Spray Drying

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-DRY-006

Spray Drying

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(statutory or otherwise) is excluded except to the extent that exclusion is
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information. Freedom under Patent, Copyright and Designs cannot be assumed.

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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 temperaturesensitive 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-PEGDRY-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
1.2
1.3

Background and purpose
Scope and limitations
Report format

1.7
1.7
1.7

2

DESIGN GUIDE

1.9

2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9

Preliminary examination of data
Rough sizing on basis of air throughput
Flowsheet considerations
Atomization method and chamber diameter
Chamber selection
Other considerations and design methods available
Programmable calculator program (Texas T159)
Psychrometric chart calculations
Example calculation illustrating psychrometric chart
design method
2.9.1 Specification of requirements
2.9.2 Solution procedure

1.9
1.9
1.9
1.9
1.11
1.11
1.11
1.14

3

FLOWSHEET CONSIDERATIONS

1.24

3.1
3.2

Is the liquid to be removed water or some other material?
Is there any volatile material present in the feed other
than water or solvent that is likely to cause problems?
Is the product or solvent toxic, carcinogenic, irritating or
the like?
Is the product a dust explosion risk?
Does the product suffer from exposure to oxygen at
drying temperatures?
Is the product sensitive to products of combustion?
Is the product permanently damaged by heat within
the exposure times applying in spray dryers?
Can the dried product be sticky, or suffer temporary
thermal damage?

1.24

3.3
3.4
3.5
3.6
3.7
3.8

1.17
1.17
1.17

1.24
1.24
1.24
1.25
1.25
1.25
1.26


3.9

3.12
3.13
3.14
3.15
3.16
3.17
3.18
3.19

Is there a maximum temperature for final discharged
product?
Could the final product be discharged with advantage at
a single point other than at the dryer itself?
Is the spray dryer the first stage of a multi-stage drying
system?
Is product particle size classification necessary or desirable?
Is heat available from an external source?
What are the limits on emissions to atmosphere?
Is heat recovery from dryer exhaust desirable?
Can the product foam in water/solvent?
Can the feed liquid foam? 1.28
Are there acoustic limits to be met?
Must special attention be paid to hygiene and cleanability?

4

CAPACITY CONSIDERATIONS

1.29

4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9

Agreement on required capacity
Evaporative load variations
Effects of transient and abnormal conditions
Shutdown and cleaning outages
Periodic maintenance requirements
Maintenance of efficient operating conditions
Effect of product properties
Plants for multiple products
Feedstock concentration

1.29
1.29
1.29
1.29
1.29
1.30
1.30
1.30
1.30

5

ATOMIZATION CHARACTERISTICS AND SELECTION

1.32

5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
5.12

Methods available
Centrifugal atomizers
Pressure nozzles
Pneumatic (two-fluid) nozzles
Maximum particle size in feedstock
Spray-air mixing
Cost comparison
Product granulometry
Erosion problems
Use of multiple atomizers
Design choices
Size distribution of spray droplets

1.32
1.33
1.33
1.33
1.33
1.33
1.33
1.34
1.34
1.34
1.34
1.34

3.10
3.11

1.26
1.26
1.27
1.27
1.27
1.27
1.28
1.28
1.28
1.28


5.13
5.14
5.15

Atomizer selection
Re-examination of product specification
Atomizer choice (summary)

1.36
1.36
1.36

6

DRYING CHAMBERS

1.37

6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
6.12
6.13
6.14

Introduction
Box-type dryers
Cylindrical dryers
Choices in chamber design parameters
Chamber designs commonly used
Applications of various chamber designs
Selection of standard size and shape
Definition of chamber volume facto
Effect of atomization and airflow on chamber design
Prevention and removal of deposits from chamber walls
Floor sweepers for flat-bottomed chambers
Designing for explosion risks in the dryer
Largest median particle size for given chamber diameter
Consideration of gas residence time in chamber sizing

1.37
1.37
1.37
1.37
1.40
1.40
1.41
1.41
1.41
1.43
1.43
1.43
1.44
1.44

7

SELECTION OF DESIGN PARAMETERS

1.45

7.1
Fixed design data
7.2
Parameters which may be varied
7.3
Detailed consideration of design variables
7.3.1 Flowsheet
7.3.2 Oxygen level
7.3.3 Condenser temperature
7.3.4 Inert gas
7.3.5 Partial recycle of exhaust gas without condensation
7.3.6 Heating means
7.3.7 Wet recycle of fines
7.3.8 Scrubber liquor
7.3.9 Cold air additions
7.3.10 Floor sweeper air
7.3.11 Atomization
7.3.12 Dryer air inlet temperature
7.3.13 Dryer air outlet temperature
7.3.14 Residence time
7.4
Design data questionnaire

1.45
1.45
1.45
1.46
1.46
1.46
1.46
1.46
1.47
1.47
1.47
1.47
1.48
1.48
1.48
1.49
1.51
1.55


NOMENCLATURE

1.58

REFERENCES

1.59

APPENDICES
App.
A.1
A.1.1
A.1.2
A.1.3
A.1.4
A.1.5
A.2
A.2.1
A.2.2

Derivation of equations in Chapter 2
Equation (2.1)
Equation (2.3)
Equation (2.7
Equation (2.4)
Equation (2.2)
Calculator program for Texas TI59
Program equations
Program listing

1.60
1.60
1.60
1.61
1.61
1.61
1.63
1.63
1.64

TABLES
Table
2.1
2.2
5.1
6.1
6.2
7.1

Values of fuel moisture rate, fmr
Psychrometric chart data state
Major characteristics of atomizers
Volume factors for dryer chambers
Recommended chamber shapes
Drying conditions used on some products

1.13
1.15
1.32
1.41
1.42
1.53


ILLUSTRATIONS
Fig
2.1
2.2
2.3
2.4
2.5
5.1
5.2
6.1
6.2
7.1
7.2

Computer program results
Psychrometric chart for once-through, direct-fired dryer example
Psychrometric chart showing effect of cold air addition
Psychrometric chart for direct-fired system with partial
exhaust recycle
Psychrometric chart showing effect of an exhaust gas scrubber
Typical product particle size distribution for three atomizer types
Extreme and typical product particle size ranges for three different
atomizer types
Chambers for centrifugal atomizers
Chambers for nozzle atomizers
Relationship between product moisture content and
air outlet temperature
Typical dryer design data questionnaire

1.14
1.20
1.21
1.22
1.23
1.35
1.35
1.38
1.39
1.50
1.56


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