Online Detection of Clear Point in Esterification Process of Polyester Production Based on Raman Spectroscopy

FT-NIR Spectrometer,fourier transform spectrometer,ftir spectrometer,ftir analysis,Fourier Transform Infrared Spectroscopy,spectral analysis,quantitative analysis,qualitative analysis,diffuse reflection,specular reflection,integrating sphere,FT-IR Spectrometer-Advanced Manufacturer,FT-NIR Spectrometers；ftir spectrometer,Optosky is a manufacturer of advanced FT-IR spectrometers, with products in stock and at affordable prices.,lntegrating Sphere,integrating sphere,specular reflection,diffuse reflection,qualitative analysis,quantitative analysis,spectral analysis,Fourier Transform Infrared Spectroscopy,ftir analysis,ftir spectrometer,fourier transform spectrometer,FT-IR Spectrometer-Advanced Manufacturer,FT-NIR Spectrometers；ftir spectrometer,Optosky is a manufacturer of advanced FT-IR spectrometers, with products in stock and at affordable prices.

author: Christ

2022-01-06

Online Detection of Clear Point in Esterification Process of Polyester Production Based on Raman Spectroscopy

Current manual sampling and analysis methods,such as refractive index measurement, chromatography,etc. ,have complex operations，time lags and human errors.Therefore,they can- not be used to evaluate the esterification process in polyester production in real time and determine the clear point of the reaction.In this article,an online Raman analysis system with 1 064 nm as the la- ser wavelength was set up to collect continuously the spectra of the esterification reaction between te- rephthalic acid( PTA) and 1,3-propanediol( PDO) to create poly( tri-methylene terephthalate) ( PTT) ,while using Savitzky － Golay algorithm to make baseline correction on the collected spectra.The princi- pal component analysis was used to extract the spectral information.Based on the principle of esterifi- cation process,a qualitative analysis method was established to determine the clear point by using the area ratio for Raman characteristic peaks of benzene-loop located near 1 604 cm - 1 and ester group lo- cated at 1 720 cm - 1.Results showed that the developed Ｒaman spectroscopy could accurately reflect the clear point in the esterification process of polyester prodction，With the advantages of good real- time performance，simple operation and rapid analysis,it provides a reference for the process control and product quality control of polyester production.
At present, in the esterification reaction process, the parameters that can be directly measured are temperature, pressure, flow, liquid level and other operational variables, while the quality indexes that can reflect the reaction degree, such as acid value, saponification value, esterification rate, etc., cannot be directly measured online.  These parameter values are usually obtained by physical or chemical analysis methods, such as refractive index method, gas chromatography-mass spectrometry or liquid chromatography.  However, these methods inevitably have time lag and human error, which cannot reveal the changes of molecular structure in the process of polyester reaction so as to evaluate the reaction process in real time.  Compared with the above methods, Raman spectroscopy has the advantages of non-contact non-destructive measurement, wide detection range and fast analysis speed, and is widely used in petrochemical, chemistry, biomedicine, materials, environmental protection, food safety and other fields.
Principle
In the esterification reaction process, the parameters that can be directly measured are temperature, pressure, flow, liquid level and other operational variables, while the quality indexes that can reflect the reaction degree,
such as acid value, saponification value, esterification rate, etc., cannot be directly measured online.  These parameter values are usually obtained by physical or chemical analysis methods, such as refractive index method, gas chromatography-mass spectrometry or liquid chromatography.
However, these methods inevitably have time lag and human error, which cannot reveal the changes of molecular structure in the process of polyester reaction so as to evaluate the reaction process in real time.  Compared with the above methods, Raman spectroscopy has the advantages of non-contact non-destructive measurement, wide detection range and fast analysis speed, and is widely used in petrochemical, chemistry, biomedicine, materials, environmental protection, food safety and other fields.

Raman spectroscopy solutions
In order to acquire Raman spectra of esterification reaction process, a miniature intermittent esterification reaction device was set up.

Fig. 1 Structure of esterification device

Online Raman detection system is composed of laser, excitation fiber, Raman probe, sampling pool, collecting fiber, spectrometer, computer and other parts. The system structure is shown in Figure 2. Laser as a light source, laser is transmitted to the Raman probe by the excited fiber, and then the Raman probe focuses on the material, and the Raman scattering light generated by the Raman probe is collected, and then through the collection of optical fiber back to the spectrometer; After the Raman scattering light is separated by spectrograph grating, detected by In-GAAS array and converted by A/D, the digital signal of the spectrum is transmitted to the computer system to obtain the Raman spectrum.

Fig. 2 Structure of online Raman detection system

In order to weaken the influence of fluorescence background light, Raman spectroscopy was used to detect the fluorescence at 1064nm. Select ATR3000-1064 Dispersive InGaAs array fiber Raman spectrometer (spectral coverage range is 200 ~ 2 000 cm-1, OPTOSKY). The spectrometer uses grating as a spectral element to output Raman scattering light to the detection surface. The InGaAs array detector receives the light intensity of each wavelength at the same time, and the Raman spectrum is obtained. Because the array detector has lower dark noise and higher quantum efficiency, it has high detection speed, high sensitivity and better signal-to-noise ratio. The laser is a semiconductor laser (central wavelength is 1 064 nm) with stable laser power. The Raman probe is customized with 1 064 nm high temperature resistant Raman probe (Rayleigh scattering cutoff depth is OD6, spectral working range is 200 ~ 2800 cm-1, probe working distance is 7.5 mm). The probe integrates laser emission and collection, and can filter some stray light and Rayleigh scattered light. The diameter of both excitation and collection fibers is 105 μm.
Test Results
The 1064 nm Online Raman detection system was used to collect the Raman spectra of the esterification process. In order to filter the spectral noise, suppress the fluorescence interference and eliminate the influence of other factors on the spectral information, the original spectrum was preprocessed.
In order to quantitatively describe the clear point of esterification reaction, curve fitting was carried out for the calculated area ratio and the change rate of the trend curve was calculated, as shown in FIG. 8B. When the change rate drops from positive to 0 and tends to be stable, the inflection point of the trend curve of the change rate is considered as the clear reaction point. Therefore, the time to reach the clear point can be estimated according to the change rate of the area ratio of aromatic ester peak and benzene ring peak, so as to judge the esterification reaction, providing a reliable theoretical basis for process control.

Fig. 3 Original spectrum of reaction mixture in esterification process

Fig. 4 Schematic for original spectrum of reaction

Fig.5 Raman spectrum of reaction mixture after baseline correction

Fig6. Scatter diagram of principle component analysis

Fig7. Trend of characteristic peak height of benzene-loop and ester group with reaction time

Fig 8. Trend of the area radio of ester group and benzene-loop over time(A) and its change rate(B)

Conclusions:
The direct esterification of PTT from PTA and PDO was monitored by on-line Raman analysis system. In order to weaken the fluorescence background of the reaction mixture, a self-developed 1 064 nm Raman detection system was used for spectrum acquisition. Fluorescence and other noise were further removed by baseline correction algorithm. Combined with the variation trend of characteristic peaks of benzene ring and aromatic ester groups over time, a qualitative analysis method was established by using the area ratio of benzene ring and aromatic ester peaks to determine the clear point of esterification process and measure the reaction degree. The analysis results are consistent with the experimental phenomenon.
The results show that Raman spectroscopy can reflect the clear point of reaction in real time. Compared with the existing manual sampling analysis method, it has the advantages of good real-time performance, convenient operation and fast analysis, and provides a new detection and analysis method for esterification process control.