On the Effect of Axial Dispersion in the Numerical Simulation of Fast Cycling Adsorption Processes

Abstract

Kesan serakan paksi dalam penyelakuan perangkaan proses penjerapan berkitar pantas telah dikaji. Objektif kajian ini adalah untuk (1) mengkaji semula cerapan Alpay [1] dalam kesan serakan paksi ke atas ketulenan oksigen dalam turus yang panjangnya 1.0 m, dan (2) mengkaji secara perangkaan kesan serakan paksi ke atas ketulenan oksigen dan kadar aliran suapan purata–kitar bagi proses jerapan buaian tekanan ultracepat (URPSA) dalam turus yang panjangnya 0.2 m. Daya pacu linear telah digunakan untuk memodel pemindahan jisim di antara partikel bahan penjerap. Penyelakuan perangkaan telah dijalankan dengan menurunkan persamaan kebezaan separa (PDEs) ke dalam persamaan kebezaan biasa (ODEs) dengan menggunakan kaedah ortogon penempatan bersama (OC). Sistem ODEs itu kemudian dikamil dengan menggunakan subrutin daripada Perpustakaan IMSL FORTRAN, di mana subrutin ini adalah sesuai untuk kamiran ODEs kaku. Untuk keadaan eksperimen Alpay, keputusan penyelakuan yang tidak fizikal telah dicerap untuk serakan paksi yang dijangka oleh kolerasi Langer. Walau bagaimanapun, tiada cerapan sebegini ditemui untuk proses URPSA. Keputusan penyelakuan kami telah menunjukkan satu penurunan sebanyak 10 peratus apabila nilai serakan paksi bertambah. Serakan paksi didapati tiada kesan ke atas kadar aliran suapan purata–kitar. Kerja masa depan adalah diperlukan untuk mencipta strategi yang sesuai untuk mengatasi kesulitan perangkaan yang telah ditemui. Kata kunci: Proses jerapan buaian tekanan, permodelan, penyelakuan perangkaan, permisahan udara, serakan paksi The effects of axial dispersion in the numerical simulation of fast cycling adsorption processes were studied. The main objectives are: (1) to re–examine the observation of Alpay [1] on the effect of axial dispersion on the oxygen purity for a 1.0 m column, and (2) to investigate numerically the effect of axial dispersion on the oxygen purity and cycle–averaged feed flow rale of an ultra rapid pressure swing adsorption (URPSA) process, which employed a column of 0.2 m in length. The linear driving force (LDF) model was employed to model the mass transfer within the adsorbent particles. Numerical simulation was carried out by discretising the partial differential equations (PDEs) in the space domain to a system of ordinary differential equations (ODEs), using the method of orthogonal collocation (OC). The ODEs were then integrated using a subroutine from IMSL FORTRAN Library, which is suitable for the integration of stiff ODEs. For Alpay’s experimental conditions, unphysical numerical results were observed for an axial dispersion coefficient predicted by Langer’s correlation. However, no such numerical difficulties were encountered for an URPSA process. Our numerical simulations showed a reduction of up to 10 percentage points when values of axial dispersions were increased. The axial dispersion was found to have no effect on the cycle–averaged feed gas rate. Future works are deemed necessary to device suitable strategy to overcome the numerical difficulties encountered here. Key words: Rapid pressure swing adsorption, modelling, numerical simulation, air separation, axial dispersion