Filtration Equipment Selection Criteria
To underpin the theoretical comparison with some concrete numbers, an example will be used to show what the two types of centrifuge are capable of under given feed conditions.
The removal of water from fine sodium bicarbonate is a classic case. Absolute dry solid throughput is 14,000 kg/h. The input solids concentration is 15%wt, and median particle size is 100 µm. Residual moisture should be kept to a minimum to reduce the load on downstream fluidized bed dryers. The product is relatively fine, so in principle both types of machine can be used under the given feed conditions. However, the pusher centrifuge cannot handle the 15%wt feed concentration which corresponds to a liquid throughput volume of nearly 90,000 kg/h. To circumvent this problem, a combination of pusher centrifuge and hydrocyclone will be used to make the comparison with the peeler centrifuge.
The hydrocyclone can easily pre-thicken the relatively heavy solids, and 50%wt at the underflow with a very fine cut point and low solids loss is not difficult to achieve.
A two-stage SZ 800/2 pusher centrifuge (Fig. 1) with basket diameter of 800 mm can easily handle 14 t/h of solids. With this fine product, the loss of solids in the filtrate should be in the neighborhood of 1 - 2%wt in relation to solids throughput. The achievable level of residual moisture is roughly 4.5%wt.
Another solution for this particular application is to use two HZ 180/7.1 Si machines (Fig. 3). The two horizontal peeler centrifuges have 1,800 mm diameter baskets, 7.1 m² of filtration surface and siphon baskets. A finer filter medium and renewable heel on this configuration reduce the loss of solids in the filtrate to 0.5%wt. Longer retention time in the basket, which however reduces hourly solids throughput, can further reduce the residual moisture content to around 3%wt. Splitting the load between two machines creates a semi-continuous process, because filling and centrifugal drying can take place in one machine while the other is being scraped out. With batch times of around nine minutes, this does not match the continuous output of a pusher centrifuge, but it is sufficient to reduce the load on the fluidized bed dryer.
In today’s world, energy consumption is another important part of the equation. Power consumption by the pusher centrifuge is only 40 kW per hour, which equates roughly to 3kW per tonne of product. The figure is about 9 kW/t for the peeler centrifuge. However, the increase in energy consumption in the dryer caused by higher residual moisture content and discontinuous feed on peeler centrifuges must also be taken into account.
The size of the investment in the machinery is another important aspect to consider. Given a basket diameter of 2 x 1800 mm for the dual peeler centrifuge solution compared to 800 mm for the single pusher centrifuge, it is easy to see why the pusher centrifuge has the advantage by a factor of 4–5, even when the hydrocyclone which is needed for pre-thickening is included in the equation. The difference between continuous vs. discontinuous feed changes the peripheral configuration, and that also has an effect on the level of investment.
In addition to the absolute level of investment, the ROI period is another important aspect which affects the overall economic viability of the system. System availability including centrifuge availability directly influences ROI. With peeler centrifuges, the system continues to run at 50% of full output during maintenance activity. If centrifugal drying time is temporarily reduced (assuming that the fluidized bed dryer has sufficient reserve), throughput can be increased in the other centrifuge, although residual moisture levels will rise.
The Krauss-Maffei pusher centrifuge comes with a fully replaceable cartridge which includes all of the rotating parts. An experienced maintenance team can have production back up and running within four hours following a maintenance call (e.g. bearing replacement). System availability remains high despite the fact that only one machine is installed.
The product itself often determines whether a peeler or pusher centrifuge will be used. If the parameters fall within a window which suits either type of machine, the factors described above play an important role in the decision process. In order to adequately assess these factors, customers should select a supplier that has an excellent machinery manufacturing track record and in-depth process expertise.
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