Spray Drying

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Process Engineering Guide: GBHE-PEG-DRY-006

Spray Drying Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Spray Drying CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 BACKGROUND TO TEST FACILITIES 3 5 DESIGN PROCEDURE 4 6 TEMPERATURE SENSITIVE PRODUCTS 4

7 FEEDSTOCK PREPARATION 5 8 FEEDSTOCK ATOMIZATION 5 9 PRODUCT COLLECTION 5 10 SAFETY ASPECTS 5 11 MICROSTRUCTURE OF DRIED PARTICLES 6 12 HYBRID DRYERS 6 13 BIBLIOGRAPHY 6 APPENDIX A CONTENTS LIST FOR SPS DRYING MANUAL VOLUME V PART 1 7 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 10



0 INTRODUCTION/PURPOSE Spray drying is a relatively expensive method of drying, but is particularly recommended when the product is temperature-sensitive (due to the very short time that the product experiences elevated temperatures) or if the typical physical form of a spray dried product (spheroids with a median diameter of up to 250 micron) is desirable. Another advantage of spray drying is that it produces a very homogeneous product from multi-component feedstocks. A prerequisite is that the feed can be pumped and atomized. Spray drying, with co-current air/feedstock flow, is used extensively within some companies for drying specialty products (dyes, pigments, intermediates, surfactants, ceramics, biodegradable polymer). Target moisture contents down to about 0.1% can be achieved if the feedstock is a dispersion with little or no material in solution. However as the proportion of dissolved material increases, and particularly when that material can form a skin on drying, then realistic target moisture contents increase. With typical dyestuff formulations it is difficult to achieve moisture contents less than 2%, and a typical target moisture would be 4 to 5%. If very low moisture contents are required, then a counter-current design of spray dryer could be considered where almost dry particles experience the highest air temperatures. However this mode of operation is not suitable for temperature-sensitive products. Alternatively, a combination of co-current spray drying and e.g. fluid bed drying should be considered. 1 SCOPE This Guide deals with spray drying including the need for trials and the availability of test facilities. 2 FIELD OF APPLICATION This Guide applies to process engineers in GBHE Enterprises worldwide.



3 DEFINITIONS For the purposes of this Guide, the following definition applies: SPS The Separation Processes Service (SPS) is a research and

consultancy organization, based at Harwell and Warren Spring Laboratory. It is active in the main operations related to separation, including comprehensive coverage of drying.

With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms. 4 BACKGROUND TO TEST FACILITIES When spray drying is identified as a possible method of drying a material, trials are carried out in-house on pilot plant facilities and also on manufacturers' test facilities to confirm that spray drying is a suitable technique for the feedstock. The smallest (laboratory scale and 0.8 metre diameter Niro Mobile Minor) spray dryers will give some idea of whether a feedstock will spray dry, but a negative result should not necessarily preclude the use of a full scale spray dryer. The Niro Production Minor dryer (1.2 metre diameter) is the smallest scale recommended to give indications of potential problems such as adherence of product to ducting walls etc. However even at this scale the particle size is unlikely to be as big as that required from a full scale dryer and the product may not be representative of product from a full scale dryer, e.g. in terms of dispensability. Large scale trials are necessary to check the quality of the spray dried product and to give some guidance of the likely size of installation necessary to achieve the desired production rate.



European Manufacturer, trial facilities: (a) Lab-scale dryers (b) Niro Mobile Minor (c) Niro Production Minor (d) Larger scale dryers There are a limited number of reputable suppliers of spray drying equipment. In Western Europe, the main suppliers are: (1) Niro Atomizer A/S Denmark (2) APV Pasilac Anhydro A/ Denmark (3) Drytec Ltd Tonbridge, Kent , U.K. (4) Barr and Murphy Overseas Ltd Maidenhead, Berks, U.K. 5 DESIGN PROCEDURE A design procedure for co-current spray dryers is given in Ref. [1] (the contents of which are listed in Appendix A), produced by SPS. Copies of Ref. [1] and may be ordered from SPS. A design procedure is included in Ref. [2]. SPS also provide several software packages (SPRY1, SPRY2A, SPRY2B, SPRY2C) to perform moisture and energy balances and preliminary sizing calculations for co-current spray dryers with a variety of different heating methods (direct and indirect firing, once-through versus total recycle of the drying gas). These are described in Ref. [3].



6 TEMPERATURE SENSITIVE PRODUCTS While spray drying is recommended for the drying of temperature-sensitive products, there is evidence that some designs of spray dryer ("tower" dryers, i.e. tall, narrow types designed for nozzle atomization) are better than others (squat form types designed for rotary atomizers). The reason is believed to be that in the squat form types, the large gas recirculation zones entrain particles (particularly the smaller ones) which therefore have prolonged residence times and become over dried. Such recirculation zones are likely to be much smaller in tower dryers. 7 FEEDSTOCK PREPARATION Feedstock preparation is discussed in, “Paste Preparation Systems for Dryers”. Some additional comments are given below: (a) Spray dryers have to have a feedstock which is fluid enough to pump to the atomizer. With dispersion feedstocks, particularly when the particles have a high aspect ratio, the limiting total solids content for a pumpable feedstock may be less than 20%. The addition of a small amount of a suitable surfactant, where this is permitted in the dry product, can greatly increase the pumpable concentration. (b) If the concentration of the feedstock is such that a small increase in total solids (e.g. by 1 or 2%) would cause a large increase in viscosity, this could interfere with the atomization process to give strands rather than droplets, and the feedstock concentration should be reduced. (c) With feedstocks containing dispersed particles, there is an upper limit to the size of these particles beyond which blockage problems arise. This upper size limit is smaller for nozzle atomizers, which have narrow channels which the feed has to negotiate, than for rotary atomizers.



8 FEEDSTOCK ATOMISATION A key aspect of spray drying is atomization of the feedstock. Atomization is covered in Ref. [5] “Atomization in Spray Dryers (Operational Manual)”, GBHE recommends mainly pressure nozzles and rotary atomizers, and to a limited extent the use of fluid nozzles. Work is underway to develop novel atomizers for spray drying, to give narrower drop size distributions and hence less dusty products. The narrower drop size distributions are likely to reduce the extent of entrainment in recirculation zones which causes prolonged residence times in the dryer. 9 PRODUCT COLLECTION Product leaves the dryer both from the rotary valve in the base of the drying chamber and from the drying air exhaust stream. The particle size distribution in the rotary valve will be coarser than in the exhaust stream. Use is often made of this classification effect to reduce the dustiness of the product, by collecting only the product from the chamber for further use or sales, while the fines in the air stream are separated off and recycled, usually by re-dispersing in the feedstock. Separation of the fines from the air stream is usually carried out by cyclone, though bag filters are used for some products where there is a high proportion of very fine particles. 10 SAFETY ASPECTS By their nature, spray drying facilities pose a risk of dust cloud explosions. The risk of explosions or fires is minimized by: (a) The use of operating conditions, particularly maximum air inlet temperatures, defined from appropriate flammability and explosion severity tests. In extreme cases, it may be necessary to operate with inert drying gas in a closed cycle. (b) Minimizing build-up of dried product in the dryer and associated ductwork. Use is often made of automatic hammers on the dryer chamber to disturb product adhering to the chamber wall. (c) The use of explosion suppression equipment where appropriate.



Explosion suppression equipment is not appropriate for large spray dryer chambers and so these are normally fitted with explosion relief panels to vent any explosion, thereby avoiding major damage. The subject of fire and explosion hazards in dryers is covered by GBHE-PEG-DRY-005 and Ref. [6]. Where the product is toxic, the dusty nature of spray dried products poses particular problems both in terms of the obvious need to protect operators from exposure to the product and also the less obvious fact that, in the event of an explosion, the opening of relief panels allowing the toxic product to be blown into the environment might not be acceptable. 11 MICROSTRUCTURE OF DRIED PARTICLES The quality of spray dried products can be affected by the microstructure of the dried particles. This is controlled mainly by the nature of the feedstock, and to a lesser extent by the drying conditions. The relationship between particle microstructure and feedstock is discussed in “Spray Drying: The Qualitative Effect of Factors Governing the Packing Density and Dustiness of Spray Dried Products”, W M L Wood (1987). An awareness of this relationship can influence the range of feedstock parameters and drying conditions to be used, which in turn can influence the final design and size. 12 HYBRID DRYERS In addition to conventional spray dryers, hybrid dryer systems are offered by some manufacturers. Such systems combine spray drying with fluid bed drying. Several manufacturers design systems with a separate fluid bed fed from the spray dryer. Niro offer a single unit, their FSD dryer, in which a fluid bed dryer is incorporated in the base of a co-current spray dryer. Such units offer improved thermal efficiency and, particularly in the case of the FSD, the opportunity to produce a granulated product with reduced dust content. The design of combined systems with a fluid bed dryer following a spray dryer can be achieved using the design guides for the individual dryer types, and optimizing the thermal efficiency by choosing appropriate moisture contents for the product exiting the spray dryer and entering the fluid bed.



13 BIBLIOGRAPHY [1] Drying Manual, Volume V (Spray Drying) Part 1, SPS, Harwell [2] Industrial Drying Equipment - Selection and Application, C M van't Land, Marcel Dekker (1991) [3] Drying Manual, Volume V (Spray Drying) Parts 3 and 4, SPS, Harwell. [4] The Spray Drying Handbook, 5th edition, K Masters, Longman (1992). [5] Drying Manual, Volume V (Spray Drying) Part 2, SPS, Harwell [6] The Prevention of Fires and Explosions in Drying, 2nd edition, J Abbott, IChemE, (1991).



APPENDIX A CONTENTS LIST FOR SPS DRYING MANUAL VOLUME V PART 1

(A Practical Guide to Selection and Design of Spray dryers)

Page No

1 INTRODUCTION 1.7 1.1 Background and purpose 1.7 1.2 Scope and limitations 1.7 1.3 Report format 1.7 2 DESIGN GUIDE 1.9 2.1 Preliminary examination of data 1.9 2.2 Rough sizing on basis of air throughput 1.9 2.3 Flowsheet considerations 1.9 2.4 Atomization method and chamber diameter 1.9 2.5 Chamber selection 1.11 2.6 Other considerations and design methods available 1.11 2.7 Programmable calculator program (Texas T159) 1.11 2.8 Psychrometric chart calculations 1.14 2.9 Example calculation illustrating psychrometric chart

design method 1.17 2.9.1 Specification of requirements 1.17 2.9.2 Solution procedure 1.17 3 FLOWSHEET CONSIDERATIONS 1.24 3.1 Is the liquid to be removed water or some other material? 1.24 3.2 Is there any volatile material present in the feed other

than water or solvent that is likely to cause problems? 1.24 3.3 Is the product or solvent toxic, carcinogenic, irritating or

the like? 1.24 3.4 Is the product a dust explosion risk? 1.24 3.5 Does the product suffer from exposure to oxygen at

drying temperatures? 1.25 3.6 Is the product sensitive to products of combustion? 1.25 3.7 Is the product permanently damaged by heat within

the exposure times applying in spray dryers? 1.25 3.8 Can the dried product be sticky, or suffer temporary

thermal damage? 1.26



3.9 Is there a maximum temperature for final discharged product? 1.26

3.10 Could the final product be discharged with advantage at a single point other than at the dryer itself? 1.26

3.11 Is the spray dryer the first stage of a multi-stage drying system? 1.27

3.12 Is product particle size classification necessary or desirable? 1.27 3.13 Is heat available from an external source? 1.27 3.14 What are the limits on emissions to atmosphere? 1.27 3.15 Is heat recovery from dryer exhaust desirable? 1.28 3.16 Can the product foam in water/solvent? 1.28 3.17 Can the feed liquid foam? 1.28 3.18 Are there acoustic limits to be met? 1.28 3.19 Must special attention be paid to hygiene and cleanability? 1.28 4 CAPACITY CONSIDERATIONS 1.29 4.1 Agreement on required capacity 1.29 4.2 Evaporative load variations 1.29 4.3 Effects of transient and abnormal conditions 1.29 4.4 Shutdown and cleaning outages 1.29 4.5 Periodic maintenance requirements 1.29 4.6 Maintenance of efficient operating conditions 1.30 4.7 Effect of product properties 1.30 4.8 Plants for multiple products 1.30 4.9 Feedstock concentration 1.30 5 ATOMIZATION CHARACTERISTICS AND SELECTION 1.32 5.1 Methods available 1.32 5.2 Centrifugal atomizers 1.33 5.3 Pressure nozzles 1.33 5.4 Pneumatic (two-fluid) nozzles 1.33 5.5 Maximum particle size in feedstock 1.33 5.6 Spray-air mixing 1.33 5.7 Cost comparison 1.33 5.8 Product granulometry 1.34 5.9 Erosion problems 1.34 5.10 Use of multiple atomizers 1.34 5.11 Design choices 1.34 5.12 Size distribution of spray droplets 1.34



5.13 Atomizer selection 1.36 5.14 Re-examination of product specification 1.36 5.15 Atomizer choice (summary) 1.36 6 DRYING CHAMBERS 1.37 6.1 Introduction 1.37 6.2 Box-type dryers 1.37 6.3 Cylindrical dryers 1.37 6.4 Choices in chamber design parameters 1.37 6.5 Chamber designs commonly used 1.40 6.6 Applications of various chamber designs 1.40 6.7 Selection of standard size and shape 1.41 6.8 Definition of chamber volume facto 1.41 6.9 Effect of atomization and airflow on chamber design 1.41 6.10 Prevention and removal of deposits from chamber walls 1.43 6.11 Floor sweepers for flat-bottomed chambers 1.43 6.12 Designing for explosion risks in the dryer 1.43 6.13 Largest median particle size for given chamber diameter 1.44 6.14 Consideration of gas residence time in chamber sizing 1.44 7 SELECTION OF DESIGN PARAMETERS 1.45 7.1 Fixed design data 1.45 7.2 Parameters which may be varied 1.45 7.3 Detailed consideration of design variables 1.45 7.3.1 Flowsheet 1.46 7.3.2 Oxygen level 1.46 7.3.3 Condenser temperature 1.46 7.3.4 Inert gas 1.46 7.3.5 Partial recycle of exhaust gas without condensation 1.46 7.3.6 Heating means 1.47 7.3.7 Wet recycle of fines 1.47 7.3.8 Scrubber liquor 1.47 7.3.9 Cold air additions 1.47 7.3.10 Floor sweeper air 1.48 7.3.11 Atomization 1.48 7.3.12 Dryer air inlet temperature 1.48 7.3.13 Dryer air outlet temperature 1.49 7.3.14 Residence time 1.51 7.4 Design data questionnaire 1.55



NOMENCLATURE 1.58 REFERENCES 1.59 APPENDICES App. A.1 Derivation of equations in Chapter 2 1.60 A.1.1 Equation (2.1) 1.60 A.1.2 Equation (2.3) 1.60 A.1.3 Equation (2.7 1.61 A.1.4 Equation (2.4) 1.61 A.1.5 Equation (2.2) 1.61 A.2 Calculator program for Texas TI59 1.63 A.2.1 Program equations 1.63 A.2.2 Program listing 1.64 TABLES Table 2.1 Values of fuel moisture rate, fmr 1.13 2.2 Psychrometric chart data state 1.15 5.1 Major characteristics of atomizers 1.32 6.1 Volume factors for dryer chambers 1.41 6.2 Recommended chamber shapes 1.42 7.1 Drying conditions used on some products 1.53



ILLUSTRATIONS Fig 2.1 Computer program results 1.14 2.2 Psychrometric chart for once-through, direct-fired dryer example 1.20 2.3 Psychrometric chart showing effect of cold air addition 1.21 2.4 Psychrometric chart for direct-fired system with partial

exhaust recycle 1.22 2.5 Psychrometric chart showing effect of an exhaust gas scrubber 1.23 5.1 Typical product particle size distribution for three atomizer types 1.35 5.2 Extreme and typical product particle size ranges for three different

atomizer types 1.35 6.1 Chambers for centrifugal atomizers 1.38 6.2 Chambers for nozzle atomizers 1.39 7.1 Relationship between product moisture content and

air outlet temperature 1.50 7.2 Typical dryer design data questionnaire 1.56

