To what extent can process time and energy be saved with modern cone mixing dryers?
Modern cone mixing dryers can save process time and energy above all where they continuously guide the product to large temperature-controlled contact surfaces, dry under vacuum at low temperature, and combine several process steps in a single closed apparatus. Reliable percentage figures, however, cannot be stated across the board: they depend strongly on the product, the solvent, the starting and target residual moisture, the vacuum level, condensation, fill level, heat-transfer surface, and the comparison process.
Shorter process times
The most important time saving arises from improved heat and mass transfer. In a modern cone mixing dryer, the product is not heated statically but continuously circulated. This repeatedly brings moist material into contact with the temperature-controlled vessel wall and, where present, with the temperature-controllable shaft, arms or helix. The contact surface is continuously renewed. Insulating product coatings, moist pockets and local temperature differences can thereby be reduced.
The result is more even evaporation. The vacuum lowers the boiling point of water or solvents, so that evaporation can begin at a lower product temperature. For sensitive products, it is therefore not necessary to work with an unnecessarily high jacket temperature. Conical vacuum contact dryers are used specifically for heat- and solvent-sensitive products precisely because of this combination of indirect heat transfer, product circulation and low-temperature operation.
A further time saving arises from mastering rheological transitions. Many products change during drying from a suspension, through a viscous or tough, plastic phase, to a dry powder. It is precisely in the sticky intermediate phase that evaporation can slow considerably in static dryers, because large lumps or poorly heated product zones form. A mixing tool keeps the mass in motion and renews the contact surfaces. Where needed, a deagglomeration stage can break up lumps in a targeted manner. This improves the access of heat and vacuum to the moist product surface and prevents the process from being governed by individual compact agglomerates.
The overall cycle is shortened further where mixing, reacting, crystallising, evaporation, drying, cooling and, where applicable, homogenising take place in the same apparatus. Every transfer that is eliminated saves not only time for discharging and charging, but also waiting time, cleaning, re-inerting, intermediate storage and potential rework. For products with strict containment or moisture requirements, this effect can be more important than the pure shortening of the evaporation time.
A reliable drying endpoint can also save time. If temperature, pressure, condensate flow, product moisture or a suitable PAT signal are monitored continuously, the batch can be ended once the target residual moisture is reached. Without a robust endpoint concept, over-drying often occurs for safety reasons. This increases both cycle time and energy consumption. Industrial studies on vacuum drying emphasise that suitable endpoint determination and process monitoring are key levers for reducing energy consumption and cycle duration.
Lower energy demand
In vacuum drying, energy is needed essentially for evaporating the solvent, for the system's heat losses, for driving the mixing tool, for generating the vacuum, for condensation and, where applicable, for cooling. A cone mixing dryer does not save the physically necessary enthalpy of vaporisation. Savings instead arise from fewer losses occurring, more efficient heat input, and the avoidance of unnecessary process time.
The apparatus-related key is indirect contact drying. Heat is introduced into the product via the temperature-controlled vessel wall and, where applicable, via heated internal mixing tools. Because the mix touches these surfaces continuously, the required heat can be transferred at a moderate temperature difference. Large, actively wetted heat-transfer surfaces improve drying performance and can reduce the required jacket temperature. This lowers heat losses while at the same time protecting temperature-sensitive products. For conical vacuum dryers, heat-transfer coefficients are cited that, depending on the product and operating state, lie in a very wide range of approximately 50 to 2,000 W/(m²·K); this wide range shows how decisive product movement and rheology are for the actual energy demand.
The vacuum enables drying at a lower temperature. This allows sensitive active ingredients, fine chemicals and specialty powders to be processed more gently. In favourable cases, low-temperature heat can additionally be used, for example warm water, recovered process heat, or a low-temperature thermal-oil circuit. Vacuum dryers are therefore particularly attractive where product quality would suffer at high temperature, or where a suitable low-temperature heat source is already available.
The drive energy of the mixing tool remains comparatively low for conical systems in many applications, because the product movement takes place predominantly through controlled circulation at low rotational speed. It is not negligible, however, particularly not during tough, plastic intermediate phases. Variable frequency drives and a rotational speed adapted to the process state can help to increase the mixing intensity only where it is actually required for heat transfer or deagglomeration. For a specific plant, therefore, not only kWh per batch but also kWh per kilogram of solvent evaporated, mixing-tool run time, specific vacuum energy demand, and condenser capacity should be balanced.
Solvent recovery does not necessarily improve thermodynamic drying efficiency, but it can significantly reduce the overall energy and cost footprint. In closed vacuum systems, vapours are routed via condensers. Recovered solvent does not need to be purchased, transported and disposed of, or only to a lesser extent. At the same time, VOC emissions are reduced. Closed, indirectly heated conical vacuum dryers are therefore used for hazardous, toxic or solvent-containing products and can enable extensive solvent recovery.
Additional levers for savings
The economically relevant savings go beyond the energy meter. A cone that discharges largely without residue reduces product losses and shortens the time until the next batch. For high-value active ingredients or fine chemicals, a reduced residual quantity at the bottom of the apparatus can have a greater economic effect than a moderate saving in heating energy.
Good cleanability reduces set-up time, cleaning media, drying time after CIP, and the risk of not being able to release a batch due to cross-contamination or residues. In multi-product plants, the largest OEE gains can therefore arise from shorter changeover times and less rework, not only from a shorter evaporation phase.
Integrated process control also reduces the number of pumps, conveying equipment, intermediate vessels, filter connections and transfer operations. This lowers auxiliary electricity consumption and personnel effort. At the same time, the risk of dust losses, moisture uptake, oxidation and contamination can decrease. These factors are particularly significant for high-value or hygroscopic products.
Assessing savings correctly
Sweeping statements such as "30 to 60 percent shorter drying" or "20 to 40 percent less energy" are not reliable without a reference process. Comparison figures can differ widely, because a vacuum drying cabinet, a static vessel, a paddle dryer, or an existing cone mixing dryer, for example, differ considerably in heat-transfer surface, fill level, vacuum, condensation, heating medium and cycle downtime. In one published laboratory study, a conical screw dryer achieved the highest drying rate under the conditions examined, but attrition and agglomeration were also observed — an indication that time savings and product protection must always be assessed together.
A robust assessment therefore compares the complete batch cycle, not just the pure time to reach the target residual moisture. What needs to be captured is:
Charging, heating up, evacuating, evaporation, post-heating and cooling, venting, discharging and cleaning.
Kilograms of water or solvent evaporated per batch.
kWh for the heating or cooling medium, mixer drive, vacuum pump, condensation and auxiliary units.
Heat losses at the vessel, piping and condensation system.
Target residual moisture, residual solvent, homogeneity, particle size distribution and product yield.
Solvent recovery rate and costs for fresh solvent as well as disposal.
Cleaning duration, set-up losses, residual discharge, and number of batches that cannot be released.
From this data, key figures such as kWh per kg of solvent evaporated, kg of product per hour, total cycle time, yield, and cost per kilogram of released product can be derived. Only then can it be properly assessed whether a modern cone mixing dryer is the economically better solution.
How amixon® assesses energy efficiency, TCO (Total Cost of Ownership) and payback of an industrial mixer
The economics of a mixer or vacuum mixing dryer are not decided by its electrical power consumption alone. What matters is energy demand, mixing and drying time, heat transfer, product loss, residual discharge, cleaning and changeover times, maintenance, and the service life actually achievable. amixon® assesses these factors as Total Cost of Ownership across the entire lifecycle and determines the decisive characteristic values through trials with the original product.
The relevant cost blocks
Over a review period of, say, ten years, five cost blocks determine the economics: energy, product loss, cleaning and changeover times, maintenance and spare parts, and depreciation over the real service life. These factors are closely interrelated. A plant with a low connected load is not automatically economical if it requires long mixing or drying times, if relevant quantities of product remain in the mixing chamber after every batch, or if product changeovers are laborious.
The specific energy demand cannot seriously be stated as a general catalogue value in kilowatt-hours per tonne. It depends on the formulation, batch size, fill level, moisture and solvent content, vacuum level, temperature, heat-transfer surface, heat losses, mixing time, rotational speed, rheology, deagglomeration requirements, and target residual moisture. For a mixing dryer, the heating and cooling medium, vacuum generation, condensation, vapour filtration and, where applicable, inerting are added on top. amixon® therefore determines the relevant data in the pilot plant with the original product and under realistic process conditions.
Energy and heat transfer
amixon® mixers work with controlled, three-dimensional product circulation and comparatively low tool circumferential speeds from around 0.8 m/s. Energy input does not occur primarily through high rotational speeds, throwing motion or strong impact stress, but through recurring product circuits. This limits mechanical energy input and supports product-gentle processing.
Short and reproducible mixing times lower the energy per batch and increase throughput per unit of installed power. In the twin-shaft mixer HM, product streams overlap and support rapid distribution throughout the entire batch volume. In the cone mixer KS, the required homogeneity can, depending on the formulation, fill level and target mixing quality, already be reached after just a few tool revolutions. What is decisive is not the theoretically shortest mixing time, but a robust process window in which the target quality is achieved reproducibly over the long term.
In amixon® vacuum mixing dryers and cone reactors, the SinConcave®/SinConvex® mixing helix can provide particularly large heat-transfer surfaces. In addition to the double-walled, temperature-controllable mixing chamber, the shaft, mixing arms and mixing helix can be designed to be temperature-controllable. These surfaces extend deep into the product volume and supplement the heat transfer via the vessel jacket.
Their effectiveness rests on their being continuously touched by the mix. The three-dimensional product circulation lifts the product upward near the wall, guides it downward again in the centre, and repeatedly brings it into contact with the vessel wall, shaft, arms and mixing helix. Contact with the temperature-controlled surfaces is constantly renewed. This allows heat and mass exchange during vacuum drying to take place efficiently, without the product being exposed to unnecessarily high temperatures or high rotational speeds.
Vacuum lowers the boiling point of water and solvents. This allows drying to take place at lower product temperatures, which protects sensitive active ingredients, fine chemicals, crystalline products, microorganisms, pigments or specialty powders. Savings do not arise from eliminating the necessary evaporation energy, but from reducing heat losses, unnecessary excess temperatures, unproductive mixing times and overlong drying cycles.
Residual discharge and changeover times
Residual discharge is a key, often underestimated efficiency parameter. Any quantity of product remaining in the mixer after discharge causes product loss, additional cleaning effort, carry-over risk, or several of these drawbacks at once. For high-value active ingredients, specialty chemicals, high-priced food ingredients or functional additives, a thorough residual discharge can be economically more important than saving individual kilowatt-hours.
amixon® mixing systems can, depending on design and product properties, enable a very thorough residual discharge. For the AM and KS series, discharge rates of up to approximately 99.98 percent are cited, and up to approximately 99.99 percent for ComDisc® discharge concepts. Such figures are design target values, not a blanket guarantee for every recipe. The discharge rate actually achievable depends on particle size, bulk density, moisture, stickiness, electrostatic charging, surface condition, product adhesion and discharge concept. It must therefore be verified with the original product.
In addition to heat transfer and mixing quality, the SinConcave®/SinConvex® mixing helix also supports thorough residual discharge. It moves the product in a controlled manner through the entire mixing chamber and, depending on the vessel geometry, conveys it toward the discharge area. In cone apparatus, gravity provides additional assistance. Good discharge reduces cleaning effort, lowers the consumption of rinsing or cleaning media, shortens the time until the next batch, and reduces the risk of cross-contamination.
Low-dead-space, readily accessible mixing chambers and inspection openings additionally shorten cleaning and changeover time. This increases the productive availability of the plant. In multi-product plants especially, every shortened product changeover improves the Overall Equipment Effectiveness and increases the quantity of released product that can be produced annually.
Maintenance and service life
Maintenance costs are influenced by construction, rotational speed, sealing concept, product abrasiveness, cleaning frequency and accessibility. A mixing tool supported only at the top avoids a lower shaft passage in the product area. This reduces the number of critical sealing and bearing points. The low-speed mode of operation additionally limits wear on the mixing tool, bearings and product-contact surfaces.
A long real service life significantly improves the economics. If an apparatus can be operated reliably over many years through preventive maintenance, available spare parts, modernisation or retrofitting, the investment is spread over more batches. amixon® reports that numerous machines have been in daily operation for more than 30 years. The specific service life, however, remains product- and process-dependent: abrasive materials, aggressive media, strong temperature changes, frequent wet cleaning, or high load cycling can accelerate wear on individual components.
Long-term spare-parts supply and the documented remanufacturability of customer-specific components support long-term, plannable operation. For a realistic TCO assessment, however, maintenance intervals, sealing costs, bearing costs, cleaning effort, and expected plant availability should still be taken into account.
Scale-up with the original product
amixon® can provide customers with expert, sound support in transferring laboratory and pilot trials to large vacuum mixing dryer plants. Scaling up thermal processes cannot be derived from geometric similarity alone. When a production plant is significantly larger than the pilot-plant dryer, factors including the actively wetted heat-transfer surfaces, fill height, thermal mass, heat losses, condenser load, vapour volume flow, filter load, and the supply of heat-transfer medium all change.
amixon® therefore carries out drying and reaction trials with the original product and records the relevant process data comprehensively. This includes, for example, the pressure and vacuum profile, product temperature, flow and return temperatures of the heating or cooling medium, condensate quantity, condensate flow, mixing-tool rotational speed, torque, fill level, product behaviour, and process time. On this basis, amixon® can establish the required heat-transfer surface, temperature control, vacuum technology, vapour filter, condenser, heating and cooling capacity, mixing-tool design, and the suitable size for the industrial target plant.
The measurement data from the drying trial are combined with process-engineering know-how and thermodynamic calculations. This allows the performance of a large production plant to be estimated in advance and fixed by design. The active heat-transfer surface is not derived from the vessel geometry alone. Product contact with the temperature-controllable mixing tools is also taken into account for different fill levels. This is important because the effective heat-transfer surface can change during drying as the fill level decreases and the product properties change.
amixon® can then implement the apparatus properties established on the basis of the trials in terms of design and manufacturing. This includes vessel geometry, size, mixing tool, temperature-controllable shaft, mixing arms and mixing helix, heat-transfer surfaces, torque reserve, cutting rotors, vacuum system, vapour filter, condenser, material selection, sealing concept, discharge unit, cleaning system, instrumentation and automation. The plant is not only sized theoretically, but tailored to the process conditions observed in the trial.
The aim of this approach is for the customer to be able to produce swiftly at the intended product quality after completion and commissioning. The plant should not have to be "run in" during production through subsequent adjustments, unclear performance reserves, or avoidable process problems. The pilot-plant trial reduces this risk, because critical process phases, such as heating up, the peak evaporation phase, rheological transitions, reaching the target residual moisture, cooling, discharge and cleaning, are examined and documented before the investment is made.
Further information on transferring trial data to large production plants is provided by amixon® in the article "amixon® helps with the sizing of large vacuum mixing dryer plants".
Trials as the basis for decision-making
Before an investment, technical and economic assumptions should be verified with the original product. At the amixon® pilot plant at the Paderborn headquarters, more than 30 test units in different sizes are available. Additional pilot facilities exist in Japan, India, Thailand, China, South Korea and the USA. The trials can be carried out with realistic fill levels, batch sizes, and the intended temperature, pressure and vacuum conditions.
Mixing quality, product protection, energy input, heat transfer, drying kinetics, temperature profile, vacuum profile, condensate flow, residual discharge, cleanability, changeover time and reproducibility are assessed. The results are documented and form the basis for the design of the production plant as well as for TCO, ROI and payback calculations. Recipes, trial results and jointly developed process data are treated confidentially.