At pulp-and-paper mills, drying the paper products is an integral component in producing the highest quality paper possible. The drying process consumes more than 60 percent of the total energy demands of the papermaking process.
Typically, drying requires investments in large equipment, capital and operating costs, and heating requirements for proper production. Fortunately, opportunities in the recovery of excess heating and operating costs can yield a more efficient, environmentally friendly operation.
Overall, pulp-and-paper mills are one of the five largest industrial consumers of energy. Yet in some cases, this industry has been slow to install plate heat exchangers due to the presence of cellulose and resin in the waste-stream and air-stream mixtures. Even small amounts of cellulose and resin will clog and foul most traditional heat exchangers.
Assembled-plate energy banks present an alternative for pulp-and-paper mills. They can be installed directly in-line with the existing exhaust. Assembled-plate energy banks are difficult to foul with the dirty exhaust streams found in the paper-drying process. Though energy prices are volatile globally, the energy savings possible in the paper industry as well as others can result in quick payback periods even with large heat exchanger purchases.
Assembled-plate energy banks are not prone to fouling because the surfaces in the heat exchanger are smooth and without crevices. Also, the heat transfer surface area is continuously washed as water condenses out of the airstream, providing a self-cleaning solution to fouling.
Energy Recovery in Pulp-and-Paper Mills
The significant performance of energy-recovery systems in the paper industry derives from the hot, moist airstream produced in the drying process. Assembled energy banks can utilize the much larger heat transfer coefficient of a condensing vapor — compared to non-condensing heat exchange — to operate at up to 10 times the efficiency of traditional heat exchange processes.
As the hot humid exhaust is blown through the exchanger, cooling liquid — typically water or glycol mixtures — causes condensation on the surface of the heat transfer plates. The method takes advantage of the much higher heating coefficient during the phase change.
While this is the primary driver of heat transfer, there is still significant transfer in
the non-condensing region as well. The cooling media, now heated, then can be utilized throughout the facility as an alternative heat source.
While moist air-to-air heat recovery can be done with extended-surface heat exchangers like finned tubes, they clog quickly with the contaminant-laden source of paper exhaust. Such exhaust contains fibers and resins that significantly affect performance. Assembled-plate energy banks do not foul in this process because all surfaces in the heat exchanger are smooth and without crevices. Additionally, the heat transfer surface area is washed continuously as water condenses out of the airstream. This provides a self-cleaning solution to fouling.
Energy and Cost Savings. Heat recovery allows the mill to avoid using another energy source, typically fossil fuels, to provide the necessary heat in accompanying areas of the mill. This presents as large and apparent cost reductions in plant operation. Energy costs are calculated based on current fuel costs and go straight to a mill’s bottom line. Using a price of $5 per million BTUs for a fuel source, 9 million BTU/hour in energy savings and 2,000 operating hours per year, the cost savings are:
2,000 hours x 9 million BTU/hour x $5/million BTU = $90,000 per year
If the cost to purchase and install the energy bank heat-recovery system is $100,000, the payback period is just over a year. This example demonstrates the potential financial benefits of energy recovery. These savings scale to the size of the recovery system and energy-reduction capacity; in some cases, the savings result in six- to seven-figure annual reductions in operating costs.
Environmental Impact. The environmental impact of heat recovery in the drying process is notable. Benefits include reduced energy consumption and greenhouse gas emissions, the removal of pollutants from the exhaust stream, and a reduction of the load on the mill’s scrubbing system. Such benefits from the implementation of waste-heat recovery remain present long after the system is put in place.
Many government entities have recognized the benefit of heat-recovery projects to the environment. This has led to federal and, in some cases, state grant programs or tax incentives for heat-recovery projects.
Assembled energy banks can utilize the much larger heat transfer coefficient of a condensing vapor compared to non-condensing heat exchanger to operate at up to 10 times the efficiency of traditional heat exchange processes.
Other Uses for Industrial Processes
While the focus has been on energy recovery specifically in the pulp-and-paper industry, assembled-plate energy banks are suited for used in many other process applications. Any hot, condensable process stream can implement these exchangers to benefit in a similar manner. Applications such as ethanol refineries, chemical processing facilities and grain processing plants are ripe for comparable savings.
Take the example of an ethanol plant. Assembled-plate energy banks can help with maximizing operating capacity by focusing on steam-limited operations such as ethanol processing. Similar to a paper mill, exhaust in the plant is wasted energy. Finding a way to capture and reuse that energy to make clean, boiler-quality steam that recirculates through the bio-refinery is the goal. Many plants would install an additional boiler to produce more steam and increase the production of ethanol. However, an assembled-plate energy bank can produce more steam with that exhaust energy and save money. This maximizes the steam capacity, which increases output without the need for additional boilers, and reduces steam-related bottlenecks.
Other examples of heat recovery in industrial processes include:
- The reheat of scrubbed flue gas in power generation.
- Brew kettle exhaust in a brewery.
- Fryer exhaust in a food processing plant.
- Drying process exhaust in a chemical plant.
Close examination of wasted heat in any plant process will likely find opportunities for an energy-recovery project.
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