DRYER DRUM MIXERby J. Don Brock, PhD., P.E.

Technical Paper T-119

ASTEC encourages its engineers and executives to author articles that will be of valueto members of the hot mix asphalt (HMA) industry. The company also sponsors inde-pendent research when appropriate and has coordinated joint authorship betweenindustry competitors. Information is disbursed to any interested party in the form oftechnical papers. The purpose of the technical papers is to make information availablewithin the HMA industry in order to contribute to the continued improvement processthat will benefit the industry.

CONTENTSINTRODUCTION.................................................................................... 3BACKGROUND ..................................................................................... 4DRUM MIX COATER-I (DMC-I) ............................................................. 6DRUM MIX COATER-II (DMC-II) ........................................................... 7NEWER DRUM MIXERS ....................................................................... 9

INTRODUCTIONAccording to all predictions the American infrastructure will go through a total

rebuilding program in the current decade. Returning our infrastructure to its oncepreeminent position will require new sewer systems, new drainage systems andre-establishing grades and drainage. An approximate 95% increase in traffic overthe past fifteen years has led to a tremendous overload of the system. Thus, thebulk of new construction will be widening, maintenance, and upgrading of thesystems. By the mid-nineties 50 to 70% of all asphalt mixes will likely contain oneor more of the following recycle products:

• Crushed concrete

• Recycled asphalt mixes

• Roofing shingles

• Oil contaminated soils

• Rubber

• Glass

Also, the addition of additives such as lime and anti-strip will be necessary.

During this decade further shortages of crude oil may occur and lead to avariations in asphalt products available for new mixes and recycling.

Asphalt in recycle material is harder than in new asphalt mix because of aging thatoccurs while it is part of the old road surface. But the quality of new mixincorporating large percentages of recycle asphalt and various other recycleproducts will have to be equal to that of virgin mix. Both the old and new liquidasphalt will have to be distributed uniformly throughout the mix. This will requiremaking mixes 20 to 30° F hotter than usual to produce a mix viscosity that will allowcompaction to desired densities.

Even though mixes using various recycled products must be run hotter, environ-mental codes will require that the asphalt facilities operate absolutely clean. Eventhough the national code allows 20% opacity (Figure 1), more and more state andlocal codes require zero opacity. The code is the same for all mixes, no matter howmuch recycle is used or the type of mix.

In order to obtain zoning and permits for a facility, its emissions must not only beinvisible, but it must be odor-free and must run quietly. Over the many years sincethe introduction of the Clean Air Act in 1973, enforcement of codes has not beenuniform. But enforcement will become much more uniform. And even thosefacilities in remote areas will be required to meet more stringent codes in the future.

To be competitive in an increasingly competitive market, asphalt facilities willhave to run higher production, have lower operating costs, and be easier tomaintain. Last, but not least, the facilities must produce mix of the highest quality.The mix must have minimum segregation, excellent mixing of both the recycle andvirgin materials, complete coating, and uniform film thickness.

BACKGROUNDOver the years asphalt plant manu-

facturers have designed equipmentto meet the needs of the industry.Since needs change continually,equipment is continually changing.

In the ’50s and ’60s batch type as-phalt plants were common. They didan excellent job producing qualitymixes. But the late ’60s and early ’70sbrought air pollution requirements.Most batch plants had production re-stricted by additional air pollutionequipment. As a result, batch plantsbecome larger and more difficult tomove.

The early ’70s brought the inventionof surge and storage bins. This, plusthe availability of electronic controls,led to the introduction of drum mixplants. They operated as continuousmix plants. They were simpler. Andthey could be much more portable(Figure 2). It was believed that theywould eliminate bulky air pollutionequipment.

In 1973 the Clean Air Act was passed.It required all asphalt plants to haveemissions of less than 20% opacity. Itmandated particulate grain loading ofless than 0.04 grains/dry standardcubic foot. Emissions from early drummixer plants were much less thanfrom batch plants. But the drum mix-ers (without collection equipment) stillcould not meet requirements of thenew Clean Air Act. Consequently,wet washers and baghouses wereadded to the drum mixers. This, un-fortunately, reduced their portability.(Figure 3).

Due to shortages and high prices ofcrude oils in the late ’70s recyclingagain became economically feasible.Drum mixers readily lent themselvesto the introduction of recycle. It simplyrequired cutting holes in the shell ofthe drum so the recycle material couldbe introduced into a cooler zone,downstream from the hot gases of theburner. (Figure 4).

Concurrent with the oil shortages,crude oils from all over the world be-gan to enter the United States. Varia-tions in the crude oils led to variationsin the hardness in the asphalts (Fig-ure 5). Recycle materials containedharder asphalt, which needed to beused with softer virgin asphalt. Often,however, softer materials were notavailable. And that led to cutting backthe harder asphalt with lighter oils toartificially soften it.

Development of the milling machinemade more recycled material avail-able. It facilitated the removal of vary-ing amounts of the road for leveling,for maintaining clearances of under-passes, and for eliminating deadweight on bridges. Each year the per-centage of recycle mix increased. Andeach year the air pollution codes be-came stricter. This led to a problemcaused by distillation, an inherentproblem in the basic drum mixer.

As shown in Figure 6, a refineryconsists of one or more distillationcolumns. They distill crude oil at tem-peratures of 600–700° F and sepa-rate it into heavier and lighter frac-tions. Many years ago it was foundthat crude oil could be distilled atmuch lower temperatures (300° F) byinjection of steam.

A drum mixer (Figure 2) is verysimilar to a refinery. The liquid as-phalt is spread out as a thin film overmany square feet of surface area inthe mix. There it is exposed to tem-peratures in the gas stream of ap-proximately 400 to 450° F.

The mix consists of large and smallparticles. The small particles havelow mass and many of them arepresent. The small particles heat upquickly to the temperature of the gasstream as shown in Figure 7.

The gas stream contains exhaustgases and steam. The amount ofsteam depends upon the moisture inthe aggregate. Higher moisture in theaggregate produces more steam.More steam produces more distilla-tion, especially if the asphalt has beenartificially softened with lighter frac-tions. These problems led to the needfor a new generation of asphalt plants.

DRUM MIX COATER-I (DMC-I)By the mid-1980s it became appar-

ent that virgin asphalts should beintroduced into the mix in a differentway because they contain increasedamounts of light oil. They needed tobe removed from the gas stream toavoid exposure to hot gases or steam.This led to the development of theDrum Mix Coater-I plant shown inFigure 8. It had a parallel flow drummixer in which the hot gases andaggregate move in the same direc-tion. It also had a mixer or coater atthe discharge end of the drum.

This plant produced an excellent product using recycle. The recycle entered intothe center of the drum where it mixed with hot virgin aggregate. The old asphaltin the recycle melted, partly coating the virgin aggregate prior to introduction ofvirgin liquid asphalt near the end of the drum. This ensured that both virgin andrecycle liquid asphalt was placed on both the virgin and recycle aggregate,resulting in an excellent homogeneous mix. But as air pollution codes became

stricter, inherent problems of theseplants and drum mixers became ap-parent:

1. These plants ran much higherstack temperatures than those withthe counterflow dryer previously usedwith batch plants. (Operators acceptedthis because the plants gave higherproduction rates.) The higher rateswere obtained by running higher gasflows than previously used oncounterflow dryers with batch plants.

2. The plants were very sensitive tomix temperatures. Running recycleoften required increasing mix tem-perature by 10 to 30° F. But increas-ing mix temperature made the plantssmoke.

3. Smoking or opacity increased ashigher percentages (above 30%) ofRAP were used. It was found thateven 30 year old recycle still con-tained varying fractions of light oil. Ifthe original asphalt was cut back witha light oil similar to a motor oil, or witha heat transfer oil, it remained in thematerial throughout the period it wason the road. Moreover, oil spilled fromautomobiles and trucks added to theoil in the pavement and showed upwhen it was used as recycle. Visibleemissions that began to occur ashigher percentages of recycle wereused led to the development of theDrum Mix Coater-II.

DRUM MIX COATER-II (DMC-II)Figure 9 shows a diagram of the

Drum Mix Coater-II. The DMC-II uses a counterflow dryer instead of the parallelflow dryer used in either the original drum mixer or a Drum Mix Coater-I.

In the original drum mixer the hot gases and the aggregate move in the samedirection within the drum as shown in Figure 10. And when it produces a 300° Fmix, the lowest obtainable exit gas temperature at the exhaust of the drum is 300°

F, regardless of drum length. Even intoday’s most efficient and modernparallel-flow drum mixers, this tem-perature reaches 310 to 330° F.

Figure 11 shows a temperature pro-file of the original parallel-flow drummixer with a center inlet. The profilealso applies to the Drum Mix Coater-I previously described. Flights wereinstalled in the front of the drums tosuperheat the virgin aggregate to atemperature as high as possible whilereducing gas This was done to re-duce temperatures to which smalland large recycle particles were ex-posed. It did improve the operation ofboth mixers when using recycle. Butit did not totally eliminate steam distil-lation from recycle material. Steamdistillation thwarted efforts to totallyeliminate all visible emissions when30% or more recycle material wasused and virgin aggregate had mois-ture levels of 5% or higher.

As shown in Figure 9 and 12, DrumMix Coater-II has a counterflow dryerand uses a mixer at the dischargeend of the dryer. And, as Figure 12shows, the virgin aggregate entersthe end where gases are discharged.Consequently, the gases travel in onedirection while the aggregate travelsin the opposite direction. This designallows much lower exit gas tempera-tures to be achieved.

Exit gas temperatures as low as180° F have been achieved using wetwashers with this type of dryer. How-ever, with baghouses, temperatures

at the dryer exit are usually controlled at about 240° F. This prevents condensationof water or acids in the baghouse. When using natural gas or low sulphur fuel oils,exit temperatures can be lowered to 220° F. Lower temperatures allow higherproduction rates while using less fuel, a more efficient plant operation.

With the Drum Mix Coater-II recycleenters into the coater or mixing sec-tion (Figure 12). The virgin aggregateis superheated. It, in turn, heats therecycle materials.

In the drum mixer (Figure 11) andDMC-I (Figure 8) approximately 90%of the recycle material heating is bythe virgin aggregate and 10% by thehot gases. But in the Drum Mix Coater-II, 100% of the recycle heating is doneby the virgin aggregate.

The major disadvantage of the DrumMix Coater-II is the short mixing timein the coater. There is no mixing priorto the coater. The hot virgin liquidasphalt, virgin aggregate and coldrecycle are injected at approximatelythe same point in the coater. Conse-quently, when high percentages of recycle material are used, the short mixing timeproduces a less homogeneous mixture. Newer versions of this plant with twincoaters allow introduction of recycle before the liquid is injected. This produces aproduct very similar to that of a Drum Mix Coater-I.

This type of plant has another disadvantage. Its drum shell gets very hot whensuperheating aggregate to 600–650° F. However, the shells can be air-cooled byvarious ingenious methods to eliminate this problem and increase efficiency.

NEWER DRUM MIXERSBy late 1986 it became apparent that a new type of plant would be needed in the

’90s. It should be capable of running high percentages or recycle. It needed to becapable of running high mix temperatures. It had to operate without polluting theatmosphere with odors or smoke (zero opacity would be required). This plant alsoneeded to have an efficiency equal or greater than existing plants. It would haveto readily use various recycle products to make a high-quality mix. These needsled to the eventual development of two new types of mixers:

1. Double Barrel drum mixer

2. A counterflow mixer with an embedded burner

Figure 13 shows one version of a counterflow mixer with an embedded burner.Figure 14 shows a profile of temperatures through the mixer. It uses a counterflowdryer to dry and superheat the aggregate. The burner is inserted approximately14' into the drum. Recycle material is injected down-stream of the burner where

it mixes with the hot virgin aggregateand virgin liquid asphalt.

But this plant has disadvantages. Ithas high shell temperatures and ashort mixing time for recycle and vir-gin materials. Moreover, it is difficultto maintain because key componentson the burner are not accessible.

Figure 15 shows the same type ofcounterflow mixer with an air cooledshell and air injection holes adjacentto the burner. Figure 16 shows aprofile of temperatures through themixer. Secondary air for the burner ispulled through the injection holes,eliminating two disadvantages of themixer shown in Figure 13.

Pulling secondary air through a outershell cools the drum and preheats theair for combustion. The preheatedsecondary air increases plant effi-ciency approximately 4% in continu-ous operation and up to 8% for stopand start operations. The holes giveaccess to key components of theburner for maintenance.

Figure 17 shows a revolutionary newtype of mixer using a combinationcounterflow dryer and drum mixer. Itis the Double Barrel® mixer. The mixeruses an enlarged coater, comparableto the one on Drum Mix Coater-II(Figure 9). The shell of the drumserves as the shaft of the coater. Thisresults in a coater 10 to 12' in diam-eter, giving an extremely large mixingarea.

Figure 18 shows a temperature pro-file of the Double Barrel drum mixer.Virgin aggregates are dried in theinner drum. They are superheated to

600°–650° F when running 50% re-cycle. After drying, the virgin aggre-gates drop through the wall of theinner drum into the outer drum. Thereit meets with the recycle material andthe two are mixed. The super hotvirgin aggregates melt the old asphaltin the recycle material and the oldasphalt coats the virgin aggregatesbefore fresh liquid asphalt is injectedinto the mix. Mixing time in this outershell is 3/4 to 1 minute. The mix timeis longer than usual, long enough toproduce a very homogeneous, high-quality mix.

The outer shell of the drum does notrotate. Because of that it provideseasy access for adding a variety ofother recycle materials to the mixwhen needed or available. With thisdesign, 90% of the heat for meltingand drying the recycle comes fromthe virgin aggregate. The other 10%comes from the heat conductedthrough the inner shell and throughthe blending blades in the mixer sec-tion. Heat conducted through the in-ner shell to the mix keeps the shellfrom overheating. The outer shell ofthe Double Barrel stays at approxi-mately 120° F at all times. All thesefactors produce a highly efficientmixer.

Any smoke or odor coming out of themixing section is pulled back throughholes in the inner drum. These are thesame holes that pass the virgin ag-gregates into the outer shell. The pol-lutants go directly into the flame of theburner where they are consumed. As a result there are no odors and the stack haszero opacity. Moreover, virgin asphalt and recycle materials are not exposed to hotgases and steam produced by drying virgin aggregate in the inner shell. So, thesegases and steam cannot distill light oils from the these materials.

As recycle material is heated in theouter drum or mixing chamber of theDouble Barrel mixer, its moisture isdriven off as steam. This produces aninert steam atmosphere in the mixingchamber, with practically no free oxy-gen. Thus, there is virtually no oxida-tion of the mix while in the mixingchamber. The unusally long mixingtime in the outer chamber gives suffi-cient time for the recycle to melt priorto the injection of new liquid asphalt.All of these favorable conditions re-sult in an extremely high quality, well-coated mix.

Figure 19 shows production ratesfor parallel-flow drum mixers with 6 to10' diameters, a mix temperature of300° F and a stack temperature of310° F.

Figure 20 shows production ratesfor counterflow drum mixers withoutpreheated secondary air or an air-cooled shell.

Figure 21 shows production ratesfor the counterflow Double Barrelmixer. It cools the shell while preheat-ing secondary combustion air.

The Double Barrel mixer plant givesan important secondary benefit. Itcauses bags in the baghouse to havemuch longer life. Because no oil froma Double Barrel mixer reaches thebaghouse, bags last 700,000 to1,000,000 tons of mix.

Conventional drum mixers strip lightoils from the liquid asphalt. The oilscoat the filter bags, shortening theirlife (Figure 22). They may last for only20,000 to 40,000 tons of mix whenrunning recycle materials.

The Double Barrel mixer has an-other advantage over conventionaldrum mixers. On the Double Barrelmixer, fines are returned to the mixerfrom the baghouse via a rotary air lockand screw conveyor. The pressuredrop between the baghouse and theDouble Barrel mixer is very low. As aresult, the airlock will last up to tentimes longer than the air lock andblower system typically used on aconventional drum mixer. (Figure 23.)

The counterflow dryer design pro-vides lower stack temperatures. Thisallows higher production rates to beachieved as shown in Figure 24.

With the counterflow mixer design asshown in Figure 13, Figure 15, andFigure 17, flame efficiency is consid-erably higher. Aggregate is hot anddry by the time it reaches the flame.Consequently, it evaporates any drop-lets of unburned fuel thereby increas-ing combustion efficiency. In a paral-lel-flow mixer cold, wet aggregateshowers through the flame and tendsto smudge out the flame. This allowsdroplets of fuel to go unburned andproduces much lower efficiency. (SeeFigure 25).

Figure 26 shows typical fuel usagefor three types of drum mixers. Notethat the Double Barrel mixer usesnearly 1/4 gallon less fuel per ton at5% moisture. Accordingly, it burnsapproximately 36,000 gallons less fuelon a plant that runs 200,000 tons peryear with average moisture of 5%.With an average moisture of 10% theDouble Barrel saves slightly morethan 1/4 gallon per ton, saving 65,000gallons in a year. When using recyclematerials the Double Barrel mixergives even higher savings.

The revolutionary Double Barrelmixer meets the needs of the future.It can run up to 50% RAP withoutvisible emissions. And it can use avariety of recycle materials. It oper-ates more efficiently and has lowermaintenance. It produces hotter mixesof highest-quality while meeting strin-gent federal and local air pollutioncodes.

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