In situ measurement of hindered settling function in decanter centrifuge by periodic segmentation technique in wireless electrical resistance detector (psWERD)

Abstract

In situ measurement of hindered settling function Hϕ as a function of particle volume fraction ϕ in a decanter centrifuge has been proposed by periodic segmentation technique in wireless electrical resistance detector (psWERD). The processes to calculate Hϕ has three steps which are (1) segmentation of measured resistance R(z,t) in three settling periods; (2) elimination of the screw conveyor’s (SC’s) adverse effect; (3) calculation of ϕ which is important to determine the in situ hindered settling velocity uhinins and calculation of the proposed uhinfit from the proposed Hϕ. In the 1st step, R(z,t) are segmented based on the phases of liquid L, screw S, and particle P under three settling periods which are pre settling (pre), transient settling (tran), and post settling (post). In the 2nd step, the segmented R(z,t) is categorized by fourier transfrom (FT) based on the frequency of the liquid fL, screw fS, and liquid-particle fLP phases with a focus on eliminating SC’s adverse effect which causes fluctuating R(z,t). In the 3rd step, the ϕz,t is obtained by applying the effective medium theory (EMT) based on the conductivity of each phase which are liquid σ¯L, particle σP, and slurry σLPz,t. Each conductivity is calculated based on the average liquid resistance R¯Lz, the static particle resistance RP under closed-pack particle volume fraction ϕcp, and the slurry (liquid-particle) resistance RLPz,t from the 2nd step. Afterward, the uhinins is obtained from the distribution of ϕ which refers to the length parameter in the settling radius rset. Hence, the uhinins is calculated by time t needed for particle to settle from ϕ0 to ϕcp in tran. The proposed Hϕ is developed by considering the influence of ϕcp and the increasing distribution of ϕ due to the particle settling which is presented by fitting parameters of k and p. Take into account the importance of single particle settling in Stokes settling velocity uSt, the proposed uhinfit is determined by calculating the uSt and fitting parameters in the Hϕ to fit the uhinins. To evaluate the psWERD, a decanter centrifuge was used in the industrial scale experiment by appliying three operational parameters which are types of SC, centrifugal forces G, and the differential speeds ω. Two types of SC which are SC1 and SC2 were used to investigate the effect of G to the in situHϕ under constant ω. While SC2 was used to investigate the effect of ω to the insituHϕ under contant G. The in situ measurement is conducted in three measurement points z which represents the particle settling condition at different locations. In order to verify the experiment results, simulation on electrical measurement in the boundary of particle sensitivity area APSA of psWERD and on particle settling behavior in centrifugal fields are done by Multiphysic software. As a result, the psWERD is successful in determining the best fitting parameters for the in situHϕ with good average root mean square error 〈RMSE¯〉≤10-3. The in situHϕ also has a good agreement to the previous work of Hϕ determined in decanter centrifuges with the similar operational condition. Finally, the Hϕ shows the potential for evaluating the in situ separation process in decanter centrifuges under specific operational conditions.