Continuous process precipitation

The Biazzi continuous process is also used. The reactants are continuously fed to a series of nitrators at 15—20°C followed by separation of the PETN, water washing, solution in acetone at 50°C, neutralization with gaseous ammonia, and precipitation by dilution with water. The overall yield is more than 95%. The acetone and the spent acid are readily recovered. 

The main disadvantage of producing EMD in the form of a soHd deposit on the anode is that the process must be intermpted to remove the anodes from the cells. Stripping the Mn02 is a labor-intensive operation. A number of continuous processes have been devised to generate the Mn02 as a precipitate that collects at the bottom of the cell where it can be removed without intermption of the electrolysis (86—89). 

The goethite process precipitates crystalline aFeO-OH (goethite) as well as PFeO-OH, aFe202, and amorphous phases. The reaction is carried out at 90°C and pH 3.0, for 4—6 h in either batch or continuous fashion, and the iron(III) ion must be kept <1 g/L. Both jarosite and goethite soHds are usually lagooned. 

Recent Developments. A considerable amount of cellulose acetate is manufactured by the batch process, as described previously. In order to reduce production costs, efforts have been made to develop a continuous process that includes continuous activation, acetylation, hydrolysis, and precipitation. In this process, the reaction mixture, ie, cellulose, anhydride, catalyst, and solvent, pass continuously through a number of successive reaction zones, each of which is agitated (92,93). In a similar process, the reaction mass is passed through tubular zones in which the mixture is forced through screens of successively small openings to homogenize the mixture effectively (94). Other similar methods for continuous acetylation of cellulose have been described (95,96). 

Sodium chromate can be converted to the dichromate by a continuous process treating with sulfuric acid, carbon dioxide, or a combination of these two (Fig. 2). Evaporation of the sodium dichromate Hquor causes the precipitation of sodium sulfate and/or sodium bicarbonate, and these compounds are removed before the final sodium dichromate crystallization. The recovered sodium sulfate may be used for other purposes, and the sodium bicarbonate can replace some of the soda ash used for the roasting operation (76). The dichromate mother Hquor may be returned to the evaporators, used to adjust the pH of the leach, or marketed, usually as 69% sodium dichromate solution. 

Calculation of the induction time is crucial, since gaining a stable and continuous process requires residence times in the mixer < precipitation induction times in order to prevent incrustation in the mixing device. The induction... 

Both continuous and batch methods may be used in methanolysis. The batch mediod requires an autoclave, crystallizer, and centrifuge and a system for the melting and distillation of the DMT obtained. In the two-stage Hoechst continuous process, waste PET is melted and fed to a reactor. Preheated methanol is added to the autoclave, which is equipped with a mixer. The conversion reaches 70-90% in the first reactor, after which the reaction stream is introduced into a second autoclave at a lower temperature near the bottom, where it rises slowly and die higher density impurities settle at the bottom. The reaction stream leaves the second autoclave and its pressure is reduced to 0.3 MPa. On further reduction of the pressure and cooling, DMT precipitates and is subsequently purified.12... 

It should therefore not be surprising that for relatively small-scale operations involving solids handling within the fine and intermediate chemicals industry, batch operation is preferred. Similarly, continuous processes that involve precipitation or crystallization, a common unit operation in fine chemicals, are rare. Small-scale examples are known, for instance, a continuous crystallization process was used by Bristol-Myres Squibb in order to improve dissolution rates and bioavailability of the product [12]. The above does indicate that not all process or parts thereof are suited for conversion from B2C, given the current technology. 

When fully hardened, the cement is resistant to erosion provided the solution has a pH above 4. However, the glass polyalkenoate cement is susceptible to erosion immediately after set because some of the matrixforming cations and anions are still in soluble form. In fact, the hardening process is one where these cations and anions continue to precipitate. For this reason these cements have to be protected, temporarily, by a varnish. 

Unimmobilized Corynebacterium propinquum (CGMCC No. 0886) cells containing a cobalt-dependent NHase were employed in either batch or continuous reactions for the production of nicotinamide from 3-cyanopyridine [24]. In the continuous process, membrane filtration separated precipitated product (>5 wt%) and the microbial cell catalyst from the reaction mixture, where the catalyst was then recovered and returned to the reactor using a continuous addition of aqueous 3-cyanpyridine to maintain substrate concentration at <20% (w/v), a final conversion of >99% was obtained. 

It is now known that teeth undergo a continuous process of demineralisation and remineralisation (see Table 2), which is driven by changes in the plaque composition [29]. In the presence of fermentable carbohydrates plaque microorganisms generate characteristic organic acids, that is, lactic and acetic [17], and these diffuse through the pellicle to the tooth surface and cause demineralisation [30]. Ions are then liberated from the mineral phase into this low pH liquid [31], and they diffuse outwards and re-precipitate at the surface layer of the demineralised lesion [32,33]. If this process is sufficiently rapid, there is a net loss of tooth mineral and irreversible cavity formation. 

Wastewaters containing complexed metals with a strong complexing agent such as EDTA, ammonia, or citrates require a two step precipitation for the metal removal. A continuous process using ferrous sulfate or ferrous chloride is as follows ... 

In a saturated solution, an equilibrium exists between the rate of precipitation of solute particles and the rate of dissolution of solute particles. The rate of precipitation equals the rate of dissolution. The shape of a crystal of solute added to a saturated solution will change after a period of time at a constant temperature and pressure, but its mass will remain the same. The equilibrium between dissolving and precipitation is dynamic, a continuous process. 

The methanol level in the precipitation vessel (25%) was optimized in studies of process conditions for the precipitation When methanol levels below 25% were used, gumming of the product was observed. The product of the above precipitation process filtered and washed well, did not agglomerate during drying, and, physically, appeared and behaved the same as micronized material. Such a continuous process was operationally attractive despite the dilution, since it shortened the time cycle (a large crystallizer was available) and avoided the labor intensive and dusty micronization process. However, the micronization process using wet API was adopted since it was deemed useable on all production sites. 

These problems are avoided if a continuous process is employed for the precipitation however, this makes higher demands on the process control. In a continuous process all parameters as temperature, concentrations, pH, and residence times of the precipitate can be kept constant or altered at will. Continuous operation is, for instance, used for the precipitation of aluminum hydroxide in the Bayer process. Bayer aluminum hydroxide is the main source for the production of cata-lytically active aluminas. The precipitation step of the Bayer process is carried out continuously. An aluminum solution supersaturated with respect to Al(OH)3, but not supersaturated enough for homogeneous nu-cleation, enters the precipitation vessel which already contains precipitate so that heterogeneous precipitation is possible. The nucleation rate has to be controlled very carefully to maintain constant conditions. This is usually done by controlling the temperature of the system to within 2-3 degrees [7]. 

The continuous process usually allows precipitation at low supersaturation conditions, since seeds are already present in the precipitation vessel. Thus, no homogeneous precipitation, which needs high levels of supersaturation, is necessary, and nucleation occurs heterogenously with the associated lower supersaturation levels. 

The weathering of minerals can be understood as a continuous dissolution-precipitation process involving them. The process can be very complicated, leading to multiphase equilibria, in which more than one solid phase, solution, and gas phase may be present. For example, primary silicates transform to secondary silicate minerals via such weathering reactions (Stumm and Wollast, 1990), as in the formation of kaolinite (AljSijC OH) from anorthite (CaAl2Si208) ... 

As mentioned in Chapter 10 on water softening, as long as the concentrations of CaCOs and Mg(OH)2 exceed their solubilities, the solids may continue to precipitate. This condition can cause scale to form, a solid that deposits due to precipitation of ions in solution. To prevent scale formation, the water must be stabilized. A water is said to be stable when it neither dissolves nor deposits precipitates. If the pH is high, stabilization may be accomplished using one of several acids or using CO2, a process called recarbomtion. If the pH is low, stabilization may be accomplished using lime or some other bases. 

It must be recognized that transfer of species from the atmosphere by dry deposition is a continuing process, whereas that in aqueous phase, through precipitation, is periodic, and strongly dependent upon meteorological conditions. 

Lead styphnate and lead azide are precipitated in correct proportion and mixed by a continuous process. 

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