Evaporators with inverted circulation loops

Abstract

The evaporator operated under atmospheric pressure and under 620 mm Hg vacuum processing saturated solutions of sodium chloride (direct solubility: solubility increases at highertemperatur e) and sodium sulfate (inverse solubility: solubility decreases at higher temperature). These solutions were evaporated with crystal circulation and also with separation of crystals (clarification) by inserting a removable baffle into the separator to remove large crystals. Solids concentration in the circulated slurry was 20257c and 5%, respectively, for the two modes of operation. Inversion and stabilization of flowwas carriedout by blowing air (steam) into the riser pipe through perforated tubes or by a circulating pump. Air flow during blowing was 156 m3/m 2 (based on pipe active area) or 1-3 wt. % of the secondary steam. This air flow resulted in stable circulation with 1.5-1.6 m/sec maximum velocity [4]. Forced circulation experiments were carried out only under atmospheric pressure with 1.5, 2.7, and 3.4 m/sec velocities. Capacity (water evaporated), circulation rate, superheat in the tubes, and the dynamics of parameter variation with time (run length) were investigated. Washing of the evaporator, when it scaled up, was done "on-stream" by reducing the concentration of the circulating solution: the capacity of the evaporator was restored after 20-30 rain. Experimental resuits are shown in Figs. 2-4. Figure 2 shows that, when air blowing is used, the time for scaling to begin (run duration) depends on the effective temperature difference, the composition of the solution, and the type of solution (direct or inverse solubility). Crystal circulation was benificial only for sodium suIfate solutions at At = 15 ~ and was detrimental for evaporating sodium chloride solutions (crystals were precipitated on the upper tubesheet). Scale formation on the heating surface was different for the two solutions: growths on the tubesheet when evaporating sodium chloride with clean tube surfaces (Fig. 4a, b) and a continuous scale formed during evaporation of sodium sulfate (Fig. 4 c). During evaporation of sodium sulfate solutionwith At = 15~ there was no scaling while the tubesheet was subjected to extensive scaling when sodium chloride was evaporated under sinailar conditions (Fig. 4b). Superheat was 3-3.5 ~ and 1.5-2 ~ respectively, for sodium chloride and sodium sulfate solutions. Similar resultswere obtained in evaporating sodium chloride solutions with air blowing, while rapid (after 2-3 h) scaling was noted with sodium sulfate solutions. Forced circulation experiments demonstrated (Fig. 3) that crystal circulation (operation with clarification) extends the operating cycle only for sodium sulfate solutions, particularly at At = 15 ~ Increasing the circulation velocity to > 3.0 m/sec (not shown in Fig. 3) did not affect the operation of the evaporator. This is evidently due to the incomplete removal of supersaturation due to reduced solution residence time in the separator. Circulation rates > 1.5-2 m/sec for sodium chloride solution do not affect the operation of the evaporator significantly. Evaporation of sodium sulfate without clarification of the solution at At =10 ~