Powder and Bulk Engineering, April 1997

Thermal hazards: How to identify and minimize them in your drying process Dr. Vahid Ebadat and James C. Mulligan Chilworth Technology, Inc.

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A fire or explosion in your dryer can injure your workers, damage your plant and equipment, and interrupt your process for a long time. Identifying such thermal hazards in your drying process means understanding dryer properties and, most impor- tantly, characterizing your powder. This article’s first sections describe potential thermal hazards. The final sections explain how to evaluate your pow- der to minimize these hazards in your drying process.

uring drying, some dryer properties and powder characteristics together can create fire and explo- D sion hazards. To help understand the hazards,

let’s take a look at how powder behaves during drying.

Heating and cooling compete during drying. Heat is added to the wet powder either directly (via conduction) or indirectly (via convection) to evaporate the liquid. But the powder also loses heat as the liquid evaporates, al- though this heat loss decreases almost exponentially over time for a constant drying temperature. Heat can also be lost to the dryer walls and other components depending on their relative temperature and the powder’s thermal conductivity.

When a heated powder accumulates, it serves as an insu- lator - that is, the powder’s heat-retention rate exceeds its heat-loss rate. This can happen when powder collects at a spray dryer’s bottom, when powder deposits on dryer ledges or walls, or when a dried powder discharges di- rectly from a dryer into a container. Such an accumulation of powder in bulk (or in a layer) is thermally insulated from the cooling occurring at the interfaces between the powder and the atmosphere or dryer walls.

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rn E. At a temperature called the onset temperature, the

exothermic decomposition process starts at the accumu- 2. lation’s center and proceeds even if no more heat is added

to the dryer. This is called self-heating. Under some con- ditions and for some powders, the onset temperature can be just a few degrees above the ambient temperature. Once self-heating starts within the powder and as long as the process isn’t stopped, several results are possible: smoldering, fire, or explosion. How hazardous each is de- pends on the decomposition’s nature and rate.

Smoldering When a self-heating powder degrades, smolders, and chars, it can be an imminent hazard or simply prevent the powder from meeting your quality standards. Smoke or fumes emitted during decomposition can pose a fire and explosion hazard. For instance, smoke can carry burning embers to nearby combustible materials or flammable at- mospheres. Some self-heating powders can also produce flammable vapors, such as when an organic powder is ex- posed to moisture and generates methane, carbon monox- ide, or both during anaerobic decomposition.

In other cases, decomposition occurs only beneath the powder surface, with no obvious signs of smoke or fumes. The process can continue for hours or even days before you detect it. In fact, you may see smoldering only when the powder is disturbed, such as when dryer trays empty or when the powder discharges into a container. Disturbing a smoldering powder can cause it to ignite suddenly because you’ve introduced air to powder that was previously under the surface, creating another fire and explosion hazard.

Fire Self-heating is a combustion process, which means that under some conditions a self-heating powder can proceed from decomposition to combustion’s ultimate conclu- sion: ignition. While disturbing a smoldering powder can

62 Powder and Bulk Engineering, April 1997

ignite it, the powder can also ignite under other conditions if the self-heating rate is high enough.

Factors affecting selfheating rate. A powder’s self- heating rate depends on exposure time, accumulation vol- ume and nature, aeration, and composition.

Exposure time. The time between the powder’s exposure to heat and decomposition’s onset is called the induction period. The period’s duration depends on your heating and environmental conditions and the powder properties. For many powders, decomposition involves relatively low levels of exothermic activity during induction, fol- lowed by extremely rapid decomposition at onset. As a result, you may not detect self-heating during routine dry- ing operations when residence times are relatively short. But if you increase the residence time (inadvertently or otherwise), the powder can decompose -possibly at a dangerous rate - to a stage you haven’t seen before, thus posing a major fire and explosion hazard.

Accumulation volume and nature. As more heated pow- der accumulates in bulk or in a layer, thermal insulation increases and the heat-loss rate from the powder’s center decreases. Thus as the powder volume increases, the powder’s self-heating tendency increases. For this rea- son, scaling up a drying process, increasing a dryer batch size, or allowing a powder layer to accumulate in a dryer can tip the balance of the heat-retentionheat-loss equa- tion, sometimes with devastating effects.

The heat-loss rate is also affected by the powder accumu- lation’s nature, such as its density or configuration. For instance, the configuration -which can be in a vessel, pile, or layer - determines how much powder contacts heated and nonheated surfaces in the dryer and how much powder interfaces with heated and ambient air.

Aeration. If oxidation drives the self-heating powder’s decomposition, the contact between the powder and air will have the strongest effect on decomposition. Thus a temperature that’s safe for one dryer type can be unsafe for another depending on whether the powder is aerated - as is the case for a powder that’s fluidized in a fluid- bed dryer but heaped in a layer in a tray dryer. Onset tem- peratures for the same powder when heated by forced airflow and by diffused airflow can vary by 50°C (90°F) or more.

Composition. Impurities in your powder can strongly af- fect its thermal stability during drying. Metal or organic contaminants such as grease or oil can greatly reduce the powder’s onset temperature. In one test, oil contamina- tion reduced an organic intermediate powder’s onset tem- perature from 380°C (716°F) to less than 220°C (428”F).’ Such contamination can result from changes in a pro-

cessed powder’s composition, such as those caused by a formula change.

Secondaryfires. While a self-heating powder is an immi- nent fire hazard, the initial fire can also cause a secondary fire. For instance, the powder’s ignition can ignite nearby flammable vapors or dust clouds. Flammable vapors can be produced in a dryer that’s drying a solvent-wet powder. But even if the powder is dissolved in water or another nonflammable liquid, flammable conditions can still re- sult if the powder releases a dust cloud after drying. Flam- mable vapors can also be released during decomposition.

When a self-heating powder degrades, smolders, and chars, it can be an imminent hazard or simply prevent the powder from meeting your quality standards.

Yet it isn’t necessary for a self-heating powder to ignite to be an ignition source for a secondary fire or explosion. Thermal radiation from the self-heating powder can be enough to ignite a nearby flammable vapor, dust cloud, or other combustible material. In some cases, a heated dryer section can also be an ignition source, such as when the dryer surface temperatures exceed the autoignition tem- perature (AIT) of a nearby vapor or minimum ignition temperature (MIT) of a nearby dust cloud.

Explosion If self-heating occurs inside an enclosed dryer, the ves- sel’s pressure will rise due to the heating of the contained atmosphere and combustion product gases. If the self- heating powder ignites a flammable vapor or dust cloud within the vessel, the combustion rate can be very fast, generate intense heat, and greatly raise the pressure, caus- ing an explosion.

In general, peak explosion pressure in a sealed system (such as a batch dryer) is limited to about 8 to 10 times the initial pressure. Some powders, such as sodium dithionite and many azo compounds, can emit gases during exother- mic decomposition fast enough to cause overpressuriza- tion of the dryer without ignition.

How to evaluate fire and explosion hazards Completely characterizing your powder before initiating your drying process is the best way to minimize dryer fire and explosion hazards. This includes evaluating the pow- der’s thermal stability (and its potential for violent gas ex- plosion); dust flammability; and vapor flammability.

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Because this article concentrates on testing the powder’s thermal stability (that is, potential for igniting a fire or ex- plosion when the powder isn’t suspended in a dust cloud but is exposed to heat), a detailed discussion of dust and vapor flammability is beyond this article’s scope. [Edi- tor’s note: See the related sidebar, “Tests for evaluating dust and vapor flammability,” for brief lists of tests and sources for further reading.]

Powder and Bulk Engineering, April 1997

You can evaluate your powder’s thermal stability with one or more of these tests?

Bulk powder

Aerated powder

Powder layer

Isothermal basket

*Dewar calorimetry

Gas evolution

Using just one test may not be enough to fully evaluate your powder’s thermal stability because that stability in a

In the Dewar calorimeiry test, a vacuum-iacketed Dewar flask is used to measure the rote and quantify of heat evolved by a powder during a chemical reaction.

66 Powder and Bulk Engineering, April 1997

dryer’s heated environment is complex and the powder doesn’t have a unique onset temperature. So take care to choose the right tests - especially those that reasonably simulate your drying operation’s parameters - and de- velop a suitable testing approach (such as running more tests if your dryer’s operating temperature will be close to the powder’s onset temperature) so the test data applies to your operation.

Also make sure you test representative samples of your powder, because small formulation changes from one batch to another can greatly affect the powder’s thermal stability. Using representative samples will ensure that you have the valid, relevant, and applicable data you need to minimize fire and explosion hazards.

No amount of careful testing can guarantee that you won’t have a dryer fire or explosion. Yet the tests are criti- cal because they help you:

Assess your powder’s potential thermal stability hazards during drying and subsequent packaging and storage.

Specify operating conditions that will reduce the haz- ards to an acceptably low level.

The following information discusses the thermal stability tests.

Bulk powder test The bulk powder test can help you evaluate your pow- der’s thermal stability when it’s heated in bulk form (such as when the powder accumulates at a spray dryer’s bot- tom). In the procedure, the tester places a 9-cubic-inch powder sample in a glass cell that has a sintered-glass bot- tom. The cell is placed in a laboratory oven whose tem- perature is ramped (increased) O.S”C/min (0.9”F/min) for a 14-hour period, exposing the sample to constantly in- creasing temperatures. The powder sample is aerated only by natural air diffusion through the cell’s sintered- glass bottom. The tester measures the sample temperature at several cell locations to detect any exothermic activity and to identify the activity’s onset temperature.

As long as it’s determined in a properly performed test, you can use the onset temperature (with a suitable safety factor) to specify the maximum safe temperature for your drying operation. However, you may need to do addi- tional tests if:

The powder’s residence time in your dryer is longer than 14 hours.

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You detect exothermic activity close to your dryer’s pro-

Powder and Bulk Engineering, April 1997

posed or actual operating temperature.

*The onset temperature is less than 200°C (392°F).

For instance, to precisely determine the onset temperature you may need to perform a series of isothermal bulk pow- der tests, in which the oven temperature is held constant, or large-scale isothermal basket tests (discussed in the later section, “Isothermal basket test”).

Aerated powder test You can use the aerated powder test to evaluate your pow- der’s thermal stability in a dryer that aerates the powder by passing heated air through it, as in a fluid-bed dryer. The aerated powder test is very similar to the bulk powder test because it uses the same powder sample size, sample cell type, and constantly increasing oven temperatures for the same period. However, in the aerated powder test, heated air flows at 0.6 Vmin through the sample during the entire test period. As in the bulk powder test, the tester measures the sample temperature at several locations in

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thermal stability during fluid-bed drying, the test doesn’t simulate fluidization conditions, which might actually re- move decomposition heat due to high airflow. Instead, the test simply increases aeration beyond that expected from natural air diffusion alone. Ideally, the test provides a rea- sonable worst-case sirnulalion of drying conditions at the end of a fluid-bed drying cycle, when less heat is removed (that is, when the dryer fan is off but induced airflow is still present). You can also perform the test isothermally to observe the powder’s thrmnal stability at a certain tem-

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(D To evaluate your powder’s thermal stability in a dryer that passes heated air over a powder layer, such as in a tray dryer, you can use the powder layer test (also called the air-over-layer test). The test simulates both the condi- tions to which the powder layer is exposed in the dryer

During a gas evolufion test, the tester monitors the temprufure und pressure of a powder sumple sealed in a glass curius fube.

and the heating of residual powder deposits that accumu- late on the dryer ledges and walls.

In the procedure, the tester loads a powder sample large enough to fill a 3-by-1.5-by -0.75 -inch metal sample tray, then inserts the tray into a tube furnace. The furnace tem- perature is ramped O.S"C/min (0.9"F/min) for a 14-hour period, exposing the sample to constantly increasing tem- peratures. Heated air passes over the sample at 4.5 Vmin. The tester measures the sample temperature at several lo- cations in the tray to detect any exothermic activity and identify the activity's onset temperature. As with the bulk powder and aerated powder tests, you can conduct this test isothermally to observe the powder's thermal stabil- ity at a particular temperature.

Isothermal basket test The isothermal basket test allows you to observe the ef- fect of scale (that is, powder quantity) on your powder's thermal stability and determine the heated powder's onset temperature more precisely. In the test, which includes several trials, the tester loads a powder sample into a cube-shaped stainless steel mesh basket and suspends the basket for up to 24 hours in an oven kept at a constant temperature. The tester measures the sample temperature at several locations within the basket to detect any exothermic activity and the activity's onset temperature. The tester repeats trials at various appropriate tempera- tures and with various basket sizes (such as 1,4,9, and 16 cubic inches) until the sample's minimum onset tempera- ture is determined.

The tester chooses the temperature for the first trial based on results of another test - typically the bulk powder test. Temperatures for subsequent trials are based on re- sults of previous trials.

Because the trials are performed with at least three differ- ent basket sizes, you can extrapolate and interpolate two types of information from the results: the maximum safe operating or storage temperature for different-size dryers and safe powder amounts and package sizes for storage and transportation. This makes the test particularly rele- vant if your powder is discharged from a dryer directly into a container without cooling the powder. You can also use the test to assess your powder's thermal stability hazards if smaller scale tests have indicated a potential stability problem, such as a lower-than-anticipated onset temperature.

Powder and Bulk Engineering, April 1997 69

Dewar calorimetry test To measure the rate and quantity of heat that's evolved by your powder during a chemical reaction - for instance, when your powder is added to a liquid to form a slurry for drying - you can use a Dewar calorimetry test. The test is one of the most accurate and reliable for measuring this data.

A temperature that's safe for one dryer type can be unsafe for another depending on whether the powder is aerated.

In the procedure, the tester loads the powder and liquid used in your chemical reaction into a vacuum-jacketed flask, called a Dewarjlask, that is fitted with an agita- tor and has temperature-sensing probes in the flask walls that send data to a computer. The flask is placed in an oven, and the oven temperature is constantly in- creased. The powder and liquid are agitated until the desired chemical reaction is achieved. The probes mea- sure the chemical reaction's heat and send the data to the computer.

As with the isothermal basket test, you can use this test to observe the effect of scale on your powder's thermal sta- bility. You can also run isothermal Dewar calorimetry tri- als if the flask's heat-loss characteristics are equal to or less than those of your dryer. To simulate your dryer's op- eration at a larger scale, you can perform the Dewar calorimetry test adiabatically -that is, while keeping the sample's heat loss at zero. In such a test, when the sample temperature in the flask rises after a chemical reaction, the computer signals the oven temperature to increase, keep- ing the flask and oven temperatures the same to maintain zero heat loss.

Gas evolution test The gas evolution test determines the onset temperature for exothermic decomposition of individual or commin- gled powders and the rate at which the decomposition gives off combustion gases (called the gas evolution rate). In the procedure, the tester seals a 10- to 20-gram powder sample into a glass Carius tube that's fitted with a re-entrant thermocouple and pressure transducer. Then the tube is heated at 2"C/min (3.6"F/min) up to 400°C

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(752°F) or a preset pressure cutoff. The tester continu- ously monitors temperature and pressure in the tube.

By analyzing the resulting data, you can determine the powder’s onset temperature. Once you know the onset temperature, you can determine the gas evolution rate by conducting isothermal trials. You can also use such trials to determine your powder’s safe storage temperatures and periods based on induction period effects. For instance, if your powder has a long induction period (that is, if exothermic decomposition starts long after the powder is exposed to heat), the powder may start to smolder or even ignite only after being stored for a certain period, such as 24 hours or 1 week. By testing the powder at regular inter- vals over a long period, you can determine how to store the powder to avoid such ignition. PBE

Powder and Bulk Engineering, April 1997 71

Suggestions for further reading J. Abbott, editor, Prevention of Fires and Explosions in Dryers, The Institution of Chemical Engineers (UK), Rugby, England, 1990.

P.F. Beever and P.F. Thorne, “Isothermal methods for as- sessing combustible powders: Theoretical and experi- mental approach,” Symposium Series 68, The Institution of Chemical Engineers (UK), 198 1.

Free information on thermal hazards testing and fire and explosion properties of bulk materials is available from the authors (contact information listed below).

Endnotes 1. Test results reported in N. Gibson, D.J. Harper, and R.L. Rogers,

“Evaluation of the fire and explosion risk in drying powders,” PZunt/Operutions Progress, Vol. 4, No. 3, July 1985, pages 181-189. Dr. Vahid Ebadat is vice president of Chilworth Tech-

nology, Inc., Princeton Corporate Plaza, 11 Deer Park Drive, Suite 204, Monmouth Junction, NJ 08852-9623; 908/274-0900- James c. Mu@Pn is a consulting en@-

2. While differential scanning calorimetry and accelerated rate calorimetry tests have other uses, data from these tests may not be relevant to thermal stability in a drying operation due to their small

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_ - _ sample sizes, insufficient sample aeration, and high scanning rates. neer at the company.