Hydroxides , residence times

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

One patent describes a continuous process involving an aqueous alkah metal hydroxide, carbon disulfide, and an alcohol (82). The reported reaction time is 0.5—10 min before the mixture is fed to the dryer. The usual residence time is on the order of hours. A study ia the former USSR reported the use of the water—alcohol azeotrope for water removal from isobutyl or isoamyl alcohol and the appropriate alkah hydroxide to form the alkoxide prior to the addition of carbon disulfide (83). 

Continuous polymerization in a staged series of reactors is a variation of this process (82). In one example, a mixture of chloroprene, 2,3-dichloro-l,3-butadiene, dodecyl mercaptan, and phenothiazine (15 ppm) is fed to the first of a cascade of 7 reactors together with a water solution containing disproportionated potassium abietate, potassium hydroxide, and formamidine sulfinic acid catalyst. Residence time in each reactor is 25 min at 45°C for a total conversion of 66%. Potassium ion is used in place of sodium to minimize coagulum formation. In other examples, it was judged best to feed catalyst to each reactor in the cascade (83). 

Typically, the saponification is run with 10% sodium hydroxide solution in a reactor cascade at 95-98°C under stringent pH control. The saponification mixture is separated in a settler. The upper phase consists of alkanes with a small proportion of chloroalkanes, which is removed by oleum refining or dehydrochlorination and high-pressure hydrogenation. The refined alkanes can be recycled to the reactor. In the aqueous lower phase are alkanesulfonates, sodium chloride, and between 4 and 8 wt % hydrotropically dissolved alkanes. An optimal separation can be approached at 95 °C, and residence times of less than 60 min if Fe(III) ions are added and pH values of 3-5 are maintained. 

Since many different gas/liquid contactors were used, the experimental conditions differed for each device and the reader is referred to the listing in the original reference [5], The liquid flows range from 10 to 1042 ml h and the gas volume flows from 180 to 25 020 ml h. The corresponding residence times were 0.01-19.58 s. The ratio of carbon dioxide to sodium hydroxide was fixed at 0.4. 

The carbon dioxide volume content was varied from 0.8 to 100 vol.-% the gas velocity changes from 0.1 to 42.9 mm s [5]. The residence time varied from 0.1 to 9.7 min 64 single streams of a liquid film thickness of 65 pm were used at a total volume flow of 50 ml h . The ratio of carbon dioxide to sodium hydroxide was fixed at 0.4. 

Thorium generally exists as a neutral hydroxide species in the oceans and is highly insoluble. Its behavior is dominated by a tendency to become incorporated in colloids and/or adhere to the surfaces of existing particles (Cochran 1992). Because ocean particles settle from the water column on the timescale of years, Th isotopes are removed rapidly and have an average residence time of = 20 years (Fig. 1). This insoluble behavior has led to the common assertion that Th is always immobile in aqueous conditions. While this is generally true in seawater, there are examples of Th being complexed as a carbonate (e.g.. Mono Lake waters, Anderson et al. 1982 Simpson et al. 1982) in which form it is soluble. 

Crystallization of magnesium hydroxide by a continuous mixed suspension mixed product removal crystallizer was conducted to make clear the characteristics of reactive crystallization kinetics of magnesium hydroxide, which was produced by the precipitation from magnesium chloride with calcium hydroxide. The following operating factors were investigated affecting the crystallization kinetics the initial concentration of feeds, residence time of reactants, feed ratio of reactants, and concentrations of hydroxide and chloride ions. 

An inductively coupled argon plasma eliminates many common interferences. The plasma is twice as hot as a conventional flame, and the residence time of analyte in the flame is about twice as long. Therefore, atomization is more complete and signal is enhanced. Formation of analyte oxides and hydroxides is negligible. The plasma is remarkably free of background radiation 15-35 mm above the load coil where sample emission is observed. 

Metallurgical waste powders containing iron and zinc oxides and hydroxides can be reduced to metals in RF thermal plasma in the presence of hydrogen. However, if the particles are agglomerated they cannot be reduced due to the short residence time in the hot plasma region. Thus, to achieve... 

One of the critical units in the production of paper is a reactor called a digester. In the kraft process this reactor is a two-phase tubular reactor in which the lignin that binds the wood chips together is broken down through a combination of chemical and thermal effects. The white liquor (aqueous solution of sodium hydroxide and hydrosulfide) and solid wood chips flow countercurrently in some zones and co-currently in others. The residence time of the pulp is about 10 h. 

Aldolization, Hydrogenation and Refining. In the second processing step, aldolization (1 0), n-butyraldehyde is added to aqueous sodium hydroxide which is recycled from the decanter. The caustic concentration in the cycle is maintained at 3% by adding makeup caustic. Residence time in the aldol reactor is typically about 60 seconds. Reaction temperature is controlled between 110°C and 120°C at pressures of 50 to 70 psi. The reactor effluent is fed to a decanter to separate the 2-ethylhexenal (EPA) from the caustic solution recycle. Water produced by dehydration of the aldol dilutes the cycle and is purged after decantation. 

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]. 

Titration curves may be misleading when the reagent and effluent reach an equilibrium pH on a time-scale comparable to or longer than the residence time in the treatment system. Unless the reactions involved are modeled, predictions based on the titration curve(s) may be quite inaccurate. The most common instance of this involves the use of calcium hydroxide reagents (slaked or hydrate lime), which is discussed below. If other slow reactions take place, considerable modeling effort may be needed, or very careful interpretation of results. 

The formation of metal hydroxide surface precipitates and subsequent residence time effects on natural sorbents can greatly affect metal release and hysteresis. It has generally been thought that the kinetics of formation of surface precipitates was slow. However, recent studies have shown that metal hydroxide precipitates can form on time scales of minutes. In Figure 3.7 one can see that mixed Ni-Al hydroxide precipitates formed on pyrophyllite within 15 minutes, and they grew in intensity as time increased. Similar results have been observed with other soil components and with soils (Scheidegger et ak, 1998 Roberts et ak, 1999 Sparks, 2002, 2005). 

The formation and subsequent aging of the metal hydroxide surface precipitate can have a significant effect on metal release. In Figure 3.8 one sees that as residence time (aging) increased from 1 hour to 2 years, Ni release from pyrophyllite, as a percentage of total Ni sorption, decreased from 23 to 0% when HNO3 (at pH 6.0) was employed as a dissolution agent for 14 days. This... 

Neutral hydroxide Si(OH)4 is predominant in the natural water, the content of anion Si(0H)30 is in a lesser degree. The continental river water discharge is responsible for 0.2 x 10 tons of soluble silicon species. The mass of Si compounds in the ocean is 4, 110 x 1tons, and the residence time of Si in the marine waters is 20,550 years. The transport of silicon from terrestrial to oceanic ecosystems is not counterbalanced by the reverse transport. In addition to the soluble species, the content of silicon in river particulate matter is about 120 /rg/L. This gives the elemental transport of 4.8 x 10 tons/yr. The total estimate of river water fluxes from the global land area to the ocean is 5.0 x 10 tons/yr. Aeolian migration of silicon is responsible for 0.47 X 10 tons per year. It means the annual global land losses (river and wind fluxes) are 5.47 x 10 tons (Dobrovolsky, 1994). 

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