Feedstock Drying: "No Silver Bullet"

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Feedstock Drying: No

Silver BulletFeature Article

Scott Jones

1/17/2012

This is a feature article Biomass Processing & Technology magazine (BPT) regarding drying

technologies for biomass & biosolid drying and dewatering.


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Conventional Dryers

As an operation of prehistoric origin, drying is one of the oldest and most common functions of

processing. It is, however, normally not associated with innovation. For many processors it is

therefore hard to justify the need for research and development (R&D) and capital equipment

dollars. It is interesting to note, however, that about 250 patentsthe titles of which contain the

words dryer, drier, or drying are issued by the U.S. Patent Office every year, according

to Arun Mujundar, coauthor of the book, Advanced Drying Technologies.

Today, there are over 200 variants of industrial dryers found in practically all major industrial

sectors. There is no silver bullet in drying technology.Drying and dewatering is a combination

of science, technology and art (or know-how based on extensive experimental observations and

operating experience) according to Mujundar. There are scores of conventional dryers with well-

established records of performance that arent necessarily optimal for the application according

to: energy consumption, product quality, safety or environmental impact.

The significant growth in drying and dewatering R&D in the western world, according to

Mujundar, was triggered by the energy crunch of the early 1970s which increased the cost of

drying several-fold within only a few years. Energy conservation still remains one of the main

concerns in drying today. It is worth noting, however, that continued efforts in innovation are

also driven by increased productivity, better product control, quality control, new products and

new processes.

According to Greg Stover, Director of Sales and Marketing with Marion Mixers, conventional

dryers tend to be physically large. Most drying technologies are very old. Many are more than ahundred years old and were designed at a time when energy and environmental considerations as

well as quality demands were not as stringent. Even the newer super heated steam dryers are

based on German inventions from the 1800s. Typical dryers found in the USA are large rotating

units, ring dryers, belt dryers, etc. Typically they are direct gas fired, which poses several

problems:

1. The flame degrades the product, causes losses in protein, changes color of product, etc.2. Fires are commonplace.3. Many dryers use high pressure steam as a heating medium. This requires certified boiler

operators, liability insurance, air permitting, etc.

4. Most dryers have enormous swings in product quality. Often, there is virtually no way todetermine what the product looks like for 25-35 minutes until it comes out the other end.

A lot of product is lost due to temperature swings, bad color, scorching, clumping, etc.

5. Large dryers are inflexible and do not respond well to production changes.6. Thermal oxidizers are often required to treat noxious components as they exit dryers.

Thermal oxidizers are one of the biggest maintenance nightmares. They break often with

steep fines if not fixed with urgency. They are also extremely costly to operate.


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Novel Drying Technologies

The majority of novel drying technologies, which evolved through a process of evolutionary

improvements, were built to offset some or all of the limitations faced in operating conventional

dryers. Noveltyper seis not adequate justification for embracing new technology; it must be

technically superior and cost-effective compared to the current technology. In some instances,

the newer technologies may offer advantages over the conventional systems. Keep in mind,

however, novel versus conventional is like comparing apples and oranges.

For drying technologies, one or more of the following attributes may call for an innovative

replacement of existing products, operations, or processes:

New product or process not made or invented heretofore.

Higher capacities than current technology permits.

Better quality and quality control than currently feasible.

Reduced environmental impact.

Safer operation.

Better efficiency (resulting in lower cost).

Lower cost (overall).

Better control, more flexibility, ability to handle different products.

As some 50,000 materials are estimated to require drying on varying scales, it is impossible to

cover in depth all possible dryer types and products in this article. These three innovative drying

systems, however, show promise as commercially-accepted alternatives to conventional dryers:

cyclonic, pulse combustion and microwave.

Pulse Combustion Drying

Pulse jet dryingis an emerging technology of significant potential since it is claimed to increase

productivity, maximize utilization of the input fuel, enhance heat transfer and reduce pollutant

emissions. (Muralidhara and Lockhart, 1985; Mujumdar, 1991; Zbicinski et al., 2002; Kudra et

al., 2003; Mujumdar and Wu, 2004; Kudra and Mujumdar, 2007).

The term pulse combustion originates from the intermittent (pulse) combustion of solid, liquid,

or gaseous fuels in contrast to continuous combustion in conventional burners. Based on pulse

combustion, an intermittent (pulse) combustion generates intensive pressure, velocity and to a

certain extent, temperature waves propagated from the combustion chamber through a tailpipe(diffuser) to the process volume (an applicator) such as a dryer, calciner or incinerator. Because

of the oscillatory nature of the momentum transfer, pulse combustion intensifies the rates of heat

and mass transfer.

The mechanism behind the operation of a pulse combustor is a complex interaction between an

oscillatory combustion process and acoustic waves that are propagated from the combustor.


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The process of pulse combustion is initiated when air required for combustion and fuel in the

form of a gas jet or a liquid spray are drawn into the combustion chamber, mixed to form an

explosive gas, ignited by a spark plug and combust instantly in an explosive-like manner.

Pulse Jet technology offers the following advantages over conventional heating systems:

Increased heat-and mass-transfer (by a factor of 2-5)

Increased combustion intensity (by a factor of up to 10)

Higher combustion efficiency with low excess air values.

Reduced pollutant emissions (especially NOx, CO, and soot) by a factor of up to 3

Improved thermal activity efficiency by up to 40%

Reduced space requirements for the combustion equipment

The major disadvantage of pulse combustion systems is the noise generated in normal operation.

The sound pressure level (SPL) emitted by the burner that resonates in the applicator depends on

the operating pressure and frequency; the higher the operating pressure, the higher the SPL.

Sound deadening measures are necessary.

Microwave drying

The mechanism for drying with microwave energy is quite different from that of conventional

drying. In conventional drying, heat is transferred to the surface of the material by conduction,

convection or radiation and into the interior of the material by thermal conduction. Moisture is

initially flashed off from the surface and the remaining water diffuses to the surface. This is often

aslow processin conventional drying and the diffusion rate is limited, requiring high external

temperatures to generate the temperature differences required.

In conventional or surface heating, the process time is limited by the rate of the heat flow into the

body of the material from the surface as determined by its specific heat, thermal conductivity,

density and viscosity.

Microwaves are not forms of heat but rather forms of energy that are manifested as heat through

their interaction with materials. Microwaves initially excite the outer layers of molecules. The

inner part of the material is warmed as heat travels from the outer layers inward. Most of the

moisture is vaporized before leaving the material. If the material is very wet and the pressure

inside rises rapidly the liquid will be removed from the material due to the difference in pressure.This creates a sort of pumping action forcing liquid to the surface, often as vapor. The result is

very rapid dryingwithout the need to overheat the atmosphere and perhaps cause case hardening

or other surface overheating phenomena.


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Mechanism of Heating

Microwave energy does not heat the room; only the desired material with no harmful greenhouse

gas emissions from the heat source. The energy is mainly absorbed by a wet material placed in

the cavity. Water, fat and sugar molecules in food materials absorb energy from the microwave

in a process called electromagnetism; a phenomena associated with electric and magnetic fields

and their interactions with each other and with electric charges and currents.

Microwaves are electromagnetic waves having a wavelength (peak to peak distance) varying

from 1 millimeter to 1 meter. 1GHz = 1,000 MHz. Frequency of microwaves lies between 0.3

GHz and 3 GHz. A domestic microwave operates at 2450 MHz (a wavelength of 12.24 cm). 90%

of American homes have microwave ovens. Industrial/commercial microwave systems typically

operate at 900 MHz (a wavelength of 32.68 cm). This range allows more efficient penetration of

the microwave through the material.

Microwave heating is most efficient on liquid water and much less on fats and sugars and frozenwater where the molecules are not free to rotate. Depending on water content the depth of initial

heat deposition may be several centimeters or more with microwave ovens, in contrast to

grilling, which relies on infrared radiation, or the thermal convection of a convection oven,

which deposit heat shallowly at the food surface.

Historically the biggest constraint to using microwave energy for industrial processing has been

the inability to heat the material with uniform energy distribution. If you have non-uniform heat

distribution the result is underexposed and overexposed material in the same batch or continuous

process. Recent advances controlling uniform heat distribution have been overcome.

Energy Savings with Microwave

There was a time when electrically powered systems were considered too costly to operate for

high-power applications. Today, with the rising costs of oil and gas, this is no longer necessarily

true. There is significant interest today in the potential energy saving from a microwave heating

system. The offsets to the current cost of electricity include the increased speed of drying, the

direct coupling of energy into the material and possible lower drying temperatures. Processors

that use steam, gas or electric heating systems are aware of the high costs of running these

systems. Consider the energy-saving benefits of microwave heating verses conventional heating:

Slash energy consumption by up to 50%.

Reduce man-hours and downtime involved in cleaning.

Minimize ancillary equipment such as heated jackets, boiling pans and heating vessels.

Shrink your heating and/or cooking footprint.

Eliminate warm up and cool down time.

Microwave energy does not heat the room only the material.


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Economics of Microwave Systems

Several criteria for successful microwave drying systems are related to reduced cost. Cost

saving may be realized through:

1. Energy savings2.

Faster batch processing

3. Operational efficiencies4. Increased throughput5. Labor reduction6. Reduction in heat load in the plant7. Reduced maintenance costs8. Reduction in product fouling9. Less floor space needed

The image above represents an innovative design of a microwave mixer that uses microwave

energy to heat, dry or cook inside the vessel in either a horizontal or vertical tank, supplied with

an agitation system. This is an example of a novel concept that has successfully made it through

pilot testing and is being commercially manufactured and sold in the food processing and

chemical manufacturing industries.

Applications/Functions/Industries

In food processing, where industrial microwave equipment has been around for over 30 years,

this technology is capable of performing multiple, custom design applications including heating,

drying, tempering and cooking in multiple stage recipes that thaw, cook and simmer in the same


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vessel. Microwave technology is a viable commercial alternative to traditional cooking and

heating methods and is being seriously considered by many food, drink and pharmaceutical

producers.

Microwave technology can accommodate virtually any vessel size. It is ideal for applications

where energy efficiency is required. Manufacturing and processing industries such as chemical,

plastic, mineral and recycling that are interested in this form of volumetric heating should

explore this technology further.

Feedstock Drying: "No Silver Bullet"

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