better education can help topopularise cryogenicgrinding (Table 1), but healso argues that for manyapplications the technologyis currently still tooexpensive. By providingbetter understanding of thefundamentals of cryogenicgrinding, he says, the newplant will reduce refrigerantconsumption whileimproving product qualityand increasing throughput.

“Of course it depends onthe product,” said Bertling,“but I would say that inconventional cryogenicgrinding we should be ableto cut liquid nitrogen consumption by20–40%. To cut costs even further,mechanical refrigeration can replacesome or even all of the liquid nitrogen –and in the latter case that means totaloperating costs, including capital charges,of just 15–20% of the cost of anequivalent cryogenic mill.”

P robing the fundamentalsConventional cryogenic grinding usesliquid nitrogen to cool the material to beground, making it brittle and so reducingthe amount of energy required by themill (see box on page 20). It alsoincreases throughput – often by 100% ormore for the same particle sizedistribution – or enables finer grinding at

cryogenics

23 November 2000

RYOGENIC grinding is a proventechnology that is extremelyeffective, especially for plastics

and rubbers, according to FrankBurmester, an engineer with industrial gassupplier Air Products in Germany. But ashe pointed out: “Unfortunately, all toooften it’s put in a box marked ‘tooexpensive’”. Now a new independentpilot plant and consultancy servicededicated to cryogenic grinding is set tochange engineers’ perceptions of thismethod of size reduction.

Based at Oberhausen in the Ruhr areaof Germany, the facility is run byFraunhofer Umsicht, a research institutionspecialising in environmental, safety andenergy technology that forms part ofGermany’s Fraunhofer Gesellschaft. The2 0 0 kg/h pilot plant, which is equippedwith six different mills, was started up on10 November.

“This plant is unique – no university orcompany in the world has such awonderful facility for cryogenic grinding,”said Burmester. Air Products is supportingthe project along with severalmanufacturers of grinding equipment, andBurmester has been closely involved.Notable features, he says, are the use ofmechanical refrigeration alongside liquidnitrogen, and a control system based on aneural network.

Jürgen Bertling is a researcher atFraunhofer Umsicht who has beenresponsible for much of the design of thenew plant. Like Burmester, he thinks that

19the chemical engineer

A new test centre for cryogenic size reduction will provide impartial advice,better technology and lower grinding costs, reports Charles Butcher

Cryogenic grinding: anindependent voice

C

Table 1: Applications of cryogenic grinding

Application Material Size range (µm) Market (t/y)

Powder coatings Polyester, acrylics, 40–50 300,000 (Europe)PU, PA, PVC

Plastisols (rotomoulding) PVC, PE, PP, PA < 500 *150,000 (Europe)

Textile coatings PA, PE 20–300

Carpet backing PE, EVA < 500

Adhesives Polyester, PU, EVA < 100

Lubricants Waxes

Recycling rubber Tyres, EPDM, NBR, SBR < 200 40,000 (Germany)

Recycling plastics uPVC

Recycling blends PE and rubber < 30

*The market figure for plastisols and textile coatings are combined Source: Fraunhofer Umsicht

▲ Figure 1: Many factors affect liquid nitrogen consumption incryogenic grinding. As a result, consumption in very similar plantscan vary by a factor of three or more

the same throughput, and it tends tonarrow the product particle sized i s t r i b u t i o n .

Liquid nitrogen consumption variesfrom around 0.7 kg for every kg ofmaterial ground, for low-value productssuch as recycled rubber, right up to2 0 kg/kg for difficult, high-value productsthat cannot be ground by any othermethod. Figure 1 shows some of thefactors affecting liquid nitrogen use.

One reason liquid nitrogenconsumption figures are so hard to pindown, says Bertling, is that different usersget different performance from similarequipment: “One plant may use 1 kg ofliquid nitrogen per kg of product, whileanother may use three or five times asmuch for the same product in an identicalmill. At the moment we are working ondifferent ideas for optimising the process.Up to now there is not very much know-how in this area; mills are an oldtechnology, but at the theoretical levelthey are quite poorly understood,especially in cryogenic service.”

Neural networks are a good way tounderstand processes that are hard tomodel analytically, so the new pilot plantis controlled by a commercial neural-netsoftware package. “The hard part hasbeen not in the software but in decidingwhat to measure and what weightingfactors to use,” said Bertling. “Aconventional cryogenic grinding process

cryogenics

23 November 200020 the chemical engineer

measures thetemperature at a singlepoint, at the bottom ofthe mill. In our searchfor optimal control weare looking at muchmore data, includingthe inlet temperaturesof the air and the solid,and the powerconsumption of themill.”

Quick-changeplantMeasuring around5 × 10 m and 12 mhigh, the pilot plantbuilding houses millssupplied by Desytec(the Desintegratormill), Hosokawa

Alpine (UPZ), Hosokawa Mikropul(Cascade prototype and ACM), JöckeringMaschinenfabrik (Ultrarotor III) andMicrotec (UTM). Capacities of theindividual mills vary from 50–500 kg/h,and their specific air consumptions spanthe range 2.5–10 m3/kg. Studying thecharacteristics of the different mills willhelp the researchers choose the besttypes for particular jobs, and mayeventually lead to completely new millsespecially designed for cryogenicgrinding.

For the moment, though, theemphasis is on optimising the design ofthe plant rather than the millsthemselves. The equipment is arrangedfor maximum flexibility (Figure 2), and ittakes just 30 minutes to change fromone mill to another, says Bertling. Tominimise emissions, exhaust air isfiltered and recirculated.

Grinding quickly, grinding coolGRINDING is a pretty inefficient process: up to 99% of themechanical energy entering the mill ends up as heat. Withindustrial mills typically in the size range 20–100 kW, cooling isa significant issue – and for materials that deform or melt whenthey are warmed, temperature rise can also be a problem.

Water or other liquids provide effective heat transfer, but notall materials are suitable for wet grinding. Indirect water coolingof the mill is of limited effectiveness because of the lack of heattransfer area.

Most dry mills therefore rely on a large flow of air ornitrogen to both cool and transport the product. But even witha large airflow, some particles can reach temperatures of up to300°C, which is often high enough to cause a significant loss ofq u a l i t y .

A clean and effective way to boost cooling is to inject liquidnitrogen at a temperature of –196°C into the product upstreamof the grinding process. An example is the Cryo-grind systemoffered by industrial gas supplier Air Products.

How low can you go?Grinding processes using liquid nitrogen fall into two types:temperature-controlled grinding and true cryogenic grinding. Inthe first type, the liquid nitrogen is injected directly into the mill,where it acts as a heat transfer medium rather than a refrigerant.Liquid nitrogen injection rate is controlled by a sensor thatmeasures the temperature of the air and nitrogen leaving themill. The ground product leaves the mill at typically 10–30°C,and at no stage does its temperature fall low enough to causee m b r i t t l e m e n t .

True cryogenic grinding is a different process designed toexploit the tendency of many materials to become brittle at lowtemperatures. In grinding plastics and rubbers, for example, thestresses needed to break up the material are dissipated byrelaxation. As a result, plastics need 10–100 times as muchenergy to grind as inorganic materials, or typically1 0 0 – 1 0 0 0 kWh/t at room temperature for particles in the sizerange 100–1000 µ m .

Rapid impacts and low temperatures hinder the ability ofpolymers to relax. The effect is pronounced: for some grades of

polypropylene, cooling from 20°C to –20°C increases stresses by an amount corresponding to a million-fold decrease incontact time during the impact. This is where cryogenic grindingcomes in.

To allow time for the material’s temperature to fallsufficiently, the liquid nitrogen is sprayed onto the feed in aspecial contactor, such as a screw conveyor, well upstream ofthe mill. Large particles are harder to chill, so the feed istypically a granulate of around 3–6 mm size.

Cooling shrinks the crystal lattice of the substance to beground, and introduces microscopic cracks that greatly reducethe amount of energy needed to cause fracture. Usefully, theheat capacity of the material decreases as the temperature falls,thus reducing the amount of liquid nitrogen needed to reducethe temperature further.

In its glassy state the material is easier to grind, sothroughput can be increased, particle size can be reduced, orboth. Wear on the mill is less, and the presence of inert nitrogenusually keeps the oxygen level below 6%, so providing usefulprotection against dust explosions.

As well as being free from damage caused by temperatureexcursions, the cryogenically-ground product typically has anarrower particle size distribution than can be obtained fromgrinding at ambient temperature. The product generally flowsmore freely, and the mill is easier to clean.

So what’s best?Not surprisingly, the best grinding technique depends on theproduct and the process. For easy-to-grind products such asmany inorganic substances, especially with large particle sizes,cryogenic grinding may not be economic, says Frank Burmesterof Air Products: “If you want to reduce particle size in this case,it can be cheaper to use the existing equipment at ambienttemperature and simply reduce the throughput.”

But for more difficult products, working at cryogenictemperatures can boost throughput by 100% or more, as well asimproving product quality. An extreme example, saysBurmester, was a metal soap whose waxy nature made it almostimpossible to grind at ambient temperature. “We increased thethroughput from 20 or 30 kg/h to 250 kg/h,” he says.

▲ The new cryogenic grinding pilot plant at Fraunhofer Umsichtuses indirect mechanical refrigeration (shown here) as well asliquid nitrogen to cool the material to be ground. As a result,operating costs may be as low as 20% of those for conventionalcryogenic grinding, claim the researchers. Photo: Jürgen Bertling

cryogenics

23 November 2000 21the chemical engineer

After better control and optimisedplant design, a new approach torefrigeration is the third route by whichthe Fraunhofer researchers plan to cutgrinding costs. Their cascademechanical refrigeration plant, whichuses the refrigerants R507 and R508 togenerate refrigerant temperatures downto –80°C and solid temperatures downto –60°C, is cheaper than using liquid

nitrogen for processes that do notrequire true cryogenic temperatures.

As well as carrying out originalresearch work at the new plant, theFraunhofer team hopes to winconsultancy work and industrial trialsfrom companies keen to optimise theircryogenic grinding processes. “I amsure cryogenic grinding is a growingmarket, especially in areas such as

rotomoulding materials, hot-melt adhesives, powdercoatings and recycling,” saysBertling, “and this facility willprovide independent advice,with no bias towardsparticular manufacturers.” Ifthe economic gains to be hadfrom improving cryogenicgrinding are anything like hispredictions, this is atechnology about which wewill be hearing a lot more inthe years to come. ■

For more information contactJürgen Bertling, FraunhoferUmsicht in Germany, tel +49 208 85 98 168,

fax +49 208 85 98 290, email bj@umsicht.fhg.de, or Frank Burmester, commercialtechnologies engineer, Air Products inGermany, tel +49 2 324 689 309, fax +49 2 324 689 426, email Burmesf@apci.com.

Websites www.umsicht.fhg.de/and www.airproducts.de

▲ Figure 2: The new cryogenic grinding pilot plant at Fraunhofer UMSICHT is designed for maximumprocess flexibility, with a wide choice of mill types. Through better control, optimised flowsheeting and theuse of mechanical refrigeration as well as liquid nitrogen, the researchers hope to cut operating costs byup to 80%