor, and polycondensation may be carried out in the fourth and fifth reactors. In a further aspect, the polyester is recovered from te fifth reactor.

[069] In another aspect, esterification may be the predominate reaction in the first and second reactors and polycondensation may be the predominate reaction in the fourth and fifth reactors.

[070] In yet another aspect, the temperature of the reaction-medium exiting the first, second, and/or third heat exchangers may be at least 25O⁰C.

[071] In yet another aspect, at least 1 , or at least 3, or at least 5, or at least 10 percent and/or up to 75, or up to 50, or up to 30, or up to 20 percent of the total energy input employed for the heating of the reaction medium may be provided by indirect heat exchange with a heat transfer medium (HTM) that is not steam. In a further aspect, the amount of the heating provided by indirect heat exchange with the HTM may be at least 1 , or at least 5, or at least 10, or at least 20 joules per gram of the polyester produced from the polyester production facility and/or up to 500, or up to 250, or up to 100, or up to 50 joules per gram of the polyester produced from the polyester production facility.

[072] In yet another aspect, the process further comprises heating the HTM by indirect heat exchange with steam.

Example

[073] This is a calculational example that quantifies the heat duty associated with PET melt product synthesis facilities. The table below sets forth calculated values for the heat duties of a PET melt product synthesis facility constructed in accordance with the inventive embodiment depicted in FIG. 2, where all heat duties are supplied by steam.

[074] In a conventional PET melt product synthesis facility, at least the first four heating duties, which sum to 152,900,000 BTU/hr (1 ,936 joules per gram; 44.8 megawatts), would be provided by a fuel-fired furnace heating low- vapor-pressure organic heat-transfer fluid (HTM).

Claims

We Claim:

1. A process for making a polyester in a polyester production facility, the process comprising:

(a) forming a reaction medium comprising at least one monomer that includes terephthalic acid (TPA) and/or an ester derivative of TPA;

(b) subjecting at least a portion of the reaction medium to one or more chemical reactions in the polymer production facility to thereby produce the polyester; and

(c) heating the reaction medium at one or more locations in the polyester production facility, wherein at least 50 percent of the total energy input employed for the heating of the reaction medium is provided by indirect heat exchange between the reaction medium and steam.

2. The process of claim 1 , wherein at least 80 percent of the total energy input employed for the heating of the reaction medium is provided by indirect heat exchange between the reaction medium and steam.

3. The process of claim 1 , wherein the amount of the heating of the reaction medium provided by indirect heat exchange with steam is at least 900 joules per gram of the polyester produced from the polyester production facility.

4. The process of claim 1 , wherein the polyester is produced from the polyester production facility at a rate of at least 400 million kilograms per year.

5. The process of claim 1 , wherein the heating of the reaction medium by indirect heat exchange with steam is carried out in one or more shell-and- tube heat exchangers.

6. The process of claim 5, wherein the reaction medium flows on the tube-side of the one or more shell-and-tube heat exchangers and steam flows on the shell-side of the one or more shell-and-tube heat exchangers.

7. The process of claim 5, wherein the reaction medium flows through the tube-side of the shell-and-tube heat exchangers at a superficial velocity of at least 1.4 meters per second, measured on a time-averaged and cross- sectional-area-averaged basis.

8. The process of claim 5, wherein the tubes of the shell-and-tube heat exchangers have an outside diameter of less than 0.003 meters.

9. The process of claim 1 , wherein at least 50 percent of the heating of the reaction medium provided by steam is accomplished by indirect heat exchange with steam having a pressure of at least 7 megapascals.

10. The process of claim 1, wherein the at least one monomer further comprises ethylene glycol.

11. The process of claim 1 , wherein the reaction medium is maintained at a temperature greater than 25O⁰C during at least one of the chemical reactions.

12. The process of claim 1 , wherein the steam employed for the heating of the reaction medium has an initial temperature of at least 25O⁰C.

13. The process of claim 1, wherein the chemical reactions comprise estehfication and polycondensation.

14. The process of claim 13, wherein at least 75 percent of the heating of the reaction medium by indirect heat exchange with steam is used to heat the reaction medium for an esterification reaction.

15. The process of claim 1 , wherein the one or more chemical reactions are carried out in a plurality of serially connected reactors, wherein the serially connected reactors include a first reactor, a second reactor, and a third reactor.

16. The process of claim 15, wherein each of the first, second, and third reactors has a respective first, second, and third heat exchanger associated therewith.

17. The process of claim 16, wherein the first, second, and third heat exchangers are shell-and-tube heat exchangers with steam on the shell-side and the reaction medium on the tube-side.

18. The process of claim 16, wherein each of the first, second, and third reactors has a respective first, second, and third vapor-liquid separation zone associated therewith, and wherein the heat exchangers and vapor-liquid separation zones associated with each of the first, second, and third reactors are located in separate vessels.

19. The process of claim 17, wherein esterification is carried out in the first reactor, and wherein the reaction medium has a conversion of at least 70 percent upon exiting the first reactor.

20. The process of claim 16, wherein esterification is carried out in the second reactor, and wherein the reaction medium has a conversion of at least 90 percent upon exiting the second reactor.

21. The process of claim 16, wherein the amount of the heating of the reaction medium provided by indirect heat exchange with steam in the first heat exchanger is from 500 joules per gram to 3,000 joules per gram of the polyester produced from the polyester production facility.

22. The process of claim 16, wherein the serially connected reactors include a fourth reactor and a fifth reactor, and wherein polycondensation is carried out in the fourth and fifth reactors.

23. The process of claim 22, wherein the polyester is recovered from the fifth reactor.

24. The process of claim 22, wherein esterification is the predominate reaction in the first and second reactors and polycondensation is the predominate reaction in the fourth and fifth reactors.

25. The process of claim 1 , wherein from 1 percent to 50 percent of the total energy input employed for the heating of the reaction medium is provided by indirect heat exchange with a heat transfer medium (HTM) that is not steam.

26. The process of claim 25, wherein the HTM is heated by indirect heat exchange with steam.