Decanter controls

Figure 7.4 Decanter controls, (a) Conventional level control (b) Buckley control structure to eliminate interaction.

When a decanter is automated, automation of a lot of the associated equipment is also necessary, together with interlocking. For instance, it would be inadequate to have a decanter operating automatically unattended if failure of the cake off-take system could occur without communication of the fact to the decanter control system. 

There are a number of types of polymer make-up system. The one represented in Figure 8.1 is the usual dual tank batch make-up system for solid-grade polymers. It basically comprises a powder hopper with a screw feeder, discharging into a stirred vessel. The volume of water is controlled by level probes in this vessel. The contents are stirred for a fixed time, to allow the polymer to dissolve and age to its full potency. After the required ageing time, it is automatically transferred to the polymer supply vessel when actuated by a low-level signal from this second tank. The polymer pump is controlled from the decanter control system. 

Centrate solids concentration measurement is an important parameter for decanter control. Several such instruments are available to measure in this range. However, one problem presented by centrate from a decanter, on many applications, is the copious production of bubbles or foam in the flow. These bubbles are read, by many instruments, as solids, thus preventing the use of such devices. De-aeration of a sample flow of the centrate has met with a modicum of success. Some decanter manufacturers developing their own instrument [2] have obtained more success. 

Conveyor torque today is an essential part of decanter control. However, direct reading of conveyor torque is very difficult to achieve. Even direct reading of pinion torque is difficult, but could be done using strain gauges on the pinion shaft. However, the most usual method is to use a calibration of the braking device. The control device for the brake will give a read-out, on request, of the braking torque. 

Another nickel cataly2ed process is described ia a Tolochimie patent (28). Reaction conditions claimed are 1—2.4 MPa (150—350 psi) at 100°C minimum. The combination continuous stirred reactor and gravity decanter uses density-driven circulation between the two vessels to recirculate the catalyst to the reaction 2one without the use of filters or pumps. Yield and catalyst usage can be controlled by varying the feed rates. 

Avoid attempts to recover simultaneously both high and low boiling nodes in high purity from mixtures of >3 components, particularly in columns that reflux compositions different from the distillate composition, ie, reflux of one phase from a decanter, as such operations may be difficult to control. 

The ratio of cycHc to linear oligomers, as well as the chain length of the linear sdoxanes, is controlled by the conditions of hydrolysis, such as the ratio of chlorosilane to water, temperature, contact time, and solvents (60,61). Commercially, hydrolysis of dim ethyl dichi oro sil a n e is performed by either batch or a continuous process (62). In the typical industrial operation, the dimethyl dichi orosilane is mixed with 22% a2eotropic aqueous hydrochloric acid in a continuous reactor. The mixture of hydrolysate and 32% concentrated acid is separated in a decanter. After separation, the anhydrous hydrogen chloride is converted to methyl chloride, which is then reused in the direct process. The hydrolysate is washed for removal of residual acid, neutralized, dried, and filtered (63). The typical yield of cycHc oligomers is between 35 and 50%. The mixture of cycHc oligomers consists mainly of tetramer and pentamer. Only a small amount of cycHc trimer is formed. 

If the process feed does not He in the Hquid—Hquid region it can be made to do so by dehberately feeding either pure or pure B to the decanter, as required. This may only be necessary during start-up or for control purposes because the recycled azeotrope has the beneficial effect of dragging the decanter composition further into the Hquid—Hquid region. 

Control philosophies applied to continuous countercurrent decantation (CCD) thick eners are similar to those used for thickeners in other applications, but have emphasis on maintaining the CCD circuit in balance. It is important to prevent any one of the thickeners from pumping out too fast, otherwise an upstream unit could be stai ved of wash liquor while at the same too much underflow could be placed in a downstream unit too quickly, disrupting the operation of both units as well as reducing the circuit washing efficiency. Several control configurations have Been attempted, and the more successful schemes... 

To the acid chloride, mechanically stirred and heated on the steam bath, is added 2.5 kg. (805 ml. 15.6 moles) of dry bromine as rapidly as it will react (Note 5). The addition requires about 12 hours. The contents of the flask are stirred and heated an additional 2 hours, transferred to a dropping funnel (Note 6), and added in a thin stream to 5 1. of absolute ethyl alcohol, which has previously been placed in a 12-1. flask provided with a stopper carrying an effleient reflux condenser, a separatory funnel, and a mechanical stirrer. The resulting vigorous reaction is controlled by external cooling. After the dibromoacid chloride has been added, the reaction mixture is allowed to stand at room temperature overnight and is then poured into 5 1. of cold water. The top alcoholic aqueous layer is decanted and extracted once with 8 1. of ether. The oily bottom layer is dissolved in the ether extract, washed first with 1 1. of a 2% sodium bisulfite solution, then with two 1-1. portions of 3% sodium carbonate solution, and finally with several portions of water. The ether solution is dried over 175 g. of potassium carbonate the solvent is distilled on the steam bath. The yield of residual ester (Note 7) amounts to 2260-2400 g. (91-97% of the theoretical amount). 

For a decanter that operates under gravity flow with no instrumentation flow control, tire height of the heavy phase liquid leg above the interface is balanced against the height of one light phase above the interface [23]. Figures 4-12 and 4-13 illustrate the density relationships and the key mechanical details of one style of decanter. 

The agitation studies for PET depolymerization were performed in the Atlas Launder-ometer. The Launder-ometer is a device for rotating closed containers in a thermostatically controlled water bath. The procedure used in these experiments was adapted from an American Association of Textile Chemists and Colorists (AATCC) standard test method. The 5% sodium hydroxide solution (250 mL) was preheated to 80°C in a 1-pint stainless steel jar. The catalysts were added in the following amounts in separate experiments TOMAC (0.04 g, 0.0001 mol) TOMAB (0.045 g, 0.0001 mol) and HTMAB (0.045 g, 0.0001 mol). The PET fiber specimens (1.98 g, 0.01 mol) were placed in the containers along with ten -in. stainless steel balls to aid in the agitation process. The jars were sealed in the Launder-ometer, whose bath was at the desired temperature (80°C). The machine was allowed to run for the allowed treatment times (i.e., 30, 60, 90, 150, and 240 min) at 42 rpm. Upon decanting, any residual fibers... 

After cooling, the microspheres were washed by decantation with petroleum ether to give a free-flowing powder. They were then sieved, dried, and stored in a freezer. Size distribution can be controlled by the stirring rate the yield is 70-90%. The process was quite reproducible with respect to yield, size, and loading distribution, if the same molecular weight of polymer was used. Less than 5% error was observed (5). 

Two separate 2.1 L reservoirs contain the catalyst and product phases and the contents are fed into the reactor through a standard liquid mass flow controller. The contents of the reactor can be sampled from a pressure fed sample tube. The pressurized liquid reactor products exit the reactor through a pressure control valve, which reduces the pressure to atmospheric, and the liquid contents are delivered to a continuous decanter where the phases separate. The catalyst phase then settles to the bottom where it is drained for recycle and reuse, while the product phase is collected into a 4.2 L reservoir. 

In any equipment where an interface exists between two phases (e.g. liquid-vapour), some means of maintaining the interface at the required level must be provided. This may be incorporated in the design of the equipment, as is usually done for decanters, or by automatic control of the flow from the equipment. Figure 5.16 shows a typical arrangement for the level control at the base of a column. The control valve should be placed on the discharge line from the pump. 

Decanters are normally designed for continuous operation, but the same design principles will apply to batch operated units. A great variety of vessel shapes is used for decanters, but for most applications a cylindrical vessel will be suitable, and will be the cheapest shape. Typical designs are shown in Figures 10.38 and 10.39. The position of the interface can be controlled, with or without the use of instruments, by use of a syphon take-off for the heavy liquid, Figure 10.38. 

The height of the liquid interface should be measured accurately when the liquid densities are close, when one component is present only in small quantities, or when the throughput is very small. A typical scheme for the automatic control of the interface, using a level instrument that can detect the position of the interface, is shown in Figure 10.40. Where one phase is present only in small amounts it is often recycled to the decanter feed to give more stable operation. 

Commercially, lead azide is usually manufactured by precipitation in the presence of dextrine, which considerably modifies the crystalline nature of the product. The procedure adopted is to add a solution of dextrine to the reaction vessel, often with a proportion of the lead nitrate or lead acetate required in the reaction. The bulk solutions of lead nitrate and of sodium azide are, for safety reasons, usually in vessels on the opposite sides of a blast barrier. They are run into the reaction vessel at a controlled rate, the whole process being conducted remotely under conditions of safety for the operator. When precipitation is complete, the stirring is stopped and the precipitate allowed to settle the mother liquor is then decanted. The precipitate is washed several times with water until pure. The product contains about 95% lead azide and consists of rounded granules composed of small lead azide crystals it is as safe as most initiating explosives and can readily be handled with due care. 

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