Increase aquafeed production through plant optimization
December 01, 2007
by Meyer Sosland
The number of different aquatic species and the tonnage of the annual "farmed" harvest continue to increase as sustainable aquaculture gains support.
It is estimated that more than 30% of the $100 billion global seafood market is from aquaculture. This equates to more than 40 million tonnes of seafood produced each year.
World demand is expected to increase by at least 3% annually over the next few years. Roughly 90% of aquaculture production occurs in the Asian region of the world.
Fresh water aquaculture accounts for 58% of the output and marine aquaculture for 42%. The various aquaculture species, both marine and fresh water, can be categorized according to the buoyancy properties of their feed.
To achieve the level of buoyancy required, specific bulk density ranges have been established for each feed for the environment in which the feed is being fed. The floating/sinking properties change with water temperature and salinity.
The recipe, the hardware and the operating parameters for the extrusion, drying, cooling and coating processes are adjusted to meet necessary buoyancy properties and fat levels in the final product.
Power inputs, measured as specific mechanical energy (SME), and moisture levels are the major operating parameters that are controlled during the extrusion process to yield the desired bulk density of the final product.
In an effort to meet increased aquaculture feed production requirements, it is important to maximize throughputs in an existing production line. Conducting a simple line audit can easily identify bottlenecks to higher throughputs.
Bottlenecks that potentially limit throughputs of an existing extrusion line are as follows:
- preconditioning capacity;
- available extruder power;
- extruder volumetric capacity;
- die open area;
- upstream/downstream unit operations.
PRECONDITIONING CAPACITY The preconditioning step initiates the heating process by the addition of steam and water into the dry mash.
Uniform and complete moisture penetration of the raw ingredients significantly improves the stability of the extruder and enhances the final product quality. Objectives of a preconditioning step are to continuously hydrate, heat and uniformly mix all of the additive streams together with the dry recipe.
The preconditioning process is relatively simple. Raw material particles are held in a warm, moist, mixing environment for a given time and then are continuously discharged into the extruder.
This process results in the raw material particles being hydrated and heated by the steam and water in the environment.
Dual shaft, intermeshing preconditioners have improved mixing in comparison to the single shaft preconditioners and have a longer average retention time of up to 90 seconds for a similar throughput. Dual shaft, intermeshing preconditioners have beaters that can be changed in terms of pitch and direction of conveying. This feature of adjustable beaters is not found on many conditioning devices.
Of all the preconditioners available today, the differential diameter/differential speed preconditioners (DDC) are the most sophisticated.
The DDC has the best mixing characteristics combined with the longest average retention times of those available.
DDC preconditioners offer retention times of up to six minutes for given throughputs comparable to the 15 to 45 seconds possible in single preconditioners or multiple-stacked single conditioners (sometimes referred to as dual conditioners). The two shafts of a DDC preconditioner are counter-rotating so that material is continuously interchanged between the two intermeshing chambers for maximum mixing.
Un-preconditioned raw materials are generally crystalline or glassy amorphous materials, which are very abrasive until they are plasticized by heat and moisture within the extruder barrel. Preconditioning prior to extrusion will plasticize these materials with heat and moisture by the addition of water and steam prior to their entry into the extruder barrel. This reduces their abrasiveness and results in a longer useful life for the extruder barrel and screw components.
Extruder capacity can be limited by energy input capabilities, retention time and volumetric conveying capacity.
While preconditioning cannot over- come the extruder’s limitations in volumetric conveying capacity, it can significantly contribute to energy input and retention time. Retention time in the extruder barrel can vary from as little as five seconds to as much as two minutes, depending on the extruder configuration. Average retention time in the preconditioner can be as long as five minutes. For some high moisture processes, the energy added by steam in the preconditioner can be as much as 60% of the total energy required by the process.
To increase preconditioner capacity or to compensate for inadequate preconditioning, one or more of the following steps can be employed:
• increasing preconditioner size;
• increasing existing preconditioner fill by one-time adjustment of beater configuration;
• adding the automatic Retention Time Control (RTC ) system;
• increasing energy inputs in the extruder.
AVAILABLE POWER TO THE EXTRUDER When power is the limitation to more throughput, the options to remove this bottleneck are more obvious. Factors to consider include the following:
• install larger extruder drive motor (more available kilowatts);
• check with extruder manufacturer to determine maximum allowable installed power based on system design limitations;
• factor in the effects of removing other bottlenecks (improved preconditioning, etc.).
Extrusion systems in the industry are available with power trains of more than 2,000 kilowatts. Lack of power is the most common bottleneck to higher production rates for existing process lines. An extrusion system operating at or above full load for most products is an indication that power is the limitation to higher throughputs.
VOLUMETRIC CAPACITY OF THE EXTRUDER Volumetric capacity is based on the free volume geometry of the extruder screw and the screw speed. Plotting the screw speed (revolutions per minute) versus potential output (kilograms per hour) indicates screw performance or efficienc. In most cases, actual output is lower than the potential volumetric capacity due to backwards pressure or leakage flow. However, when the extruder is designed with a cooled, grooved inlet feed throat and barrel sections, the output can be higher than the expected, calculated volumetric capacity of the screw.
A bottleneck due to volumetric capacity is usually manifested by the extruder operating in a "choked" or full condition.
Barrel fill will be great enough to plug or partially plug the barrel steam and water injection ports. In extreme cases, infeed material will visibly fill the extruder inlet and overflow the throat.
Force-feeding devices are sometimes disguised as tools to increase volumetric capacity, but their main function is to eliminate product bridging in the extruder inlet due to poor mixing in the preconditioning stage.
The volumetric capacity for an extrusion line can be increased in one or more of the following ways:
• installing a larger extruder screw diameter;
• increasing extruder screw speed;
• configuring the extruder with screw geometries designed for maximum conveying efficiency;
• utilizing grooved barrel liners;
• controlling extruder barrel temperatures with heating/ cooling systems.
OPEN AREA OF DIE ASSEMBLY A specific die open area is required to develop the proper back pressure and barrel in the extruder during processing. This open area requirement remains rather constant for a product having a distinct buoyancy. If the die area is insufficient, products may expand too much and extruder loads are excessive as a result of increased barrel fill.
Increasing the die open area to increase throughput potential is a straightforward relationship. Many die design techniques are employed to increase the number of die openings and the total die open area.
The most common arrangement of die orifices is on the die face, which is axially positioned with the extruder center line. A substantial increase in the number of die orifices can be realized when they are arranged on the periphery of the die extension in a pattern that is radial to the extruder centerline.
DOWNTIME AND USABLE PRODUCT Reduced downtime is often overlooked as a bottleneck to higher plant capacities. An extrusion line that has a throughput of 10 tph potentially loses five tonnes of product for every 30 minutes of downtime. A certain amount of downtime is unavoidable due to scheduled maintenance, product changeover and other plant functions such as fumigation and sanitation.
Many feed manufacturers believe they operate their lines 24 hours a day, seven days a week and are surprised to look at end-of-the-year production records that can indicate up to 20% actual downtime. Practices that can be implemented to reduce downtime are as follows:
• adjusting production schedules for
• installing hardware tools that have quick-change features;
• designing control systems for compressed startup/shutdown modes;
• training production personnel to reduce downtime;
• implementing preventive maintenance programs;
• designing system hardware for maintenance and cleaning accessibility.
In addition to downtime reduction, increasing usable product is a significant opportunity where off-spec product may run as high as 8% of total production. Considerations for increased levels of usable products include the following:
• automated retention time control in preconditioners to reduce startup/shutdown wastes;
• screw element and liner designs to give positive conveyance;
• high extruder speeds and variable speed drives to shorten process response times;
• online, automated control of SME and recipe analysis;
• automated extruder control systems that compress startup/shutdown modes;
• experienced and trained production personnel to control process;
• process flows that handle the product gently;
• systems to recycle under-processed material and off-spec product.
UPSTREAM/DOWNSTREAM PROCESSING It is easy to focus on the extrusion operation and the potential bottlenecks.
However, most bottlenecks occur along the process flow in areas other than the extruder.
An audit to increase plant production levels should include an evaluation of each unit operation along the entire flow. Potential bottlenecks could be found in one or more of the following areas:
All unit operations along the process line must be properly sized to avoid a flow bottleneck. As each bottleneck is identified and eliminated, a new, secondary bottleneck will likely appear. A different bottleneck may be identified for each product that is manufactured in a given process line.
This auditing process can continue indefinitely, but at each step it is necessary to do a cost/benefit analysis to determine if the economics are favorable. WG
This article is based on a presentation by Galen Rokey, Wenger International, Inc., at the Aquafeed.com conference, Aquafeed Horizons 2007, which took place during Victam International, Utrecht, Netherlands in May 2007. Rokey may be contacted at GalenR@Wenger. com. For more information about Aquafeed.com Conferences, visit www.aquafeed.info.