Desolventiser toaster dryer coolers (DTDCs) are used for recovering solvent from meal in the oil seed industry.
Meal is separated from the solvent using a desolventiser toaster, or, desolventiser toaster dryer cooler. Often the desolventiser toaster, dryer cooler, and desolventiser toaster dryer cooler, are abbreviated simply as ‘DT', ‘DC’ and ‘DTDC’ respectively. A DTDC is simply a DT and DC combined. The machine chosen for solvent recovery depends upon design considerations and the availability of space.
Desolventiser Toaster (DT) Process
After the miscella and meal has left the extractor, the residual solvent must be recovered from both.
For the purposes of this article, we will assume a DTDC is used. The amount of residual solvent in the meal varies depending upon the original oleaginous material, but we will assume a residual amount of 25-35%, as is typical for soybean meal after passing through the extractor.
The DTDC consists of a long cylindrical shell with trays separating the various stages. The first trays are known as the predesolventizing trays. Below these trays are the counter-current trays, sparge tray and steam drying tray. Each tray has rotating agitators known as sweeps or arms. The agitators move the meal across the tray until it exits through a discharge port and onto the next tray. The agitator arms are fixed to a central shaft that is driven by a large electric motor via a single gearbox.
Steam is used both directly and indirectly to heat the meal and increase its moisture content; this is known as toasting. During the predesolventizing stage, the meal will be indirectly heated from approximately 60°C to approximately 70°C, for 20 to 30 minutes. This heating of the meal causes the solvent to partially evaporate from the meal. Because heat is transferred via conduction, the meal layer is kept shallow to increase the contact surface area and aid heat transfer. The meal layer on each predesolventizing tray is normally between 10 to 20cm thick. It can be expected that between 10 to 25% of the entire meal solvent is evaporated in the predesolventizing stage.
Note that predesolventizing trays are always located in the top of the DTDC and are designed to allow vapours from below to exit through the top discharge port. The number of predesolventizing trays employed depends upon design, a larger diameter tray means less trays, and thus the height of the DTDC can be reduced.
Meal falls from the lowest predesolventizing tray to the uppermost counter-current tray. Counter-current trays use direct steam to heat the meal and increase its moisture content. Each counter-current tray is perforated to give the steam direct access to the meal layer on each tray.
A DTDC will usually have a maximum of four counter-current trays mounted below the pre-desolventizing trays. The meal layer on each counter-current tray is approximately 100 to 120cm thick. The meal on the counter-current trays is now classed as ‘wet’ and exits the counter-current section with a moisture content of between 17 to 22%, and a temperature of approximately 100°C.
The sparge tray provides approximately 70% of the total heat required during desolventisation. Steam is introduced through apertures directly into the meal onto the upper surface of the tray. The steam flow rate determines the apertures size.
After desolventisation, the meal leaves the desolventiser toaster and enters the drier cooler. For soybean meal, the DT exit temperature will be approximately 110°C with a moisture level of approximately 20%.
Drier Cooler (DC) Process
The DC only has two primary objectives. The first objective is to reduce the moisture content of the meal in order to conform to industry standards; approximately 12% moisture is desirable. The second objective is to reduce the meal temperature; approximately 30°C is desirable.
The DC is similar in design to the DT. It is vertically orientated and has an insulated shell. Meal passes down the DC tower in the same manner as the DT, but it is cooled as it progresses downwards. DCs utilize three different tray designs, these are:
- Steam Drying Trays
- Air Drying Trays
- Air Cooling Trays
With steam drying trays, steam enters the trays and is maintained at approximately 10 bar(g). The steam in the trays indirectly heats the meal and evaporates the moisture within the meal. Water vapour from the DC process is almost always recovered. Care must be taken not to overheat the meal as this damages its properties and reduces its monetary value.
It is possible to have DCs with no steam drying trays, although it is also possible to employ up to five steam drying trays for a single DC.
The temperature, moisture level and toasting time all have an effect upon the quality of the meal and so must be tightly controlled. Meal with poor nutritional properties is deemed low quality and will consequently fetch a lower market price.
The desolventized meal can be sold to the feed industry, or, power industry. Meal sold to the feed industry will be pelletised on site or at another location. The power generation industry uses meal for the firing of steam boilers.