Pneumatic Conveying & High Accuracy Batch Weighing
Traditionally, manual batching methods have been used for material handling and feeding ingredients to the mixing process. However, many process industries around the world are now opting to automate their operations in order to increase productivity, save time and improve overall product quality as well as process safety.
In the automation of these plants, Fresco Systems implements pneumatic conveying systems for major, minor and micro ingredients as well as integrating high accuracy automated batching and weighing systems into the mixing process.
By utilizing automated transfer methods for the raw ingredients along with highly accurate means of batching, process industry manufactures can realize lower overall manufacturing costs, lower overall manufacturing time as well as increased savings on individual raw ingredients as a result of reduced waste.
The production of any blended product typically involves the intermediate process steps of transfer and weighing or “batching “of individual ingredients based upon their weight percentage in a blend. Depending on this percentage, ingredients are usually categorized as majors, minors and micros.
In many cases, the transfer and weighing of these majors, minors, and micros to the blending step can be a manual and labour intensive process. The automation of this process includes the transfer of the raw ingredients to the batching system, where either Gain-in-Weight (GIW) or Loss-in-Weight (LIW) batch systems are used to accurately and efficiently deliver the combination of raw ingredients to the mixer.
Major ingredients can arrive to the plant in a variety of forms, such as railcar, truck or bulk bag systems. These major ingredients are transferred to specialized storage silos. From there they are then conveyed to the specific weigh batch stations as required for the blend. These transfer systems can easily be integrated into a plant PLC system featuring recipe control for multiple ingredients and processes, in order to provide maximum control and flexibility.
The transfer of major ingredients into and within a plant is generally achieved via various types of conveying systems. Pressure Differential (PD) trucks and railcars use positive pressure to unload material, whereas other types of delivery to the batching process can often be achieved by either positive pressure or vacuum pneumatic conveying.
The mode of transfer of ingredients is dependent upon a wide variety of process parameters, including material characteristics, distance to be transferred, required rate of transfer, and the type of source and destination containers and/or process.
• Pressure Differential (PD) trucks Transfer
Upon the arrival of a PD truck at the plant, a flexible hose is connected from the PD truck to the conveying line. If the truck does not have a built-in blower, a second line is connected to a pressure blower on site. The system operator selects the desired destination (e.g. silo 1 for starch or silo 2 for flour) on the truck unload control panel.
When the system is started, the blower pressurizes the PD truck and pushes material out of the truck via positive pressure through the conveying line and directly into the silo. Many times, an inline magnet is installed in the conveying line to remove any metal particles which may be present in the conveyed material. When the high level sensor in the silo is activated, the operator closes the material flow gate on the truck and allows the system to purge the conveying line before finally stopping the operation.
• Dilute Pneumatic Transfer: Vacuum or Pressure
Other possible sources of ingredient delivery include boxes, sacks, bulk bags or super sacks. In all of the ingredient transfer steps, pneumatic conveying systems can be used to transfer these ingredients. These systems can utilize either positive pressure or vacuum dilute phase conveying.
Positive pressure conveying systems are typically used to transport bulk materials over long distances and at high throughputs. Applications which involve pressure conveying often include loading and unloading of large volume vessels such as silos, railcars, trucks, and bulk bags.
Vacuum (negative pressure) systems are generally used for lower volumes and shorter distances. One of the advantages of vacuum systems is the inward suction created by the vacuum blower and reduction of any outward leakage of dust. This is one of the reasons why vacuum systems are often used in higher sanitary or dust containment applications. Another advantage of vacuum systems is the simple design for multiple pickup points. It should be noted, however, that the distances and throughputs possible with a vacuum system are limited due to the finite level of vacuum that can be generated from the prime mover being the exhauster blower. Sometimes a combination of pressure and vacuum conveying designs is used for a process, taking full advantage of the benefits and efficiencies of each technology.
Batch Weighing Principles
After transfer from the material source, the ingredients are usually delivered to the batching station. This station may consist of volumetric metering devices, such as screw feeders or valves, which deliver the product to a hopper mounted on load cells.
This method is called Gain-in-Weight (GIW) batching. Alternatively, the station may consist of gravimetric feeding devices, such as screw or vibratory feeders, mounted on load cells or scales, which deliver the product to the process by means of Loss-in-Weight (LIW) batching. As outlined below, in some cases where small amounts of micro ingredients are required for an overall large batch, both methods are employed: LIW feeders for the micros and minors, and GIW batchers for the major ingredients.
• Gain-in-Weight Batching
In GIW batching volumetric metering devices sequentially feed multiple ingredients into a collection hopper mounted on load cells. Each feeder delivers approximately 90% of the ingredient weight at high speed, slowing down towards the end of the cycle to deliver the last 10% at a reduced rate to ensure higher accuracy. The GIW controller monitors the weight of each ingredient and signals each volumetric feeder to start, increase or reduce speed, or stop accordingly. Once all the ingredients have been delivered, the batch is complete and the mixture is discharged into the process below.
It should be noted that this type of batching method is sequential for each ingredient, and therefore generally results in a longer overall batching time than with LIW batching.
• Loss-in-Weight Batching
LIW batching is used when the accuracy of each individual ingredient weight in the completed batch is critical or when the batch cycle times need to be very short.
Gravimetric feeders operating in batch mode simultaneously feed multiple ingredients into a collection hopper. Adjustment of the delivery speed (on/off, fast/slow), lies with the LIW feeder controls and the smaller weighing systems deliver highly accurate batches of each ingredient.
Once all the ingredients have been delivered, the batch is complete and the mixture is delivered to the process below. Since all ingredients are being delivered at the same time, the overall batch times as well as further processing times downstream are greatly reduced. This method of batching is often used for micros (such as trace elements and vitamin fortification) due to the high accuracy requirements and cost of these ingredients.
Batch Weighing with Scale Hoppers
Scale hoppers are receiving hoppers suspended on load cells for ingredient batch weighing. The material resides in the scale hopper until the precise weight and or combination of materials is achieved. Weighing accuracies of +/- 0.5% of the full scale capacity can be expected. Once the desired weight has been achieved and the mixer calls for material a discharge valve opens and the material in the scale hopper is discharged into the process below.
In conclusion, properly weighing and accurately delivering the ingredients to the mixing process without manual intervention can result in a number of process advantages, including fewer mistakes, better accuracy, lower bulk costs, improved product quality and savings in manufacturing costs.
The highly experienced engineering staff of Fresco Systems can provide a wide variety of design and layout options in both ingredient transfer and delivery to help manufacturers lower process costs while improving efficiency and product quality.