Centrifuges decanter clarifiers

Centrifuge decanters are apparatus of a relatively elementary design. They are used for a coarse separation of particles, for example, in order to increase concentration in sludge. They are also encountered in the medical field for separating blood constituents such as platelets and leukocytes. In order to clarify a fluid with a high degree of quality, other apparatus need to be employed, such as centrifugal separators, which we shall discuss in section 17.5. 

Pharmaceutical Removal of suspended matter is a frequent application for MF. Processes may be either clarification, in which the main product is a clarified liquid, or solids recovery. Separating cells or their fragments from broth is the most common application. Clarification of the broth in preparation for product recovery is the usual objective, but the primary goal may be recovery of cells. Cross-flow microfiltration competes w l with centrifugation, conventional filtration by rotary vacuum filter or filter press and decantation. MF delivers a cleaner permeate, an uncontaminated, concentrated cell product... 

For high-performance liquid chromatography (HPLC) analysis samples (0.5 mL) were clarified by centrifugation at 14000 Gav for 5 min and the supernatant was decanted, filtered through a 0.2 yim in-line syringe filter and analysed directly by chiral HPLC (see below). 

For isolated material the reaction mixture was clarified by centrifugation at 14000 Gav for 5 min and the supernatant decanted and extracted with dichloromethane. The dichloromethane phase was dried (MgS04) and concentrated in vacuo to yield the title compound as a colourless oil (4 mg, 43 %). 

Figure 3.5 Clarifying decanter (horizontal scroll centrifuge). Source Courtesy of Westfalia Separator Ltd.

Decanters are frequently used in conjunction with disc-stack-type centrifuges in the pre-preparation of clear juices and juice concentrates, where the initial decanter treatment results in a partially clarified juice with a low level of suspended solids. This is followed by a clarification stage using a disc-stack whereby the solids are thrown outwards from the through-flow juice stream into a solids-holding space and automatically discharged therefrom as and when an optimum level of solids is reached (see Figure 3.6). 

Hence, in products from D to I, which were the supernatants obtained after dilution, decantation, and centrifugation of previous products, respectively, significant drops in protein and lipid levels were observed in comparison with the initial product composition (Table 21.12). Only samples H and I had a protein level different from the other samples (Table 21.12). The decrease of protein content was of 9% and 8%, respectively, for these products. This suggests that a part of the protein in the supernatants has precipitated. This could be explained by their lower ionic strength (conductivity of 0.4—0.8 mS/cm). Products D, E, F, and G had similar protein contents. In a general manner, the acidification by EDBM followed by a dilution resulted in the preservation of more than 90% of the protein present initially. For the fat content, the more important decrease was observed for products E and F with a respective reduction of their lipid contents of 73% and 66%. In second position are products H and I with 49% and 45% respective reduction of lipids levels (Table 21.12). These different precipitation levels might be explained by the initial fat contents of the products. Effectively, products E and F obtained from samples B and C contained between 0.72% and 0.78% of fat while products H and 1 obtained from products Bi and Ci have fat levels between 0.47% and 0.55%. Samples dilution following the EDBM step resulted in a reduction of lipids from 0.78% to 0.21%, a decrease of nearly 73% of initial content in WPC lipids. It also resulted in clarified supernatants with a low level in hpids and with the majority of the proteins present initially. 

Clinitest (Bayer Corporation, UK) was used to verify that the ferment had gone to completion. The ferments were centrifuged and the wine decanted from the lees. A sample (20mL) of each wine was stored in the dark at room temperature for aging studies. The lees (approx. 5mL) were extracted with 2xl0mL methanol/formic acid (90 10) with agitation. The methanol was removed by rotaiy evaporation, the remaining solution was made up to SmL with water and pH adjusted to approx 3.5 with sodium hydroxide (29). Samples were analysed immediately after the completion of fermentation and then on a weekly basis by HPLC. The clarified wines were also monitored by UV-Vis spectrophotometry. 

Gelvatol mounting medium (see Note 3) Mix 2.4 g of polyvinyl alcohol with 6 g of glycerol, 6 mL of deionized water, and 12 mL of 200 mM Tris-HCl (pH 8.0) in a beaker on a stir plate for several hours until dissolved. Transfer the solution into a 50-mL polypropylene tube, heat the solution to 50 °C for 10 min, and then clarify by centrifugation at 5000 x g for 15 min. Decant the supernatant into a new 50-mL polypropylene tube and add 625 mg of l,4-diazobicyclo-[2.2.2]-octane (DABCO). Invert the tube to dissolve DABCO. Store 1 mL aliquots of mounting medium in a frost-free freezer at -20 or -80 °C. DABCO is essential as it reduces photobleaching of fluorescence. 

Space available for equipment is frequently a key factor, especially when retrofitting existing operations. For locations with considerable real estate available, large gravity decanters, API separators, or circular clarifier vessels are applicable. For space-limited situations, use of parallel plates can greatly reduce the separator area and volume, sometimes by a factor of 10-20. High-spe disk centrifuges require the smallest... 

Hemfort, H. and Kohlstette, W. (1984). Centrifugal clarifiers and decanters for biotechnology. Technical Scientific Documentation Number 5, Westfalia Separator AG, Oelde, Germany. 

Figure 2.2 Decanter centrifuge (from Alfa-Laval, 1978). 1, Hollow drive shaft with stationary inlet tube 2, erosion protected solids discharge ports 3, tapered beach section of rotor for discharge of solids 4, solids deposited on rotor wall 5, screw conveyor 6, pond of clarified liquid 7, exchangeable overflow weirs 8, conveyer drive shaft from gearbox.

Tn all but the last 10 or 15 years the majority of decanter applications have been limited by their ability to clarify the liquor. Scale up of these centrifuges Would be based on their clariKcation ability. Thus, a lot of the earlier development has been concentrated on enhancing clarification capacity. Bowl speeds have been increased, bowls lengthened and solids discharge areas increased to ensure solids choking does not inhibit clarification volume. Apart from these obvious methods there are some mechanical modifications which can help,... 

As far as possible, then, clarification aims at a complete separation of solids from the liquid stream. The next purpose, by contrast, aims specifically to leave some solids in the exit liquid. In the classification of solids by a decanter, a slurry of solid particles of mixed particle size, or, less often, of mixed densities, is treated in such a way that a specific fraction is removed as separated solid, leaving a well-defined fraction of the original solids still in suspension. This mode of operation is particularly relevant to the processing of kaolin (china clay), and it also finds a place where the decanter is used to remove oversize material, ahead of a more efficient clarifier, which might interfere with the final separator s operation (e.g. which might block the nozzles of a disc centrifuge). The decanter is a very efficient means of effecting classification by particle size. 

Consider the smallest particle in the feed sludge that has to be separated, the cut point size. d. This particle has a density, ps, and settles in a liquor of density, pl. and viscosity, The feed slurry enters the decanter at a rate of Qf, at a pond radius, ri, at point X at time t=0. By the time the particle traverses the clarifying length of the centrifuge, L, in time t-tg, the particle must settle to a radius T2, at point Y, the bowl internal radius, if it is to be collected by the conveyor. The centrifuge rotates at a constant angular... 

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