batch vs. continuous by E. Gail Mize and Greg Renegar

technical Paper t-145

T-145 baTch vs. conTinuous

asTEc encourages its engineers and executives to author articles that will be of value to members of the hot Mix asphalt (hMa) industry. The company also sponsors inde-pendent research when appropriate and has coordinated joint authorship between industry competitors. information is disbursed to any interested party in the form of technical papers. The purpose of the technical papers is to make information available within the hMa industry in order to contribute to the continued improvement process that will benefit the industry.

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CONTENTS

INTRODUCTION ............................................................................................... 2

ADVANTAGES OF CONTINUOUS mIx TEChNOlOGy ................................... 4

FINAl ANAlySIS .............................................................................................. 7

APPENDIx ...................................................................................................... 10

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I. INTRODUCTIONOn many occasions, it becomes necessary for a producer of hot mix asphalt paving materials to determine whether to invest in another batch plant or to change to a continuous type plant. A continuous plant takes on several confi gurations.

The older continuous plants are parallel fl ow drum mixers where the materials proceed in the same direction as the hot gases. These devices, illustrated in Figure 1, either have a center entry port for RAP or not, depending on the customer's requirements.

A later vintage of this plant type is the counterfl ow drum mixer with the burner head placed part way into the drum to allow mixing with liquid asphalt to occur behind the burner, out of the gas stream (Figure 2). This prevents some hydrocarbon emis-sions and increases fuel effi ciency over the parallel fl ow drum mixer.

The most recent and most techno-logically advanced version of this is Astec’s Double Barrel® drum mixer, which has a counterfl ow style drum. It has increased mixing time due to the mechanical method of mixing similar to a batch plant. The Double Barrel drum mixer depicted in Fig-ure 3, provides the highest level of thermal effi ciency of any continuous mixer on the market today, and has the greatest ability to handle RAP effi ciently. Mechanical mixing of 45 to 60 seconds provides a very uni-form product of the highest quality. The internal counterfl ow dryer in the

F1

F2

F3

3

F4

Standard Surface Area Calculation

Sieve Size Percent Passing S. A. Factor Surface Area 3/4” 100 - 2.0 3/8” 90 - - No. 4 75 2 1.5 No. 8 60 4 2.4 No. 16 45 8 3.6 No. 30 35 14 4.9 No. 50 25 30 7.5 No. 100 18 60 10.8 No. 200 10 160 16.0 Total 48.7

The total surface area of all sieve sizes is 48.7 square feet per pound

Double Barrel drum mixer gives the best thermal efficiency available today in a continuous plant.

Most people think, erroneously, that the batch plant is able to deal with inconsistent blends of aggregates better than the continuous plant. To understand why this is not the case, one must become familiar with the mix design process. This paper explains why the continuous mixer will handle the job as effectively as the batch process.

First and foremost is the underlying principle of mix design theory. In mix design, a calculation is made to determine the surface area that needs to be coated with asphalt cement, also called the binder. A normal film thickness of approximately ten microns is used so that the quantity of asphalt for this mixture can be determined. Various additives (polymers, fibers) have been used to increase film thickness without drainage. The amount of surface area determines the amount of asphalt cement that is to be used. In a batch plant, the amount of material that is used in the mix is predominantly from Bin 1. In most cases the Bin 1 material is from forty to fifty percent (by weight) of the total mixture. With this in mind, let us look at several different mix designs and determine the surface areas based on material that is less than the normal bottom deck screen size in a batch plant. This screen opening size is normally 3/16 inch (5 mm).

The gradation of the aggregate or blend of aggregates employed in the mix is used to calculate the surface area of the aggregates. This calcula-tion consists of multiplying the total percent passing each sieve size by a “surface area factor” as set forth in Figure 4. Add the products thus obtained and the total will represent the equivalent surface area of the sample in terms of square feet per pound. It is important to note that all the surface area factors must be used in the calculation. Also, if a differ-ent series of sieves is used, different surface area factors are necessary.

The noteworthy surface area factor for Portland cement is 300 and for hydrated lime is 400.

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II. ADVANTAGES OF CONTINUOUS MIX TECHNOLOGYThis paper will demonstrate that the vast majority of the surface area of a mix is not controlled by the sizing screens in a batch type facility. What this basically means is that the important surface area considerations (and thus the quantity of asphalt cement to be used in a mixture in a hot mix facility) is a function of what exists in the cold feed and is uncontrol-lable thereafter.

Let's clarify the differences in the way aggregate is processed, with respect to gradation, in a batch and continuous plant. In a batch plant, sized aggregate is placed in cold feed bins and blended to some degree depending on the particular plant, before being sent to the dryer. When the aggregate exits the dryer, the aggregate blend is carried to a large screen and separated into four or more blends. The mix gradation called for in a particular batch is created by "pulling" an appropriate amount of aggregate from each of the bins, commonly called hot bins, since they contain hot aggregate. It is generally assumed that a batch plant can accommodate inconsistent aggregate blends in the cold feed due to the screening operation that takes place at an intermittent point in the process. This paper explains why this is not so. In contrast, a continuous plant blends aggregates at the cold feed system as the computer controls "tell" each cold feed bin feeder exactly how much aggregate is required to satisfy the mix design requirements. The blending at this point in the process is very precise. This mixture is then dried, mixed with liquid AC (bitumen), and transferred to a surge bin or storage silo. If RAP is added, it is first fractionated into hard-to-segregate sizes and treated just like the virgin ag-gregate. It is placed into two or more RAP feed bins and precisely blended before being mixed with the hot virgin aggregate. The virgin aggregate and RAP are mixed together before the virgin AC (bitumen) is added. Of course, a consistent mix depends on consistent aggregates in the cold feed bins, but a batch plant must also have consistent aggregates at the cold feed end of the process in order to produce a consistent product, as this paper will prove. Since consistent aggregates are required for both a continuous and batch process, the continuous plant becomes the plant of choice with respect to initial cost, operating cost, maintenance cost, portability, and RAP processing capabilities.

Now let us look at four different mix designs that are standard designs in the State of Georgia. These designs are standard designs used by the Georgia Department of Transportation. A maximum density curve as well as a surface area calculation is included for all four mix designs.

The surface area calculation for Georgia Base Mix is as shown in Figure 5. In this Georgia Department of Transportation Base Mix, 10.4 percent

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of the surface area (SA) is above the No. 8 sieve and 89.6 percent of the surface area is No. 8 sieve size and smaller. Therefore, it is safe to say that in this mix, a batch plant screen has control of about 10 per-cent of the surface area and the rest is under the control of the cold feed bins. In short, in both the batch plant and the continuous mixer, the cold feed proportioning control system controls 90 percent of the surface area in this mix.

Let’s now look at a Georgia Depart-ment of Transportation “B” Modified Binder mix.

In this Georgia Department of Trans-portation Modified Binder Mix, 10.3 percent of the surface area (SA) is above the No. 8 sieve and 89.7 percent of the Surface Area is No. 8 sieve size and smaller (Figure 6). Therefore, it is safe to say, once again, that in this mix a batch plant screen has control of about 10 per-cent of the surface area and the rest is under the control of the computer blending system of the cold feed bins.

F5

Surface Area Calculation for a GA Base Mix

Sieve Size Percent Passing S. A. Factor Surface Area +No. 4 - 2 2.0 -No. 4 47.0 2 0.9 No. 8 36.0 4 1.4 No. 16 26.0 8 2.1 No. 30 20.0 14 2.8 No. 50 15.0 30 4.5 No. 100 9.0 60 5.4 No. 200 5.5 160 8.8 Total 27.9

F6

Surface Area Calculation for “B” Modified Binder Mix

Sieve Size Percent Passing S. A. Factor Surface Area 3/4 92.5 2 2.0 3/8 74.5 - - No. 4 58.0 2 1.2 No. 8 46.0 4 1.8 No. 16 33.0 8 2.6 No. 30 24.0 14 3.4 No. 50 17.5 30 5.3 No. 100 10.0 60 6.0 No. 200 5.5 160 8.8 Total 31.1

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Lets now look at a Georgia De-partment of Transportation Open Graded Surface “D” Mix.

In this Georgia Department of Trans-portation Open Graded “D” Mix, 21.3 percent of the surface area (SA) is above the No. 8 sieve and 78.7 percent of the surface area is No. 8 sieve size and smaller (Figure 7). Therefore, in this sample mix, a batch plant screen has control of about 20 percent of the surface area. The rest is under the control of the cold feed bin blending system.

Now let’s look at the last mix design to be presented in this section. It is a Georgia Department of Transporta-tion Dense Graded Surface “F” Mix.

In this Georgia Department of Trans-portation Dense Graded Surface “F” Mix, 9.7 percent of the surface area (SA) is above the No. 8 sieve and 90.3 percent of the surface area is No. 8 sieve size or smaller (Figure 8). Therefore, it is safe to say, once again, that in this mix a batch plant screen has control of about 10 per-cent of the surface area and the rest is under the control of the cold feed bin blending system.

The above four mixes represent a wide range of all the mixes produced in the hot mix business. When con-sidering the surface area control of the batch plant versus a cold feed computer blending system, which is always part of a modern continuous plant, it is easy to understand why the continuous plant now dominates the plant type chosen by contractors

F8

Surface Area Calculation for Dense Graded Surface “F” Mix

Sieve Size Percent Passing S. A. Factor Surface Area 3/4 100.0 2 2.0 3/8 95.0 - - No. 4 65.0 2 1.3 No. 8 47.0 4 1.9 No. 16 36.0 8 2.9 No. 30 26.0 14 4.2 No. 50 19.5 30 5.9 No. 100 12.0 60 7.2 No. 200 5.5 160 8.8 Total 34.2

F7

Surface Area Calculation for Open Graded Surface “D” Mix

Sieve Size Percent Passing S. A. Factor Surface Area +No. 4 100.0 2 2.0 -No. 4 30.0 2 0.6 No. 8 7.5 4 0.3 No. 16 6.0 8 0.5 No. 30 5.0 14 0.7 No. 50 4.0 30 1.2 No. 100 3.5 60 2.1 No. 200 3.0 160 4.8 Total 12.2

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and operators in the United States today. It is also easy to understand that the continuous plant will eventually take over as producers realize that the continuous plant is a more energy efficient, less expensive to maintain, and provides the control necessary at the cold feed bins to consistently produce mix that meets the most stringent specifications.

III. FINAL ANALYSISIn the final analysis, what is in the cold feed bins is what will be produced with either a continuous or batch process.

So what are the batch plant advantages?

1. Multiple mixes can be produced in small quantities. However, the continuous plant can also produce multiple mixes with multiple long term storage silos.

2. To meet specifications that require a batch plant.

3. A marketing advantage when properly applied in selling a multi-tude of different mix designs to many different market segments.

The primary reason for the purchase of a batch plant today is a lack of understanding of the continuous process. This is coupled with too little knowledge of how the batch process works. How the continuous plant per-forms and its tremendous ability to make a uniform product is now widely known in North America, the world's largest market for hot mix asphalt.

The screens in the batch plant may also create a problem with final mix gradation. It is caused by changing screen efficiency, or override. This occurs when the bed thickness varies on the screen deck. This fluctuation is a function of the feed rate to the screen, the material characteristics, the number of decks and the material depth on the cloth.

The Appendix on page10 represents a standard screen sizing chart. All the manufacturers publish a chart similar to this with similar factors for screening of aggregates. This chart, like all the others, utilizes an equa-tion in which you insert the various factors from the table to arrive at the tons per hour and efficiencies of the screen. The equation rearranged is:

Area of Deck(S) x Screening Factors(A x B x C x D x E x F) = Through Flow Tonnage(T)

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After a screen is in place on a batch plant it is obvious that the area of the deck cannot change. Therefore, the deck area becomes a constant. See Appendix for factors A through F. Factor A is determined by the cloth size, which will not change after sizing. Factor B will not change when the feed from the cold feed is kept in the same proportions. Factor D will not change for the same reason that Factor B will not change. Factor E is not used. Factor F will not change since you will be using the same deck for comparison.

On a batch plant running the same cold feed blend, the only variable fac-tor in Factors A through F is Factor C, which is the screening efficiency. Therefore, when the Through Flow Tonnage is varied, the only factor that can change on an operating batch plant is Factor C.

Simply put, the equation looks like this:

S (constant) x C = T

Consequently, C varies with T and the screening efficiency goes up and down with the tons per hour that is fed to the deck. The bottom deck is the critical deck on a batch plant since what is placed into Bin 1 is controlled by this deck. By examining Factor C and the desired efficiency columns in Appendix A it becomes clear that the amount of Bin 1 material in Bin 2 can vary greatly when the feed rate to the bottom deck changes.

By assuming that the production rate through the dryer doubles, it is easy to see that as much as 20 percent of Bin 1 material can be placed into Bin 2. This will affect the surface area in the mix since Bin 1 material is placed in the mix when Bin 2 is discharged into the weigh hopper. The amount of asphalt cement should be changed since the gradation of the mix changed. It fact, the specification band (range) of the mix design could be effected for some sizes of aggregates.

Varying the gradation in the bins means full width bin samples should be taken from the hot bins to make certain that the correct gradation is being removed from the hot bins. In a typical North American style batch plant, provisions are made between the hot bin gates and the aggregate weigh hopper to take a full width bin sample to verify the gradation of materials in the hot bins. In European style plants there is often no provision for full width sampling or any sampling of the hot bins. In the United States, the state Departments of Transportation that specify the design formulas have a mandatory requirement for full width hot bin sampling. Lack of hot bin sampling would lead one practiced in the art of mix design to assume that

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the continuous plant would certainly be a better choice than a batch plant that has no provision for checking accuracy on a frequent basis.

Another area of concern in a batch plant that has large hot bins is Bin 1 segregation. The Astec pat-ented partition in the hot bin mini-mizes segregation (Figure 9). Note the location of the partition. It allows the material on the sloping surface of Bin 1 to feed in proportion to the coarser material that passes through the screen later and collects on the other side of the partition. A less at-tractive solution is to place a sloping plate or baffle under the screen to move the fines over to the center of the bin (Figure 10). This works to a more limited degree. In the case of very large hot bins, the first solution is definitely the best.

In conclusion, a higher quality product due to less variation can be produced from a continuous plant. State of the art computer controls, accurate belt scales, asphalt (bitumen) metering sys-tem, and the cold feed blending system will assure that a con-sistent product is produced. In addition, recycling, an economic requirement of the future, is able to be handled much better in the continuous facility. It dramatically lowers mix cost and reuses an otherwise waste product.

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