Powder and Bulk Engineering, January 1996 3 1

Engineering study: Solving a blending problem

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Vernon A. Fauver Dow Chemical USA

If your equipment isn’t doing the job it was designed to do and you’re losing product and production capacity as a result, it may be time to replace or retrofit the equip- ment. But how do you find the right solution? This article is a step-by-step account of a chemical plant’s engineer- ing study to solve their batch blending problem. The plant engineers’ study included their own investigation of the problem, blender tests at an equipment manufac- turer’s test center, the plant’s statistical analysis of the test data, and the blender manufacturer’s support at startup. While this story covers a batch blending process, you can apply the principles to almost any bulk solids processing or handling application.

atch consistency is particularly important for a plant that produces specialty chemicals or other complex, high- B value products. Such products are often made in batch

reactors, and the plant may produce several similar products or grades of a single product. To control batch-to-batch variation of product properties within statistical limits, the plant typically blends the final reactor product into larger lots before shipping.

One plant in Dow Chemical’s Michigan Division operates this way. Until recently, the plant blended various batches of its spe- cialty chemical powder into larger lots in an ll-foot-diameter, 2,000-cubic-foot-capacity proprietary gravity silo blender, shown in Figure 1. The unit was installed in 1966. A dilute- phase pneumatic conveying system transferred individual batches from transfer hoppers into the blender, where the batches formed layers of slightly different properties. Once the blender was fully loaded, the powder flowed through it by grav- ity and discharged to the pneumatic conveying system, which circulated the powder back to the blender’s top for another cycle. After blending, the lot was discharged and packaged in small bags.

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thoroughly mix the lot. But over the years the time required for thorough mixing had increased to 6 to 8 hours, requiring at least eight turnovers. Eventually, as the plant began to manufacture higher quality products, even this time wasn’t enough. While the lot tested within specifications in the blender, the packaged

2. powder’s assay differed from bag to bag; additional circulation didn’t reduce the variation. Increasing product demand was also requiring shorter blend times.

The gravity silo blender was designed to circulate the powder for the equivalent of two turnovers - a 2-hour process - to

32 Powder and Bulk Engineering, January 1996

Finding the problem’s source Plant engineers studied the gravity silo blender to determine why blend times had increased and why the bagged powder was inconsistent. First, they compared the existing blender’s silo and components with the original design drawings to deter- mine if the equipment had been modified or damaged. This re- vealed that a baffle inside the silo was damaged. After replacing the baffle, throughput improved slightly, although not to the 2- hour design goal. But blend consistency didn’t improve.

Seeking a solution When the Dow engineers consulted the gravity silo blender manufacturer, the manufacturer recommended a retrofit to con- vert the blender to the manufacturer’s current gravity silo blender design. But the retrofit would require extensive struc- tural changes to the silo, and the result would be a slow blender that still depended on circulation to achieve mixing.

So the plant engineers looked at other options. Their require- ments were:

The new blender must provide a fully random blend.

*The new blender must achieve good mixing in the shortest possible time.

The new blender must be installed with minimal downtime.

The project must be completed at minimal capital cost.

First, the engineers referred to two published articles on blenders. One paperkompared the performance of seven blenders and showed that a pulse blender using gas (typically air) pulses to fluidize and blend ingredients mixed faster than a ribbon blender or a baffled rotating drum. The paper’s authors concluded that the pulse blender, ribbon blender, and baffled rotating drum mixed slightly faster than a double-cone or orbit- ing screw mixer. [Editor’s note: Find more information on these and other mixers and blenders in articles listed under “Mixing and blending” in Powder and Bulk Engineering’s comprehensive “Index to articles,” December 1995.1

The other paper’ presented a detailed comparison of a pulse blender, an orbiting screw mixer, a double-cone blender, and a Z-blade mixer. The authors of this paper determined that the pulse blender was three times faster than the double-cone unit and eight times faster than the orbiting screw mixer. Both pa- pers reported that the pulse blender’s capital and operating costs were higher than those of the other units.

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Next, the plant engineers considered Dow’s previous experi- ence with various blenders. Dow had successfully used many

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ribbon blenders and double-cone blenders, so the engineers were confident they could achieve predictable, reliable mixing by replacing the plant’s existing silo blender with a ribbon blender or double-cone blender. Although in other plants Dow operated ribbon blenders the same size as the plant’s gravity silo blender, a ribbon blender was more expensive than the silo blender. A double-cone blender wasn’t available in the silo blender’s capacity.

Powder and Bulk Engineering, January 1996

The plant would also have to install a new blender in parallel with the existing unit to minimize downtime during installa- tion. Because adjacent space was limited, the new blender would have to be located at some distance from the existing unit, which would add additional conveying system costs.

The plant engineers took a closer look at the pulse blender be- cause it blended many times faster than the other units with similar mixing results. The pulse blender could also be retrofit- ted to the existing blender’s silo. Although the retrofit would re- quire downtime, it appeared to be a simpler and less expensive project.

But Dow had removed two high-pressure pulse blenders from other plants because they didn’t blend well. The blenders used pulses of 200- to 300-psi air from high-pressure-drop nozzles to circulate and blend powders. So despite the high-pressure

pulse blender’s other advantages, the engineers hesitated to se- lect the unit.

The plant engineers also learned that another Dow Michigan Division unit and a Dow subsidiary had just installed low-pres- sure pulse silo blenders for mixing similar specialty chemical powders. The model3 they installed uses 15- to 30-psig air pres- sure to control the blending pulses.

A closer look at how the low-pressure pulse blender works

The low-pressure pulse unit’s blending head, shown in Figure 2, attaches to a silo’s cone bottom by means of a coupling. The unit consists of a cone valve, 16 pneumatically operated injec- tor valves around the head’s perimeter, a high-pressure mani- fold linked to an air supply, and a discharge flange leading to the batch outlet. A control system is linked to electric timers that control the injector valves.

During the filling cycle, shown in Figure 3a, the cone valve rests on the blender bottom and seals the outlet as ingredients are loaded into the vessel one at a time, forming layers. During the blending cycle, shown in Figure 3b, the timers control the injector valves according to a preset on-off cycle. The valves re- lease pulses of low-pressure compressed air upward and around

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the cone valve and into the powder, moving in a slightly circu- lar pattern. As each air pulse enters the vessel, the material bed is fluidized; when the pulse stops, the fluidized bed collapses completely.

Powder and Bulk Engineering, January 1996

The cone valve not only helps the air flow upward into the pow- der but directs the powder flow during blending. Pulsing con- tinues until the ingredients are fully blended. Depending on the application, the air pulses are filtered as they’re vented from the vessel top and exhausted from a dust collector.

During the discharge cycle, shown in Figure 3c, the cone valve raises, opening the outlet to discharge the blended powder. The raised cone valve serves as a flow insert that prevents powder segregation and ratholing.

Making the final selection After discussing the installation with the subsidiary, the plant engineers decided that the low-pressure pulse blender per- formed much better than the high-pressure unit.

Then the engineers compared relative mixing times and capital costs for the low-pressure pulse blender, retrofit gravity silo blender, ribbon blender, and double-cone blender, as shown in Table I. This, along with the mixing data gathered from the arti- cles, convinced the engineers to try the low-pressure pulse blender. They decided to make test runs with their specialty chemical powder at the blender manufacturer’s test center.

Testing the blender Dow sent six bags of powder, each with a different assay, to the blender manufacturer’s test center. Here the powder was pneu- matically conveyed into the vessel of a low-pressure pulse blender test unit, where the powder from each bag formed one of six layers in a 3-foot-deep bed.

The test unit consists of an 11-cubic-foot clear plastic vessel with an 18-inch-diameter pulse blending head. The vessel has a top cover that opens for sampling. The test unit’s plastic walls permit viewing of the blending action.

After loading the powder, the test center staff ran preliminary tests to see how the powder would blend. For each test, the staff

38 Powder and Bulk Engineering, January 1996

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sults. Figure 4 shows the mixing improvement over time for three typical test runs.

The blender manufacturer’s test center staff took more samples in each test run during discharge to see if the blend was segre- gating. The standard deviations for the three runs during dis- charge were0.21,0.15, and0.32.

The tests showed two things:

used different air pressures and on-off pulse cycle times based on experience with blending similar powders. The air pressure that worked best was from 20 to 25 psig; the pulse cycle time that worked best was a 0.5-second air pulse followed by a 2- second settling period.

The test center staff then ran several tests at 20 to 25 psig with the 0.5-second on, 2-second off cycling. At l-minute intervals throughout each test, the staff used a thief sampler to take sam-

four different levels, for a total of 16 samples per test interval.

The lOW-Pressure pulse blender mixes thoroughly and quickly.

ples Of the powder from each quadrant Of the bed at The unit’s cone valve permits consistent discharge without segregating the blend.

The Dow engineers took the samples to their own lab for a mix- ing homogeneity analysis. They based the analysis on the mix- ing index slu,, where s is the sample composition’s standard deviation and crR is a completely random mixture’s standard de- viation.*For a given blender load, uR is constant, so a plot of s versus time adequately shows the degree of mixing.

For each test, in which the material bed consisted of the six lay- ers of powder with different assays, the assays’ standard devia- tion was 37.4. After 1 minute of blending, the 16 samples’ standard deviation was 1.3. After 2 minutes, the standard devia- tion was also 1.3, and after 3 minutes, it was 0.82. Blends of powders with a narrower range of assays produced similar re-

Designing and installing the blender Based on the test results, the blender manufacturer produced a full-scale blender design for the Dow Michigan plant. The plant engineers specified that the low-pressure pulse blending head must fit the existing 11-foot-diameter silo. At the time of the tests, the largest model in use was an 18-inch-diameter unit in- stalled at a Dow subsidiary on a 7-foot-diameter, 400-cubic- foot-capacity silo. During the design of the Michigan unit, the blender manufacturer commissioned a 36-inch-diameter blend- ing head on a 10-foot-diameter silo. Dow agreed to go with the new, larger blending head. So the Dow engineers designed are- inforced, 36-inch-diameter mounting flange to accept the pulse blending head’s coupling.

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Dow’s field service shop installed the flange and pulse blend- ing head on the existing silo’s cone bottom. The installed unit is shown in Figure 5, which also shows the support posts the shop added to help the structure withstand the pulsing thrusts and the load from the collapsing fluidized bed. The pulse blending head was also linked by a 6-inch gas supply pipe (at the figure’s upper right) to a gas surge tank.

Powder and Bulk Engineering, January 1996

As shown in Figure 6, a dust collector was installed above the silo both to release the pulse gas with minimal back pressure to the vessel and to collect dust from the dilute-phase pneumatic conveying system that loads the silo. A butterfly valve is lo- cated at the blender inlet to control powder feeding during the filling cycle. The control system’s control panel is located about 10 feet from the blender.

Because the plant engineers wanted to eliminate any possibil- ity of a dust explosion, they chose nitrogen instead of air to sup- ply the blending pulse. Because the blender required only 20- to 25-psig pulses to blend the specialty chemical powder, nitro- gen could be supplied from the plant’s nitrogen main. However,

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2. the 36-inch-diameter pulse blending head requires about 70 scfs of gas for each pulse’s duration, which means that a large volume of gas must be available to the blending head. The engi- neers were concerned that this would excessively drain the plant nitrogen supply so they added a large gas receiver near the

minimizing any effect on the nitrogen supply to the rest of the

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plant’s nitrogen inlet. The receiver maintains the system pres- sure by averaging the nitrogen supply between pulses, thus

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Starting up and operating the low-pressure pulse blender

1985. A technician from the blender manufacturer fine-tuned the blender’s control system over the f is t 3 days. The blender

The low-pressure pulse blender was started up in December

ran full-size batches from the start, and the plant engineers set- tled on the optimal blending cycle within 1 week. Since then, the blender has run smoothly.

During the filling cycle, specialty chemical powder batches of various assays feed via the pneumatic conveying system and

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2. butterfly valve into the silo, where each batch forms a layer.

42 Powder and Bulk Engineering, January 1996

During the blending cycle, the electric timers automatically sig- nal the injection valves to release nitrogen into the vessel in short pulses over a 20-minute period. During the discharge cycle, the cone valve raises and the blended lot is conveyed to a hopper for bagging.

The low-pressure pulse blender typically blends 1,000 to 1,250 cubic feet of powder per lot. Although the blender releases pulses over 20 minutes, most lots appear to be mixed after 10 to 12 minutes, a drastic reduction from the 6- to 8-hour or longer cycles required for circulating lots in the old gravity silo blender. Samples taken from the bagging line downstream from the pulse blender show a standard deviation of about 0.08; the old blender’s bagged samples had an average standard devia- tion of 2.7. The pulse blender’s assay variation across several lots matches that observed during testing, showing that the lots are thoroughly mixed.

Dow is so confident of the pulse blender’s performance that the engineers now take and analyze a full set of samples from the bagging line just once a week. As a result, analysis costs are now less than one-quarter those of the old blender.

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2. The low-pressure pulse blender requires about 10 percent of the mixing time needed by a ribbon blender that mixes similar

cost, including the nitrogen supply, is about 12 percent that of the ribbon blender. And as predicted, total project costs are about 33 percent of the estimated cost for replacing the silo

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powders in another Dow plant. The new blender’s operating

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Editor’s note: How you can apply this information Dow’s engineering study to find a blending equipment solution is an approach you can apply to various equipment problems in your bulk solids handling or processing plant.

Before looking for new equipment, carefully investigate the problem’s source by comparing your existing equipment’s de- 7 i

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sign drawings and other data with the equipment itself. The equipment may have been modified or damaged over the years.

2. Check the literature for studies investigating or comparing sim- ilar equipment models. What are the capital and operating costs

for each? How does their performance compare? How much floor space or headroom does each require?

When selecting new equipment or a retrofit solution, consider your plant’s or industry’s experience with similar equipment types and equipment manufacturers. Find an extensive list of equipment manufacturers grouped by equipment type in Pow- der and Bulk Engineering’s 1995-96 Reference & Buyer’s Re- source (August 1995). Also check the customer service record of any manufacturer you’re considering. Then work with the manufacturer’s test center to determine how the new equipment will work for your application. The test center staff has experi- ence with the equipment and in getting the best results with op- erating conditions and materials like yours.

Analyze the test data to determine how well the equipment solves your problem. Work with the manufacturer to design the full-scale equipment based on the test data. Then enlist the

manufacturer’s aid when installing and starting up the equip- ment. Establishing a close working relationship with the manu- facturer at this crucial stage will help ensure the equipment continues to successfully meet your needs.

References 1. Miles, J.E.P., and Schofield, C., “Performance of several in-

dustrial mixers using non-segregating free-flowing pow- ders,” Transactions of the Institution of Chemical Engineers, Vol. 48, pages 85-89.

2. Ashton, M.D., and Valentin, F.H.H., “The mixing of powders and particles in industrial mixers,” Transactions ofthe Znsti- tution ofchemical Engineers, Vol. 44, pages 166-1 88.

3. Blendcon pulse blending head, Dynamic Air, St. Paul, Minn. Similar equipment is available from several manufacturers. Find them listed under “Mixers and blenders” in Powder and Bulk Engineering’s 1995-96 Reference & Buyer 5. Resource (August 1995).

Vernon A. Fauver is an environmental associate at Dow Chemical USA, Environmental Services, 1261 Building, Mid- land, MI 48667; 51 7/636-5668. He holds BS and MS degrees in chemical engineering from Purdue University in West hfayette, Ind. This article is adapted from a paper the author presented at the 1994 Powder and Bulk Solids Conference in Rosemont, Ill.

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