(145e) Performances of Advanced Structured Packings Produced By 3D Foam Printing for Gas-Liquid Multiphase Contactors in FGD Process Intensification

Multiphase contactors are widespread in chemical engineering processes and applied for the synthesis and purification of compounds for petrochemical, food, chemical and pharmaceutical industries, and many others targeted applications, e.g. flue-gas cleaning. Packed-bed towers are largely adopted for the process intensification of gas-liquid and liquid-liquid processes, thanks to their capacity to promote interfacial mass transfer rate. Moreover, they allow exploiting the liquid-phase chemical reactions at their highest level, and a reasonable pressure drop costs. Over the last 50 years, several studies^1-3 investigated the performance of different types of random and structured packings for different applications, correlating their performances with the geometric characteristics of commercial packings provided by major vendors, e.g. Sulzer, Koch Glitch, Raschig, etc.

In the last decade, 3D printing technology has increasingly spread in industrial applications to substitute conventional manufacturing processes and to allow faster cheaper, and in-situ production of commercial products⁴. Recently, this technology has been adopted to produce several process equipment, such as gas and liquid distributors, mixers, catalytic monoliths and packings for packed towers^5,6. For packings, the main benefit of 3D printing is the possibility of producing plastic items in an extremely flexible manner, to meet different application needs and allow a process intensification, thanks to an easier tailor-made design for specific plant size and operating conditions. Besides, the wide range of 3D printing materials and the possibility to perform different types of printing add further flexibility to the production of packings for chemical reactors and separators.

In this paper, we report an extensive study on the performance of a Y-type corrugated packing produced by 3D foam printing. The packing was tested in a desulfurization chemical absorption tower (Wet-FGD) chosen as a model liquid-gas multiphase reactor due to its key role in clean coal technologies, for the reduction of sulfur compounds emissions. Experiments aimed at evaluating the pressure drops, mass transfer rates (i.e. the liquid and gas side mass transfer coefficients) and the SO₂ removal efficiencies of a pilot-scale scrubber equipped with the new packing operated in the typical conditions of wet-FGD processes for coal-combustion plants. The experimental results were compared with those of a commercial Y-series metallic packing, Mellapak 250Y (M250.Y) by Sulzer Chemtech. The pressure drops for Mellapak 250Y were recovered by the experiments of Brunazzi and Paglianti⁷ who tested the M250.Y in both dry and wet-mode using an air/water system at 25°C and 1 atm. The mass transfer coefficients were calculated with the Brunazzi and Paglianti³ model which is based on an experimental dataset on absorption columns equipped with M250.Y and Sulzer BX. Finally, the M250.Y desulfurization efficiencies were calculated by process simulations in ASPEN PLUS operating under the same experimental conditions of the present work. The simulations follow the same approach proposed in Flagiello et al.⁸: a rate-based block was used to simulate a counter-current flow absorption column, then a hydrodynamic and fluid dynamic model was implemented based on the data provided by Brunazzi and Paglianti^3,7. Finally, the equilibrium model of Flagiello et al.⁸ for SO₂ solubility in water was used.

Methodology

The experiments were performed in a fully instrumented pilot-scale scrubber⁸ equipped with a DN100 column and filled with 892 mm (Z) of structured packing, designed via CAD and manufactured using 3D foam printing technology. The column operates in counter-current flow with the gas stream fed at the bottom. The packing prototype was designed based on the typical configuration of corrugated sheets, which form the classical packing structure. All geometric parameters were fixed trying to obtain a nominal area (a_n = 240 m²/m³ without enhancement due to the printing technology and foaming) and void fraction (ε_p = 0.96) as close possible to that of the benchmark (Mellapak 250Y: a_n = 250 m²/m³ and ε_p = 0.97). The experimental activity was divided into three sets of experiments, detailed below.

- Set A consists in measuring of specific pressure drops in dry (ΔP_d/Z) and wet (ΔP_w/Z) mode of the prototype using an air-water system at 25 °C. Dry mode tests were performed by varying the airflow, in the range 5 - 160 m³/h (gas load factor F_G = 0.20 - 6 Pa^0.5). Wet mode tests were carried in the same gas conditions with water flow rates variable between 40 and 160 L/h (liquid load factor F_L = 5.10 - 20.38 m/h).

- Set B consists in experimental tests to measure the contribution of purely physical mass-transfer coefficients of both gas and liquid-side (k_G,SO2a_e and k_L,SO2a_e) provided by prototype. To this end, we performed SO₂ absorption experiments either with NaOH aqueous solutions (0.05 M) and distilled water acidified with HCl (0.1 M)⁶. Experimental tests (25 °C and 1 atm) with NaOH aqueous solution have been used to estimate k_G,SO2a_e coefficient while those with HCl aqueous solution allows determination of k_L,SO2a_e coefficient.

- Set C consists in the FGD experiments from model flue gas at 32 m³/h (F_G =1.25 Pa^0.5) constant flow rate with either 500 or 1000 ppm_v SO₂ concentration, as a reference case, in order to evaluate the separation efficiency of the prototype. The liquid stream was an alkaline tap water (collected in the North-West area of Naples) at pH 7.6 and its flow rate was varied from 40 to 160 L/h (F_L = 5.10 - 20.38 m/h).

Selected Results

The experiments showed that the pressure drop (Set A) of the 3D foam printed packing increases with the gas load at similar rates as compared to the commercial Mellapak 250Y. These results were mainly due to the similar values of surface area, void fraction and hydraulic diameter. Despite the prototype channels were narrower, both the packings showed similar flooding limit and could operate up to pressure drops in the range of 6.0 - 6.5 mbar/m, hence covering gas loads between 2.8 - 3.8 Pa^0.5 corresponding to the liquid loads investigated.

The desulfurization tests (Set C) revealed very promising performances: in fact, for 500 ppm_v of inlet SO₂ concentration tested, at a high liquid load (above the F_L = 16 m/h), the separation efficiency (> 95%) was similar to the figure retrieved for the Mellapak 250Y from ASPEN PLUS simulation. For the lowest liquid loads, a marked improvement of the separation efficiency for the 3D printed packing was observed especially for the experiments at F_L = 5.10 m/h, where efficiency of about 28% more was achieved. Moreover, a 42% increase was obtained at 1000 ppm_v of inlet SO₂. This mirrors a faster SO₂ absorption rate that is mostly related to the liquid phase mass-transfer coefficient: in fact, the mass-transfer characterization (Set B) showed higher transfer rates provided by the packing prototype, i.e. for k_G,SO2a_e up to 10% more and k_L,SO2a_e more than 20 - 45%. This improvement likely depends on the greater specific surface transfer area generated by the surface foam packing with an enhancement of approximately 30% but also by improved wettability of the packing plastic foamed surface as assessed by dedicated tests to measure the liquid-solid contact angle for the prototype. Another relevant point is the optimization of the packing elements including the baffle rings and of its specific mechanical design, which allows improving the liquid-film dispersion reducing the liquid maldistribution on column walls and between packing elements.

Key Conclusions

The use of 3D foamed printed packings allows an intensification of desulfurization processes by exploiting specific surface properties, also assuring same pressure drops. In general, they enable higher degrees of flexibility and customization in gas-liquid multiphase reactors and separators.

Literature Cited

1. Billet, R., and M. Schultes, “Predicting mass transfer in packed columns,” Chem. Eng. Technol., 16, pp. 1–9, (1993).
2. Olujić, Ž., et al., “Predicting the efficiency of corrugated sheet structured packings with large specific surface area,” Chem. Biochem. Eng. Q., 18, pp. 89–96 (2004).
3. E. Brunazzi, and A. Paglianti, “Liquid-film mass-transfer coefficient in a column equipped with structured packings,” Ind. Eng. Chem. Res., 36(9), pp. 3792–3799 (1997a).
4. Kalsoom, U., et al.,“Current and future impact of 3D printing on the separation sciences,” Trends in Analyt Chem., 105, pp. 495–510 (2018).
5. Miramontes, E., et al.,“Additively manufactured packed bed device for process intensification of CO₂ absorption and other chemical processes,” Chem. Eng. J., 388, pp. 124092 (2020).
6. Parra-Cabrera, C., et al.,“3D printing in chemical engineering and catalytic technology: Structured catalysts, mixers, and reactors,” Chem. Soc. Rev., 47, pp. 209–230 (2018).
7. E. Brunazzi, and A. Paglianti, “Mechanistic pressure drop model for columns containing structured packings,” AIChE journal, 43(2), pp. 317–327 (1997b).
8. Flagiello, D., et al.,“Characterization of mass transfer coefficients and pressure drops for packed towers with Mellapak 250.X,” Chem. Eng. Res. Des., 161, pp. 340–356 (2020).