KR810001745B1 - Direct esterfication process for use in production of polyethylene terephthalate - Google Patents

Abstract

This method describes the continuous and direct esterification of terephthalic acid with an excess of ethylene glycol under the existence of ester reaction product having low d.p. at the increased temp. Here, ethylene glycol is recovered by treating reaction heat vapor and recirculated to be esterificated. Then, this vapor is supplied to the combined spray condenser-refinery tower and the heat content of this vapor is used to distil the water and org. impurity of glycol which is recovered from the bottom of the tower. This esterification is carried out by flowing back the aq. effluent condensate to this tower.

Description

Continuous Direct Esterification Method

The figure shows a schematic of an apparatus useful in the method of the present invention.

The present invention relates to continuous direct esterification useful for the preparation of poly (ethylene terephthalates) from ethylene glycol and terephthalic acid, and more particularly to the recovery and reuse of volatile volatile ethylene glycol in etherification reactions.

Direct esterification methods useful in the preparation of poly (ethylene terephthalates) include esterification of terephthalic acid with ethylene glycol, accompanied by ester polycondensation. The water formed in this reaction is also evaporated and removed.

In a continuous process, terephthalic acid is mixed with excess ethylene glycol to form a slurry or paste and then heated to form a low molecular weight reaction product. This reaction is carried out by mixing the slurry with the low molecular weight reaction product heated to the desired reaction temperature. References: US Patent No. 3,590,072 to Leybourne III, US Patent No. 3,676,485 to Lewis et al, US Pat. No. 3,697,579 to Balint et al, Chapman et al. Described in US Pat. No. 3,927,982 to Chapman et al. The latter three patents mentioned above exemplify reactors having an internal heater with a reaction product circulating from the reaction vessel to the reactor via a heater and with the slurry supplied to the circulating product before the heater. All or most of the esterification reaction then takes place upon the passage of glycol and terephthalic acid through the heater, where the reaction vessel mainly contributes to the separation of the vapor from the reaction product. The resulting low molecular weight polyester is fed from the reactor to a continuous polymerization step to form a high molecular weight polymer suitable for melt spinning into polyester yarn in a conventional manner.

Excess ethylene glycol used for the slurry feed is evaporated in the reactor with a small amount of organic impurity vapors (e.g. acetaldehyde, 2-methyl-1, 3-dioxolane and 1,4-dioxane) formed in water vapor and reaction Remove These steams are pulverized to recover ethylene glycol for reuse. A method for purifying ethylene glycol is described in US Pat. No. 3,367,847 to Pierson, but a simpler and less expensive method is required.

The reactor vapor also contains reaction products that are evaporated or contained. The patent of Lewis et al describes the use of a partial condenser outside the reaction vessel, which operates at a temperature low enough to condense the gasified monomer into a liquid but not low enough to condense ethylene glycol. . The patent also states that distillation columns can be used in place of partial condensers when large losses of monomer are required (paragraphs 4, lines 65-73), but there is no mention of recovery of ethylene glycol from steam.

It is important to remove the ester product from the vapor before recovering vaporized ethylene glycol. The more volatilized organic impurities formed in the reaction and the ester product at the temperature used to separate ethylene glycol from water form precipitates that cause disturbances in the recovery operation.

The present invention provides a continuous direct esterification process wherein ethylene glycol is continuously recovered from the reaction vapor and reused in a slurry of ethylene glycol and terephthalic acid fed to a direct esterification reaction to produce an ester product. All ethylene glycol in the reaction vapor is recovered in a single combination spray condenser-rectification column and recycled to the feed slurry without further purification. The rectification is carried out so that the heat of reaction containing the reaction steam supplies all the heat required to achieve proper distillation of water and organic impurities from the recovered ethylene glycol.

This continuous direct esterification process involves a slurry feed of ethylene glycol and terephthalic acid at a molar ratio of glycol 1.5-4 (preferably 1.8-3.0) to terephthalic acid 1.9 heated to 280 ° -315 ° C. in the reaction product to esterify terephthalic acid. It is preferred that it is such a type that hot ester vapors which form ester products having an average degree of polymerization of 2 to 10 and which contain small amounts of ester products and ethylene glycol with organic impurities and water are separated from the reaction products. This improved process involves directing the hot reaction steam to the spray zone at the bottom of the tower, condensing the vapor obtained from the top of the tower with an aqueous distillate, and about 40 to 70 at the top of the tower to act as reflux. Recycling the wt% aqueous distillate causes the aqueous distillate to contain less than 0.5 wt% (preferably less than 0.05 wt%) ethylene glycol and collect glycol condensate at the bottom of the distillation column containing less than 10% water. Spray hot reaction vapor with some glycol condensate at 140 ° to 180 ° C. (preferably 155 ° to 175 ° C.) in the spray zone to maintain a constant temperature and remove ester product and large amounts of glycol from the vapor. And recycling the glycol condensate to the slurry feed.

The rectification tower has between 5 and 20 plates (preferably about 12 plates). The reflux is preferably adjusted to maintain a constant temperature at the bottom end within a range of 100 ° to 140 ° C. The reaction vapor is sprayed into the reaction at 1 to 25 kilograms (preferably 1 to 8 kilograms) of the solids per kilogram of slurry feed.

The column is preferably operated so that the glycol melt contains 2-7% by weight to reduce the formation of ether compounds from the glycols in the esterification reaction.

The composition of the reaction vapor depends on the temperature and pressure of the molarol and esterification reaction of ethylene glycol in the sludge feed.

The reaction is preferably carried out at about atmospheric pressure. Typical reaction steams contain 47 to 84 parts by weight of ethylene glycol, 53 to 16 parts by weight of water, 10% or less of ester product, and 2% or less of volatile organic impurities. Most water and volatile organic impurities are raised as aqueous distillates and the temperature of the condensate in almost all ethylene glycol and ester production heat tanks 58 is high because the condensate is used to cool the steam feed from the reactor. Heat tank temperatures below about 155 ° C form undesirable amounts of solids in the heat tank. Undesirable glycol ether compounds are formed at high rates at high temperatures. The preferred maximum heat tank temperature for the various slurry feed moles is approximately:

2G / TPA 1.8 2.0 2.0 2.4 2.6 2.8 3.0

Heat Tank Temperature (℃) 160 164 167 169 171 171 173 175

Sufficient heat is removed from the system by the condenser 42 for operation under these conditions without additional cooling of the heat tank condensate. Proper cooling of the vapor feed in the spray zone may be achieved when the condensate is used at a spray rate of 1 to 25 kg per kg of slurry fed to the reactor.

EXAMPLE

10003.6 kg of ethylene glycol (2G) per hour was mixed with 485.3 kg (1.070 lb) of terephthalic acid (TPA) per hour using an apparatus of the type shown in the figure to form a slurry having a 2G / TPA molar ratio of 2.5. This slurry is then fed to a recycle pipe in a reactor with an external heat exchanger.

The slurry enters the bottom of the heat exchanger and is heated at 305 ° C. with the ester reaction product. Reactor pressure was 2.5 psig (17.2 kpag). Esterification of terephthalic acid to form an ester product having a relative viscosity (HRV) of 2.9 (corresponding to an average degree of polymerization of about 7), which is subsequently polymerized to a HRV of 21.5 to spun into a filament for fibers. To form polyester with excellent properties. The polyester filaments were spun, stretched and cut to produce 4.25 denier staple fibers with high quality grades for life test and diethylene glycol content.

Excess glycol and other vapors are separated from the ester product in the reactor and fed to the spray zone at the bottom of the tower used for the recovery of ethylene glycol. These hot feed steams are sprayed onto 595 pounds (5500 pounds) of soft tank condensate per hour to cool the steam to 163 ° C and to wash the sublimation and containing ester product which forms a solid at low temperatures, which is then subjected to 39.4 in the bubble cap top. Pass upwards through 12 plates of cm (15 1/2 inch) diameter. A reflux at 95 ° C. was provided to the peak plate at a rate of about 140 kg (308 pounds) per hour to recover almost 100% of ethylene glycol in the reactor vapor. This reflux rate was controlled by a temperature signal from the bottom (plate). As the temperature increases, the reflux rate increases to maintain the heat tank condensate at a temperature of about 157 ° C., and the reflux rate decreases as the temperature decreases. The temperature was then measured in the liquid on the plates of the tower.

Plate plate plate

(1) 120 ° C (4) 102 ° C (10) 100.5 ° C

(2) 106 (8) 101 (12) 100

The upper vapor from the apex plate has approximately the following weight * composition.

Water 98.13 2-Methyl-1,3-dioxolane 0.76

Ethylene Glycol 0.016 1,4-Dioxane 0.043

Acetaldehyde 1.05

The condensed upper steam is used for reflux and the remaining distillate is disposed of at 113.4 kg (250 pounds) per hour. Uncondensed steam is discharged into a waste scrubber using 68 kg (150 pounds) of 8.2 pH mol per hour to rise to a mixture pH of about 6.0 (>). Low acidic pH facilitated waste treatment handling by reducing degradation of 2-methyl-1,3-dioxolane.

Ethylene glycol in the feed steam was recovered as a heat tank condensate of approximately the following weight percent composition.

Ethylene Glycol 89.5 Solid 4.5

Water 5.6 diethylene glycol 0.4

This glycol condensate is recovered at a rate of about 283 kg (625 pounds) per hour and recycled for use in the slurry fed to the reactor without any stagnation. The “solid” content of the condensate results from the sublimation and contained ester product washed in the feed steam by spraying on the condensate. Spray treatment effectively eliminated obstruction of the tower.

Relative viscosity (HRV) is the viscosity ratio of 0.8 gm solution of polyester dissolved at room temperature in 10 ml of 800 ppm H ₂ SO ₄ hexafluoro isopropanol to the viscosity of H ₂ SO ₄ -containing hexafluoro isopropanol itself. These were measured at 25 ° C. with capillary viscosimeter and expressed in the same units.

Claims (1)

Continuous esterification of terephthalic acid with excess ethylene glycol at elevated temperature in the presence of low-polymerization ester reaction products, wherein the process comprises treating the heat of reaction steam to recover ethylene glycol and recycling the ethylene glycol to esterification. In which the steam is combined into a combined spray condenser-rectifier and the heat content of the steam supplies the heat necessary to distill water and organic impurities from the glycol recovered as a condensate at the bottom of the tower and a portion of the glycol. Using as part of the spray condenser against this vapor and condensing the aqueous effluent to circulate a portion of them back to the upper part of the tower to act as reflux of the tower.

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