Caustic solution evaporation plant

Abstract

The invention relates to a caustic solution evaporation plant for concentrating low concentrated caustic feed comprising a feeding line for low concentrated caustic solution, an evaporator receiving the low concentrated caustic solution from the feeding line, the evaporator being run at a pressure of less than 500 mbar and a temperature of less than 130°C on the tube side, a heat exchanger arranged in the feeding line for low concentrated caustic solution receiving product caustic solution from the evaporator for heat transfer, a discharge line for discharging product caustic solution comprising a heat exchanger for cooling the product caustic solution, a feeding line to feed steam to the evaporator providing a pressure of at least 1.9 bar and a temperature of at least 130° C on the shell side, a scrubber receiving vapours generated in the evaporator to be washed with vapour condensate from the evaporator, at least one blower receiving washed vapours from the scrubber for compression, the at least one or each blower being fed with vapour condensate from the evaporator for vapour steam saturation, a discharge line for excess vapour condensate, a line for feeding compressed vapour steam from the at least one blower to the shell side of the evaporator, a cooling circuit for cooling product caustic solution comprising a tank for cooling fluid and a heat exchanger for cooling water, and a cooling water line providing cooling water to the cooling circuit.

Description

[0001] BTRS0095 22.08.2025 AK / pd

[0002] Caustic solution evaporation plant

[0003] The invention relates to a caustic solution evaporation plant for concentrating low concentrated caustic feed solution to a level of 50 or more percent. The plant is specifically intended for concentrating caustic soda solution.

[0004] Caustic soda solutions (NaOH solutions) are mostly obtained by electolysis in a membrane process as an 30% to 33% aqueous solution. It is known to concentrate such solutions in evaporation plants to a NaOH concentration of 50% by weight. The same holds for caustic potash solutions / KOH solutions.

[0005] Bringing the initial caustic solution to a temperature that is suited for further concentration requires high energy input. It is therefore desirable to optimise the energy flow within such a evaporation plant and to use the energy within the system internally for heating the feed.

[0006] Accordingly, the caustic solution evaporation plant of the invention comprises a feeding line for low concentrated caustic solution, an evaporator receiving the low concentrated caustic solution from the feeding line, the evaporator being run at a pressure of less than 500 mbar and a temperature of less than 125°C on the tube side, a heat exchanger arranged in the feeding line for low concentrated caustic solution receiving product caustic solution from the evaporator for heat transfer, a discharge line for discharging product caustic solution comprising a heat exchanger for cooling the product caustic solution, a feeding line to feed steam to the evaporator providing a pressure of at least 1 .9 bar and a temperature of at least 130°C on the shell side, a scrubber receiving vapours generated in the evaporator to be washed with vapour condensate from the evaporator, at least one blower receiving washed vapours from the scrubber for compression, the at least one or each blower being fed with vapour condensate from the evaporator for vapour steam saturation, a discharge line for excess vapour condensate, a line for feeding compressed vapour steam from the at least one blower to the shell side of the evaporator, a cooling circuit for cooling product caustic solution comprising a tank for cooling fluid and a heat exchanger for cooling water, and a cooling water line providing cooling water to the cooling circuit.

[0007] One key feature of the present plant is the mechanical vapour recompression for providing saturated steam for heating the evaporator.

[0008] The mechanical vapour recompression serves to increase the pressure and the dew point of the vapours coming from the evaporator to use their enthalpy of condensation for heating the same evaporator stage. Several turbo fans mounted in series allow the evaporation of 32% NaOH solution to 50% NaOH solution.

[0009] The turbo fan is a highly evolved, bespoke unit. It is normally supplied as a package which can be tailored to meet the customer’s specification in the most effective, efficient way possible.

[0010] The turbo fan is developed to meet the rigorous demands of the Mechanical Vapour Compression process. This technique, developed in 1980, recycles the vapour created during the evaporation process and re-uses the energy it contains. Mechanical vapour recompression (MVR) found a role in a broad range of processes where a product goes through an evaporation stage. By using MVR evaporators to provide distillate, high levels of water recycling and energy efficiency can be achieved. Designed to work in extreme conditions, the proposed turbo fans have shown excellent safety, reliability and performance records.

[0011] A high level of heat recovery will be applied to minimize the steam requirements pre-heating the feed caustic using the hot product caustic as well as the vapour condensate.

[0012] In the present plant a feeding line feeds pre-heated low concentrated caustic solution to the evaporator, preferably a falling film evaporator. The evaporator is run at a pressure of less than 500 mbar and a temperature about 120°C on the tube side. Normally, the 30 to 33% and NaOH caustic feed solution is concentrated to a level of about 50%. Pre-heating is achieved by means of a heat exchanger making the heat of the product solution available for the feed. In the heat exchanger the feed solution is preheated to a temperature of about 90 to 100°C. Additionally, the caustic feed solution can be preheated by vapour condensate in a plate heat exchanger prior to the heated by the product solution. Before leaving the plant the product solution is further cooled down in a heat exchanger to a cooling circuit lowering the temperature to 40 to 50°C.

[0013] The plant further comprises a feedline to feed steam to the evaporator providing a pressure of at least 1 .9 bar and a temperature of at least 130°C on the shell side. Inside the evaporator tubes caustic solution is evaporated to the final concentration of about 50%. While the tube side of the evaporator is operated under a moderate vacuum, the shell side has a moderate pressure of at least 2.0 bar.

[0014] In a scrubber the vapours generated in the evaporator are washed with vapour condensate from the evaporator.

[0015] The plant comprises at least one blower for pressurizing the vapours exiting the scrubber. At the same time, the vapours in the at least one blower are saturated by injection of condensate from the evaporator. In particular in small and medium sized plants, there are used more than one blower, in particular 3 to 6 are arranged in a row. Preferably, the blowers are turbo fans. The compressed vapour steam from the least one blower is fed to the shell side of the evaporator.

[0016] Vapour condensates from the evaporator and from the scrubber are discharged after having transferred their heat to the process.

[0017] The plant additionally comprises cooling circuit for cooling product solution. The cooling circuit comprises a tank for cooling fluid and a heat exchanger, which is connected to a cooling water line. In addition, the cooling circuit may comprise a heat exchanger to take up heat from the vapour condensate to be discharged from the plant. The cooling liquid is collected in a tank and circled within the circuit by means of a pump.

[0018] The evaporation plant of the invention may comprise a tank for condensed vapours exiting the shell side of the evaporator. The condensed vapours are used to saturate the vapours compressed in the blowers.

[0019] The plant of the present invention is designed primarily for the concentration of sodium hydroxide solution. However, with few modifications regarding the working temperature and pressure, it may also be used for the concentration of potassium hydroxide.

[0020] The invention is illustrated by the attached flow diagram. The following process description refers to a 1 stage NaOH evaporation plant, as per Basic Diagram D-317 207.

[0021] The 30 to 33% NaOH caustic feed solution is pre-heated by a plate heat exchanger (HE-1101) using the hot product caustic to about 90 - 110°C. To avoid the requirement of continuously feed of make up steam at low caustic feed temperature (below 70°C), the caustic can be preheated by vapour condensate in a plate heat exchanger (HE-1102) prior to entering the heat exchanger HE-1101.

[0022] The preheated caustic enters the falling film evaporator (EV-1101) from the top. Inside the evaporator tubes the caustic is evaporated to the final concentration of 50%. The tube side of the evaporator is operated under vacuum of 400 to 425 mbar a, while the shell side is operated under moderate overpressure of 2.0 to 2.5 bar a.

[0023] To ensure a proper turndown operation and long lifetime of the equipment, the evaporator is equipped with a recirculation line. At partial load the automated recirculation of product caustic maintains the optimal film thickness.

[0024] The product caustic is cooled by the feed caustic in HE-1101 . To reach the final product temperature of 40 to 50°C it is further cooled by circuit cooling water in plate heat exchanger HE- 1103.

[0025] The generated vapours are led to a vapour scrubber (SC-1101), where they are washed and saturated by the injection of vapour condensate. The condensate spray is not only used to wash and saturate the vapours, but also partially flashes, increasing the quantity of available vapours for mechanical vapour recompression.

[0026] The vapours are compressed by a first blower (V-1101). Then they are saturated again by the injection of condensate before being further compressed by the following blowers (V-1102, V- 1103, V-1104 and V-1105). After each blower an injection of condensate saturates the vapour stream. After the last saturation step, the vapours are used for heating the shell side in the evaporator EV-1101 .

[0027] To achieve the desired flexibility of the plant capacity a vapour recirculation on the MVR section is provided. The vapour condensate from the evaporator EV-1101 is used to desuperheat and wash the vapours in SC-1101 before being cooled in HE-1104 and HE-1102 and finally sent to battery limit. To protect the equipment from damages related to low-quality cooling water, an internal cooling circuit is used. The cooling water enters HE-8100 where it is used to cool the circuit cooling water to the desired process temperature. The circuit cooling water is used to cool the product (HE-1103), the condensate (HE-1104) and for the lubrication oil system of all blowers. Then the circuit cooling water is collected in the tank T-8100 and pumped via P-8100 to the heat exchanger HE-8100 where it is cooled back to the desired temperature.

Claims

Claims1 . A caustic solution evaporation plant comprising- a feeding line for low concentrated caustic solution,- an evaporator receiving the low concentrated caustic solution from the feeding line, the evaporator being run at a pressure of less than 500 mbar and a temperature of less than 130°C on the tube side,- a heat exchanger arranged in the feeding line for low concentrated caustic solution receiving product caustic solution from the evaporator for heat transfer,- a discharge line for discharging product caustic solution comprising a heat exchanger for cooling the product caustic solution,- a feeding line to feed steam to the evaporator providing a pressure of at least 1 .9 bar and a temperature of at least 130° C on the shell side,- a scrubber receiving vapours generated in the evaporator to be washed with vapour condensate from the evaporator,- at least one blower receiving washed vapours from the scrubber for compression, the at least one or each blower being fed with vapour condensate from the evaporator for vapour steam saturation,- a discharge line for excess vapour condensate,- a line for feeding compressed vapour steam from the at least one blower to the shell side of the evaporator,- a cooling circuit for cooling product caustic solution comprising a tank for cooling fluid and a heat exchanger for cooling water, and- a cooling water line providing cooling water to the cooling circuit.

2. The evaporation plant of claim 1 , wherein the evaporator is a falling film evaporator.

3. The evaporation plant of claim 1 or 2 comprising 3 to 6 blowers arranged in line.

4. The evaporation plant of one of the preceding claims comprising a tank for condensed vapours exiting the shell side of the evaporator.

5. The evaporation plant of one of the preceding claims comprising a heat exchanger arranged in the vapour condensate discharge line for cooling always cooling fluid from the cooling circuit.

6. The evaporation plant of one of the preceding claims, wherein the evaporator is pressurised to 2.0 to 2.5 bar on the shell side.

7. The evaporation plant of one of the preceding claims, wherein the evaporator has a temperature on the tube side of 100 to 130°C.

8. The evaporation plant of one of the preceding claims, wherein the caustic solution is caustic soda solution.

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