Original Research PaperA systematic framework to monitor mulling processes using Near Infrared spectroscopy
Graphical abstract
Introduction
Inorganic metal-oxide supports offer unique properties for the immobilization of metal complexes, such as mechanical strength, surface area and porosity [1]. Alumina is one such inorganic support which has been widely used for several decades in the catalyst industry with applications in the petroleum [2], petrochemical [3], pharmaceutical [4] industries and biofuels [5]. The presence of the alumina support [5] and even the structure of alumina can significantly affect the performance of catalysts, as indicated by Luo et al. [3].
Catalysts supported on alumina, which are synthesized by ammonia precipitation, exhibit higher activity and selectivity to isobutene than the corresponding catalyst synthesized from hydrochloric acid reflux. Due to its importance, there is a large demand for alumina supports in the catalyst industry. However, the large-scale manufacturing of extrudable alumina with desired particle and pore size distribution, and surface area is still a challenging task in the current catalyst production scenario due to difficulties in handling powders.
Usually, the production of alumina is processed in three steps: mulling, extrusion and calcination. Mulling is similar to granulation but is applied in catalyst manufacturing and incorporates wetting, mixing, primary particle size reduction and particle shaping, all within a single unit operation. It improves flowability, reduces dustiness and increases bulk density. Moreover, several highly favored properties including high porosity and large surface area, which brings about faster reaction rates, are introduced by mulling. This effect is achieved by decreasing primary particle size with the help of peptizing agents and high shearing action. A well-mulled paste after undergoing extrusion, to gain the desired shape, is subsequently calcined. The process of calcination removes any residual moisture and mainly volatile fractions leaving behind pores in these alumina supports that allow for the impregnation of catalyst. A set of process variables, namely temperature, moisture content, pH, impeller speed and chopper speed, are known to affect the quality of the final product [6], [7], [8]. Therefore, it is a challenging process to optimize and scale up based on the process variables alone. Extrusion is a commonly used shaping process in many industries, as well as in the catalyst industry, since extruded catalyst supports are much easier to handle and recycle. The main condition that the extruded materials have to fulfill is having sufficient plasticity [9]. Alumina is a non-plastic material and, therefore, extrusion of alumina pastes requires processing additives such as binder and lubricant agents to impart plasticity and flow characteristics, which is another reason for the need of mulling in catalyst support production. In the mulling process, water, with the possibility of a peptizing agent such as nitric acid, can be used as the binder, wherein the binder to solids ratio has significant effect on the plasticity. This makes the monitoring of water/moisture content extremely important. Much less attention however has been paid to the monitoring of this moisture content.
Generally, batch processing is a commonly used method since the machinery for batch production is already installed and is more flexible, in terms of both machine application and productivity adjustment. However, continuous processing is more popular since it helps with stabilization of product quality as well as reduction of manufacturing time and cost of goods. It has been well established in the chemical, cosmetics and food industry [10], [11], [12]. For continuous process monitoring, non-destructive monitoring techniques are generally used, especially vibrational spectroscopic methods such as Near Infrared (NIR) and Raman, which are combined with multivariate calibration routines for use as in-line process analyzers [13], [14], [15]. These monitoring techniques offer several advantages over conventional wet chemistry techniques including non-invasiveness, little or no sample preparation and rapid measurements [16], [17]. Recent examples where process monitoring has been utilized include raw material dispensing, chemical reactions, granulation, drying and powder blending [18], [19], [20], [21], [22], [23], [24]. Similarly, a PAT (process analytical technology) based monitoring is illustrated from an industrial perspective by Chandra et al. [25]. Moreover, several works also incorporate NIR for feedback process control [26], [27]. All of these applications show unique advantages and feasibilities of using vibrational spectroscopy as real time PAT tools.
The –OH stretching vibration is a very strong absorption band in the NIR region and is thereby used for water content measurement, resulting in the successful use of NIR in food and chemical manufacturing [15], [28], [29], [30]. Analysis of samples in the form of solids, liquids and pastes are simpler using NIR as sample preparation is minimal. Since moisture content plays a key role in the mulling process, this paper discusses the use of NIR as a suitable PAT tool in process monitoring. During the mulling process, material characteristics such as particle size distribution and powder surface properties such as smoothness and roughness could slightly vary. The effect of these variations on the moisture content measurement is still an open area of research. Accuracy and precision of measurement also significantly depends on the location and installment of the sensor into the manufacturing plant. Therefore, prior to sensor integration, optimal conditions such as sensing location distance between sensor and sample, characteristics of sensing interface, and data acquisition specifications need to be identified. However, because of the unavailability of a systematic method, the identification of the optimal sensor conditions is still based on a heuristic approach.
Critical operation parameters of the NIR spectrometer such as scan number, acquisition time, distance between sample and detector, sample thickness and sensor locations in order to obtain accurate measurements have been identified in this study. The effect of particle size and water content of the particles on NIR measurements have also been investigated since they change the physical properties of material surface which would impact the NIR absorption. The methodology discussed is generic and can be used for any combination of process and PAT tools.
Section snippets
Materials and equipment
A specific grade of alumina, CATAPAL boehmite alumina, was used in this study, which was manufactured by Sasol, and the distilled water was generated by a Millipore Milli-Q Water Purifier.
A Design of Experiment (DOE) analysis was performed using the Minitab 16 statistical software. For the calibration model, the alumina and water were mixed in a vortex mixer from VWR International. The batch mulling experiments were carried out in a KG-5 high shear granulator from Key International, Inc. and
Pre-treatment of raw spectra data
The plots of the raw spectral data collected by varying the working distance between sample and spectrometer, are given in Fig. 5. In the NIR region, the –OH band is usually observed clearly at 1895 nm (–OH stretch free water) and 1428 nm (OH 1st overtone) [31]. Based on this, a wavelength range from 1399 to 1493 nm was selected for water content analysis. As shown in this figure, the absorbance value decreases as the distance between sample and detector increases. Also, it can be seen in the
Conclusions
The possibility of application of near infrared spectroscopy, a technique used for substance discrimination and quantification based on perpetual molecular motion, on batch mulling is discussed. To achieve the above goals, adequate calibration models are necessary. Many models for calculating liquid content at different conditions were created. All the liquid content models passed the validation test and have higher R-square and low RMSE values, which indicate adequate and reliable models.
The
Acknowledgements
This work is supported by Rutgers Catalyst Manufacturing Science and Engineering Consortium. The authors would like to acknowledge SASOL for the donation of their CATAPAL B alumina. The authors would also like to acknowledge Krizia M. Karry for useful discussions and Yinmingzi Liu, Aditya Donepudi and Meera Patel for their generous help in performing experiments.
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