MM 2: Adsorption Mini Manual
MM 2:
Adsorption Mini Manual 1 INTRODUCTION 1.1 Scope 1.2 Definitions 1.2.1 Physical adsorption and chemisorption 2 ADSORBENT MATERIALS 2.1 Introduction 2.2 Activated Carbons 2.3 Inorganic Adsorbents 2.3.1 Silica gel 2.3.2 Activated alumina 2.3.3 Clay materials 2.3.3.1 Montmorillonite based 2.3.3.2 Pillared clays 2.3.4 Transition metal cyanometallates 2.4 Molecular Sieves 2.4.1 Carbon molecular sieves 2.4.2 Zeolites 2.4.3 Low aluminium (hydrophobic) zeolites 2.4.4 Phosphate-based molecular sieves 2.5 Polymeric Adsorbents 2.5.1 Polystyrene and polyacrylic adsorbents 2.5.2 Carbonised polymers 2.5.3 Polymers with tailored pores 2.5.4 Other polymeric materials 2.6 Other Adsorbent Systems 2.6.1 Bone charcoal 2.6.2 Biopolymeric materials 2.6.3 Miscellaneous materials 2.7 Degradation of Adsorbents 2.7.1 Mechanical degradation 2.7.1.1 Mechanical fouling 2.7.1.2 Fracturing and Attrition 2.7.1.3 Osmotic shock 2.7.2 Chemical deterioration 2.7.2.1 Coking 2.7.2.2 Ageing 2.8 Selection of Adsorbent Materials 3 THE STATE OF THE SCIENCE 3.1 Sorbent-Sorbate Interactions 3.1.1 Physical interactions 3.1.1.1 Van der Waals forces 3.1.1.2 Electrostatic interactions 3.1.2 Chemisorption 3.1.3 Other effects 3.1.3.1 Steric factors 3.1.3.2 Entropy effects 3.1.3.3 Adsorbent heterogeneity 3.1.4 Interactions applying to specific adsorbent types 3.1.4.1 Zeolite molecular sieves 3.1.4.2 Activated carbons 3.1.4.3 Silica gel 3.1.5 Selectivity 3.2 Equilibria of Adsorption 3.2.1 Equilibria - single component systems 3.2.1.1 The adsorption isotherm 3.2.1.2 Adsorption at low surface coverage 3.2.1.3 The Langmuir isotherm 3.2.1.4 Other models for isotherms 3.2.1.5 Isotherm hysteresis 3.2.1.6 Measurement of isotherms 3.2.1.7 Characterisation of adsorbents 3.2.2 Equilibria - multicomponent systems 3.2.2.1 Measurement of multicomponent isotherms 3.2.2.2 Models of multicomponent systems 3.2.2.3 Qualitative treatment of liquid systems 3.2.3 Computer simulation (Monte Carlo) techniques 3.3 Kinetics of Adsorption 3.3.1 External film mass transfer 3.3.2 Intraparticle mass transfer 3.3.2.1 Linear driving force approximation 3.3.2.2 Non-linear driving force approximations 3.3.3 Applicability of the LDF approximation to cyclic PSA processes 3.3.4 Application of the LDF approximation to multiple resistances 3.3.5 Identification of controlling mechanisms 3.3.5.1 Packed beds - literature correlations 3.3.5.2 Packed beds -theoretical interpretation of the breakthrough curve 3.3.5.3 Batch systems - short-time and long-time rate data 3.3.5.4 Mass transport in macropores 3.3.5.5 Effect of turbulence or agitation in batch systems 3.3.5.6 Effect of concentration in batch systems 3.3.5.7 Effect of adsorbate size and shape in batch systems 3.3.5.8 Effect of adsorbent particle size in batch systems 3.3.5.9 Apparent activation energy 3.3.5.10 Interruption tests 3.4 Heat Transfer Effects 3.4.1 External film resistance to heat transfer 3.4.2 Intraparticle resistance to heat transfer 3.4.3 Heat transfer to a vessel wall 3.4.4 Axial thermal conduction in a packed bed 3.4.5 Axial thermal conduction in a vessel wall 3.4.6 Overall rate of heat transfer through a vessel wall 4 STATE OF THE TECHNOLOGY 4.1 Process Configurations 4.1.1 Fixed and moving beds 4.1.2 Batch processes 4.1.3 Fixed bed processes 4.1.4 Moving bed processes 4.1.4.1 Pulsed bed adsorbers 4.1.4.2 Solids in plug flow adsorbers 4.1.4.3 Fluidised bed adsorbers 4.1.4.4 Rotary bed adsorbers 4.1.5 Simulated moving bed adsorbers 4.1.6 Regeneration methods 4.1.6.1 Thermal swing adsorption (TSA) 4.1.6.2 Pressure swing adsorption (PSA) 4.1.7 Chromatographic processes 4.2 Application of Adsorption Processes 4.2.1 Gas drying 4.2.2 PSA for air separation 4.2.3 Natural gas sweetening 4.2.4 Hydrocarbon separations 4.2.5 Solvent Recovery / VOC Removal 4.2.6 Water treatment 4.2.7 Summary 4.3 Instrumentation and Control 4.3.1 Introduction 4.3.2 Control of adsorption processes 4.3.2.1 Time cycles 4.3.2.2 Composition / Concentration 4.3.2.3 Temperature front sensing 4.3.2.4 Pressure 4.3.2.5 Flowrate 4.3.2.6 Measurement of level 4.4 Hazard and Safety Equipment 4.4.1 Pressure vessel hazard 4.4.2 Fire hazard 4.4.3 Electrical hazard 4.4.4 Chemical hazards 4.4.5 Instrumentation hazard 4.5 Economics and Process Selection 5 DESIGN PROCEDURES 5.1 Data Requirements 5.2 Stagewise Batch and Continuous Contacting 5.2.1 Single stage 5.2.2 Cross-current multistage 5.2.3 Counter-current multistage 5.3 Differential Continuous Contacting 5.4 Fixed Beds 5.4.1 Rigorous methods 5.4.1.1 Thermal effects 5.4.1.2 Isothermal operation 5.4.1.3 Isothermal, equilibrium controlled systems 5.4.1.4 Isothermal, rate controlled systems 5.4.1.5 Non-isothermal and multicomponent systems 5.4.1.6 Cyclic fixed bed process design 5.4.2 Constant pattern behaviour 5.4.3 Short-cut and scoping methods 5.4.3.1 Length of unused bed (LUB) 5.4.3.2 Mass transfer zone length (MTZL) 5.4.3.3 Empty bed contact time (EBCT) 5.4.3.4 Bed depth service time (BDST) 5.4.3.5 Transfer unit approach (NTU and HTU) 5.4.3.6 Capacity and breakpoint 5.5 Hydrodynamics 5.5.1 Pressure drop 5.5.2 Fluidisation 5.5.3 Bed crushing 5.5.4 Axial dispersion 5.5.4.1 Gases and vapours 5.5.4.2 Liquids 5.5.4.3 Porous systems at low velocities 5.6 Scale-up and Pilot-plant Studies 5.7 Performance Evaluation 6 RESEARCH AND DEVELOPMENT REQUIREMENTS 6.1 Adsorbent Design, Tailoring and Modification 6.2 Thermodynamic and Kinetic Data and Models 6.3 Process Design and Analysis 6.4 Technology, Equipment and Process Engineering 6.5 Process Applications 7 REFERENCES 8 NOMENCLATURE 9 APPENDIX 1 - EQUIPMENT MANUFACTURERS 10 APPENDIX 2 - ADSORBENT SUPPLIERS (UK) 11 APPENDIX 3 - ADSORBENT PROPERTIES Terms and Conditions of Use | The major sections are: Part 1 Introduction Contents of Manual Adsorption (MM 2) is concerned with the distribution and attachment of fluid molecules, termed the adsorbate molecules, onto the surfaces of a microporous solid, termed the adsorbent. A broad range of commercially available adsorbents are described. For the effect to have any commercial significance as a method of separation, the adsorbent is required to have a very high internal surface area and volume, and to provide good access to such surfaces from outside. Internal surface areas can be as high as 2000 m2/g but the range is more usually restricted to 300 to 1200 m2/g in order to preserve good mechanical strength. Adsorption is in virtually all cases an exothermic phenomenon. An adsorption process is defined as the separation of the adsorbate components of a gas or a liquid mixture by the transfer of one or more components to the surface of the microporous adsorbent. Adsorbed components are held to the surface by intermolecular forces which are considered in detail, including van der Waals and electrostatic forces. They may be subsequently removed or desorbed, thereby allowing the adsorbent to be reused and possibly allowing the adsorbed components to be recovered. The attractive forces in adsorption are generally, but not always, weaker than those of chemical bonds and desorption techniques are reviewed, such as:
The desorption, or regeneration, step is a very important part of the overall process; since it normally determines the overall energy requirements of an adsorption process, as well as the purity of products which can be achieved in subsequent adsorption steps. Continued reuse of an adsorbent is normally required in order to reduce to a minimum the inventory of expensive adsorbent materials. However, in a few cases, desorption is not feasible, and the adsorbate must be removed by thermal destruction or another chemical reaction, or the adsorbent is simply discarded. The actual mechanism of separation in an adsorption process is not always wholly that of adsorption at a solid surface. In fact several distinct selectivity mechanisms are considered in detail, namely:
Applications of adsorption are listed and include:
The distinction by composition between a bulk separation and a purification is not clear. However, it is necessary to know in the rigorous design of an adsorption process whether the bulk fluid flowrate can be assumed constant. In the majority of adsorption processes, the adsorbent needs to be regenerated periodically for reuse. Four basic methods can be used, but in commercial processes it is normal to have a combination of the following:
These are reviewed. A comprehensive list of adsorption processes is presented. The most commonly used systems contain fixed beds of the adsorbent through which the fluid is passed. Whilst continuous countercurrent arrangements are generally the most favoured for other separation processes, attempts to operate adsorption processes in such a manner have generally met with technical failure. This is because, until recently, adsorbent materials have not been able to withstand attrition in moving bed or fluidised bed equipment. In a fixed bed, a mass transfer zone of adsorption develops initially near the entrance and moves progressively with time towards the exit at which "breakthrough" begins to occur. Regeneration of the adsorbent is then required. In order to operate a fixed bed process in such a way as to produce a steady-state supply of product it is usual to have at least two fixed beds operating out of phase in a cyclic operation, although single bed, steady-state product flow configurations are possible and practicable. Volume II Part 1 Introduction. Scope, Definitions, Physical adsorption and chemisorption. Materials (Activated carbons, Inorganic adsorbents, Molecular
sieves, Polymeric adsorbents, Other adsorbent systems) Volume II Part 3 State of the science. Sorbent-sorbate interactions, physical interactions, chemisorption, other effects, interactions applying to specific adsorbent types, selectivity, equilibria of adsorption (equilibria - single component systems, equilibria - multicomponent systems, computer simulation (Monte Carlo) techniques) kinetics of adsorption (external film mass transfer, Intraparticle mass transfer, applicability of the LDF approximation to cyclic PSA processes, application of the LDF approximation to multiple resistances, identification of controlling mechanisms, packed beds - literature correlations, packed beds -theoretical interpretation of the breakthrough curve, batch systems - short-time and long-time rate data, mass transport in macropores, effect of turbulence or agitation in batch systems, effect of concentration in batch systems, effect of adsorbate size and shape in batch systems, effect of adsorbent particle size in batch systems, apparent activation energy, Interruption tests) Heat transfer effects (external film resistance to heat transfer, Intraparticle resistance to heat transfer, heat transfer to a vessel wall, axial thermal conduction in a packed bed, axial thermal conduction in a vessel wall, overall rate of heat transfer through a vessel wall) Volume II Part 4 State of the technology. Process Configurations (fixed and moving beds, batch processes, fixed bed processes, moving bed processes, simulated moving bed adsorbers, regeneration methods, thermal swing adsorption (TSA), pressure swing adsorption (PSA), chromatographic processes) Application of adsorption processes (gas
drying, PSA for air separation, natural gas sweetening,
hydrocarbon separations, solvent recovery / VOC removal, water
treatment) Volume II Part 5 Design procedures. Data requirements Volume II Part 6 Research and development requirements. Adsorbent design |