Spent acid phase

Figure 52 presents a phase diagram for the system alkylbenzene sulphonic acid - sulphuric acid -water. Before dilution, the sulphonation mixture is represented by a point (P) close to the bottom of the diagram, near or on the lower perimeter of the area of immiscibilily. In diluting with water the composition of the sulphonation mixture "moves up" into the area of immiscibility see dotted line P-L. After dilution into the area of immiscibility, the mixture of alkylbenzene sulphonic acid, sulphuric acid and water will separate into two phases a spent acid phase with very little sulphonic acid (about 0.3%) and a sulphonic acid phase. The composition of the latter is that of the intersection of the appropriate tie line (say the tie line indicated as 80% spent acid composition) with the left hand perimeter of the area of immiscibility (point R). The composition of the spent acid phase is given by the intersection of the same tie-line with the zero percent sulphonic acid line (see point S), since the small amount of sulphonic acid in the spent acid phase is neglected for the purpose of this phase diagram. From S, Q and R, all on the 80% spent acid tie-line, the composition of the sulphonic acid phase and the amounts of sulphonic and spent acid phases can be calculated. 

The sulphonic acid phase leaves the separator at the top, the spent acid phase at the bottom. 

Good separation between the sulphonic acid and the spent acid is important, in order to minimise the loss of sulphonic acid in the spent acid phase. Also, the higher the sulphuric acid content of the sulphonic acid phase, the less acceptable the latter is for many of its applications. Furthermore, the higher the sulphonic acid content of the spent acid, the less acceptable this is to its prospective buyers, because of foam formation in fruther processing. 

It is obvious that the nitrated product must be separated from the acid in equil with it (spent acid). If the product and the spent acid form two immiscible liq phases, eg, NG, EGDN, or molten TNT, separation is effected by gravity or centrifuging. If the product and spent acid form a solid and a liq phase, eg PA, NC or PETN, separation is effected by centrifuging (PA NC) or filtration (PETN). If the nitration is carried in the vapor phase (NM), separation is effected by distillation... 

Next, the dilution product is allowed to separate into two phases and the lower, aqueous acid layer is removed as spent acid. Much of the color produced during the sulfonation is contained in the spent acid. The upper layer in the separator, which contains around 90% alkylbenzenesulfonic acid (the remainder being primarily dissolved sulfuric acid), is also removed from the separator and neutralized with sodium hydroxide solution or with sodium carbonate solution to yield the active paste. When sodium hydroxide is used a considerable amount of heat must be removed. This is done in the third loop reactor of this process, the neutralization heat exchanger (also known as the neutralizer). If sodium carbonate solution is used, the neutralization is much less exothermic and no heat exchanger is needed at this stage. 

Traditionally, aromatic nitrations have been performed with mixtures of concentrated nitric and sulphuric acids leading to considerable effluent problems associated with the spent acid. A heterogeneous alternative for. sulphuric acid has been reported (Bertea et al., 1995), namely dealuminated Mordenite, which is sufficiently robust to be able to catalyse the vapour-phase nitration of benzene with 65% aqueous nitric acid. 

Because of its large reactor volume, the auto-refrigerated process can operate at very low alkene space velocities of about 0.1 h-1 LHSV (WHSV ca. 0.03 h 1). This design helps in increasing the octane number of the product and lowering acid consumption. The reaction temperature is maintained at about 278 K to minimize side reactions. Spent acid is withdrawn as 90-92 wt% acid. The isobutane concentration in the hydrocarbon phase is kept between 50 and 70 vol%. 

To the separator, an overflow for spent acid (18a) is fitted, by means of which the height of the boundary layer between the two liquid phases can be regulated. Spent acid flows down to a pressure-egg (19), from which it is discharged by means of compressed air to a tank (18) (only a part of the pipe is shown) and then to setling tanks as in the combined process, (p. 95). 

Success of a continuous operation will depend on the possibility of easy phase separation (such as of NG from spent acid), but when such separation is difficult and requiring a long settling out period (such as RDX in presence of formaldehyde nitric acid), the continuous method become difficult and dangerous... 

Reprocessing is based on liquid-liquid extraction for the recovery of uranium and plutonium from used nuclear fuel (PUREX process). The spent fuel is first dissolved in nitric acid. After the dissolution step and the removal of fine insoluble solids, an organic solvent composed of 30% TriButyl Phosphate (TBP) in TetraPropylene Hydrogenated (TPH) or Isopar L is used to recover both uranium and plutonium the great majority of fission products remain in the aqueous nitric acid phase. Once separated from the fission products, back-extraction combined with a reduction of Pu(I V) to Pu(III) allows plutonium to be separated from uranium these two compounds can be recycled.2... 

Use of sensors to measure gas phase NO2 concentration, electrical conductivity of the reaction mass, and gas phase temperatures at several critical points in semi-continuous nitration reactors permits safe operation of nitration processes [10]. The use of non-aqueous titration analysis in the control of nitration processes in explosives manufacture is discussed [11]. Counter-intuitively, safety of spent acids from nitrate ester production is decreased by lowered nitric acid content. This is because the runaway reaction is oxidation of alcohols, kinetically easier than that of the dissolved nitrate esters from which the alcohols are reformed by hydrolysis [16]. 

When nitration has been completed the reaction mixture is transferred by compressed air to a cast iron separator with a capacity of 15 m3, where it is allowed to remain at rest for 6 hr to separate into two liquid phases. Then the spent acid is conveyed to a special cast iron tank, where it stays for a few days to recover the temainder of the MNT, which is collected from the surface and added to the toluene to be mononitrated. The spent acid, having a composition ... 

The output consists of 0.1173 kg/h of the organic phase and 0.3014 kg/h of spent acid. These two streams can be considered as follows ... 

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