The economics of feed drying

As a feed processor, you know your drying operation has a significant impact on your bottom line. Drying is a very energy-intensive operation. That’s why most processors diligently track the cost of fuel used in their drying operation. Are you aware, however, that 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 which could significantly improve your bottom line.

Energy consumption

The cost of energy used in operating 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 realise that money is being spent to remove moisture in the drying operation. This energy is essentially needed to cover four heat loads in the drying operation. They are:

1) Warming up the product,

2) Evaporating water from the product,

3) Heating the make-up air; and

4) Steady-state heat losses.

Extruded feed, for example, leaves the extruder at a high temperature and very little energy is used in warming up the product. The pellets cool as they are conveyed to the dryer. Typically, the pellets enter the dryer above 65^°C. Therefore, most of the energy in the drying operation is used to evaporate water. The energy required to change liquid water into vapour (latent heat of vaporisation) 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 feed dryers, or any other convection dryer. Only vacuum dryers operating under an extreme vacuum can reduce this value, but these dryers are not practical for extruded feeds. However, you may be able to reduce the energy consumed in evaporating water by reducing the amount of water that the dryer must remove. Table 1 shows the tremendous impact that inlet and outlet pellet 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 considerations, but do all of your extruder operators operate the extruder the same way? If one shift runs the extruder ‘wetter’ than the next shift, you are wasting energy by adding excessive water to your product only to spend money on the dryer to remove the water.

Heating the make-up air

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. 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. 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. In an extruded feed line, preheated make-up air can be drawn from your cooling operation. Spent cooling air used to cool the pellets is still relatively dry and can be used as make-up air to the dryer.

Improper dryer operation

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 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.

Figure 1- Dryer efficiency.

Unless your dryer is very 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.

So, how much does it cost to run an extruded feed dryer? Typically, dryers will use between 3,000 kJ/kg and 4,500 kJ/kg of water evaporated. A very poorly-operated or poorly-designed dryer might even use much more. Based on a typical value of 3,250 kJ/kg of water evaporated, an extruded feed line producing 75,000 tons per year will typically use about 5 x 10^¹⁰ 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 (approx. 227,800 Euro). Naturally, this cost will change depending on the type and cost of fuel used in the drying operation.

Figure 1 and our experience in evaluating extruded feed dryers show that many dryers in the industry are using over 50% more energy than they should due to improper design or operation. This represents a tremendous potential savings for the feed producer. Imagine saving US$100,000 per year (approx. €91,100), simply by adjusting a few dampers and making minor changes in the way you operate your dryer. Believe it or not, many feed producers have done just that, after learning how to properly run their dryers.

Moisture uniformity

Most extruded feeds are sold by weight. For this reason, if the product is over-dried, it is as if you are sending water at the value of your product – which should have gone to packaging—straight out your exhaust stack. Suppose that your extruded feed cannot 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 your product to 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 feed line as in the previous example, producing 75,000 tons per year. A 3% lost 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 (approx. €364) per ton, it is US$900,000 (approx. €820,000). At US$600 per ton (approx. €547), it is US$1.35 million (approx. €1.23 million). The numbers are staggering.

The bottom line is that you cannot afford to be over-drying your product. A well-designed, well-adjusted feed dryer should be able to dry your product to within ±0.75% or better. When was the last time you checked the moisture uniformity on your dryer?

Maintenance and sanitation

Many extruded feed producers 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 cleaning and maintaining the equipment? A dryer producing 10 tons per hour of extruded feed can be producing well over US $100,000 (approx. €91,865) worth of product each day. If you are spending even three hours a week of unplanned maintenance or cleaning downtime, you have lost 30 tons of final product per week, which equates to US $650,000 (approx. €597,220) per year of lost production, using the above parameters. The cost of this downtime must be considered when looking at the economics of your drying operation. In conclusion, we can say that drying is critical and many producers will significantly reduce their drying costs and increase their production rates by taking a closer look at their drying operation.