Evaporation equipment for caustic alkali solution

Abstract

The caustic solution evaporation apparatus according to the present application comprises: a feed line for a low-concentration solution; an evaporator receiving the low concentration solution of the feed line, operating on the tube side at less than 500 mbar and less than 130 DEG C; a heat exchanger in the feed line of the low-concentration solution for receiving the product solution from the evaporator for heat transfer; a product solution discharge line including a heat exchanger for cooling the product solution; a line feeding steam to the evaporator providing at least 1.9 bar and at least 130 DEG C on the shell side; the scrubber is used for receiving steam of the evaporator and washing steam condensate of the evaporator; a blower receiving scrubbing vapor from the scrubber for compression, which is supplied to vapor condensate of the evaporator for vapor saturation; an excess vapor condensate discharge line; a line that supplies compressed vapor from the blower to the evaporator shell side; a product solution cooling circuit comprising a cooling fluid tank and a heat exchanger for cooling water; and a line that provides cooling water to the circuit.

Description

Technical Field

[0001] The present application relates to a caustic solution evaporation device for concentrating a low-concentration caustic feed solution to a level of 50% or higher, and is particularly used for concentrating caustic soda solution. Background Art

[0002] Caustic soda solution (NaOH solution) is primarily obtained by electrolysis in membrane processes as a 30% to 33% aqueous solution. It is known to concentrate this solution in evaporation plants to a NaOH concentration of 50% by weight. The same applies to caustic potash solution / KOH solution.

[0003] Bringing the initial caustic solution to a temperature suitable for further concentration requires a high energy input. It is therefore desirable to optimize the energy flow within such evaporation equipment and to use the energy within the system internally to heat the feed. Utility Model Content

[0004] Therefore, a caustic solution evaporation device of the present application comprises: a feed line for a low-concentration caustic solution; an evaporator receiving the low-concentration caustic solution from the feed line, which operates 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 feed line for the low-concentration caustic solution, receiving the product caustic solution from the evaporator for heat transfer; a discharge line for discharging the product caustic solution, which comprises a heat exchanger for cooling the product caustic solution; a feed line for feeding steam to the evaporator, which provides a pressure of at least 1.9 bar on the shell side. a pressure and a temperature of at least 130°C; a scrubber receiving the steam generated in the evaporator for scrubbing with steam condensate from the evaporator; at least one blower receiving the scrubbed steam from the scrubber for compression, which is fed with steam condensate from the evaporator for steam saturation; a discharge line for excess steam condensate; a line for feeding compressed steam from the at least one blower to the shell side of the evaporator; a cooling circuit for cooling the product caustic solution, which includes a tank for cooling fluid and a heat exchanger for cooling water; and a cooling water line providing cooling water to the cooling circuit.

[0005] A key feature of this equipment is mechanical vapor recompression to provide saturated steam for heating the evaporator.

[0006] Mechanical vapor recompression is used to increase the pressure and dew point of the vapors coming from the evaporators in order to utilize their condensation enthalpy to heat the same evaporator stages. Several turbofans installed in series allow evaporation of 32% NaOH solution into 50% NaOH solution.

[0007] Turbofans are highly developed, custom-made units that are typically supplied in kit form, allowing them to be tailored to meet customer requirements in the most effective and efficient way possible.

[0008] Turbofans were developed to meet the stringent requirements of mechanical vapor compression processes. Developed in the 1980s, this technology recovers the vapor produced during the evaporation process and reuses the energy it contains. Mechanical vapor recompression (MVR) plays a role in various processes where products pass through evaporator stages. By using MVR evaporators to provide distillate, high levels of water recycling and energy efficiency can be achieved. Designed to operate under extreme conditions, the proposed turbofan has demonstrated an excellent safety, reliability, and performance record.

[0009] A high level of heat recovery will be applied to use the hot product caustic as well as steam condensate to minimize the steam requirement for preheating the feed caustic.

[0010] In this apparatus, a feed line feeds a preheated, low-concentration caustic solution to an evaporator, preferably a falling-film evaporator. The evaporator operates at a pressure of less than 500 mbar and a tube-side temperature of approximately 120°C. Typically, a 30-33% NaOH caustic feed solution is concentrated to a level of approximately 50%. Preheating is achieved via a heat exchanger that makes the heat of the product solution available to the feed. In the heat exchanger, the feed solution is preheated to a temperature of approximately 90-100°C. Alternatively, the caustic feed solution can be preheated by steam condensate in a plate heat exchanger before being heated by the product solution.

[0011] Before leaving the plant, the product solution is further cooled in a heat exchanger to a cooling loop, which reduces the temperature to 40-50°C.

[0012] The apparatus also includes a supply line for supplying steam to the evaporator, which provides a pressure of at least 1.9 bar and a temperature of at least 130° C. on the shell side. Inside the evaporator tubes, the caustic solution evaporates to a final concentration of approximately 50%. When the tube side of the evaporator is operated at a moderate vacuum, the shell side has a moderate pressure of at least 2.0 bar.

[0013] In the scrubber, the steam generated in the evaporator is scrubbed with steam condensate from the evaporator.

[0014] The system includes at least one blower for pressurizing the steam exiting the scrubber. Simultaneously, the steam in the at least one blower is saturated by injecting condensate from the evaporator. In particular, in small and medium-sized systems, more than one blower is used, particularly three to six blowers arranged in a row. Preferably, the blower is a turbofan. The compressed steam flow from the at least one blower is fed to the shell side of the evaporator.

[0015] The steam condensates from the evaporator and from the scrubber are discharged after transferring their heat to the process.

[0016] The apparatus also includes a cooling circuit for cooling the product solution. The cooling circuit comprises a tank for the cooling fluid and a heat exchanger connected to a cooling water line. The cooling circuit may also include a heat exchanger to absorb heat from the steam condensate to be discharged from the apparatus. The cooling liquid is collected in a tank and circulated through the circuit by a pump.

[0017] The evaporation apparatus of the present application may include a tank for condensing steam leaving the shell side of the evaporator.The condensed steam is used to saturate the steam compressed in the blower.

[0018] The apparatus of the present application is primarily designed for concentrating sodium hydroxide solutions. However, with minor changes in the operating temperature and pressure, it can also be used for concentrating potassium hydroxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present application is illustrated by the accompanying drawings, in which:

[0020] Figure 1 It is a flowchart used to illustrate the present application. DETAILED DESCRIPTION

[0021] The following process description relates to a 1-stage NaOH evaporation plant, e.g. according to basic diagram D-317 207.

[0022] The 30-33% NaOH caustic feed solution is preheated to about 90-110°C using hot product caustic through a plate heat exchanger (HE-1101). To avoid the need to continuously feed make-up steam at low caustic feed temperatures (below 70°C), the caustic can be preheated by steam condensate in a plate heat exchanger (HE-1102) before entering heat exchanger HE-1101.

[0023] The preheated caustic soda enters the falling film evaporator (EV-1101) from the top. Inside the evaporator tubes, the caustic soda is evaporated to a final concentration of 50%. The tube side of the evaporator operates at a vacuum of 400-425 mbar a, while the shell side operates at a moderate overpressure of 2.0-2.5 bar a.

[0024] To ensure proper regulation and long life of the equipment, the evaporator is equipped with a recirculation line. Automatic recirculation of the product caustic maintains an optimum film thickness at part load.

[0025] The product caustic is cooled by the feed caustic in HE-1101. To reach a final product temperature of 40-50°C, further cooling is carried out in plate heat exchanger HE-1103 with loop cooling water.

[0026] The generated steam is introduced into the steam scrubber (SC-1101) where it is scrubbed and saturated by injecting steam condensate. The condensate spray is used not only to scrub and saturate the steam but also partially flash evaporates, increasing the amount of steam available for mechanical vapor recompression.

[0027] The vapor is compressed by the first blower (V-1101). It is then saturated again by the injection of condensate before being further compressed by the subsequent blowers (V-1102, V-1103, V-1104, and V-1105). After each blower, the injection of condensate saturates the vapor stream. After the final saturation step, the vapor is used to heat the shell side of the evaporator EV-1101.

[0028] In order to achieve the desired flexibility of the plant capacity, steam recirculation is provided on the MVR section.

[0029] The steam condensate from evaporator EV-1101 is used to cool and scrub the steam in SC-1101 before being cooled in HE-1104 and HE-1102 and finally sent to the battery end.

[0030] To protect the equipment from damage associated with low-quality cooling water, an internal cooling circuit is used. The cooling water enters the 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), condensate (HE-1104), and the lubricating oil system for all blowers. The circuit cooling water is then collected in tank T-8100 and pumped via P-8100 to the heat exchanger HE-8100, where it is cooled back to the desired temperature.

Claims

1. A caustic solution evaporation device, characterized in that: The evaporation equipment comprises: - Feed lines for low concentration caustic solutions, an evaporator receiving the low-concentration caustic solution from the feed line, the evaporator operating on the tube side at a pressure of less than 500 mbar and a temperature of less than 130° C., a heat exchanger arranged in the feed line for low-concentration caustic solution, receiving the product caustic solution from the evaporator for heat transfer, - a discharge line for discharging the product caustic solution, comprising a heat exchanger for cooling the product caustic solution, a feed line for feeding 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 the steam generated in the evaporator for scrubbing with steam condensate from the evaporator, at least one blower receiving scrubbed vapour from the scrubber for compression, the or each blower being fed with vapour condensate from the evaporator for vapour saturation, - a discharge line for excess steam condensate, - a line for feeding compressed steam from the at least one blower to the shell side of the evaporator, - a cooling circuit for cooling the product caustic solution, comprising a tank for the cooling fluid and a heat exchanger for the cooling water, and - A cooling water line which supplies cooling water to the cooling circuit.

2. The evaporation device according to claim 1, characterized in that The evaporator is a falling film evaporator.

3. The evaporation device according to claim 1 or 2, characterized in that The evaporation device includes 3 to 6 blowers arranged in series.

4. The evaporation device according to claim 1 or 2, characterized in that The evaporation apparatus includes a tank for condensed vapor exiting the shell side of the evaporator.

5. The evaporation device according to claim 1 or 2, characterized in that The evaporation device comprises a heat exchanger arranged in the steam condensate discharge line for continuously cooling the cooling fluid from the cooling circuit.

6. The evaporation device according to claim 1 or 2, characterized in that The evaporator is pressurized to 2.0-2.5 bar on the shell side.

7. The evaporation device according to claim 1 or 2, characterized in that The evaporator has a temperature of 100-130° C. on the tube side.

8. The evaporation device according to claim 1 or 2, characterized in that The caustic solution is a caustic soda solution.

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