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Preparing the cereals for grinding: BLENDING AND DAMPENING

06 February 20149 min reading
Dr. eng. Tanase TANASE Tecnocereal srl Abstract: Within the cereals cleaning and conditioning plants, various technological operations takes place, which have the primary purposes both grain mass cleaning of foreign bodies and the dust adherent on the kernels surfaces as well as getting a load of wheat that will behave in a more “appropriate” way to grinding in order to obtain the desired products. By “as appropriate”, we understand the process stability, constant quality finished products and the economic performance of the system evaluated by extractions and energy consumption. Of all these operations, we will make in this paper a short overview on two of them: the Blending of Grain and Water Conditioning. Both operations have a direct effect on technological and economic performance of the plant and also conditioning the constant quality of the finished products. Addressing issues was achieved through the practical purpose of these operations by reviewing the existing resources available to the mill operator to perform these operations. 1. INTRODUCTION 1.1. Blending of Cereals: By the term “Blending of Cereals” we understand achieving a blend of different sorts of cereals, in a controlled manner, after which, the resulting blend have certain quality parameters. Unlike the homogenization operation performed for flour, in this context, the blending means actually the leveling of quality parameters of the grain mass reaching the grinding department by mixing two or more kinds of grains in well-controlled percentages. The need to obtain, at the end of the grinding section, constant quality products, requires such an approach. The bakers expects that the miller to deliver them constant quality raw materials or products with tolerances on quality parameters in limits that can be tolerated by the bakery production line and without affecting their finished products quality. Considering that we have “n” cereal varieties, with different quality parameters ci, which must be mixed, the question that arises is to determine how they can be achieved in a lot of grain which in the end of the milling operation showing expected quality flours. The blending percentages are firstly established by the laboratory department. The desired quality is to be requested by the commercial department. Important to note is that the system must be provided in mixed cell no. equal to no. of the cereals varieties that must be mixed. The equation that represents the basis for mixtures of grain is: where: • ca is the weighted average quality index for the batch which is to be prepared for grinding, expressed in the specific units of the quality index factored into the analysis • qi is the quantity from the wheat of sort “i”, in [kg] • ci is the weighted average quality index of the quantity qi and it is expressed in the specific units of that quality index 1.2. Conditioning with water: Due to the multiple advantages it brings the grinding process, the operation of water conditioning has become a fundamental operation in modern grindin facilities. Today, a cereals grinding plant without water conditioning systems cannot be designed. Water Conditioning usually involves achieving of the following: • dispensing of water in continuous flow to the wheat stream and their homogenization • tempering of the dampened wheat 2. OPERATING PROCEDURES 2.1. Blending of Cereals: Dosage of the different sorts can be made by one the following ways: • volumetric dosing • gravimetric dosing 2.1.1. Volumetric dosing requires the presence of the volumetric dosing machines, in the technological flow of the cleaning section, at the outlets of the daily bins. The simplest form of this is a standard air-lock, provided with variable speed drive unit. You must also know the volumetric capacity/rotation thereof. A dedicated form of these appliances is the Volumetric Dosing Machine. They essentially consist of dosing cylinder, divided into several sections, usually 5 or 6, each with a specific volumetric capacity/revolution. They can be driven as single unit (fig.1) or in battery of maximum 4-5 units (fig.2). The capacity of such Dosing Machines vary from some hundreds of [kg/h] to 15-20 [tons/h]. For safety operation, the system must be provided with minimum level sensors on the outlet of the daily bins . 2.1.2. Gravimetric dosage is a much modern method based on electronically managed devices with automatic operation. Multiple advantages compared to volumetric dosing system, namely: • Higher dosing accuracy because the adjustment of the flow is continuous regardless of variations of moisture, hectoliter weight or angle of repose of cereal • simultaneously start and stop of the dosing sequence • Possibility of the total automation by PC and dedicated software • Registering of the total/partial transited amounts of cereals • Possibility of working based on dedicated recipes software • Lower specific consmptions The most common Gravimetric Dosing Machines, are working on one of the following principles: • Assessment of the momentary capacity by “impact plate” technique, ponderal dosers • Assessment of the momentary capacity by “loss-in-weight” technique, differential dosers The devices that works in “impact plate” technique, are mainly composed of (fig.3): • A continuous inlet section adjusting mechanism. This can electrically or pneumatic driven • An Impact Plate, high abrasion resistant, mounted on a force transducer, usually a load cell, for to measure the downforce exerted by the flowing cereal. Based on internal working alghoritms and using a dedicated interface, the analogical signal is then converted into digital and further processed by the on-board or remote electronic unit • A electronic unit for the entire management of the dosing process, complete of different accessories for to program and display of different information and alarms and further optional communication with a central PC station for recipe management, etc. By implementing such a system with multiple machines connected to a central PC unit or specialized, batches can be prepared on the basis of programmed recipes database. The devices working in the “Loss-in-weight” technique, are more complex than the ponderal dosers and can be used both to dose a mass flow or a volume flow of product. As the working principle, the system monitors the weight of the product inside a pan installed on load cells and using a continuous adjustable gate like item 2 in fig. 3, product flow is metered scheduled. Optional, the system can monitor also speed of filling/emptying the weighing hopper, giving also some other information relevant to the product like the hectoliter weight. 2.2. Conditioning with water: The Automatic Dampening Systems for Cereals were introduced on the market around 1980. Such a system is mainly composed of: • Measuring track, that continuous measure the parameters of the incoming flow (capacity, hectoliter weight, temperature, initial moisture) • Electronic unit for the management of the dampening operation • Water dosage panel • Water and Cereals mixer, provided with product detecting box By the position of the Measuring Track to the Mixer, the system can operate as: • Feed-forward, measuring the parameters of the incoming cereals in the front of the mixer (fig.4) • Feed-back, measuring the parameters of the dampened cereals after the mixer The mill operator programs the desired final moisture of the commodity at the electronic unit. In systems like those in fig. 4, the Measuring Track column continuous transmit information to the electronic unit, which continuously calculates and adjusts the flow of water currently required to be added to the flow of grain. On systems measuring flow characteristics of grain after dampening, moisture measurement occurs after adding water, after which the electronic unit continuously adjusts water flow according to programming done by mill operator. Grain moisture measurement is performed in continuous flow through several methods: • by measuring the capacitance in the measuring cell, the dry wheat • microwave measuring • use of nuclear magnetic resonance (NMR) Depending on the manufacturer, the systems have different configurations and options. Modern systems of conditioning and preparation of cereals are provided with multistage dampening systems. Depending on the initial and final grain moisture, type of finished products, working capacity and other characteristics of the raw material, the cleaning-conditioning plant can be fitted with 2 or 3 steps of dampening. An important mention is that a very special role for flour color plays the dampening operation before 1-st break. In either case, the underlying formula to calculate required water flow for the dampened grain flow, is the following: where: • W is the water momentary required mass flow to be added at a certain moment in the cereal mass flow Gi, in [kg/s] • Gi is the momentary cereal mass flow, measured in feed-forward technique, in [kg/s] • mf is the final moisture of the resulted dampened and tempered cereal, scheduled by the mill operator on the electronic unit, in [%] • mi is the initial moisture content of the incoming product mass flow Gi, in [%] Rest time can vary widely from 6-8 h to 24 or even 36 h. Sizing the volume of the resting bins will be based on the estimated time for rest, so that will add a bin to change the flow from one cell to another. There are configurations of the cleaning-conditioning section that uses a single cell for each dampening stage. In this case, the miller will have to assess the operation of cleaning-conditioning plant in order to meet times of rest after each dampening step. Special attention should be paid to the outlet of the resting bins. It must be equipped with systems to ensure FIFO (First In - First Out). Different constructions and configurations are available on the market (systems with multiple openings, with truncated cone, etc.), all with the same goal, namely the bin to be discharged in FIFO principle. A system that does not respect this principle creates difficult problems in further processing due to the unevenness of moisture in grain mass. There are also systems in which it is considered the differential wetting of grain, making the selection by one of their characteristics: grain size, hectoliter weight, and so on, just to get a mass of grain with as much as possible uniform moisture level.
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