The concept of superheated steam drying is not new. New Zealand has had a plant involved in the drying of green crops, in particular alfalfa, using natural geothermal steam for several decades. However, the elevated temperatures and the drying method of the new technology is what makes the airless dryer unique.
The timing is right for this new technology. Because drying operators are demanding energy efficiency, increased product sterility and purity, and ease of environmental compliance, the technology offers a viable alternative to these problems faced by the rendering industry. Increasingly, restrictive environmental controls favor airless drying because it avoids capital spending on strategies to deal with the environmental impacts of the drying.
A Long Journey
The development began in 1996 when Meat New Zealand commissioned AgResearch (formally the Meat Industry Research Institute of New Zealand) to investigate the benefits and viability of using superheated steam as a drying medium within the rendering industry. Neil Clarke, Meat New Zealand general manager of research and development, agreed that the timing is right for the airless dryer.
“The BSE [bovine spongiform encephalopathy] scare in the Northern Hemisphere means you’re now required to sterilize product,” he said. “The dryer operates at high temperatures but the product’s nutritional value is not lost. And the odors are carried off in the steam, which is condensed into water, which is a lot easier to deal with than getting the volatile compounds out of air.”
Dr. Grant Schou, AgResearch, headed the initial project, and by early 1997, a pilot plant was built and trials begun on the drying of meat and bone meal, gel bone, and associated products. This not only involved clinical work, but also a pilot plant operating in several rendering operations to fully evaluate the technology. By mid-1998, the results identified improved energy efficiencies, greater yields and nutritional content of product, and significant environmental benefits in comparison with traditional drying technologies. The project now needed a commercial partner who could design and fabricate a full-size plant as well as market the technology.
At about that time, Pinches Industries of Melbourne, Australia, was finalizing its acquisition of Keith Engineering. Keith had an enviable presence in both the Australian and New Zealand rendering industry throughout the 1970s and 1980s. While Keith Australia had maintained and further enhanced its local position, Keith New Zealand had suffered badly in the local market. The airless drying project had the right mix of risk, innovation, and new product development needed to rebuild the Keith name in New Zealand and to further enhance the company internationally. In 2001, Keith Engineering New Zealand signed a commercialization agreement with Meat New Zealand to represent the technology internationally, including the design, build, marketing, sales, and all warranties.
The task then was to find a prospective buyer. There was interest in the process from the mainstream multi-site meat processing companies, but the comment was often that they did not want to be a guinea pig. At the same time, discussions had started with Lowe Corporation who had been supportive as far back as 1998 in seeing the process brought to commercial realization.
Lowe Corporation’s involvement was paramount. They were interested in the increased yield of gelatine-bearing gel bone from their all-bovine processing plant in Hawera, New Zealand. Lowe had conducted earlier gel bone trials at AgResearch using the superheated steam pilot plant, which confirmed superior product, and it was an area that they were keen on developing further.
In addition, Lowe has an enviable reputation as an innovative leader in new meat industry processes and therefore understood the risks involved in the implementation of new technologies. Previously, Graeme Lowe had pioneered low temperature rendering and hot boning techniques in the New Zealand sector. In mid 2001, Lowe Corporation placed an order with Keith to supply the first full-scale airless dryer to their Hawera rendering operation.
The Airless Drying Process
The process uses superheated steam at temperatures up to 841 degrees Fahrenheit (°F) (450 degrees Celsius (°C)) to dry at atmospheric pressure. It does not require a boiler to operate except at start-up when an initial raw charge of saturated steam is introduced from the site supply for approximately 180 seconds to initially fill the dryer.
With a true airless drying environment, the dryer internal is sterile and produces a faster drying rate than conventional air or contact dryers. Results from the AgResearch trials identified that superheated steam dryers dry at a faster rate while using less raw energy at temperatures above 410°F (210°C).
The absence of oxygen produces unique benefits, including the minimization of fire and/or risk of explosion and no burnt or overheated product, impacting the product quality. The process does not require any form of biofiltration or odor control. Nitrogen oxide levels are markedly reduced and associated discharge to air, land, or stream is negligible. Environmentally, the process is well suited to the rendering sector.
Lowe’s Hawera dryer is constructed entirely of food-grade stainless steel, including all ducting, fans, cyclone, and valves, ensuring that the sterile nature of the airless dryer is not compromised. The dryer utilizes two separate closed loops – combustion and drying. The separation between the two loops is via a high efficiency heat exchanger.
The combustion loop, which produces heat energy from a two-megawatt gas burner, heats up one side of the heat exchanger. This effective separation ensures that the burnt products of combustion do not contaminate the product to be dried, which results in a more sterile drying environment. The combustion loop recycles a high percentage of its heat in order to maximize its operating efficiencies. The volume of exhaust gases emitted is limited and matches the amount of clean fresh air required to maintain a burner flame. The control of the combustion loop, including burner setting and speed of the three fans – combustion air, exhaust, and recirculation – are controlled from the central process logic controller (PLC).
The drying loop recirculates the superheated steam via a 37 kilowatt (kW) process fan. Superheated steam is conveyed via 700-millimeter ducting through a dust cyclone, process fan, and heat exchanger before entering a cascading rotary drying vessel measuring some 14.5 meters in length and 1.8 meters in diameter.
The energy required to maintain circulation within the loop is considerably less than if the fan was circulating hot air due to the mass of the elevated steam vapor being considerably lower than air. Therefore, the fan is speed controlled via the PLC control. The fan presents the cooled steam from the preceding pass at 284°F (140°C) to the heat exchanger where it is reheated to a maximum of 840°F (450°C) from where it is introduced to a rotary cascading drum along with the moist material to be dried. The actual temperature is being constantly varied depending on the evaporation rate being achieved within the dryer. This ensures that the amount of heat energy presented closely matches the optimum required to achieve the desired drying rate.
The flow through the drum is maintained at a controlled speed to ensure that it circulates efficiently with the moist material, which is picked up and “curtained” via specially designed flytes, ensuring that a broad approach of material is presented across the drum on every revolution. The steam flow is then exited at a constant 284°F (140°C) and passed through the cyclone, removing all dust before being presented back to the heat exchanger for reheating.
The process of evaporating moisture means that more steam leaves the dryer on each pass than enters it. This is bled off before the heat exchanger and is presented to a condenser unit where the waste heat is converted into hot water that is reused within the plant.
Pressure variation is constantly maintained across the entire loop and varies little from true atmospheric pressure. Maintaining an effective seal so that hot steam vapor does not escape or that outside oxygen is prevented from entering, is controlled by the speed of the recirculating steam fan in conjunction with high temperature drum seals and the product feed and discharge auger speed. Knife gate valves also protect the entry and exit product feeds as well as at strategic points around the steam ducting.
The design allows for controlled expansion and contraction to protect against stress build-up. The entire process is well insulated and clad, providing a high degree of heat loss protection. This has two beneficial effects: 1) raising the overall efficiency of the drying loop to 85 percent, which has contributed to impressive fuel conservation figures; and 2) where the dryer has been shut down overnight, the retained heat insures a quick restart, again requiring less energy.
The drum grows both in length and in circumference when at operating temperature. Innovative expansion devices ensure that the steam seals remain effective across all temperatures and that the rotary drum does not alter its centerline position from cold to hot state. These arrangements also ensure that the mesh of the drive and drum gears are precisely maintained.
The rotary drum is powered by a 15 kW electric motor via a planetary gearbox to give drum speeds of between three and nine revolutions per minute. The drum is supported by two tires that ride on four-carrier wheels mounted on the chassis.
The control system is the heart of the process and has been specifically designed for this technology, drawing upon expertise gained with alternative processes in equally demanding environments such as dairy and cheese processing and timber treatment. At any one time, up to 12 sensors are monitoring flows and temperatures and making subtle setting changes to the burner outlet, process fan speed, and feed augers to ensure that only the required amount of heat energy is delivered to the drying vessel that matches the heat loading of the wet product being introduced. The maximum delay or lag time in making these modifications is seven seconds – the time the superheated steam needs to travel the length of the drying vessel.
The heart of the control system is an Allen Bradley PLC that in turn controls four advanced proportional integral derivative control loops. The system allows for full recording, trending and reporting a record of quality control, and proof that sterilization criteria has been met.
Commissioning
The commissioning of the plant included the proving of sterilization, vital for the commercial value of the dried product. The criteria for establishing sterilization is proving 260°F (115°C) for 60 minutes (or equivalent). This was achieved using a low-grade hospital radioactive isotope – technetium 99 – to prove residence times, and both bacillus cereus and bacillus stearothermophilus spores for proving the sterilization for all meat and bone meal. The proving of sterilization was enhanced due to the sterile environment that the airless process produces.
Additionally, a full energy audit was completed. The design parameters expected to show a 20 percent energy saving. In reality, this is closer to 35 percent based on similar throughputs of conventional drying method. This is expected to increase with further refinements, including the utilization of the waste heat from the combustion loop exhaust.
A full product audit was also completed. This showed that the commercial yield from the gel bone was over 250 percent improved from that previously obtained, with nutritional values of meat and bone meals correspondingly higher.
There is one last audit still to complete, an environmental impact assessment to rate its environmental friendliness. The process has almost no smell or odor and noise is at a minimum. Visitors have had to be shown that the unit contains rendered by-product inside its drying loop as they have not encountered the traditional rendering dryer smells. The unit has a porthole-type viewing window and illumination that allow operators to observe inside the drying drum.
Current Situation
The airless dryer at Lowe’s Hawera plant has been operating for some 3,000 hours since the official handover October 2002. The unit has proved very reliable and outages have been at a minimum during this time. Plant operators have quickly come to terms with the operation, and the control system allows varying steps of operator access depending on skill level.
The continuing development is ongoing. The original pilot plant has undergone a full rebuild to allow it to try alternative materials, both in and outside of the rendering industry. A second trials plant is being designed for an Australian operation, which will allow for product testing on both sides of the Tasman Sea. High on the list is the airless drying of blood product and specialty meals, including liver meal and aquaculture feeds. Current work being undertaken at Keith’s Auckland facility on the drying of waste sludges has the potential of further benefiting the meat industry as well as alternative industry groups.
Max Morley can be reached by phone in Auckland, NZ, at (09) 266 9005, or by e-mail at mmorley@pinches.co.nz.
June 2003 Render