Caustic concentration and flaking plant

Abstract

The invention concerns a caustic concentration and flaking plant comprising a caustic solution feeding line, a first stage evaporator (EV-2101) receiving membrane cell liquor with a 30 to 50 wt.- % caustic from the feeding line and operating at a reduced pressure of 80 to 120 mbar, a second stage evaporator (EV-2201) receiving enriched caustic solution with 45 to 60 wt.-% from the first evaporator (EV-2101) and operating at atmospheric pressure, a third stage evaporator (EV-2301) receiving further enriched caustic solution with 83 to 89 wt.-% caustic from the second stage evaporator (EV-2201) and operating at a reduced pressure of 120 to 210 mbar for KOH and 550 to 700 mbar for NaOH, the first stage evaporator (EV-2101) being heated by the vapours generated by the second stage evaporator (EV-2201), the second stage evaporator (EV-2201) and the third stage evaporator (EV2301) being heated by thermal oil provided by a heater (H- 6301), a closed loop with a circulation pump (P-6301) for circulating thermal oil between the heater (H-6301), and the second and third stage evaporators (EV-2201, EV-2301), a first condenser (C- 7301) receiving vapours generated by the first stage evaporator (EV-2101) and a second condenser (C-7302) receiving vapours generated by the third stage evaporator (EV-2301), a vapour condensate tank (T-7301) collecting vapour condensate from the first (C-7301) and second (C-7302) condenser and condensed heating vapour from the first stage evaporator (EV-2101), a caustic exit line feeding molten 92 to 99 % caustic to a flaking unit comprising at least one flaking machine (F-3111, F-3112).

Description

[0001] Caustic Concentration and Flaking Plant

[0002] The invention relates to a caustic concentration and flaking plant, especially for KOH and NaOH, comprising a caustic solution feeding line, preferably caustic soda or caustic potash, a first stage evaporator receiving membrane cell liquor with a 30 to 60 wt.-% caustic solution from the feeding line and operating at a reduced pressure, a second stage evaporator receiving enriched caustic solution with 45 to 60 wt.-% caustic from the first evaporator and operating at atmospheric pressure, a third stage evaporator receiving further enriched caustic solution with 80 to 89 wt.-% caustic from the second stage evaporator and operating at a reduced pressure, the first stage evaporator being heated by the vapour generated by the second stage evaporator, the second stage evaporator and the third stage evaporator being heated by thermal oil provided by a heater, a closed loop with a circulation pump for circulating thermal oil between the heater, and the second and third stage evaporators, a first condenser receiving vapour generated by the first stage evaporator and a second condenser receiving vapour generated by the third stage evaporator, a vapour condensate tank collecting vapour condensate from the first and second condenser and condensed heating vapour from the first stage evaporator, a caustic exit line feeding molten 92 to 99 wt.-% caustic to a flaking unit comprising at least one flaking machine.

[0003] Caustic soda solutions (NaOH solutions) or caustic potash solutions (KOH solutions) are mostly obtained by an electrolysis membrane process as 30% to 50% aqueous solution. It is a common practice to further concentrate such solutions in concentration plants to a practically anhydrous melt for further processing into flakes, pellets or prills.

[0004] Known evaporation plants for preconcentration of aqueous caustic solutions to be concentrated up to 60% in the case of KOH and 70% in the case of NaOH operate frequently with three falling film evaporators in a "one-pass through" mode with an integrated vapour separator. The first two of the three falling film evaporators normally work under vacuum, the third one at atmospheric pressure or even at overpressure up to 6 bar abs. Low concentrated caustic solution is fed to a first evaporator, which is heated by the vapours generated by a second evaporator. The enriched caustic solution from the first evaporator is fed to the second evaporator for further concentration, the second evaporator being heated by the vapours generated by the third evaporator. The product caustic solution from the second is fed to the third evaporator, which is heated by pressurised steam. Product caustic solution, steam and generated vapour are passed through heat exchangers to retain the heat within this preconcentration plant.

[0005] Pressurised steam used in such processes requires the shell side to be pressure safe up to at least 16 bar. In addition, steam of more than 200°C must be provided. If no steam sources are available in the plant, this requires a steam generator. High-pressure steam requires safety and sealing provisions, which are costly. In particular, where no inherent steam source is available in a plant, a heating system independent from steam is necessary. However, this would require a new design for many of the components and lines within the plant.

[0006] Smaller preconcentration plants of known technology operate with only 2 stages of falling film evaporators, where the second atmospheric pressure stage with higher concentrated caustic is heated with external energy to generate the vapour for the first vacuum stage heating.

[0007] The first stage evaporator of the invention may be substituted by a preconcentration plant of known technology splitting the first stage to 2 or 3 sub-stages applying falling pressures from the higher number stage to the lower number stage and using the vapour of the higher number stage to heat the lower number stage as in known evaporation plants for this concentration range. In other words, the first stage evaporator may be split to 2 to 3 sub-stages with rising operation pressure from sub-stage to sub-stage to use the vapor of the higher sub-stage for heating the lower substage.

[0008] The highest number sub-stage is heated with external energy from heating oil. In case of availability of overpressure vapour from the third stage of the invention, the latter maybe added to the heating vapor of the second sub-stage of a in 3 sub-stages-preconcentration plant.

[0009] Concentrating caustic solution from about 40% to almost 100% molten NaOH or KOH requires high temperatures - melting at about 360°C - and specific materials that withstand the corrosive power of these concentrated caustic solutions. The preferred material is nickel or its alloys or sintered silicon carbide. In addition, there is the problem of the chlorate contents of the membrane cell liquor leaving the electrolysis process. The process parameters in a caustic concentration plant must take account of all these specifics. It is an object of the present invention to provide a plant for the concentration of caustic solutions, especially of KOH and NaOH, which can be run at low or ambient pressure, avoids largely the use of steam and uses / recovers energy from the heating system and the product line, wherever possible. This objective is met with the plant of claim 1 .

[0010] The caustic concentration and flaking plant of the present invention makes use of three evaporators for concentrating 30 to 50% membrane cell liquor to 92 to 95% and preferably to 92 to 99% molten caustic, which is converted into flakes in a flaking unit. The first evaporator preferably is a falling film evaporator, which is heated by the vapours generated in the second evaporator.

[0011] The first evaporator stage may be substituted by a 2 or 3 stage-preconcentration plant of known technology, where the vapor from the higher stage is the heat source for the lower stage evaporator with lower pressure than the higher stage and with heating oil as heat source for the highest sub-stage.

[0012] The 2 or 3 sub-stages substituting the first evaporator are also preferably falling film evaporators.

[0013] The caustic solution fed into the first evaporator of the invention is concentrated to 45 to 60% by weight and preferably to 55 to 60% by weight. This solution exiting the first evaporator is fed into the second evaporator, which is a rising film evaporator, and concentrated therein to 83 to 86% by weight, preferably to 83 to 89% by weight. The second evaporator is heated by thermal oil provided with a temperature of 350 to 400°C, preferably 350 to 430°C.

[0014] In a third evaporator, also a rising film evaporator, the solution discharged from the second evaporator is further concentrated to 92 to 95% by weight, preferably to 92 to 99% by weight, which means that the product is molten caustic with a small residue of water. The third evaporator is also heated by thermal oil provided with a temperature of 350 to 430°C, preferably 350 to 400°C.

[0015] The second stage evaporator and the third stage evaporator being heated by 400°C-class thermal oil of typically Diphenyl and Diphenyl-oxide type or by 430°C-class thermal oil of typically polydimethylsiloxane type provided by a heater.

[0016] In case of 430°C-class thermal oil, the pressure range in the third stage evaporator may be extended to the overpressure region up to 1 ’729 mbar abs. for NaOH to be concentrated up to 98.5% respective to 1 ’262 mbar for NaOH to be concentrated up to 99% or 1 ’382 mbar for KOH to be concentrated up to 92%. The overpressure vapour may be used for heating the first stage of the plant enhancing the total energy efficiency. In case of KOH to be concentrated up to 95%, the maximum pressure range can be extended only to 702 mbar abs.

[0017] The thermal oil used for heating the second and third evaporator is circulated in a loop and heated centrally in a heater by means of fuel or electricity. Both, the second and third evaporator, are supplied with fresh thermal oil of 350 to 430°C, preferably 350 to 420°C, in parallel. The thermal oil leaving the second evaporator is used to preheat product caustic solution from the first evaporator in a heat exchanger. Spent thermal oil from the second and third evaporator are combined and pumped to the heater for reheating.

[0018] The heating of the first evaporator is provided by vapours generated in and leaving the second evaporator which have a temperature considerably above 100°C. Vapours generated in the first evaporator are condensed and collected in a tank before being discharged. The spent heating vapour of the first evaporator is also collected in this tank. The vapour generated in the third evaporator is condensed in a condenser and thereafter fed to the condensate tank to be discharged.

[0019] The evaporators are operated at specific pressure regimes. The first stage evaporator has a pressure of less than 280 mbar, preferably less than 150 mbar, more preferably 80 to 120 mbar, and in particular 90 to 105 mbar. More specific, the first stage evaporator has a pressure of 60 to 220 mbar for NaOH and 60 to 280 mbar for KOH, preferably 80 to 120 mbar abs.

[0020] The second stage evaporator runs at a pressure of about one bar, while the third stage evaporator is operated on the product side under a vacuum in the range of 80 to 710 mbar depending on the caustic substance and its concentration to be reached, preferably 160 to 200 mbar for KOH and 500 to 750 mbar for NaOH.

[0021] The third stage evaporator receiving further enriched KOH caustic solution with 80 to 89 wt.-%, preferably 83 to 86 wt.-% KOH caustic from the second stage evaporator and operating at a reduced pressure of 513 to 718 mbar abs. for NaOH to be concentrated up to 98.5% at a reduced pressure of 513 to 530 mbar abs. for NaOH to be concentrated up to 99%.

[0022] The third stage evaporator operates at a reduced pressure of 83 to 61 1 mbar abs. for KOH to be concentrated up to 92% and 83 to 279 mbar abs. for KOH to be concentrated up to 95%, The product leaving the first stage evaporator takes up heat from product leaving a second stage evaporator in a heat exchanger and is further heated in an additional heat exchanger by thermal oil, which leaves the second stage evaporator. If necessary, high temperature thermal oil from the loop can be fed in to bring up the temperature of the product stream up to around the boiling temperature at the inlet of the second stage evaporator.

[0023] The product leaves the second stage evaporator by gravity at the top and transfers part of its heat to the product leaving the first stage evaporator in a heat exchanger, as mentioned above. This prevents flash evaporation at the inlet of the third stage evaporator because of the reduced pressure in that evaporator.

[0024] Alternatively, the product of the second stage evaporator may be expanded to the pressure of the third stage evaporator in a flasher, where the temperature of the liquid drops to the evaporation temperature of the third stage enhancing its caustic concentration by sudden evaporation of some of its water content to maintain the enthalpy equilibrium. In this case, the liquid exiting the first stage is only heated up with the thermal oil.

[0025] After concentration, the product leaves the third stage evaporator at the top by gravity and is fed to a flaking unit where it is cooled down in at least one flaking machine to provide flakes to be packed in a conventional manner. Each flaking machine is cooled by a cooling medium, which itself is cooled by cooling water. If more than one flaking machine is involved, a distribution vessel takes up the product leaving the third stage evaporator for distribution to the individual flaking machines.

[0026] As is usual in high concentration caustic plants, at least part of the equipment is made from nickel, nickel alloy or silicon carbide, which withstands the corrosive power of the caustic solution, which normally contains certain amounts of chlorate. In that case it is preferred to feed sugar solution or other corrosion inhibitors into the brine to reduce the chlorate. Alternatively, sodium respective potassium bisulphite or sodium respective potassium boron hydride, hydrazine, formic acid or formate salts may be used as reduction agent for this purpose.

[0027] To prevent the caustic solution or caustic melt from solidifying in the lines between the evaporators and the flaking unit, it is preferred to correspondingly extend the thermal oil loop to heat up such lines, in particular the product feeding line to the third stage evaporator, the product exit lines from the second stage and third stage evaporators and the feeding lines to and within the flaking unit. The thermal oil loop is provided with hot thermal oil by means of a heater. The heating medium may be fuel or electricity to provide the necessary temperature. The thermal oil loop preferably involves also a storage tank with a filling pump and / or an expansion vessel.

[0028] The vapour generated in the third stage evaporator is fed to a condenser and thereafter collected in the central tank of the plant before being disposed of. The vapours generated in the second stage evaporator are used as a heating medium to heat the brine fed into the first stage evaporator, the spent heating medium being collected in the central tank. The vapour generated in the first stage evaporator is condensed in a condenser, condensate being fed to the central tank.

[0029] The flaking unit provided with caustic melt comprises preferably two flaking drums cooled in a separate cooling loop with a cooling medium, preferably water, the cooling loop itself being water cooled. The flaking drums are of conventional design, the caustic flakes produced therein are packed in bags for further handling.

[0030] Key of the invention is the use of a thermal oil heating loop involving two rising film evaporators. As caustic melts at a temperature of about 360°C to 415°C, the thermal oil used in the heating loop is required to be stable at such temperatures. Aromatic or mineral oils or derivatives thereof and silicone oils are able to provide such temperatures. In order to avoid oxidation, it is expedient to exclude oxygen and in the case of silicone oil also humidity to avoid hydrolyzation, operating the loop under an inert atmosphere, e.g. under nitrogen. A suitable loop temperature range is 350 to 400 °C, preferably 350 to 430°C, and in particular 360 to 400°C for 400°C-class thermal oil and 380°C and 430°C for 430°C-class thermal oil. Thermal oil returning from the evaporators generally has a temperature of about 275 to 320°C.

[0031] An important feature is the pressure drop in the third stage evaporator, as compared to the second stage evaporator, which allows a further concentration of the brine at the heating loop temperature.

[0032] The invention is further described in the attached drawing, which is not intended to limit the scope. In the drawing, the terms PC, FC, LC and TC stand for pressure control, flow control, level control and temperature control, respectively. The term A / B indicates the presence of two redundant devices.

[0033] The incoming caustic solution is first supplied with the required amount of sugar solution or one of the other above mentioned reduction agents via tank T-8301 A / B and pump P-8301 A / B to eliminate the chlorate content of the brine. Chlorate in combination with high concentrated and hot caustic solution corrodes nickel, a metal frequently used in the plant of the invention.

[0034] The 30 to 50% membrane cell liquor is fed to the first stage evaporator, a falling film evaporator (EV-2101) operating on the product side under a vacuum in the range of 80 to 120 mbar abs and preferably of 90 to 105 mbar abs. During a single pass through the evaporator, the caustic solution is evaporated to about 45 to 60% or 55 to 60% alternatively. The generated vapours are fed to a surface condenser (C-7301), where they are indirectly condensed by cooling water. Alternatively, a mixing condenser can be used as well, where the vapours are condensed directly into the cooling water and lead as mixed water stream to the battery limits.

[0035] Inerts are sucked off by either a dry running vacuum pump, a closed loop water-ring vacuum pump (P-7302), or a steam ejector vacuum system. The closed loop vacuum pump is often chosen to minimize the waste-water amount. The circulating water will be cooled in a heat exchanger, for example a plate heat exchanger, by either chilled water or cooling water.

[0036] The caustic solution is discharged from the bottom part of the first stage evaporator (EV-2101) by means of a pump (P-2101) and is passed through two heat exchangers (HE-1511 and HE-1521). On passing these heat exchangers, the caustic solution is warmed up to a defined temperature using the heat from caustic solution and thermal oil out of the second stage evaporator (EV-2201). The temperature after the preheaters is set to around the boiling temperature of the caustic solution at the conditions of entering the second stage evaporator.

[0037] To reduce the nickel corrosion of the equipment and thus the nickel pick-up in the caustic flakes, a corrosion inhibitor solution (sugar solution or the above-mentioned alternatives) is dosed to the caustic stream, as indicated above.

[0038] During a single pass through the second stage evaporator, a rising film evaporator (EV-2201) operating on the product side at atmospheric pressure, the caustic solution is evaporated to about 83 to 89% and preferably to about 83 to 86%. The vapours hereby generated are used to heat the first stage evaporator (EV-2201).

[0039] The second stage evaporator (EV-2201) is heated by thermal oil. The thermal oil leaving the second stage evaporator is then used in heat exchanger (HE-1521) to partially preheat the caustic solution between the first and second stage. The caustic solution is discharged from the upper part of the evaporator by gravity and is passed through heat exchanger (HE-151 1). By passing this heat exchanger, the caustic solution is cooled down using the heat of the caustic solution out of the first stage. The cooling of the caustic solution after the second stage is to prevent flash evaporation at the inlet of the third stage evaporator (EV- 2301), which is run at reduced pressure.

[0040] The cooled caustic solution is led to the bottom part of the third stage evaporator, a rising film evaporator (EV-2301) operating on the product side under a vacuum in the range of 140 to 210 mbar abs and preferably in the range of 140 to 160 mbar abs. During a single pass through the evaporator, the caustic solution is concentrated to 98 to 99% NaOH or 92 to 95% KOH. The hereby generated vapours are led to a surface condenser (C-7302), where they are indirectly condensed by cooling water.

[0041] The third stage evaporator (EV-2301) is heated by thermal oil. The thermal oil leaving the third stage evaporator (EV-2301) is returned to be reheated.

[0042] The caustic melt is discharged from the top of the third stage evaporator (EV-2301) by gravity and flows to a seal pot or distribution device (V-2312). From there the caustic melt flows also by gravity to conventional flaking machines (F-3111 , F-3121), where it is processed to cold flakes by cooling water.

[0043] In case of using surface condensers, all vapour condensate is collected in the vapour condensate tank (T-7301) and is pumped to battery limits by means of the vapour condensate pump (P-7301). If a mixing condenser is used, the related vapour condensate is then sent to the battery limits together with the cooling water of this mixing condenser. In this case, the condensate of heating the first stage evaporator (EV-2101) will be sent to battery limits as vapour condensate.

[0044] The thermal oil from the third stage evaporator is mixed with the thermal oil from the second stage evaporator (after passing HE-1521) and circulated by a pump (P-6301) through a forced flow heater (H-6301), where it is warmed up to the required temperature of about 350 to 430°C, preferably between 350 and 400°C.

[0045] The caustic melt lines and the related equipment are equipped with a heat tracing system with the same thermal oil. By passing the evaporators, the heat exchanger and the tracing system, the thermal oil is cooled down to about 275 to 320°C. The thermal oil circuit is designed as a closed loop, equipped with an expansion vessel (V-6302) with nitrogen blanketing. The system comprises also a storage tank (T-6301) with a filling pump (P-6302). The forced flow heater (H-6301) can either be of an electric type or equipped with a burner system to burn natural gas, hydrogen or fuel oil. In a preferred embodiment, both the expansion vessel and the storage tank of the thermal oil loop are heated with steam to keep and provide thermal oil at an elevated temperature. Additionally, as shown, the thermal oil loop may be extended to the flaking unit to keep the caustic melt leaving the third stage evaporator liquid. This part of the thermal oil loop extends to the distribution device V-2312 and to the flaking units heating the transport lines of the melt. The thermal oil flow of these extensions is represented in the drawing by broken lines.

Claims

Claims1 . A caustic concentration and flaking plant comprising- a caustic solution feeding line,- a first stage evaporator (EV-2101) receiving membrane cell liquor with a 30 to 50 wt.-% caustic from the feeding line and operating at a reduced pressure of 80 to 120 mbar,- a second stage evaporator (EV-2201) receiving enriched caustic solution with 45 to 60 wt.-% from the first evaporator (EV-2101) and operating at atmospheric pressure,- a third stage evaporator (EV-2301) receiving further enriched caustic solution with 83 to 89 wt.-% caustic from the second stage evaporator (EV-2201) and operating at a reduced pressure of 120 to 210 mbar for KOH and 550 to 700 mbar for NaOH,- the first stage evaporator (EV-2101) being heated by the vapours generated by the second stage evaporator (EV-2201), the second stage evaporator (EV-2201) and the third stage evaporator (EV2301) being heated by thermal oil provided by a heater (H-6301),- a closed loop with a circulation pump (P-6301) for circulating thermal oil between the heater (H-6301), and the second and third stage evaporators (EV-2201 , EV-2301),- a first condenser (C-7301) receiving vapours generated by the first stage evaporator (EV-2101) and a second condenser (C-7302) receiving vapours generated by the third stage evaporator (EV-2301),- a vapour condensate tank (T-7301) collecting vapour condensate from the first (C-7301) and second (C-7302) condenser and condensed heating vapour from the first stage evaporator (EV-2101),- a caustic exit line feeding molten 92 to 99 % caustic to a flaking unit comprising at least one flaking machine (F-3111 , F-3112).

2. A caustic concentration and flaking plant according to claim 1 , characterised in that the first stage evaporator is split to 2 to 3 sub-stages with rising operation pressure from substage to sub-stage to use the vapor of the higher sub-stage for heating the lower sub-stage.

3. A caustic concentration and flaking plant according to claims 1 or 2, characterised in that the first stage evaporator (EV-2101) and in case of substitution of the first stage evaporator (EV-2101) by a conventional preconcentration plant of 2 or 3 sub-stages also these sub-stages are falling film evaporators.

4. A caustic concentration and flaking plant according to claim 1 , characterised in that the second stage (EV-2201) and third stage evaporator (EV-2301) are rising film evaporators.

5. A caustic concentration and flaking plant according to claim 1 , characterised in that the thermal oil heater (H-6301) is heated by fuel or electricity providing thermal oil with a temperature of 350 to 430°C.

6. A caustic concentration and flaking plant according to claim 1 , characterised in that product caustic solution entering the second stage evaporator (EV-2201) is passed through a heat exchanger (HE1521) taking up heat from thermal heating oil.

7. A caustic concentration and flaking plant according to claim 6, characterised in that product caustic solution leaving the first stage evaporator (EV-2201) is passed through a preheater (HE-1511) taking up heat from product caustic solution leaving the second stage evaporator (EV-2201).

8. A caustic concentration and flaking plant according to claim 6, characterised in that product caustic solution leaving the second stage evaporator (EV-2201) is expanded to the pressure of the third stage evaporator (EV-2301) in a flash vessel with vapour connection to the heating chamber of the first stage evaporator (EV-2101), transferring the excess heat to vapour and concentrating the caustic solution.

9. A caustic concentration and flaking plant according to claim 4, characterised in that product caustic is discharged from the second (EV-2201) and third stage evaporator (EV- 2301) by gravity.

10. A caustic concentration and flaking plant according to claim 1 , characterised in that the thermal oil loop comprises an expansion vessel (V-6302) and a storage tank (T-6301).

11. A caustic concentration and flaking plant according to claim 1 , characterised in that the condensers (C-7301 ,7302) comprise each a water-cooling system and an inert gas exit.

12. A caustic concentration and flaking plant according to claim 1 , characterised in that it comprises a sugar or bisulphite or boron hydride or hydrazine or formate dosing and feeding system (T-8301) feeding aqueous solution of these nickel corrosion preventing agents into the caustic by means of a metering pump (P-8301).

13. A caustic concentration and flaking plant according to claim 1 , characterised in that the flaking unit comprises a distribution vessel (V-2312) distributing the product caustic melt from the third stage evaporator (EV-2301) to a first (F-3111) and a second (F-3121) flaking machine.

14. A caustic concentration and flaking plant according to claim 13, characterised in that the flaking unit comprises a water-cooled cooling loop for cooling the flaking machines (F- 3111 , F-3121), the cooling loop comprising a tank (T-3101) for cooling medium and a circulation pump (P-3101).

15. A caustic concentration and flaking plant according to claim 13 or 14, characterised in that the flaking machines (F-311 1 , F-3121) discharge caustic flakes to packing units (W-4111 , W-4121).

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