Solid-liquid mass transfer to hollow pellets in a trickle bed

Abstract

Local, instantaneous liquid-to-solid mass-transfer coefficients to both the internal (inside a hole) and external surfaces of hollow, nonporous pellets are measured in a trickle-bed reactor by an electrochemical technique for various gas (N 2 ) and liquid (water) flow rates and hole sizes in both the trickle and pulsing flow regimes. This information is needed for predicting the volume efficiency of hollow pellets. In the trickle-flow regime, partial wetting causes large variations in the mass-transfer rates among pellets with identical holes in the same packed bed. In the pulsing regime, the gas-flow rate has a negligible impact on the mass-transfer rate, while the product of the Sherwood number and the fractional wetting, ΦSh, increases with increasing liquid flow rate and hole diameter. The mass transfer is not uniform on the pellet surface, and the ratio of internal to external mass-transfer coefficients is approximately two-thirds. A correlation of the time-averaged mass transfer at both the pellet hole and external surfaces was developed. Mathematical simulations of the effectiveness and volume efficiency factor of pellets with several holes show that the nonuniformity in the mass transfer has a small impact unless the external mass-transfer resistance becomes the rate-limiting process