drying technologies of the future

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DRYING TECHNOLOGIES OF THE FUTUREArun S. Mujumdar aa Department of Chemical Engineering, McGill University, Montreal, CanadaVersion of record first published: 25 Apr 2007.

To cite this article: Arun S. Mujumdar (1991): DRYING TECHNOLOGIES OF THE FUTURE, Drying Technology: An InternationalJournal, 9:2, 325-347

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DRYING TECHNOLOGY, 9 ( 2 ) , 325-347 (1991)

DRYING TECHNOLOGIES O F THE FUTURE'

Arun S. Mujumdar Department of Chemical Engineering

McGill University Montreal, Canada

ABSTRACT

This paper reviews the recent advances and general trends in drying technologies

of industrial interest. Numerous emerging technologies are listed with emphasis on

the following five areas: Carver-Greenfield Process for drying of sludges.

superheated steam drying, pulse combustion drying, high intensity drying techniques

for paper and the novel impinging stream dryers. Potential application areas for the

new technologies are identified.

Keywords and Phrases: novel dryers; high intensity drying; Carver-Greenfield

process; steam drying; pulse combustion drying; impinging stream drying.

Drying is the most common and most energy-consuming industrial operation.

With literally hundreds of variants actually used in drying of particulate solids, pastes,

continuous sheets, slurries or solutions, it provides the most diversity among the

' This paper was presented as a Plenary Lecture at the CHISA'90 Congress (10th International Congress of Chemical Engineering, Equipment Design and Automation), Prague, Czechoslovakia, August 1990

325

Copyright 0 1991 by Marcel Dekker. Inc.

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326 MUJUMDAR

chemical engineering unit operations. It is also one of the least understood

operations. There is much "know-how" but not enough "know-why" in many aspects

of drying technologies. One often hears of dryers being "built" rather than "designed.

Design of most dryers therefore still depends extensively on pilot tests coupled with

extensive experience often reduced to various "rules of thumb. New technological

developments such as those in biotechnology, advanced materials, ceramics etc. will

not diminish the significance of drying technology; indeed, the novel processes and

stringent product quality requirements will make additional and difficult demands on

the designer of drying equipment. Current practice is to adapt or even force-fit

existing dryer designs to meet the new demands - in many cases rather remarkably

well.

Much of the drying R & D in the past two decades was motivated by the "energy

crisis" of the early 1970's and the extrapolated high cost of energy during the 1980's

which did not actually materialize. While the energy cost of drying low unit value

products continues to be an important consideration, current R & D is driven by the

needs - and often idiosyncrasies - of new processes and increasingly rigorous quality

requirements e.g. in biotechnological products used in the pharmaceutical industry,

and in the manufacture of superconducting materials or advanced ceramics by novel

spray drying processes to obtain the desired electrical and magnetic properties. Some

of the new processes and products may not be amenable to dehydration using

conventional dryers without introduction of significant modifications. In many

industrial processes use of conventional technology to dry a new, modified product

can result in quality and/or capacity problems. It is best to start with the dryer

selection process "from square one" to be sure the final selection is optimal and cost-

effective.

Drying technologies have evolved rather slowly over the past five decades. In the

past decade many innovative dryers - mainly modifications of basic types - have been

introduced in the market. Several new concepts are still at pilot or laboratory stage.

Earlier technologies have reached maturity and most have reached or approached

the "saturation level". Further improvement in their performance can be achieved,

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DRYING TECHNOLOGIES OF THE FUTURE 327

if at all, at significant incremental expense and/or effort which may not be

economically justified. Novel drying technologies may be expected to emerge when

current technologies have been developed to their limit.

It is extremely difficult and highly risky to predict what the drying technologies

of the next century will be but I have accepted the challenge. 1 will try to extrapolate

the current developments and trends in drying R & D to forecast what I see,

obviously in a biased manner, as drying technologies of the future. My evaluation

is strictly a personal viewpoint based on my contacts with the international drying

community and the literature available. Kudra and Raghavan (1990) have produced

an excellent concise guide of relevant literature sources for the novice as well as the

specialist. Numerous assumptions are implicit in this exercise which often are

specific to certain products and how they are produced e.g. in paper drying it is

assumed that the existing wet process will continue to be the dominant papermaking

process rather than a dry-forming process which will diminish the role of drying

significantly.

In general the trends in drying technology will continue to be in the direction of:

higher energy efficiency

enhanced drying rates leading to more compact dryers

better control for enhanced quality and optimal capacity

improved product quality

reduced environmental emissions

safer operation by elimination (or management) of fire and explosion

hazards

flexible systems which can be used for drying of several different products

at varying production rates

multi-processing capabilities e.g. combine drying with chemical reactions,

agglomeration, cooling/heating, coating, blending, classification etc. in one

unit.

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MUJUMDAR

development of "designer dryers" to impart special physical characteristics

to products e.g. in the manufacture of advanced ceramics, superconducting

materials.

development of dryers employing multiple heat sources which can be

tailored to meet requirements of specific products (e.g. Dostie et al.).

development of dryers which avoid unnecessary ,product handling by

combining operations such as dissolution or hydrolysis (e.g. the sol-gel

process for high purity ceramics) in the same vessel where drying is

accomplished.

GENERAL TRENDS

Dryers can be classified according to the physical form of the product, the manner

in which it is handled within the dryer, the mode of heat input, operating pressure

etc. Convection (or direct) type are the most common e.g. rotary, fluidized bed,

spray, flash, impingement, conveyor dryers etc. Contact (conduction or indirect)

dryers are often operated under vacuum or at low gas-flow rates to remove the

moisture being evaporated. Indirect dryers have better thermal utilization

efficiencies since heat loss in the inert gas is minimized or eliminated. In direct

dryers heat loss in the exhaust can be as high as the heat of vaporization unless effort

is made to recover exhaust gas energy albeit at some expense. Infrared heating is

employed in certain special dryers e.g. drying of coatings, printed webs etc. More

recently the advantages of volumetric heat supply by microwave or radio frequency

(RF) radiation fields have been well recognized. In dielectric drying energy is

absorbed selectively by the polar molecules of water thus obviating the need to

follow Fourier's law of heat conduction to introduce heat into the interior of the wet

solid via generation of significant thermal gradients in the material.

Combined mode drying methods are in principle superior to those employing only

one mode of heat transfer. Already there is a trend towards use of combined

directlindirect dryers e.g. fluidized bed dryers with immersed heat exchanger panels

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for drying of resins, impingement and RF dryers for coated webs, and impingement

and microwave dryers for extruded pasta. This trend is expected to continue

whenever it is applicable. Solids that tend to stick to and foul heat exchanger

surfaces cannot be dried in such equipment except with solids backmixing. Supply

of indirect heat can reduce the size of the equipment by up to 50 per cent, especially

for heat sensitive materials.

Another clear trend is towards combining two different types of dryers when the

drying characteristics of the material warrant such a combination. In the past only

one type of dryer was selected. The combination of a flash dryer followed by a

fluidized bed dryer, for example, is ideal to remove surface moisture from particulate

solids within a few seconds and then allow more residence time in a smaller fluidized

bed to remove the internal moisture content. In this case there is no need for solids

backmixing since the particles are generally fluidizable once their surface moisture

is removed in the flash dryer. Similarly, a smaller spray dryer unit may be followed

by a fluidized bed dryer to remove the internal moisture from the relatively wet spray

dryer discharge solids. If carried out only in a spray dryer the size and cost of the

spray dryer is much greater. Such "spray-fluidizers" are now increasingly popular.

The after-dryer could be a vibrated bed dryer or a fluid bed with indirect heating

depending on the product characteristics.

I expect that drying systems will increasingly include two or more dryer types if

the drying kinetics and product characteristics justify this. Such systems are

inherently more flexible and may be justified on the grounds of flexibility in

processing and manufacturing. Scale-up, design and optimization of the system,

however, will be more complex.

Control of dryers is another area where I expect significant improvements. On-

line solids moisture measurement is necessary for control of quality, agglomeration,

size distribution etc. of certain materials during drying. Non-contact instantaneous

local moisture content measurement of fast moving webs is important in monitoring

and control of paper drying processes. Developments in the area of on-line moisture

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330 MUJUMDAR

measurement of high temperature dusty gases and of solids will permit better control

of dryers of the Future. I expect expert systems will he developed commerciaHy to

select, design, analyze and control some of the more commonly used dryers.

Mathematical models will be coupled with heuristic rules based on know-how to

improve scale-up, design and analysis procedures for dryers. Unfortunately, the

know-how is not only dryer-type dependent but also product-dependent. So the odds

against development of a general expert program applicable even to one type of

dryer are quite high since different products often behave strikingly differently in the

same equipment. Modelling of product quality remains an illusive issue which

deserves greater R&D.

As far as the sburce of energy for drying is concerned, there is unlikely to be any

change. Fossil fu.els will continue to be the main source of thermal energy. Solar

energy may be employed in developing countries blessed with high levels of

insolation. Use of electricity will be confined to provide fan power or mechanical

conveying of product in the dryer. Electricity as a source of thermal energy may he

justifiable only if electricity is relatively inexpensive and the "net" energy consumption

of the drying process is low e.g. in steam drying if the exhaust steam is recompressed

for reuse.

It is important to note that stricter environmental emission standards will lead to

the need to design and operate more efficient dryers. Fossil fuel-fired dryers are

responsible for staggering amounts of CO, emissions. In future, I predict that there

will he legislative limits on CO, emissions per unit product produced.

NEW DEVELOPMENTS

In the second half of this paper I would like to discuss some specific drying

technologies that, in my opinion, show significant potential for an appreciable level

of replacement of some of the current drying technologies within the next decade.

In most cases further R & D is required and is in progress. In particular, because

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of constraints of space I have chosen to mention the following as they offer some

unique advantages in terms of the energy consumption, product quality etc.

(1) Carver-Greenfield process e.g. for drying of sludges

(2) Superheated steam drying e.g. flash, fluid bed, impinging jets etc.

(3) Pulse combustion drying for sludges, slurries, pastes etc.

(4) High-intensity drying processes for drying of paper

( 5 ) Impinging stream drying for particulates, slurries and sludges

However, this list is by no means all-inclusive. Numerous other drying

technologies are currently at various stages of development and, I am sure, several

of them will be - unless they already are - successfully commercialized. Among them

I would like to mention the following processes.

freeze spray drying processes involving addition of non-aqueous solvents

or polymers to obtain desired quality in the product.

drying of impinging droplet sprays on heated surfaces.

drying of sprays and dispersed solids in microwave fields.

fluid bed drying of continuous sheets or slabs using beds of inert dessicant

particles e.g. drying of leather using fluidized beds of indirectly heated

silica gel particles.

immersion drying by mixing and then de-mixing of wet particles with dry,

indirectly heated dessicant particles e.g. drying of grains using agitated

beds of natural zeolite particles. (Vibrated beds may also be used for this

purpose)

intermittent drying e.g. supplying heat (by convection, radiation or

microwave) only intermittently rather than continuously to save energy and

minimize damage to heat-sensitive materials.

nonthermal drying (strictly dehydration processes) using osmotic potential

e.g. drying of fruit slices by contacting with high sugar concentration syrups

(e.g. Raoult et al.).

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- drying of slurries, solutions etc. in fluidized, spouted, vibrofluidized or

agitated beds of inert particles.

- drying in modified spouted or fluidized beds e.g. mechanically spouted

beds, fluidized beds with built-in compartmentalized conveyors to ensure

true plug flow operation, centrifugal fluidized beds; beds fluidized by

impinging jets; mechanically stirred fluid beds etc.

- supplementary heating of rotary, fluid bed, agitated bed, spouted dryers

etc. of particulates by piping in microwave power to reduce drying times.

Inductive heating of moving surfaces (e.g. vessel walls of rotating vessels

or rotating agitator paddles) to introduce indirect heating and enhance

thermal performance.

- use of acoustic/ultrasonic radiation to enhance both external and internal

heat and mass transfer rates during drying especially in conjunction with

microwave, infrared or convective drying of dispersed, heat-sensitive

materials.

- use of membrane processes (membranes selective to passage of water

vapour) to recover the latent heat in the water vapor contained in low

temperature dryer exhausts without use of large heat exchangers.

- greater use of optimized two or more stage dryers of different types to

maximize energy utilization efficiencies as well as enhanced product

quality (e.g. flashlfluid bed dryers, spraylfluid bed dryers,

through/impingement dryers etc.).

- greater use of efficient dewatering techniques such as electroosmotic

dewatering to reduce load on thermal dryers.

The list is endless. One can obtain a good overview of current R & D activities

in the field by scanning the proceedings of the biennial International Drying

Symposia (IDS) initiated in 1978 as well as Drying Technology - An International

Journal. The tri~ly international forum provided by IDS meetings will continue to

allow exchange of information across geopolitical, disciplinary, linguistic and

industrial sectorial boundaries. As correctly noted by Keey (1978) there is significant

diffusional resistance to transfer of knowledge across linguistic and professional

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barriers. Such resistances act to slow the overall progress of drying technology as a

discipline. It is not unusual to come across very similar theoretical and/or

experimental studies published concurrently in different parts of the world. For

effective utilization of the limited resources available to promote and foster drying

R & D, it is essential to keep all channels of communication open and thus avoid

duplication of effort "to reinvent the wheel". I am pleased to observe that such a

trend began in this field over a decade ago and all indications point to even greater

level of "perestroika" and "glasnost" in the coming years.

Dewatering and drying of waste sludges is becoming an increasingly important

area of concern globally. The pulp and paper industry in the USA alone produces

over 2 million tons of primary sludge solids and some 0.30 million tons of biological

sludge solids per year. Most of the sludge is disposed of in landfills (Miner, 1986)

but increasingly there is a trend towards utilizing the calorific value of the waste and

burn it in boilers.

As early as the late 1940's an innovative dehydration process was proposed which

involved use of a nonvolatile liquid medium as a carrier for dispersed wet solids to

be dried in effectively multiple-effect evaporators. This is the so-called Carver-

Greenfield process. In multiple-effect evaporation external heat is supplied to the

last stage (effect) and the vapor produced there is used as heat source to the

preceding evaporator. The energy utilization can be further enhanced by mechanical

vapor recompression (MVR) of the vapor from an evaporator and then using it as

the heating medium in the same evaporator (Pluenneke and Cmmm, 1986).

In the Carver-Greenfield process first commercialized in 1961 to dry ground

slaughterhouse waste, the ground meal is contained an indigenous carrier oil (viz.

tallow). This liquid mixture was processed in a two-stage multiple-effect evaporator.

The tallow and dried solids were separated by centrifuging. The tallow was recycled

to maintain an oil-to-product ratio of 8:l. Some 60 plants of this type still operate

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in USA. Pluenneke and Cmmm (1986) have reported on a highly efficient new

version using . a light oil which can be recovered by evaporation rather than by

mechanical pressing. Light oils also extract oils present in the feed material e.g.

greases and fats from sewage sludge, oils from peat and lignite drying etc. Further,

an MVR system is included to obtain energy consumption of the order of 50 kJ/kg

water evaporated.

In the most recent version of the MVR-C-G process the feed is very dilute (2%

solids) to quite high in solids content (50%). Drying of such sludges is accomplished

in one stage. The solids size must not exceed 7.5 mm along any axis. Existing plants .

do not typically use MVR yet. Falling film evaporators can be used for low solids

concentration while forced flow is required at high concentrations. Up to five stages

(effects) are used in various C-G installations to dry sewage sludge.

Among current applications of C-G process are: drying of activated sludge from

wood pulp plant, wool processing waste from a textile manufacturing facility, instant

coffee processing waste, dairy product waste, sewage sludge etc. Products like lignite

and peat have been dried successfully at the pilot level with energy consumption in

the order of 135 kJ/kg water evaporated. The process is particularly attractive for

drying of peat. I expect that this process and its variants will find new and major

applications around the world as engineers and managers concerned with decision-

making will become familiar with the technology and as experience accumulates on

its industrial operations.

(2) Suoerheated Steam D ~ i n g

Among the principal advantages of steam drying are: cost-effective use of exhaust

steam from the dryer, lack of fire/explosion hazards, reduced potential for oxidative

damage of products ( e g discoloration, browning reactions etc.), generally improved

product quality, generally higher drying rates leading to more compact dryers etc.

Among the disadvantages are: limited industrial experience, difficulties in feed

discharge of materials without excessive air leakage, higher product temperature

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(100' C for atmospheric steam dryers removing surface moisture) etc. The higher

heat capacity of steam as compared to air reduces the mass flow rate required to

deliver a given amount of thermal energy. Also, its higher thermal conductivity

implies higher heat transfer coefficients as compared to air. The lower dynamic

viscosity of steam, however, implies longer stopping distances in steam flow due to

reduced drag; this results in higher fluidization velocities and larger chamber

dimensions for a spray dryer, for example. (See Mujumdar (1990) for a Full review

of steam drying technologies including an evaluation of their market penetration

potential.)

For heat-sensitive products lower pressure operation is possible. At least one

German company markets high pressure steam dryers for beet pulp although the

overall economics appear to be highly unfavorable even allowing credit for

significantly improved product quality. Lower pressure operation for handling heat-

sensitive solids (e.g. silkworm cocoons) is also a possibility.

Published results on industrial/pilot/laboratory scale steam drying of large

tonnage products such as coal, pulp, bark, peat, paper, timber, wood particles, beet

pulp etc. are reviewed and summarized by Mujumdar (1990). It is noteworthy that,

in principle, any direct (convection) dryer could be converted to superheated steam

operation. Pilot scale tests on steam spray dryers have been reported. Potter (1985)

and Kumar and Mujumdar (1990) have presented reviews of steam drying principles

and practice.

In general, the thermal energy consumption in direct dryers can be reduced by

supplying a part of the thermal energy requirement indirectly e.g. by contact

(conduction) heat transfer from dryer walls or heated panels or tubes immersed

within a bed of particulate solids. This also reduces the size of the dryer and

associated ductwork, size of gas-cleaning equipment, size of the fan etc. due to

reduced volumes of gas handled. Furthermore, use of mechanical vibration to

pseudo-fluidize beds of wet particles.allows operation of the dryer for particulates at

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336 HUJUMDAR

low gas flowrates. Compared to conventional fluidization such vibrated fluid beds

handle the solids gently and reduce attrition rates without loss in heat/mass transfer

rates. Thus a steam vibrated fluid bed dryer with immersed heat exchange panels

is expected to result in reduced energy consumption, smaller dryer and better product

with more effective gas cleaning.

Cost-effective utilization of the dryer exhaust steam - after cleaning or as is - is

the key to the success of such technology despite several other advantages already

quoted. Following schemes can be envisaged for the utilization of exhaust steam

which is produced at the same rate as the evaporation rate in the dryer:

(a) Use directly as process steam elsewhere in the plant (withlwithout

cleaning, need for cleaning is dryer- and product-specific).

(b) Recycle a part after reheating in a heat exchanger (may be electrical

heater); excess as process steam. (Consider fouling of heat exchanger

tubes etc.)

(c) Recycle after compression, or compression and reheating a part of the

exhaust stream depending on dryer inlet steam conditions.

(d) Partial recycle after compression and/or reheating and use excess steam

to preheat the feed to about 100' C so that condensation in dryer can be

avoided thus reducing the residence time (and hence the dryer size)

needed.

( d Use two or more stages of drying and achieve essentially a multiple-effect

operation (like in evaporators) with attendant steam economy.

Superheated steam could be compressed prior to reuse by thermocompression

(using steam jet ejectors ) or by mechanical compressors (e.g. centrifugal or screw

type). Both mechanical vapour recompression and thermocompression are quite

complex with many variables affecting payback and operating costs. For very large

drying systems and with new developments in steam compressor technology, it is

hoped that mechanical vapor recompression will provide a cost-effective means of

energy recovery and electricity utilization in steam dryers. Similar conclusions should

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apply to other types of dryers as well. Cleaning of steam for reuse in the dryer itself

or elsewhere in the process remains a problem area.

Mujumdar (1981) first proposed the use of superheated steam for drying of paper.

Based on his concept, a new drying process called Swift (Steam A t h flow -through)

is being developed at McGill University for drying of permeable grades of paper such

as newsprint. Conceptually this process utilizes high velocity superheated steam jets

impinging on the wet web supported on a rotating perforated vacuum roll; a part of

the jet flow is drawn through the web which accomplishes a significant fraction of the

total drying. The drying rates have been shown to be up to ten times higher than

those achieved in conventional multi-cylinder dryers. There is quality enhancement

over certain operating ranges. Note that steam impingement can be combined with

contact drying. Further, pure steam blow-through d~ying of light paper grades such

as tissue or towelling deserves attention from transport phenomena as well as quality

standpoints. Since through drying essentially amplifies any moisture profile entering

the dryer, it is also important to examine cost-effective ways for profile correction e.g.

use of microwave, R F or IR energy in conjunction with conventional through drying.

No work has been reported in the open literature on these potential modifications

of the through drying process. In general, steam drying (if it is feasible for the

product) is likely to be more energy efficient and results in a better product than air

drying.

(3) Acoustic and Pulse Combustion D ~ n g

Although reports on high-intensity acoustic fields on intensification of heatlmass

transfer appeared some 60 years ago, acoustic drying at near ambient temperature

of heat-sensitive products has not yet emerged from the laboratory stage. Low

efficiencies (less than 20%) of conversion of energy to acoustic radiation are

primarily to blame. As for the mechanisms of enhancement, several have been

postulated (e.g. repeated elevated and reduced pressure fields within the porous

medium; enhanced turbulent flow of air at the surface of the moist body etc.). It is

hypothesized that liquid viscosity is lowered in ultrasonic fields leading to increased

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liquid diffusivity in solids. It is also postulated that small air bubbles within

capillaries which normally impede moisture movement, pulsate under action of

temperature variations due to the alternating compression and rarefaction in sound

waves.

Acoustically-enhanced rotary, tunnel, fluidized bed dryers have been designed and

tested at laboratory scale or pilot scale in the USSR, Japan, USA etc. However,

their low utilization of acoustic energy made them unacceptable on the industrial

scale.

Acoustic fields can be used to generate sprays in a spray dryer. If the suspension

or solution to be dried is admitted into the sound generation zone of a gas-jet

generator, the shock waves produced by the generator not only atomize the solution

but also ensure a high evaporation rate.

Borisov and Gynkina (1972) used sound concentrators to enhance the drying rate

many-fold in a pneumatic dryer fitted with several elliptical concentrators. A gas-jet

sound generator was mounted at the focus of each concentrator. Frequencies of 18-

19 kHz at sound levels as high as 177 dB at a relatively low air flow rate were used.

Products such as polystyrene emulsions were dried within 2-3 seconds. Energy and

quality data were not reported.

For high value, heat-sensitive products (e.g. pharmaceuticals, some new

biotechnology products) acoustic drying seems to provide an attractive alternative.

For example a thin layer of particulate material can be transported through an

intense acoustic field with the help of a low frequencyvibrating trough. Experiments

also show that low temperature drying of gelatinous emulsion coatings on sheets (e.g.

photographic films) can be accomplished in sound fields. Borisov and Gynkina

(1972) report that at 17'C at a frequency of 6.6 kHz and pressure level of 160 dB,

a 35 mm film coating could be dried in 4-5 minutes while convective drying at 35 '

C takes up to 10 minutes to accomplish the same drying.

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Acoustic drying may influence the quality of the product although there is hardly

any systematic literature on it. There are indications in the literature that the

strength of ceramic molds dried in a sonic field (24' C, 156 dB, 10 H z drying time

2 h) had better flexural strength than if dried in hot air for 8 hours.

In general acoustic drying works better in removing the surface moisture in the

first drying rate period. For relatively thin or dispersed materials the pulsating

component of velocity is important in affecting the heat and mass transfer rate.

Rough calculations show that the actual energy consumption in acoustic drying is

three to four times that in conventional thermal techniques mainly because of the

low efficiency of sonic radiators (-25%). It is therefore worth considering only for

difficult-to-dry, expensive and low-tonnage materials. In some cases the safety

aspects and product quality may offset the cost of acoustic drying.

In view of the above, attempts have been made to combine acoustic drying with

other methods e.g. combination with convection or dielectric drying, for example. In

convection drying the relative benefits of acoustic drying fall off with rise in

temperature, however. Borisov and Gynkina (1972) quote some interesting data

showing the beneficial effects of combining microwave drying with acoustic radiation

in drying of ceramic disks. This combines the advantages of sonic drying in removing

the surface moisture with those of microwave drying in removing the internal

moisture. Combining infrared radiation with acoustic radiation has also been shown

to be beneficial for drying of asbestos, ceramic sheets etc. If the final surface

temperature were held the same the drying rate under the influence of sound field

(7 kHz, 158 dB) was three times as high as infrared alone since the radiative flux

must be reduced significantly to meet the surface temperature constraint.

Instead of generating very high frequency pressure fields using gas jets, another

approach to enhance heat/mass transfer rates from dispersed particles is to subject

them to relatively low frequency (-100 Hz) but high amplitude oscillations generated

in the exhaust of pulse combustion (PC) chambers. Indeed, the very high turbulence

intensities encountered in PD disperses pasty products and atomizes slurries without

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340 MUJUMDAR

any supplementary dispersion or atomization devices. (Olsen and Holmes, 1986).

Although work on PC drying started over a decade ago, only recently are such

systems available commercially (Bepex, 1990). Depending on the product they are

competitive with some flash drying and spray drying systems. Up to 50% reductions

in operating costs are claimed. For drying of pumpable slurries and solutions,

particularly those of heat-sensitive materials, pilot tests suggest improved product

quality, reduced maintenance and reduced energy consumption. The very short

residence time at very high temperature results in relatively cool product. Capital

costs may be up to 40% lower than those of corresponding spray dryers; no atomizer

is required. The sound pressure levels generated within the PC chamber are up to

150 dBA but outside the, dryer (3 inches from outer surface) the noise level is only

00 dBA. There is a definite difference between the product properties (including

size distribution) obtained in PCD as opposed to spray drying. Hence, pilot testing

is essential. It is expected that new developments will emerge as this relatively new

technology matures.

Nomura et al. (1989) used PC exhaust for convective drying of planar surfaces.

While high drying rates were obtained in laboratory tests, the heat transfer

coefficients obtained are of the same order as those obtained in high velocity

impingement. Other than the fact that no fans are needed to move the gas flow the

advantages of PC drying are not fully utilized here.

(4) Hieh Intensitv Dwine of Paoer

Conventional papermaking processes are essentially massive dehydration

processes which start with a fibre in water suspension with a consistency of about 0.5

per cent (solids) and end with a paper with 95-98 per cent fibre. The consistency

increase from about 40 per cent to the final level must be carried out by thermal

drying. A wide variety of dryers are used industrially; the most common ones, in

order to their industrial use, are: multi-cylinder (82%), impingement (7.6%). Yankee

(for tissue, towelling, 4%), infrared (3%). flash (I%), vacuum (1%) and dielectric

( ~ 0 . 1 % ) . The drying rate on multi-cylinder dryers is typically 10-25 kg/m2h while

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DRYING TECHNOLOGIES OF THE FUTURE 34 1

that on modern Yankee dryers can be as high as 150 kg/m2h. Up to 60 steam-heated

cylinders (-1.6 m diameter) are needed on a modern newsprint machine. The large

space requirements and capital-intensiveness of the dryer section coupied with a

"steam economy" as high as 1.5 (kg steam used per kg water evaporated), provide

good incentives for development of new high intensity drying systems to replace the

current multi-cylinder dryer - a design concept that was first developed in England

in the early nineteenth century and used wood to heat the cylinders. Bell et al.

(1991) have reviewed the practical aspects of paper drying in some depth.

Several new drying concepts have emerged in the last two decades but none has

entered the commercial arena yet. As noted earlier, at McGill University, we are

attempting to develop the " S W I l T hteam~thflow-through) process which utilizes

a combination of superheated steam jet impingement and through drying. Very high

drying rates can be attained, as demonstrated by laboratory scale tests and simulation

studies. However, major technological innovations will be needed to handle the fast-

moving web particularly at the inlet and exit ends of the dryer without excessive air

infiltration. No adverse effects are expected as far as product quality is concerned.

The Institute of Paper Chemistry (IPC) in the USA has approached the problem

from a different angle. The so-called "high intensity" drying processes developed at

IPC refer to hot-surface drying under conditions of temperature and pressure

(applied to the sheet) such that the heated surface of the web attains a temperature

above the ambient boiling point. The bulk flow of steam through the web results in

rapid internal heating of the web and enhanced vapor removal unimpeded by

external mass transfer resistances. Although not proven unambiguously, it is

plausible that some of the liquid water is forced out without phase change, which

may contribute to improved thermal efficiency. Clearly, high intensity drying requires

high heat transfer rates to the web which can be attained by raising the cylinder

surface temperature and by improved web-surface contact under very high pressure

loading (using so-called press roll or "extended nip") considerably in excess of what

is used in conventional presses. It is proposed to use high-temperature gas burners

to heat the cylinder (rather than condensing steam). The effects of high temperature

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- high pressure combination on the physical properties of the paper need to be

evaluated carefully. Using surface temperatures of 200' C - 400' C and applied

pressures of 2000 - 7000 kPa, drying rates of up to 5000 kg/m2h can be achieved in

the so-called "impulse drying" process. Note that such high evaporation rates are

achieved over very short periods of time and over small areas of the dryer, and have

only been demonstrated in static laboratory scale experiments. Major challenges face

development of this concept, particularly in the area of heat transfer (extremely high

heat fluxes at high temperatures) as well as materials which can withstand over

extended periods the high temperature - high pressure - environment. Nevertheless

it is important to develop such processes for the future.

There are several new developments taking place around the world in the search

for a better way to dry paper. Indeed, the activity is extensive enough to justify an

International Symposium on Alternate Drying Methods for Pulp and Paper which will

be held in Helsinki, Finland in June 1991. I believe that the paper dryer section of

the future will be an optimized combination of the various new concepts being

developed independently by R & D teams in several countries. Each concept by

itself has some strong merits but also some formidable technological barriers to its

implementation in practice. Use of elevated temperatures/pressures, superheated

steam and even microwaves or R F energy to improve moisture profile, are all ideas

that need to be incorporated in a logical fashion. The rapidly evolving field of

advanced materials will no doubt lend a helping hand in the near future. Sprague

(1985) has presented an extensive review of the high intensity dy ing processes for

paper currently being developed.

( 5 ) ~moineing Stream DNWS

Impinging Stream Dryers (ISD) is a class of dryers for particulates or pastes in

which most of the moisture removal occurs in an impingement zone which develops

as a result of "collision" of two oppositely directed high velocity streams at least one

of which is a two-phase flow containing wet particles or liquid droplets. Due to the

hydrodynamic characteristics of impinging gas strearrls the impingement zone is one

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of high turbulence intensity and flow/particle oscillations which enhance heat/mass

transfer and hence reduce drying times. Because of the high gas velocities, particle

loading ratios and high loss of momentum in the impingement zone, the pressure

drop of ISD's is also high as compared to that of conventional pneumatic dryers.

At the outset ISD's must be distinguished from the well-known impingement

dryers using hot air to dry webs or slab-like materials (e.g. paper, textiles, wood,

plywood, coated webs, films) or the so-called 'jet-zone" dryers wherein layers of

particles on a conveyor belt or a vibratory deck are "pseudo-fluidized by a

multiplicity of high velocity hot air impinging jets (Mujumdar, 1987, Tarnir, 1991).

ISD's are almost unknown in the English language literature and no laboratory scale

or industrial applications seem to be reported except for two recent papers by Tamir

and Kitron (1987). Flow and turbulence phenomena in single phase impingement

of two opposing jets have been studied extensively for their applications in

combustion and gas mixing. It is presumably logical to start with these fundamental

studies and extend them to two-phase jets including heat and mass transfer from

particles. However, this approach has apparently not been pursued yet.

ISD's can be classified into a large number of variants on the basis of the number

of streams impinging, angle of impingement, shape of the ducts carrying the streams,

number of streams carrying the particles (or inert particles coated with slurries,

suspensions or pastes to be dried), flow characteristics of streams (e.g. swirling, non-

swirling and direction of swirl of the two streams which may co- or counter-rotate)

etc. Several of these have been studied in the laboratory. Data are not always

reported in full. So, often it is difficult to make conclusive statements about the

usefulness of some of the variants. The shape, size and motion of the impingement

zone is clearly dependent on the aforementioned factors. It is not obvious which one

of the variants gives better performance in a given application. A proper microscopic

model of the turbulent hydrodynamics of two-phase flows in ducts is needed as a

basis for good scale-up.

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344 MUJUMDAR

Elperin (1961, 1972) was the first to propose the use of the ISD concept for

highly enhanced heatlrnass transfer between suspended particlesfdroplets and gas.

Tamir and Kitron (1987) reviewed the early Soviet works as well as their own more

recent and very extensive .studies on impinging stream contactors for various

operations in chemical engineering such as drying and heating, mixing, dust trapping,

agglomeration and coating of particles, grinding and pulverization of solids,

combustion, absorption and desorption, emulsification, chemical reaction etc.

ISD devices could be in competition with conventional fluid bed, spouted bed,

vibrated fluid bed, flash dryers and sometimes even with spray and spin-flash dryers.

The main advantages of ISD's are their high volumetric evaporation capacity, ability

to break lumps, small size, ease of control etc. On the other hand, high power

requirement for air blowing and possible erosion problems are their main limitations.

According to the Soviet literature ISD units are used industrially (co-axial

withfwithout swirl) to dry crystalline solids (e.g. lysine), pastes (e.g. sewage sludge),

granular materials (e.g. pharmaceuticals) as well as microbial products. In the last

case the short contact times are important in maintaining product quality. Product

disintegration and classification can also be achieved within the ISD; this is one of

its advantages. At least one new installation deals with drying of sludges in a

superheated steam ISD (Meltser, 1990).

Kudra and Mujumdar (1989) have attempted to classify various possible ISD

configurations although only a few of them appear to have real potential for

industrial application. While the concept is very attractive from a strictly

fundamental viewpoint, numerous other factors must be examined. In particular,

problems of product quality, product attrition, erosion of tubes, possible product

deposition in the ducts etc. need to be evaluated carefully before ISD's can find

widespread industrial acceptance. Various geometric configurations, modular/staged

designs, various heating modes etc. need to be investigated in practical as well as

fundamental studies. Scale-up of ISD's remains a complex issue which deserves

attention.

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DRYING TECHNOLOGIES OF THE FUTURE

CLOSURE

An attempt has been made here to identify novel drying technologies that show

potential for extensive applications within the next two decades. Development of

new processes and new products, introduction of more stringent environmental

regulations etc. will undoubtedly trigger development of novel dryer technologies in

the future. It is clear that a significant level of R & D, must be maintained - both

academic and industrial - to ensure that the developments in drying of solids will

keep up with the rapid changes taking place in other fields. The current situation

with regard to escalating energy costs, stringent emissions standards (particularly in

the developing countries) and consumer demand for better product quality, is bound

to provide strong stimulus for industrial R & D in drying. Hopehlly this will lead

to better industry-academia interaction and help bring new dryer concepts to the

plant scale sooner.

REFERENCES

Bell. D.O. Seyed-Yagoobi, J. and Fletchner, L.S., 1991, Recent Developments in Paper Drying, in Advances in Drying, Vol. 5, Ed. A.S. Mujumdar, Hemisphere, N.Y..

Bepex Corp., Minneapolis; MN, USA, Unison Drying System, Technical Bulletin, 1990.

Borisov, y.Y. and Gynkna, N.M., 1972. Acoustic Drying, in Fundamentals and Applications of Ultrasonics, Ed. L.D. Rozenberg, Plenum, N.Y. 19, pp. 381-469.

Dostie, M., Seguin, J.-N., Maure, D., Ton-That, Q.-A. and Chatigny, 1989. Preliminary Measurements on Drying of Thick Porous Materials by Intermittent Infrared and Continuous Convection Heating, Drying'89, Ed. AS. Mujumdar and M. Roques, Hemisphere, N.Y., pp. 513-519.

Elperin, I.T., 1961. Heat and Mass Transfer in Impinging Streams, Inz. Fiz. Zuhrnal. 6, 62-68.

Elperin, LT., 1972. Transport Processes in Impinging Jets (in Russian), Nauka Tekhnica, Minsk, 273 p.

Keey, R.B., Keynote Lecture, 1st International Drying Symposium, Montreal, Quebec, 1978.

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Kudra, T. and Raghavan, G.S.V.. 1990. Concise Guide to Drying Literature, in Drying of Solids, Ed. AS . Mujumdar, Sarita Prakashan, India.

Kudra, T. and Mujumdar, AS., 1989. Impinging Stream Dryers for Particles and Pastes, Drying Technology, 7. 219-266.

Kudra, T., Mujumdar AS. and Meltser, V.L., 1990. Impinging Stream Dryers: Principles, Practice and Potential in Drying of Solids, Ed. AS. Mujumdar, Sarita Prakashan, New Delhi, India, pp. 17-32.

Kumar, P. and Mujumdar, A.S. in Drying of Solids, 1990. Sarita Prakashan, 175 Nauchandi Grounds, Meerut, India.

Meltser, V.L., 1990, Personal Communication.

Mujumdar, 1981, Invited Paper, 100th Anniversary Commemorative Issue, Titaguhr Paper Mills Ltd., India.

Mujumdar, AS., 1987. Impingement Drying. pp. 461-474. In: Mujumdar, A.S. (Ed.) Handbook of Industrial Drying, Marcel Dekker, Inc., NY and Basel.

Mujumdar, AS. CEA Report No. 817 U 671, 1990. (Contact: Exergex Corp., 3795 Navarre, Brossard, Quebec, Canada, J4Y 2H4).

Nomura, T., Nishimura, N., Hyodo, T., Tago, Y., Hasabe, H. and Kashiwagi, T., 1989. Heat and Mass Transfer Characteristics of Pulse Combustion Drying Process, Drying'89, Ed. AS. Mujumdar and M. Roques, Hemispher/Tayor Francis, N.Y., pp. 543-549.

Olsen, K.G. and Holmes, J.G., 1986. Spray Drying and Competing Technologies in the Chemical Industry, in Drying'86, Vol. 2, Ed. AS. Mujumdar, Hemisphere/Springer Verlag, N.Y., pp. 838-843.

Pavasovic, V., Stefanovic, M. and Stefanovic, R. Osmotic Dehydration of Fruit, 1986. in Drying'86, Vol. 2, Ed. AS. Mujumdar, Hemisphere, N.Y., pp. 761-764.

Pluenneke, K.A. and Crumm, C.J.. 1986. An Innovative Drying Process with Diverse Applications, in Drying'86, Vol. 2, Ed. AS. Mujumdar, Hemisphere, N.Y., pp. 617- 624.

Potter, O.E., Drying'85., Ed. R. Toei and A.S. Mujumdar, Hemisphere, N.Y., 1990

Raoult, A.-L. e t al., Osmotic Dehydration - Study of Mass Transfer in Terms of Engineering Properties, 1989. in Drying'89, Ed. A.S. Mujumdar and M. Roques, Hemisphere, N.Y. pp. 487-495.

Sprague, C.H., 1985. High-Intensity Drying Processes, DOE/CE/40738-TI, NTIS- PR-360, Department of Energy, Washington, D.C., 71 p.

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DRYING TECHNOLOGIES OF THE FUTURE 34 7

Tamir,, A. and Kitron, A., 1987. Application of Impinging streams in Chemical Engineering Processes - A Review, Chem. Eng., Comm., 50, 241-330.

Tamir, A., 1991. Impinging Stream Contactors: Fundamentals and Applications, Advances in Transport Processes, Ed. A.S. Mujumdar and R.A. Mashelkar, Hemisphere, N.Y.

BIBLIOGRAPHY

Interested readers will find useful relevant material as well as extensive literature

citations on recent developments in solids drying in the following citations:

Mujumdar, A.S. (Ed.) 1987. Handbook of Industrial Drying, Marcel Dekker, N.Y.

ibid, Advances in Drying (Vol. 1-5), 1980-91, Hemisphere, N.Y.

ibid, DRYING series (1980, (2 vols.), 1982, 1984, 1985, 1986 (2 vols.), 1987, 1989, 1991). Hemisphere, N.Y.

The second enhanced edition of the Handbook of Industrial Drying is in preparation for expected publication in 1993 (Marcel Dekker, N.Y.).

Also, more recent refereed literature on drying and dewatering can be found in Drying Technology - An International Journal.

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drying technologies of the future

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