Shengfu Zhang

Hydrodynamic behaviour of three-phase (gas—liquid—solid) airlift reactors

Abstract The hydrodynamic behaviour of three-phase (gas—liquid—solid) airlift (TPAL) reactors have been studied using internal-loop reactors and the air—water—glass beads system. A hydrodynamic model has been developed which considers the reactor to be comprised of riser and downcomer regions. The uneven distribution of the solid particles between these regions leads to the effective densities of the liquid—solid mixtures being different between these regions. This effect is used to explain reactor stalling at low superficial gas velocities, and also to predict the effect of increasing concentration of solid particles and increasing solid particle settling velocities on reactor performance. The model can be used to predict both critical gas velocity for solids suspension and recirculating liquid velocity in the reactor. This hydrodynamic model has been tested experimentally using TPAL reactors ranging in diameters from 0.11 to 0.155 m, and in height from 1.0 to 2.0 m. The effects of superficial gas velocity, solid particle loading, particle diameter, draft tube diameter and draft tube height on the liquid recirculation velocity in the riser section (ULR) and critical gas velocity at stalling (UGcrit) were investigated experimentally. ULR was found to increase with draft tube height and to decrease with draft tube diameter. Increasing solids loading or settling velocity increased UGcrit. The set of conditions under which the TPAL reactors stalled were found to be the same whether approached via a decreasing gas velocity and a fixed quantity of solids, or a increasing quantity of solids and a fixed gas velocity. The hydrodynamics model predicted ULR well (within 30%) and UGerit well (within 10%) when simplifying assumptions regarding the head loss coefficients and downcomer gas holdup were made. At gas velocities above 0.03 ms−1, the assumption that the gas holdup in the downcomer region was negligible (made in the absence of suitable information on this parameter) became invalid, and the hydrodynamic model predicted too-high recirculation velocities.

Determination of pollutant diffusion coefficients in naturally formed biofilms using a single tube extractive membrane bioreactor

A novel technique has been used to determine the effective diffusion coefficients for 1,1,2-trichloroethane (TCE), a nonreacting tracer, in biofilms growing on the external surface of a silicone rubber membrane tube during degradation of 1,2-dichloroethane (DCE) by Xanthobacter autotrophicus GJ10 and monochlorobenzene (MCB) by Pseudomonas JS150. Experiments were carried out in a single tube extractive membrane bioreactor (STEMB), whose configuration makes it possible to measure the transmembrane flux of substrates. A video imaging technique (VIT) was employed for in situ biofilm thickness measurement and recording. Diffusion coefficients of TCE in the biofilms and TCE mass transfer coefficients in the liquid films adjacent to the biofilms were determined simultaneously using a resistances-in-series diffusion model. It was found that the flux and overall mass transfer coefficient of TCE decrease with increasing biofilm thickness, showing the importance of biofilm diffusion on the mass transfer process. Similar fluxes were observed for the nonreacting tracer (TCE) and the reactive substrates (MCB or DCE), suggesting that membrane-attached biofilm systems can be rate controlled primarily by substrate diffusion. The TCE diffusion coefficient in the JS150 biofilm appeared to be dependent on biofilm thickness, decreasing markedly for biofilm thicknesses of >1 mm. The values of the TCE diffusion coefficients in the JS150 biofilms <1-mm thick are approximately twice those in water and fall to around 30% of the water value for biofilms >1-mm thick. The TCE diffusion coefficients in the GJ10 biofilms were apparently constant at about the water value. The change in the diffusion coefficient for the JS150 biofilms is attributed to the influence of eddy diffusion and convective flow on transport in the thinner (<1-mm thick) biofilms.

Membrane aromatic recovery system (MARS): lab bench to industrial pilot scale

This article describes a novel process for recovery of aromatic amines and phenolic compounds form wastewaters, the membrane aromatic recovery aromatic system (MARS). Laboratory work on wastewaters containing aniline and phenol will be presented, including data demonstrating removal and recovery of each chemical in a sufficiently pure form to allow recycling into a chemical production process. This article also describes successful scale-up and operation of the process through pilot trials at Solutia, UK. Process economics are discussed and data showing the potential for application of the process to a wide range of organic chemicals are presented.

Chapter 8 – Membrane Aromatic Recovery System (MARS) – A new process for recovering phenols and aromatic amines from aqueous streams

Phenolic compounds (chemicals such as phenol and its derivatives) are used in phenolic resins, polycarbonates, biocides and agrochemicals. Aromatic amines (chemicals such as aniline and its derivatives) are used in a wide range of consumer products, including polyurethane foam, dyes, rubber chemicals and pharmaceuticals. The factories that manufacture and/or use these types of chemicals often create aqueous waste streams containing significant concentrations (0.1-10wt%) of these chemicals. Both aromatic amines and phenolic compounds are toxic and many of them are also carcinogenic. Tightening legislation in many countries calls for dramatic reductions in emissions of these species. A variety of processes have been proposed for treatment of these aromatic amine or phenolic compound containing wastewaters. Off-site disposal (landfill, deepwell injection) and biodegradation result in the compounds, which have typical values in the range US

Extractive membrane bioreactors for detoxification of chemical industry wastes: process development

0.75-US

Pilot scale application of the Membrane Aromatic Recovery System (MARS) for recovery of phenol from resin production condensates

20 per kg, being lost. These compounds have high boiling points and low vapour pressures. Hence, processes that rely on liquid-gas phase transition, such as distillation and pervaporation, have high-energy requirements. The use of adsorbents, such as activated carbon [1-4] or resins [58], is usually expensive due to difficulties and complexity in the regeneration stage. Problems associated with the use of solvent extraction [9-13] arise with phase separation [14] and contamination of the wastewater with solvent [15] due to the intermediate polarity of the compounds, which require moderately water soluble solvents.