Which sustainability metrics (energy, CO2, water) can be meaningfully reported for mixing processes?
For industrial mixing processes, specific, product-related sustainability metrics are particularly suitable, supplemented by absolute energy, emissions and water figures. This combination makes it possible to classify processes across time periods, recipes, plants and sites, identify efficiency potential, and integrate data into recognised management and reporting systems. The prerequisite for robust comparisons is a clearly defined system boundary, a clear functional unit — for example a tonne of saleable product or a batch — and consistent measurement and calculation methods.
For the energy assessment of a mixing process, specific energy consumption is a central metric. It is usually expressed as electrical energy in kWh per tonne, kilogram or cubic metre of saleable product. With batch mixers, energy consumption per batch can additionally be useful. What is decisive is that not only the power consumption of the mixing drive is considered, but also — depending on the system boundary chosen — auxiliary equipment such as dosing elements, conveying technology, vacuum generation, compressed air, extraction, temperature control and control technology. Only in this way can the actual energy demand of the mixing step be assessed in a traceable manner.
The mixer's electrical power consumption in kW as a time profile supplements the specific energy consumption. Load profiles make start-up peaks, idle phases, unusual loads and process-related power fluctuations visible. Particularly meaningful is examining the share of idle, set-up, cleaning and standby energy in the total energy consumption of the mixing step. High values can indicate avoidable waiting times, unfavourable shift handovers, long product changes, unsuitable recipe sequences or inefficient control parameters. With highly viscous media, dispersion processes or intensive mixing tasks, the specific stirring energy in kWh/kg or Wh/L can additionally be recorded.
If a mixer is heated, cooled, dried or used for exothermic processes, the thermal energy demand should also be assessed separately. Useful metrics are, for example, the heat or cooling demand in kWh or MJ per batch or per tonne of product. Steam, thermal oil, electric heating, cooling water, chilled water or other energy carriers can be taken into account here. A metric for energy recovery is particularly useful where usable energy is actually recovered from exhaust air, cooling circuits, thermal media or process heat and applied in another process step. With classic dry powder-mixing processes without notable heat sources, such a metric often has only limited informative value.
For CO₂ and greenhouse gas emissions, the mixing step should be clearly distinguished from the entire product footprint. Direct emissions from fuels used on site for heat, steam, thermal oil or directly fired units are recorded as Scope 1. Indirect emissions from purchased electricity, steam, heat and cooling are reported as Scope 2. The GHG Protocol distinguishes here between the location-based method, which works with average emission factors of the respective electricity grid, and the market-based method, which takes into account certain contractual or supplier-specific procurement information. In markets with suitable contractual instruments and data, both Scope 2 perspectives should be reported traceably.
A central product-related metric is the CO₂ intensity of the mixing step, for example in kg CO₂e per tonne of saleable product, per kilogram or per batch. It enables comparison of different modes of operation, plants or sites, provided the system boundary, functional unit, electricity emission factor, reporting period and calculation method match. Metrics such as kg CO₂e per operating hour can be additionally helpful, but are less meaningful on their own, because they do not take into account the quantity actually produced. A product carbon footprint should only be reported if its system boundary is clearly described. A gate-to-gate PCF relates to the defined production step, while a cradle-to-gate PCF can additionally include significant upstream emissions from raw materials, additives, packaging, auxiliary materials and transport. Particularly with emission-intensive raw materials, these upstream emissions are often more relevant than the power consumption of the mixer itself.
Avoided emissions should not be mixed with accounted Scope 1, Scope 2 or Scope 3 emissions. They are suitable as a separate project or comparison metric, for example with a retrofit, a recipe optimisation, an improvement in residual discharge, or a change of energy carrier. The prerequisite is a transparently documented reference scenario with the same functional unit, identical system boundaries, clear assumptions, comparable operating conditions and traceable emission factors.
Water-related metrics should distinguish between water withdrawal, water consumption and wastewater discharge. Water can be used in the mixing process as a recipe component, cooling medium, cleaning medium or auxiliary material. Water withdrawal should be recorded in m³ or litres per year and additionally in relation to a production unit, for example m³ per tonne of product. A useful breakdown distinguishes process water, cleaning water, cooling water and other consumption. Water consumption describes the quantity of water not returned to the same or a comparable environment. In principle, it can be determined as water withdrawal minus water discharge and changes in water storage. GRI 303 therefore treats water withdrawal, water discharge and water consumption as related disclosures.
With frequent product changes or stringent hygiene requirements, cleaning-water consumption per cleaning cycle or per product change is particularly relevant. It can be reported in litres or cubic metres per CIP, WIP or manual cleaning cycle, and additionally in relation to the quantity of product produced. In addition to the water quantity, the consumption of cleaning chemicals should also be recorded if these are material to the environmental impact or the costs. Wastewater volume can be reported as m³ per tonne of product or per batch. Depending on the industry and discharge conditions, additional relevant load parameters such as chemical oxygen demand, biological oxygen demand, solids load, pH value, conductivity or other product-specific parameters are useful.
A water reuse or recycling rate can be reported if treated water is actually reused in permitted applications. It must be clearly stated whether this involves an internal recirculation loop, or use for cleaning, cooling or other auxiliary processes. The mere circulation of cooling water is not automatically reuse in the sense of an additional resource saving. The significance of water consumption should also be considered in the context of the site. Withdrawals in water-stressed regions can be ecologically more relevant than significantly higher quantities at water-rich sites. A classification using recognised water-stress indicators is therefore useful. GRI 303 requires separate consideration of water withdrawals in water-stressed areas.
Supplementary process metrics help to make visible the causes of high energy, CO₂ or water consumption. These include overall equipment effectiveness, unplanned downtime, scrap and rework rates, residual quantities after discharge, cleaning duration, cleaning frequency and batch-to-batch consistency. Low plant availability or defective batches increase resource consumption per unit of saleable product, because energy, water and raw materials are used without corresponding value being created. Good residual discharge and stable process operation can reduce product losses, cleaning effort and rework.
For meaningful sustainability reporting, absolute annual figures and specific product metrics should be reported together. Absolute figures show the total environmental impact of a site or company. Specific figures make efficiency changes visible even when production volume, product mix or operating hours fluctuate. A practicable set of metrics therefore comprises at least total energy consumption and energy intensity per tonne of product, Scope 1 and Scope 2 emissions, the CO₂e intensity of the defined mixing step, water withdrawal, water consumption, wastewater volume, cleaning water per product change, and relevant metrics on scrap, rework and residual discharge.
Data acquisition should, wherever possible, be automated via suitable, traceably calibrated electricity, heat, water and, where applicable, wastewater meters. Connection to the process control system, energy management system, higher-level control and production systems, ERP or maintenance software facilitates assignment to batches, recipes, production quantities and causes of downtime. For consistent and auditable disclosures, system boundaries, functional units, reporting periods, measurement methods, data gaps, emission factors, calculation methods and responsibilities should be documented. The classification can be guided by the GHG Protocol, ISO 50001, ISO 14064, GRI 302, GRI 303 and GRI 305 as well as, for companies subject to reporting obligations, the relevant requirements of ESRS E1 for climate and ESRS E3 for water and marine resources.
How amixon® solves this task
At amixon®, sustainability means considering energy use, raw-material yield, cleaning effort, maintenance and service life together. The actual environmental and economic impact, however, always depends on the product, recipe, batch size, fill level, mixing task, hygiene requirements and mode of operation. amixon® therefore assesses key process parameters on as application-specific a basis as possible — for example through trials with the original product in the pilot plant or through measurements in later operation.
The energy efficiency of a mixer is significantly determined by the mixing principle, tool geometry, rotational speed, fill level and required mixing time. In many applications, amixon® mixers operate with low circumferential speeds of the mixing tools. This can enable gentle product turnover and limit the mechanical energy input. The optimal rotational speed and the actual power requirement, however, are always matched to the respective mixing task. With the Gyraton® mixer GM for large batches, the focus is on a precise, product-gentle mixing task achievable with a comparatively low connected load. The specific connected load and the specific energy consumption per batch or tonne of product depend on the product, fill level, recipe, mixing time and required homogeneity.
Short mixing times can reduce energy consumption per batch, provided the required mixing quality is achieved reproducibly. With the KoneSlid® mixer KS, amixon® states an optimal mixing quality after around 20 to 30 tool revolutions. This figure is product-specific and cannot readily be transferred to other mixer types, recipes or fill levels. For a robust assessment, the specific energy consumption should therefore be determined through pilot-plant trials or operational measurements. Meaningful metrics are, for example, kWh per batch, kWh per tonne of product and power consumption over time.
A key lever for the efficient use of raw materials is the most complete possible discharge of the mixer. Low residual quantities can reduce product losses, additional cleaning effort and, with product changes, also the risk of cross-contamination. Depending on the product, mixer type and design, amixon® mixers can achieve high discharge rates. With ComDisc® technology, discharge rates of up to 99.997 percent and higher are possible under suitable conditions. The achievable residual discharge, however, must be assessed with the respective original product, because flow behaviour, moisture, particle size, bulk density and adhesion tendency have a significant influence.
Mixing chambers with minimal dead space, good accessibility and product-appropriate cleaning can reduce the demand for water, cleaning chemicals and manual intervention. In applications involving liquid mixing-in, finely distributed dosing matched to the product can help to limit local build-up. Whether this results in shorter cleaning times or reduced consumption of cleaning agents depends on the recipe, hygiene requirements, change frequency and the respective cleaning method. With temperature-sensitive powders, a gentle mixing principle can help to limit unwanted temperature input. Whether this actually allows cooling energy to be saved should be assessed by measurement for the specific process.
A long economic service life can also contribute to resource conservation. Robust construction, suitable materials, good maintenance accessibility, spare-parts supply and retrofit options help to operate mixing plants over many years and adapt them to changed requirements. amixon® supports operators with maintenance, spare-parts supply, modernisation and retrofit. According to amixon®, spare parts are stocked, among other places, at sites in Germany, the USA and Japan, in order to limit downtime as far as possible. Modernisation can, for example, involve drives, control systems, sensors, safety functions, dosing technology, cleaning or process documentation.
For traceable sustainability reporting, application-related metrics are decisive. These include in particular the specific energy consumption per batch or tonne of product, power consumption over the course of the process, the residual quantity after discharge, the product-loss rate, water and cleaning-agent consumption per cleaning cycle or product change, the consumption of wear parts, and the plant's service life. Depending on the automation concept, corresponding operating and process data can be recorded and documented on a project-specific basis and used for energy, CO₂, resource or OEE evaluations.
Before an investment, the specific mixing task can be trialled in the amixon® pilot plant with the original product. More than 30 test units in different sizes are available for this at the main site in Paderborn. In addition, amixon® maintains technical centres in the USA, China, Japan, India, Thailand and South Korea. The trials can be carried out with application-realistic fill levels, batch sizes and the intended temperature and pressure ranges. Mixing quality, product protection, energy input, dischargeability, cleanability and reproducibility are, among other things, assessed. The documented results help operators select a suitable design and assess technical and economic risks before an investment.
A short mixing time is not the primary goal in every application. Where product quality, product protection and a low electrical connected load are the focus, a longer, deliberately slow-paced homogenisation can be advantageous. The Gyraton® mixing silo follows this principle: the mixing tool can operate at a very low rotational speed and circulate the bulk material gently. This allows the required drive power to be kept low in relation to the large batch quantity. Gyraton® mixers are available for batch sizes of up to 100 m³ and are suitable for dry, moist and wet bulk materials. The specific mixing time and motor power are each designed on the basis of the product, batch size, fill level and required mixing quality.