The Economics of Drying.

Transcription

1 The Economics of Drying. How To Improve Bottom Line Profits in Processing Innovations for a better world.

2 The Economics of Drying How To Improve Bottom Line Profits in Processing Drying is an energy intensive operation that can have a significant impact on your bottom line. That s why most processors diligently track energy costs. But did you know your dryer could be costing you 10 times as much in terms of lost production compared to energy costs? You may be sending over 3% of your production out the exhaust stack without even knowing it. Downtime for dryer cleaning and maintenance may also be costing you more than you know. This article will describe the economics of drying in more detail and outline three operational areas that could significantly improve your bottom line. Energy Consumption The simple cost of energy used to operate the dryer is the most obvious cost of drying. One need only look at the larger burners in the dryer, or at the boiler providing steam to the dryer, to realize that money is being spent to remove moisture in the drying operation. This energy is needed to cover four heat loads in the drying operation. They include: Warming up the product; Evaporating water from the product; Heating the make up air; and Steady state heat losses. Most of the energy in the drying operation is used to evaporate water. The energy required to change liquid water into vapor (latent heat of vaporization) is about 2,350 kj/kg of water evaporated (the value varies very slightly with temperature). This is a thermodynamic constant and you cannot change this value in typical dryers, or any other convection dryer. Only vacuum dryers operating under an extreme vacuum can reduce this value. Table 1. Impact of product moisture on drying load Inlet Moisture Outlet Moisture Water Removal Per Ton of Product Produced % Increase in Water Removal 24% 10% 184 Kg 27% 10% 233 Kg 27% 30% 10% 286 Kg 55% 30% 7% 329 Kg 79% Table 1 shows the tremendous impact that inlet and outlet moisture can have on the drying load in your dryer. Even a small increase in product inlet moisture from 24% to 27% on a wet weight basis increases the load on the dryer by 27%. Naturally, your inlet moisture must be driven by product quality requirements.

3 The third heat load category, heating the make up air, is typically the second highest heat load. However, in a poorly operated or poorly designed dryer, this heat load can be the largest. This is the area where you as a dryer operator and your dryer supplier can have by far the most impact. Your dryer needs a steady supply of dry make up air. Without this influx of air, the process air in the dryer would become saturated with moisture and unable to dry your product. The trick is in knowing how much air to bring into the dryer. Excessive make up air will lead to excessive energy consumption. This is because the make up air enters the dryer at a relatively low temperature, and after picking up water in the dryer, eventually leaves the dryer at a higher exhaust temperature. Clearly, energy is consumed in heating the make up air to the exhaust temperature. It is important to understand that the exhaust air in your dryer is basically the make up air leaving the dryer along with the evaporated water. If you want to reduce the amount of make up air in most dryers, you will typically reduce your exhaust, either by closing the exhaust damper or turning down the exhaust fan. Energy Consumption (%) 200% 180% 160% 140% 120% 100% 80% Proper Operation Dryer Efficiency Air Preheating Exaust Exaust and Beds 50 % to Shallow Figure 1. Exaust and Beds 50 % to Shallow and Inlet Moisture 2% too High The temperature of the exhaust and make up air also affects the energy used to heat the make up air. If the make up air is preheated before being introduced into the dryer, less energy will be used by the dryer to heat this air. Improper dryer operation, such as excessively shallow bed depths (shorter retention time in the dryer) or internal air bypasses can lead to high exhaust temperatures, thereby increasing the energy used to

4 heat the make up air to the exhaust temperature. Figure 1 illustrates the significant impact that heating the make up air and various operating parameters can have on your dryer s overall energy consumption. Unless your dryer is poorly insulated, losses from the dryer should be low. If, however, you have a dryer design with large areas of un insulated or poorly insulated surfaces, thermal losses from your dryer could also be costing you. Typically, dryers will use between 3,000 kj/kg and 4,500 kj/kg of water evaporated. A poorly operated or poorly designed dryer might even use much more. Based on a typical value of 3,250 kj/kg of water evaporated, a line producing 75,000 tons per year will typically use about 5 x 10¹⁰ kj/year. If natural gas is used as the heat source in the dryer, and if the cost of natural gas is 5 x 10 ⁶ US$/kJ, the yearly cost of natural gas to produce this tonnage is US $250,000. Naturally, this cost will change depending on the type and cost of fuel used in the drying operation. Figure 1 demonstrates an industry dryer using over 50% more energy than it should due to improper design or operation. This represents a tremendous potential savings for the processor. Imagine saving US $100,000 per year, simply by adjusting a few dampers and making minor changes in the way you operate your dryer. Believe it or not, many processors have done just that, after learning how to properly run their dryers. Moisture Uniformity Because most products are sold by weight, a processor can easily maximize the value of the cheapest product ingredient water, without fear of producing an unstable and non saleable product. For this reason, if you product is over dried, you are sending water at the value of your product which should have gone to packaging straight out your exhaust stack. Suppose that your product can t contain over 10% moisture limit and the product coming out of your dryer has a moisture variation of ± 3% moisture on a wet weight basis. This means you must dry at 7% moisture, on average, in order to ensure that no product is over the specified 10% moisture limit. The result is a 3% loss in production compared to drying the product only to 10% moisture. If your dryer could be made to dry more uniformly, you could raise your discharge moisture and get more out of the production line with no additional cost or additional dry ingredients. The value of this lost production due to non uniform drying can be surprising. Consider the same processing line as in the previous example, producing 75,000 tons per year. A 3% loss in production from this line represents 2,250 tons per year of lost production. What is the value of this lost production? At US $400 per ton, it is US$900,000. At US $600 per ton, it is US $1.35 million. The numbers are staggering. The bottom line is that no one can afford to be over drying a product. A well designed, well adjusted dryer should be able to dry a product to within ±0.75% or better. When was the last time you checked the moisture uniformity on your dryer? Maintenance and Sanitation Many processors struggle to keep old, high maintenance equipment running. The cost of purchasing replacement parts for the dryer may be well documented, but what about the cost of downtime for

5 cleaning and maintaining the equipment? A dryer producing 10 tons per hour can be producing well over US $100,000 worth of product each day. But if you re spending three hours a week of unplanned maintenance or cleaning downtime, you are losing 30 tons per week. This equates to US $650,000 per year of lost production using the above parameters. Therefore, the cost of this downtime must be considered when looking at the economics of drying. Drying is a critical component in any processing line. It not only determines the quality of your finished product, but it significantly affects the cost to operate. Most processors are well aware of the energy costs associated with drying. However, many don t track the cost of over drying due to non uniform drying, or the cost of downtime for maintenance and sanitation. The good news is processors can significantly reduce drying costs and increase production rates by taking a closer look at the drying step. If you want to find untapped potential savings in your dryer operation, a field engineer specializing in dryer optimization can give your dryer a full mechanical and process evaluation, with improvements that can increase capacity, minimize energy usage or improve product uniformity. Bühler Aeroglide has a long history of teaching customers how to identify and implement process improvements. The company s field engineering group has provided expertise and evaluation services to processors around the world for over 15 years, driven by three guiding principles: educate, evaluate and enhance. Education is provided through Drying Theory Put to Practice, a series of drying theory and dryer maintenance seminars that explore the basics, including parameters, balancing, evaluating, mechanical inspections and troubleshooting with a dryer simulation. By learning how to adjust and regulate the process for the desired outcome, participants find hidden areas of opportunity. To find a seminar near you, please visit Drying Theory Put to Practice. # # #