Wet Hydrolysis of Waste PET Resin with Sulfuric Acid

INTERNATIONAL JOURNAL OF CHEMICAL

REACTOR ENGINEERING

Volume 8 2010 Article A150

Wet Hydrolysis of Waste PolyethyleneTerephthalate Thermoplastic Resin with

Sulfuric Acid and CFD Simulation for HighViscous Liquid Mixing

Mohammad Reza Moghbeli∗ Sahar Namayandeh†

Seyed Hassan Hashemabadi‡

∗Iran University of Science and Technology, mr [email protected]†Iran University of Science and Technology, [email protected]‡Iran University of Science and Technology, [email protected]

ISSN 1542-6580Copyright c©2010 The Berkeley Electronic Press. All rights reserved.

Wet Hydrolysis of Waste Polyethylene TerephthalateThermoplastic Resin with Sulfuric Acid and CFD

Simulation for High Viscous Liquid Mixing

Mohammad Reza Moghbeli, Sahar Namayandeh, and Seyed HassanHashemabadi

Abstract

In this research, wet hydrolysis of polyethylene terephthalate (PET), as usedin beverage bottles, has been carried out using sulfuric acid. The hydrolysis reac-tion has been investigated in a laboratory glass reactor, provided with an anchormixer and a digital torque meter. The hydrodynamics of viscous liquid in stirredreactor has also been studied by computational fluid dynamics simulation. Theresults of simulations give the operational conditions for optimum mixing intothe designed agitated reactor. In the stirred reactor, the used chopped PET pieceswere degraded into its constituent monomers, terephthalate acid (TPA) and ethy-lene glycol, in an acidic medium. The hydrolysis of product was characterizedby (FT-IR) spectroscopy, while the efficiency of TPA production was measuredby (UV-Vis) spectrophotometer. The effects of different parameters such as tem-perature, acid concentration and reaction time on PET degradation have been in-vestigated. The experimental results show that the optimum reaction conditionoccurred while the sulfuric acid to used PET ratio is 4:1 at a reaction temperatureof 90◦C and reaction time of five hours.

KEYWORDS: PET degradation, acidic hydrolysis, polyethylene terephthalate(PET), terephthalic acid (TPA), ethylene glycol (EG), computational fluid dynam-ics

Erratum

The original citation for this article published on November 9, 2010 was as follows:

Namyandeh, S.; Hashemabadi, Seyed Hassan; and Moghbeli, M.R. (2010) “Wet Hydrolysis ofWaste Polyethylene Terephthalate Thermoplastic Resin with Sulfuric Acid and CFD Simulationfor High Viscous Liquid Mixing,” International Journal of Chemical Reactor Engineering: Vol.8: A150.Available at: http://www.bepress.com/ijcre/vol8/A150

The citation was corrected on November 18, 2010 as follows:

Moghbeli, Mohammad Reza; Namayandeh, Sahar; and Hashemabadi, Seyed Hassan (2010) ”WetHydrolysis of Waste Polyethylene Terephthalate Thermoplastic Resin with Sulfuric Acid and CFDSimulation for High Viscous Liquid Mixing,” International Journal of Chemical Reactor Engi-neering: Vol. 8: A150.Available at: http://www.bepress.com/ijcre/vol8/A150

Introduction

PET is a thermoplastic resin of industrial importance (Kaplan, 2000). PET is semi-crystalline polyester with high strength, transparency and safety. Large quantities of PET are consumed in the manufacturing of beverage bottles, food packaging, video and audio tapes. Recycling of these used PET products is an important issue because of environmental concern (Ertas and Guclu, 2005, Cheng et al. 2010).

The recycling of waste PET can be carried out in many ways. Three main approaches for recycling PET are mechanical, chemical, and auto-clave recycling (Scheirs, 1999; Paszun and Spychaj, 1997; Shukla and Harad, 2005; Zope et al., 2003). One of the methods for PET chemical recycling is hydrolysis where crashed PET reacts with aqueous acid, aqueous alkaline and/or neutral environment. This results in terephthalic acid (TPA) and ethylene glycol (EG), which can be used as raw material in many industrial processes (Brandrup et al., 1996, Yoshioka et al. 2001). Alkaline hydrolysis of PET is usually carried out with the use of alkaline solution of NaOH or KOH (Karayannidis et al., 2002). Acidic hydrolysis is one of the traditional methods for de-polymerization of polyesters. In acidic hydrolysis, nitric acid or sulfuric acid was used; however, the use of sulfuric acid (98%) for hydrolysis was more common (De Carvalho et al., 2006). The only disadvantage of this method is the corrosion caused by acidic solution. The main advantage of this method is the fact that the process can take place at ambient pressure, thereby giving favorable capital and operating costs.

In this paper, acidic hydrolysis of PET beverage bottles was carried out in a stirred reactor, and the influence of different parameters such as temperature, acid concentration and reaction time on PET hydrolysis has been investigated.

Experimental Works

Materials

Post-consume PET from soft drink bottles were crashed in the form of 8×8 mm2 small film pieces. Commercial sulfuric acid (98% purity), potassium hydroxide (KOH), and distilled water were used in this experimental work.

1Moghbeli et al.: Wet Hydrolysis of Waste PET Resin with Sulfuric Acid

Published by The Berkeley Electronic Press, 2010

Figure 1: Schematic of experimental setup of PET hydrolysis reactor and torque monitoring

Hydrolysis Process

Experiments were carried out in a cylindrical glass reactor (6 cm diameter and 9 cm height). The reactor was equipped with Heidolph Overhead Stirrer and a nitrogen inlet. Mixing fluid characteristics has a significant influence on the selection of mixing device. Because of varying viscosity in the mixing reaction the anchor stirrer blade made of Teflon with 3 cm width and 4.5 cm height was selected and the rotational speed was 100 rpm. Figure 1 shows the schematic of reaction setup for PET hydrolysis. At the beginning of the experiment 20 g of PET pieces were added to 4 ml distilled water and various amount of sulfuric acid (98% purity): 15, 30, 37.5, 45, and 52.5 ml. The reaction media was highly exothermic so the water is first loaded to the reactor, and hereafter the acid was added. The experiments were carried out at 40, 60, 70, 80, 90 oC. After elapsed de-polymerization reaction time (depolymerization time is referred to the period after the process is completed) the brown highly viscous product was diluted with 26 ml distilled water, and the mixture was subjected to good agitation. Terephthalate acid (TPA) is a combustible white powder, insoluble in water, soluble in alkalies, with density 1.51 (g/cm3) at 20 oC and sublimes above 300°C. Here, the sediment TPA was separated from the solution phase by filtration. The filtrate was then washed with distilled water three times. After washing, in order to increase the pH of the sediment from 6 to 11, eighteen grams of KOH (85%) was added to the sediment in a beaker. The solution was then filtered, and the

2 International Journal of Chemical Reactor Engineering Vol. 8 [2010], Article A150

http://www.bepress.com/ijcre/vol8/A150

acidity of the filtrate was increased by adding 18 ml of sulfuric acid. TPA was precipitated from new acidic solution and subsequently filtered. The white precipitated TPA on filter media was dried at 105 oC in a vacuum oven.

Characterization

The most important characterization of hydrolysis product includes crystalline white powder, burns easily and poisonous. In order to study the existence of TPA, after doing reaction on samples, the FT-IR test (by Thermo Nicolet Nexus 870) was carried out.

Cary® 100 UV-Vis spectrophotometer (Varian Inc.) was applied to measure the TPA content in the sediment product prepared from PET acid hydrolysis. For this purpose, the NaOH solution (2 N) was added to the sediment powder in order to prepare 10 ppm aqueous solution. The amount of absorbance peak at wave length 240 nm was the criterion to measure the TPA content in the product. A calibration curve was prepared based on the standard solutions of neat TPA with various concentrations (0.05, 0.1 and 0.15 gram TPA in 10 cc caustic soda 2 N). The following calibration correlation has been obtained:

Y= 0.22X + 1.793 (1)

Where Y and X are the absorbance value with arbitrary unit and TPA concentration respectively. Hydrodynamics of High Viscous Liquid Mixing

In order to develop low energy consumption and high efficiency stirrer, the hydrodynamics of mixing in acidic hydrolysis of PET was simulated by CFD techniques. The hydrolysis reaction takes place in a stirred reactor equipped with an anchor mixer.

CFD Simulation

The main issue when simulating stirred vessels is that the problem geometry varies with time due to the relative motion between the impeller and the baffles. Several methods have been used in the literature to treat that kind of problems, namely the IBC, the MRF and the SG methods (Gentric et al. 2005). For the relative velocity formulation of MRF formulation, the governing equations of fluid flow for a steadily rotating frame can be written as follows (Ranade, 2005):



Conservation of mass:

( ) 0utρ

ρ∂

+∇ ⋅ =∂

(2)

Conservation of momentum:

( ) ( ) ( )2r r r r ru u u u r p Ftρ ρ ρ ω ω ω τ

∂+∇⋅ + × + × × = −∇ +∇ +

∂ (3)

The momentum equation contains two additional acceleration terms: the Coriolis acceleration, and the centripetal acceleration.

Depending on the type of flow conditions (i.e. laminar/turbulent, steady state/transient) then appropriate boundary conditions can be defined and implemented. For example, in the case of steady state, the time dependency of variables such as velocity is set to zero. As for the boundary condition, the imaginary cylindrical wall which is assumed around the impeller is considered to be moving with the impeller being stationary (named Multi Reference Frames, MRF method). The reactor walls are subjected to no slip condition and constant temperature. The flow regime in this simulation is laminar because of high viscosity of mixing fluid. The viscosity of fluid during the reaction has been assumed constant. Three-dimensional laminar flow and mixing predictions have been carried out using MRF method. A tetrahedral mesh is used for the geometry and unstructured (or irregular) grid size has been employed, with cell sizes comparable to those of the reactor, giving a total cell number of 358,800. The governing equations for continuity and momentum, Eqs. 2 and 3, have been solved by finite volume technique (Patankar 1980, Versteeg and Malasekera 1995). The steady state simulation was carried out by using in house CFD code.

Results and Discussion

High Efficiency Mixing

Figures 2A and 2B show the influence of rotational speed on velocity magnitude contour. The minimum mixing occurs near the wall, but an approximately homogenous flow can be found in central core. Figures 3A and 3B illustrate the axial and tangential velocity, respectively.



Figure 2: Velocity magnitude contour at z=2 cm (A: N=100 rpm & B: N=120 rpm)

(A)

(B)



Figure 3: Effect of impeller rotational speed on velocity at z=4.5 cm (A: Axial velocity & B: Tangential velocity)

(B)

(A)



Inspection of the results reveals that near the wall flow goes down (negative velocity) and in the central core the flow direction is opposite. Increasing of rotational speed enhances the axial and tangential velocities in all regions. In order to investigate the optimum impeller velocity, a mixing index was defined. This index is the value of volume average velocity over the tank. The volume average velocity is calculated by dividing the summation of the product of the velocity and cell volume by the total volume of the cells. As this value is closer to maximum velocity, it will show that a better mixing is occurred. Maximum velocity is simply worked out from speed of the agitator but the volume average velocity is computed from the averaging of the simulated whole flow field. In the opinion of the authors the criteria employed here, i.e. the difference of the two velocities, is the best way of identifying the mixing with more hydrodynamic homogeneity. Also the energy consumption through the fluid mixing is important. As it is known, the fluid kinetic energy is proportional to consumed mixing power. Table 1 shows the results of a comparison between the average and maximum velocities as well as the produced kinetic energy. From the results, it can be concluded that the impeller velocity of 100 rpm has the minimum difference (i.e. 71.30%) between the maximum and average velocities, with more favorable power as typified by the extent of the prevailing kinetic energy, as compared to rotational impeller speed of 120 rpm. The optimum rotational speed creates a more homogeneous flow regime inside the tank and thereby providing a better mixing within the system.

Table 1: Comparison between the average and maximum velocity.

N (rpm*) umax (m/s) uave (m/s) max

max

100aveu uu−

× Kinetic Energy

(kJ/m3)

60 0.14 0.036 74.29 719

80 0.18 0.048 73.33 1278

100 0.23 0.066 71.30 2003

120 0.28 0.07 75 2884

* Round per minute



Figure 4: FT-IR spectra of the end-use PET

Experimental Results

Figure 4 shows the FT-IR (Fourier Transform Infrared Spectroscopy) spectra of waste PET before the acidic hydrolysis reaction. As it is shown, the chemical structure of PET in the wavelength between 3300 and 3700 cm-1, appearing the OH group, is clear. The wavelength between 3000 and 3100 cm-1 shows an aromatic compounds CH stretch (Donald et al. 2008). The wavelength between 2800 and 3000 cm-1 indicates the presence of hydrocarbon (hydrogen and carbon). In the wavelength range between 1600 and 1700 cm-1 the carbonyl group was demonstrated. In the wavelength between 600 and 800 cm-1 the aromatic ring was shown. Figures 5A and 5B show the FT-IR spectra of the white produced powder and pure TPA, respectively. The powder has been created using 45 ml of sulfuric acid at reaction temperature of 90 oC and reaction time of 5 hours. The spectra for produced powder and neat TPA show good similarity. From the spectra of produced powder, the wave length between 1600 and 1700 cm-1 shows the presence of carbonyl group and the wave length between 1000 and 1600 cm-1 shows the presence of aromatic ring, all of which confirming the presence of TPA in the final precipitated powder.



(B)

Figure 5: A) Typical FT-IR spectra of produced sediment powder from waste PET hydrolysis, B) FT-IR spectra of neat TPA prepared from Merck Co.

(A)



The Effect of Acid Concentration

The effect of sulfuric acid concentration on TPA yield at different temperatures is presented in Table 2. The displayed results highlight that if sulfuric acid quantity is in 30cc - 45cc range for 20g of PET, then TPA will be produced. The rate of hydrolysis of PET at 40 oC increases from 22.5% to 25% when the volume of acid was changed from 30 to 45 ml at the reaction time of one hour. The results show that by increasing the reaction time and acid concentration, the performance of TPA production through the hydrolysis of de-polymerization reaction, as expected, increases. Statistical analysis of tabulated data reveals that for a given temperature changing sulfuric acid concentration from 30 to 45 cc, depending on the fixed value of reaction time, the increase in TPA yield can, for the conditions studied, be between 1.2 and 10%. This increase in TPA yield is also similar when sulfuric acid concentration is kept fixed but the reaction temperature increased, as evidenced in Figures 6 & 7 (in next section).

Table 2: Effects of sulfuric acid concentration and temperature on TPA yield

TPA Yield (%) Sulfuric Acid (ml)

Tem

pera

ture

° C

Reaction time (hour)

15

10 5

3

1

34 32.4 29.9 27.2 22.5 30.0 40 35 32.8 30.5 28.5 23.3 37.5

35.7 33.8 32 0. 29 0. 25.0 45.0 53 0. 49.8 47.6 44.6 41.2 30.0

60 53.4 50 0. 48 0. 45 0. 42.5 37.5 53.9 52.2 49 0. 45.5 43.1 45.0 70.2 67.3 63.2 58.3 53.2 30.0

80 70.4 68.1 64.5 59.8 54.5 37.5 71.1 68.9 66.2 61.2 55.9 45.0 92.5 90.7 85.1 77.2 70.8 30.0

90 92.8 91.1 86.3 78.9 73.4 37.5 94.8 91.8 88.0 79.9 76.5 45.0



The Effects of Temperature on Reaction Yield

The effect of temperature on the yield of TPA is shown in Figure 6. Five different reaction temperatures were examined in the range of 40 - 90 °C. A significant increase in PET degradation with increasing reaction temperature was observed. The experimental results show that at a constant reaction time, while the amount of sulfuric acid increases the effect of temperature on TPA yield is pronounced. The results depict that at a constant amount of sulfuric acid increasing the reaction time caused the effect of temperature on the yield to increase. Furthermore, the reaction time of 15 h resulted in the maximum yield but by increasing the reaction time from 5 to 15 hours, the efficiency of TPA production is not considerable.

Figure 6: Effect of temperature on the A yieldPT using 45 ml sulfuric acid.

The Effect of Reaction Time

The effect of reaction time on the yield of TPA production is shown in Figure 7. Five different reaction times were examined (in the range of 1-15 hours). The results demonstrate that by increasing reaction time from 1 hour to 5 hours, the efficiency of TPA product will increase but by promoting the reaction time from 5 to 15 hours, the TPA production efficiency growth is not considerable.



Figure 7: The effect of reaction time on TPA yield using 45 ml sulfuric acid

The results also demonstrate that for a given temperature increasing the amount of

sulfuric acid and reaction time enhances the yield of the TPA. Moreover, the reaction temperature of 90 °C had the maximum yield when compared to other reaction temperatures.

Conclusion

Acidic hydrolysis of PET beverage bottles has been studied in a stirred reactor enabling the influence of different parameters such as temperature, acid concentration and reaction time on PET hydrolysis to be investigated. The experimental results highlighted that the optimum reaction condition can occur when the acid to PET ratio is 4:1 at a reaction temperature of 90 oC and reaction time of 5 hours. It has been established that the influence of temperature on the acidic hydrolysis of waste PET is more significant than other degradation process parameters. The results revealed that by increasing the reaction time from 1 hour to 5 hours, the efficiency of TPA product increases proportionally but by increasing the reaction time from 5 to 15 hours, the TPA production efficiency growth is not considerable.



Notation

F External Force in Eq. (3)

N Round per minute (rpm)

r r-coordinate

p Pressure

t Time

T Temperature

u Velocity

X TPA concentration

Y Absorbance value

z Height of Reactor

Greeks

ρ Density

τ Stress

ω Angular Velocity

Subscripts

ave Average

max Maximum

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Wet Hydrolysis of Waste PET Resin with Sulfuric Acid