The effect of conditioning on pellet quality

Residence time

Another factor influencing pellet quality is the residence time of the feed mash in the conditioner. Within a conditioner, residence time is normally altered by changing the level of fill. Normally this is done by changing the angle of the paddles on the mixer shaft. Feed mash that is kept in the conditioner for a prolonged time normally has an increase in pellet durability (Huang et al.,).

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Moisture diffusion into the feed particles is a relatively slow process compared to the diffusion of heat (Guerin., ). Therefore, particles that have been conditioned longer are less brittle and more plastic compared to particles with a shorter conditioning period, where the interior of the particle is still dry and brittle. More plastic particles result in better compaction of the particles to pellets. If sufficient moisture is still at the outside of the feed mash particles during pelletizing, binding is improved. Over conditioning leads to soft, pliable particles but lacks formation of bonds between particles due to absence of liquid bridges (Thomas et al., ). Liquid bridges are easy to form, weak bonds between particles. During cooling of the pellets these liquid bridges are transformed into strong solid bridges due to the removal of moisture from the pellets during which solid material is deposited at the contacting points and builds a strong bond between feed mash particles. This leads to a stronger pellet.

High pressure conditioning

In modern feed mills, besides normal conditioning high pressure conditioning is performed. Most commonly an expander, double pelleting or a BOA compactor is used in this process. High pressure conditioning uses mechanical energy to exert shear and increase the density of the feed mash, thus altering the structure. These changes in structure increase particle binding and decrease pellet porosity after pelleting which also results in higher quality pellets (Thomas et al., ). Additional effects of high-pressure conditioning are a further decrease in the size of the feed mash particles and decreasing the energy consumed of the pellet press. However, the total energy consumption for feed production including the expander, boa or the additional pelletizer will increase, usually by about 15%.

Take home messages

Conditioning is an important factor in creating good quality pellets. Optimizing pellet quality by altering conditioning parameters can be done by: making sure the steam is of high quality, increasing the temperature of the feed mash, prolonging the residence time in the conditioner and/or adding high pressure conditioning to the pelleting line.

Frequently checking the quality of the steam added into the conditioner is important. These checks encompass the whole steam delivery system, boiler functioning, effectiveness of the steam traps, the pressure drop over the steam reduction system and finally a visual check of the steam added to the conditioner.

There are several situations in which increasing conditioning temperature is beneficial: the current conditioning temperature is low, the steam is of a good quality, and the nutrients in the feed mash are not affected by degradation due to the higher temperature. The increased temperature plasticizes polymer feed components such as starch and protein which will improve pellet quality, increase capacity and reduce specific energy consumption.

Increasing the residence time of the feed mash in the conditioner affects the position of water in the feed mash particles. This in turn may lead to better plastication which may improve pellet quality. Too short a residence time will lead to excess moisture at the outside of the particle; this will lead to blockage of the pelletizer. Optimizing residence time to match particle size is therefore relevant to obtain the best possible pellet quality.

High pressure conditioning will increase pellet quality at the expense of increasing the mechanical energy used in the production process.

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Conditioning
The purpose of conditioning is to optimize the nutritional quality of feed. Conditioning enhances the Pellet quality and productivity of pellet press. This is achieved in feed milling by adding or including any process to mash feed after mixing and before pelleting.
Addition methods:
1) Steam addition
2) Water addition
Processes:
1) Expanding feed molecule through Expander
2) Compacting feed molecule through Compacter
3) Pre-pelleting
Basically, the conditioning process, condition the mash feed to become pellet feed. This article covers the topic of Conditioning through Steam addition. The process of adding moisture and heat through steam to the mash feed is called Steam Conditioning. Steam adds heat and moisture to mash feed. Addition of heat and moisture leads to some starch gelatinization. Because of gelatinization, binding of raw material happens. Secondly, it improves digestibility of nutrients.
The function of a conditioner is to provide proper contact and mixing of steam with mash feed. The process flow of Conditioning process is shown in Picture 1
The mash feed with initial moisture and temperature is an input to conditioning process. Steam is another input. Understanding the nature of these two inputs are critical to achieve good quality conditioning. In practical situation, initial moisture varies due to storage practices of raw material and environment conditions. The mash temperature will be few degrees above ambient temperature. Saturated steam is recommended for conditioning purpose.Steam with 100% vapour is called Saturated steam. The other inputs are conditioner, feed type and method of conditioning. Conditioner is basically a dynamic or continuous mixer. Hence it mixes the steam with mash feed uniformly. The heat and moisture should be uniform throughout conditioned mash feed. Testing CV of conditioner is a good practice to ensure the homogeneity of moisture and heat.
Good quality steam is applied through nozzles on mash feed. When saturated steam enters conditioner, the steam vapour cools and condensation of liquid occurs on the surface of feed particle. Both moisture and heat are migrated from the surface to core of feed particle. This migration is possible because of moisture gradient between the surface and interior of feed particle.
The output of a conditioning process is the conditioned mash feed with optimum moisture with target temperature.
Challenges in Conditioning
As a rule of thumb, each 1% moisture addition to mash feed (through saturated steam), increases 12.5&#;C temperature. Keeping thermodynamic statement in mind, the feed mills face following two major challenges in conditioning process:

1. The raw material moisture varies significantly over a period of a time. The variation in raw material moisture creates variation in initial moisture of mash feed.
2. The initial temperature of mash feed before entering conditioner is also varies significantly between summer and winter season and even between day and night in some seasons.

Further two scenarios can be visualized to understand the challenges in conditioning:

Scenario 1&#;When mash feed has high initial moisture and low feed temperature (during cool climate), target temperature cannot be reached. But before that optimum moisture is met.
Initial moisture = 13% (high); Ambient temperature = 20&#;C; Target temperature = 83&#;C
For above condition,

• 3% moisture addition through steam will take feed temperature to 57.5&#;C {(12.5X3) +20}and moisture to 16%
• 4% moisture addition through steam will take feed temperature to 70&#;C and 17% moisture

Target temperature cannot be reached. Less heated mash feed causes frictional heating at pellet die. It reduces pellet mill capacity and die life.
Scenario 2 &#; When grain moisture is low and or warm climate condition, optimum moisture cannot be achieved without exceeding target temperature
Initial moisture = 9% (low/dry); Ambient temperature = 40&#;C; Target temperature = 83&#;C
For above condition, target temperature is easily achieved. But the possible moisture addition through steam will be 3.44%. This will take the feed moisture to 12.44%. The optimum moisture cannot be achieved.
Initial mash feed moisture and environmental conditions are critical to conditioning process. Based on those factors, conditioning process should be optimized. Close watching of initial moisture and ambient conditions are important.
Critical check points in Conditioning
1 Particle size
To achieve optimum conditioning performance, fine grinding is recommended. The surface area of fine ground particles is more compared with coarse particles. As a result, the heat and moisture can penetrate to core of feed particles.
2 Initial mash feed moisture
This is explained in scenario 1 & 2 of above section. Since initial mash feed moisture is critical to conditioning process, it is recommended to monitor the moisture level on continuous basis and take appropriate strategy to achieve optimum moisture and target temperature.
3 Steam quality
Based on vapour content in steam, it is classified as

1. Saturated steam &#; 100% vapour held at a temperature and pressure at its vaporization point
2. Super-heated steam &#; 100% vapour held at temperature, greater than that of vaporization temperature
3. Wet steam &#; consists of both vapour and free water

Steam should be in saturated vapour state before entering to conditioner. This is achieved by Pressure Relief Valve (PRV). The PRV reduces high pressure steam to low pressure steam (1.5 to 2.5 kg/cm2). As a result, high pressure steam turns into low pressure dry steam.Steam quality is described based on dryness of steam. Steam should be as dry as possible. 80% dryness fraction is considered as good quality steam. The steam will be in super-heated state in boiler.

4. Residence time

Feed ingredients like grains, protein meals and other common ingredients are typically good insulators. Hence it takes longer time to transfer heat and moisture from steam to feed molecule. The time duration by which the mash feed exposed to steam, inside the conditioner is called Residence time or Retention time. Longer residence time allows better steam distribution and moisture penetration into feed particles. It improves the efficiency of conditioning. The residence time can be increased by 1) altering pick angles of conditioner paddles and 2) reducing the shaft speed of conditioner. Pellet quality and throughput (TPH) of pellet press is optimized at 30 to 90 seconds residence time

5. Degree of fill

Mash feed level in conditioner should be about 70% of available volume of conditioner.
The Pellet Durability Index(PDI) and Specific energy (kW/Ton) of pellet press are significantly influenced by conditioning process. Effective conditioning depends on properly designed, maintained and operated steam supply system. Research studies shows that conditioning contributes, 20% of pellet durability in overall feed milling process. So, by achieving optimum conditioning performance, the overall feed quality and feed mill performance can be enhanced.
by M Kanagaraj, Catalyst Techvisor Private Limited

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