The amount of energy consumed by a typical extrusion operation makes it an obvious target for reduction.
Petfood companies from giants like Mars Petcare US and Nestlé Purina PetCare to specialists like Canidae Pet Food and Natura Pet Products have been opening or retrofitting exciting eco-friendly facilities to great fanfare over the past few years. But executives from these and other manufacturers would say that making petfood more sustainable usually starts with the basics, including energy-intensive processing.
“Prior to sustainability becoming the hot buzzword, it was called cost efficiency,” said Terry Block, president of Nestlé Purina’s North American division. “As you reduce energy and water, you reduce costs, and we have been on that curve for over a decade.”
The amount of energy consumed by a typical extrusion operation makes it an obvious target for reduction. According to Tjitze Smit, petfood division manager for Graintec A/S, the extrusion process can use up to 380 kilowatt hours (kWh) of energy per ton of final product (Figure 1). The drying part of the process accounts for the largest use of energy (Figure 2).
Recovery of dryer exhaust and other processing air can save 20 to 100 kWh of thermal energy per ton of final product, Smit said in his Petfood Forum Europe 2011 presentation, “Energy recovery and odor reduction in petfood production.” Often the most cost-effective solutions integrate technologies to reduce not only energy use but also odor and carbon dioxide emissions.
Smit suggested four potential solutions for recovering energy from the drying process:
- A cross flow air/air heat exchanger allows for up to 50% recovery;
- An air/water heat exchanger offers up to 40% recovery;
- An air-to-water exchanger combined with a high-temperature (80 – 110° C) heat pump can effect up to 70% energy recovery and potentially more than 50% odor reduction; and
- An air/water heat exchanger combined with recirculation of dryer air may offer only up to 20% energy reduction but also odor reduction above 85%.
Thermal and mechanical energy are the major inputs for extrusion, with the typical operation using a 1:2 ratio of mechanical to thermal energy, according to Galen Rokey, process technology manager for Wenger Manufacturing. During a Wenger seminar for petfood producers in the UK in June 2011, he said this ratio affects utility and equipment wear costs and influences sustainability.
The costs are significant, Rokey added, with mechanical costing about 2.5 times more than thermal for the same unit of energy. Wenger has found these averages applicable in almost every region of the world:
- Thermal energy = US$7.32/million kilojoule (kJ);
- Mechanical energy = US$18.06/million kJ; and
- Equipment wear = US$1.25/ton for every 22 kWh/ton specific mechanical energy.
Thermal and mechanical energy are the major inputs for extrusion.Rokey shared an exercise in which the 1:2 ratio was altered by reducing thermal energy by 25% and 43%, with the latter resulting in a 1:1 mechanical:thermal ratio. This changed the energy inputs to the extrusion process, dramatically driving up utility costs and, to a lesser extent, equipment wear costs (Figure 3).
Figure 1. Energy consumption during extrusion: The feed extrusion process consumes a significant amount of electrical and thermal energy, said Tjitze Smit of Graintec A/S in his Petfood Forum Europe 2011 presentation, “Energy recovery and odor reduction in petfood production.”
The bottom line: To maximize efficiency, try to increase thermal energy inputs—look at steam in the preconditioner barrel—and decrease mechanical energy, especially within the extruder.
Steam heat efficiency is key, too. David Degelau, engineer and certified steam system specialist for Hydro-Thermal Corp., detailed the energy losses in a typical industrial heating system in his Petfood Forum 2011 presentation, “Process heating: temperature stability under dynamic flow rates” (Figure 4):
- Stack losses – 10-30% of energy input;
- Blow-down losses – 5-10% of boiler output;
- Condensate losses from leaks or non-functioning trap systems – up to 10% of energy input;
- Flash losses – 5-10% depending on system pressures as hot condensate is reduced; and
- Condensate heat losses – less than 2% of energy input.
According to Degelau, direct steam injection (DSI) can mitigate these losses. “DSI uses 100% of the steam’s heat energy by adding steam directly to the processing fluid.” This direct contact heating, he added, delivers benefits such as:
- Reduced steam consumption;
- Significantly lower energy costs (25% or more);
- Near-instantaneous energy (heat) transfer;
- Low maintenance (the system is self-cleaning);
- Handling of fluids that are difficult to heat, such as highly viscous or abrasive slurries;
- Small size, requiring no production floor space;
- Consistent, precise discharge temperature, with control possible to within 1° F; and
- No condensate return required.
To help determine what type of heating system is best for your operation, Degelau suggested doing an energy comparison. Several energy calculators are available online, including ones from the US Department of Energy, Hydro-Thermal and Sempra Energy.
Waste reduction and recovery can also make processing more sustainable, experts from Extru-Tech Inc. said in the February 2009 issue of The Extru-Technician. The first step is to review operational procedures and look for clear waste targets—for example, nonessential extruder water injection.
To recover as much of the remaining waste as possible, start by calculating the amount of rework you can reintroduce into the raw ingredient stream without creating more waste or off-spec product. Base this calculation on extrusion capacity rates and the product formulation, Extru-Tech said, considering all aspects such as starch gelatinization, appearance, durability, density and nutrition.
Dry waste (fines generated during extrusion, with moisture less than 13%) is best recycled by grinding and adding back to the raw material batch mixer at rates of 0-5%, according to Extru-Tech. The company offers a proprietary waste recovery system (WRS) for wet waste, which is typically created during system startup, shutdown or process interruptions and collected from the preconditioner downspout and extruder die. A WRS can continuously convey, liquefy and inject the recovered material as slurry directly into the preconditioner without any operational changes to the extruder, Extru-Tech said.
Waste reduction and recovery can also make processing more sustainable.In a case study of a plant with a WRS facilitating wet waste from an extrusion line producing premium dog food, Extru-Tech said its findings showed a collective annual savings of US$123,455: US$2,906 of energy savings, US$3,844 of water savings and recovered materials worth US$116,455.