Aspen Technology Aspen Process Manual Contents www.aspentech.com   support.aspentech.com 
Web site | Process Manual > Solids and separation > Crystallization | Word index

CR 4: Crystallization Design and Modelling

CR 4: Crystallization Design and Modelling
     Part 1: Design of Well-mixed Batch Crystallizers
         1. INTRODUCTION
             1.1 Aims of the Report
             1.2 Advantages and Limitations of the Batch Crystallizer
         2. DESIGN GUIDE
             2.1 Introduction
             2.2 Overview of the Design Procedure
             2.3 Specimen Design Calculations
         3. PRELIMINARY PROCESS AND EQUIPMENT SELECTION
             3.1 Types of Process
             3.2 Types of Equipment
         4. SPECIFICATION OF EQUIPMENT
             4.1 Introduction
             4.2 Vessels
             4.3 Agitation
             4.4 Fluidization
             4.5 Pumping
             4.6 Heat Exchange
         5. PERFORMANCE PREDICTION
             5.1 Introduction
             5.2 Heat and Mass Balances
             5.3 Supersaturation Balance
             5.4 Seeding
             5.5 Crystal Number (Population) Balances
             5.6 Crystallization Kinetics and Solubility
             5.7 Statistics of Crystal Size Distributions
             5.8 Algorithms
         6. SELECTION OF OPERATING CONDITIONS
             6.1 Introduction
             6.2 Uncontrolled Operation
             6.3 Controlled Cooling
             6.4 Controlled Evaporation
             6.5 Controlled Precipitation
         7. IMPLEMENTATION OF CONTROL
             7.1 Seeding
             7.2 Operating Conditions
         8. NOMENCLATURE
         9. REFERENCES
     Part 2: Design of Continuous Crystallizers
         1. OVERVIEW OF DESIGN PRINCIPLES
             1.1 Mass Balance.
             1.2 Energy Balance.
             1.3 Selection of Crystallizers.
             1.4 Fluid Dynamics.
                 1.4.1 Suspension of crystals.
                 1.4.2 Circulation and mixing.
             1.5 Heat Transfer.
                 1.5.1 Heat transfer coefficients.
                 1.5.2 Heat transfer area.
             1.6 Mass Transfer.
                 1.6.1 Mass transfer coefficients.
                 1.6.2 Effective crystal growth rates.
                 1.6.3 Maximum and optimum supersaturation.
             1.7 Crystallizer Volume.
         2. DESIGN CRITERIA.
             2.1 Mass and energy balances.
             2.2 Types of crystallizers.
             2.3 Selection criteria.
                 2.3.1 Crystal suspension.
                 2.3.2 Heat transfer.
                 2.3.3 Encrustation.
                 2.3.4 Evaporative surface.
                 2.3.5 Droplet entrainment.
                 2.3.6 Production rate.
                 2.3.7 Residence time.
                 2.3.8 Crystallizer volume.
                 2.3.9 Combining the criteria.
         3. DESIGN STRATEGY.
             3.1 Calculate Crystallizer Volume.
             3.2 Evaluate Suspension and Mixing Requirements.
             3.3 Perform Heat and Mass Transfer Calculations.
             3.4 Encrustation and Similar Problems.
             3.5 Mixing and circulation rate.
             3.6 Determine Requirements for Additional Features, e.g. Classification, Fines Destruction.
         4. CLASSIFICATION AND SELECTION OF CRYSTALLIZERS.
             4.1 Crystallizers with Low Attrition Rates.
             4.2 Crystallizers with High Attrition Rates.
             4.3 Cooling Crystallizers.
             4.4 Evaporative Crystallizers.
         5. KINETIC PARAMETERS AND PRODUCT QUALITY.
             5.1 Growth Rates.
             5.2 Nucleation rates.
             5.3 Product Quality.
                 5.3.1 Median crystal size.
                 5.3.2 Crystal size distribution (CSD) and coefficient of variation (CV).
                 5.3.3 Crystal purity.
         6. FINES TREATMENT AND PRODUCT CLASSIFICATION.
             6.1 Fines Treatment.
             6.2 Classified Product Removal.
             6.3 Classified Product Removal with Fines Destruction.
         7. MAXIMUM OUTPUT DURING STABLE OPERATION.
             7.1 Single-stage Crystallizer.
             7.2 Multistage Crystallizers.
                 7.2.1 Example: eight-stage countercurrent crystallizer.
         8. PRECIPITATION AND DROWNING-OUT CRYSTALLIZERS.
             8.1 Reaction Crystallization.
             8.2 Residence, Reaction, Induction and Mixing Times.
             8.3 Supersaturation and the Kinetic Parameter.
             8.4 Seeding, Dilution and Ostwald Ripening.
             8.5 Median Crystal Size.
             8.6 Design and operating conditions.
         9. REFERENCES.
         10. GLOSSARY OF PRINCIPAL SYMBOLS.
     Part 3: Agitators and Pumps
         1. INTRODUCTION
             1.1 How to use this document
             1.2 Relevant dimensionless numbers
                 1.2.1 Agitators
                 1.2.2 Pumps
             1.3 Common conversion factors
         2. FUNCTION OF AGITATORS / PUMPS IN CRYSTALLIZERS
             2.1 Liquid mixing
                 2.1.1 Mass transfer
                 2.1.2 Heat transfer
             2.2 Solids suspension and mixing
                 2.2.1 Suspension
                 2.2.2 Classification
                 2.2.3 Mass transfer to crystals
                 2.2.4 Heat transfer to/from crystals
                 2.2.5 Nucleation
                 2.2.6 Agglomeration
                 2.2.7 Encrustation
             2.3 Gas contacting
                 2.3.1 Bubble breakup
             2.4 Power requirements
             2.5 Slurry transport
                 2.5.1 Pumping requirements
                 2.5.2 Abrasion
                 2.5.3 Solids suspension
                 2.5.4 Pipework
             2.6 Mechanical considerations
                 2.6.1 Vibrations/deflections
             2.7 Prediction of agitation effects
             2.8 Further work
         3. TYPES OF AGITATOR
             3.1 Axial flow agitators
                 3.1.1 Pitched blade turbines
                 3.1.2 Propellers
                 3.1.3 Helical ribbons
                 3.1.4 Multiple agitators
             3.2 Radial flow agitators
                 3.2.1 Bladed turbines
                 3.2.2 Disc turbines
                 3.2.3 Multiple agitators
             3.3 Swirl flow agitators
                 3.3.1 Anchor/blades
             3.4 Other agitators
                 3.4.1 External recycle/liquid jet
                 3.4.2 Fluidic mixer
                 3.4.3 Natural convection
                 3.4.4 Gas sparging
             3.5 Agitator positioning
                 3.5.1 Centre axis
                 3.5.2 Off-centre
                 3.5.3 Vertical position
                 3.5.4 Side entry
             3.6 Internal geometry
                 3.6.1 Baffles
                 3.6.2 Draft tube
                 3.6.3 Contoured base
                 3.6.4 Settling zones
             3.7 Mechanical considerations
                 3.7.1 Shaft seals
                 3.7.2 Agitator drives
                 3.7.3 Materials of construction
         4. TYPES OF PUMPS
             4.1 Dynamic pumps
                 4.1.1 Centrifugal pumps
                 4.1.2 Special effect pumps
             4.2 Positive displacement pumps
                 4.2.1 Reciprocating
                 4.2.2 Rotary
             4.3 Glandless drives
             4.4 Reliability
         5. SELECTION OF AGITATION SYSTEMS
             5.1 Main agitation parameters
             5.2 Specific requirements
             5.3 Agitator selection
             5.4 Additional equipment
             5.5 Agitation requirements
             5.6 Scale-up of agitation systems
             5.7 Economic assessment and comparison
         6. SELECTION OF PUMPS
             6.1 Selecting pump type
             6.2 Estimating liquid properties
             6.3 Estimating system characteristics
             6.4 Comparing pump characteristics with system characteristics
             6.5 Use of multiple pumps
                 6.5.1 Pumps in series
                 6.5.2 Pumps in parallel
             6.6 Scaling of pump performance
             6.7 Other design considerations
                 6.7.1 Materials of construction
                 6.7.2 Allowance for pump wear
                 6.7.3 Seals
                 6.7.4 Selection of drive system
                 6.7.5 Control system
                 6.7.6 Pipework design
         7. GLOSSARY AND NOMENCLATURE
             7.1 Equipment
             7.2 Operation
             7.3 Nomenclature
             7.4 Sub/superscripts
         8. REFERENCES
     Part 4: Practical Issues of Crystallizer Scale-up
         1 INTRODUCTION.
             1.1 Definition of Scale Up.
             1.2 Concepts and Scope.
             1.3 Summary of Chapter Contents.
         2 GENERAL PRINCIPLES OF SCALING.
             2.1 The Need to Consider the Entire System.
                 2.1.1 Participation of the designer in the pilot design and operation.
                 2.1.2 Particle size distribution.
                 2.1.3 Disposal of spent mother liquor.
             2.2 Material and Energy Balances over the Crystallizer.
             2.3 Phase, Solubility and Operating Diagrams.
             2.4 Control of Crystal Size.
             2.5 Crystallization and Precipitation.
             2.6 Agitator Scale up.
             2.7 Batch or Continuous Operation.
                 2.7.1 Size of the full scale plant.
                 2.7.2 Impurities.
                 2.7.3 Crystal morphology.
                 2.7.4 Evaporative crystallization.
             2.8 Data Requirements.
                 2.8.1. Necessary data.
                 2.8.2. Useful data.
                 2.8.3. Data of general assistance.
         3. BATCH CRYSTALLIZATION.
             3.1. Batch Process Definition.
                 3.1.1. Batch and semi-continuous processing.
                 3.1.2. Methods of producing supersaturation.
             3.2. Summary of Scale Up Procedure.
             3.3. Data Requirements.
                 3.3.1. Small scale operating data.
                 3.3.2. Complete description of upstream and downstream operations.
                 3.3.3. Physical and thermodynamic characteristics of the materials in the process.
                 3.3.4. Control of supersaturation.
             3.4. Batch Size.
                 3.4.1. Combination of crystallization and other operations in a single vessel.
                 3.4.2. Adaptation of process to existing equipment.
                 3.4.3. Vessel volume.
             3.5. Vessel Geometry.
                 3.5.1. Constant volume, non-evaporative crystallizers.
                 3.5.2. Evaporative crystallizers.
                 3.5.3. Heat transfer.
             3.6. Seeding.
             3.7. Filling and Emptying the Crystallizer.
         4. CONTINUOUS CRYSTALLIZATION.
             4.1. Key Factors in Scaling.
                 4.1.1. Crystal hold up time.
                 4.1.2. Heat transfer.
                 4.1.3. Vapour disengagement.
                 4.1.4. Foam breaking.
                 4.1.5. Crystal breakage and attrition.
             4.2. Summary of Scale Up Procedure.
             4.3. Data Requirements.
             4.4. Scale Up Procedure.
                 4.4.1. Crystallizer slurry volume.
                 4.4.2 Crystallizer geometry.
                 4.4.3. Cooling heat exchangers.
                 4.4.4. Heating the circulating flow.
                 4.4.5. Entrainment.
                 4.4.6. Location of the feed point.
                 4.4.7. Classification.
             4.5. Encrustation Control.
                 4.5.1. Start up.
                 4.5.2. Minimization of encrustation.
                 4.5.3. Original design features.
         5. MELT CRYSTALLIZATION.
             5.1. General Characteristics.
                 5.1.1. Definition of melt crystallization.
                 5.1.2. Outline description of typical systems.
                 5.1.3. Separation of crystal products.
                 5.1.4. Batch versus continuous operation.
             5.2. Data Requirements.
             5.3. Scale Up Procedures.
         6. AGITATOR SCALE UP.
             6.1. Functions of Agitation.
             6.2. Flow Rate and Supersaturation.
             6.3. Suspension of Crystals.
             6.4. Feed Points.
             6.5. Scaling of Agitators.
             6.6. Scaling of Mixers.
             6.7. Circulation Pump Scale Up.
                 6.7.1. Characteristics of circulating pumps.
                 6.7.2. Circulation pump selection.
         7. SCALE UP VERSUS SCALE DOWN.
             7.1. Types of Processes.
                 7.1.1. Cooling with heat exchangers.
                 7.1.2. Cooling by evaporation.
                 7.1.3. Evaporation with heat addition.
             7.2. Effects of Impurities.
                 7.2.1. Single crystals.
                 7.2.2. Batch or continuous crystallization.
             7.3. Conclusions.
         8. OPERATING DIAGRAMS.
             8.1. Batch Crystallization.
                 8.1.1. Cooling without evaporation.
                 8.1.2. Cooling with evaporation.
                 8.1.3. General comments.
             8.2. Continuous Crystallizers.
                 8.2.1. Draft tube crystallizers.
                 8.2.2. Forced circulation crystallizers.
             8.3. A Reactor / Crystallizer.
             8.4. Drawbacks of Operating Diagrams.
         9. CASE STUDIES.
             9.1. A Solubility Problem Affecting Scaling from Batch to Continuous Crystallization.
                 9.1.1. Description of the situation.
                 9.1.2. Solution of the problem.
                 9.1.3. Lessons learned.
             9.2. Accumulation of Impurities.
                 9.2.1. Description of the situation.
                 9.2.2. Solution of the problem.
                 9.2.3. Lessons learned.
             9.3. Scaling From a Very Small Pilot Plant.
                 9.3.1. Description of the situation.
                 9.3.2. Solution of the problem.
                 9.3.3. Lessons learned.
         10 NOMENCLATURE.
         11 BIBLIOGRAPHY.
     Part 5: Control Systems for Crystallizers
         PREFACE
         1. INTRODUCTION
             1.1 Problems of Crystallization
                 1.1.1 Size and Distribution of Crystal Sizes
                 1.1.2 Crystal Quality
                 1.1.3 Vessel Fouling
                 1.1.4 Relief of Supersaturation and Product Yield
             1.2 Motivation for CSD Control
             1.3 Summary of Report
         2. MANIPULATION OF CSD
             2.1 Changes in Solids RTD
                 2.1.1 Accelerated Fines Removal (FR)
                 2.1.2 Accelerated Product Removal (Classified Product Removal, CPR)
             2.2 Solids vs Liquid Phase RTD (DDO)
             2.3 Summary of CSD Manipulation
             2.4 Crystallization Modifiers
             2.5 Over-all Process Conditions
         3. CSD DYNAMICS
             3.1 Historical Perspective
             3.2 CSD Stability
                 3.2.1 The MSMPR Configuration
                 3.2.2 Summary of MSMPR Stability Predictions
                 3.2.3 Size-Dependent Removal Configurations
                 3.2.4 Practical Strategies for Coping with CSD Instability
             3.3 CSD Transients
         4. CSD CONTROL
             4.1 The CSD Control Problem
             4.2 Previous CSD Control Studies
             4.3 Current Experimental CSD Control Studies
         5. REFERENCES
         6. NOMENCLATURE
     Part 6: Procedures for Crystallizer Scale-up
         1. THE CONCEPT OF SCALE-UP
             1.1 Introduction
             1.2 Scale-up procedures
             1.3 Summary of existing design methods for idealised flow patterns
                 1.3.1 MSMPR
                 1.3.2 Plug flow crystallizer
             1.4 Summary of contents
         2. SCALE-UP OF NUCLEATION
             2.1 Present methods
             2.2 Future work
         3. CRYSTAL GROWTH KINETICS
             3.1 Introduction
             3.2 Crystal growth in MSMPR crystalllizers
             3.3 Crystal growth in fluidized bed crystallizers
             3.4 Future work
         4. SCALE-UP OF HYDRODYNAMICS
             4.1 Introduction
             4.2 MSMPR crystallizers
                 4.2.1 Flow patterns in tanks
                 4.2.2 Stirrer speed for suspension
             4.3 Fluidised bed crystallizers
             4.4 Future work
         5. SUGGESTED COMPREHENSIVE DESIGN METHOD
             5.1 Mixed models
             5.2 Experimental studies
         6. CONCLUSIONS
         7. REFERENCES
         8. NOMENCLATURE
         9. APPENDIX 1: Numerical example of design of an MSMPR crystallizer
     Terms and Conditions of Use

Volume IV covers the design and modelling of crystallization plant.

Volume CR IV  Part 1 Design of well-mixed batch crystallizers.

Batch crystallization has the advantage of using relatively simple and flexible equipment. Both the equipment and the operating procedure should be designed to give the best conditions for making the product. Before starting to design the crystallizer, the designer must know the required specification of the product, and obtain information on the crystallization kinetics of the material. From this, he can select the method of supersaturation generation (e.g. cooling or evaporation) and hence the type of equipment to use. The equipment can now be specified in more detail, including vessel size, agitator/pumping requirements, and heat exchanger duty. The operating policy can then be specified to achieve a particular product.

The report also considers the use of seeding, and variable cooling and/or evaporation rates to improve product quality.


Volume CR IV  Part 2 Design of continuous crystallizers.

This report provides guidance on all aspects of the design of continuous solution crystallizers. The selection of crystallizer type to fulfil specific objectives is reviewed in the context of material constraints. Design strategies are presented for control of product size distribution and purity using experimentally determined information including kinetics. The report deals with multiple stage and well as single stage continuous crystallizers.

Volume CR IV  Part 3 Agitators and pumps.

Agitation plays a vital role in crystallization processes. It affects:

  • Heat transfer to cooling or heating surfaces such as vessel walls, cooling coils or heat exchangers.
  • Mass transfer from between crystals and solutions, between the supersaturation generation region and the bulk contents of the vessel, and at the molecular scale within the mother liquor.
  • Suspension of solids either as a near homogeneous slurry, or stratified in some manner according to the crystallizer design.
  • Nucleation/attrition processes by direct physical damage to crystals and generation of a turbulent flow field.

The report describes the various mechanisms relating to the above processes. Various types of agitators and pumps are reviewed and their characteristics highlighted. A selection guide is also presented for both in-vessel agitators and pumps for emptying or circulation of crystallizer contents.


Volume CR IV  Part 4 Crystallizer Scale-up.

Volume IV.4 provides a comprehensive review of the issues affecting scale up and highlights pitfalls to be avoided. In the report areas where scale-up caries a large risk are highlighted and methods to reduce this risk are suggested in many cases. A major strength of this report is its breadth of coverage as such it compliments the older SAR 22 which provides a step by step guide to the scale up of an MSMPR crystallizer. Both reports are written on the assumption that some bench-scale or pilot plant crystallization trials will be performed to provide data for the design of the full-scale equipment. The reports discuss methods for scaling-up different aspects of the crystallization process from measurements at laboratory or pilot scale to full-scale plant.

For nucleation rates, scale-up will depend on the relative importance of the various nucleation mechanisms which occur simultaneously within the vessel. These will, in turn, depend on such parameters as the physical properties of the crystals and liquor, the supersaturation levels and the vessel geometry and agitation levels. Growth rates will depend both on the distribution of supersaturation and on the fluid/particle interactions. The vessel hydrodynamics control the rate of mixing of both the liquid and solid phases in the vessel as well as the criteria for suspension of the crystals.


Volume CR IV  Part 5 Control systems for crystallizers.Classification procedures, such as fines destruction/removal or classified product removal, can be used to control the crystal size distribution from continuous crystallizers; they can also eliminate problems such as cycling of the product size. Prediction of the stability of crystal size distribution is mathematically very complex, and this report summarizes the impact of fines removal/destruction, residence time control and product classification on crystal size distribution. The various methods of implementing the control are discussed, along with such factors as the effects of transients on the product crystal size distribution. The preface to the report contains a summary of the main conclusions without going into detail, allowing easy assessment of the likely applicability of these techniques to the crystallization in question.
Volume CR IV  Part 6 CRYCON 1.0 - USER GUIDE. Crycon is a continuous crystallizer dynamic simulation program developed by SPS. The program will simulate the performance of a continuous crystallizer, generating detailed information on the state of the crystallizer throughout the crystallization.
Volume CR IV  Part 7 Development of a batch crystallization process. Often those charged with the development of a crystallization process are not crystallization specialists. Typically the task may fall to a development chemist whose primary focus and principal skill lies in the area of chemical synthesis rather than the product isolation. This document is designed to address the needs of this specific group. The reader is taken through the stages of generating the first crystals of a difficult to crystallize material, on to polymorph and solvate screening and salt form selection. The next stage, selection of a superstauration generation route, is handled by gathering basic solubility data of sufficient quality to be able to identify potential starting points and isolation points based on the possible supersaturation generation routes of; cooling, drowning out, evaporation and reaction (pH shift, salt formation, coupling etc). The next stage involves a scoping crystallization in which the metastable zone width is determined from the proposed starting point. The scoping experiment also provides material that can be characterised both in terms of solid-liquid separation performance and specific properties of the crystalline product. At each stage practical guidelines are provided for the conduct of the simple experiments needed to gather the key data necessary to select the supersaturation generation technique(s) and the crystallization conditions. These experiments are designed to be quick and simple and to yield data of a quality sufficient to allow the initial decisions to be made. The manual part is written with the development of a scaleable and operable process in mind and the impact of scale up on alternative crystallization strategies is examined at key points throughout the manual. A subsequent manual part is proposed for troubleshooting batch crystallization processes.