JPH0225672B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a deaeration method for a flash evaporation type seawater desalination apparatus using a final stage deaeration method. [Background Art] In evaporative seawater desalination equipment (hereinafter referred to as MSF), deoxygenation of make-up seawater is very important from the perspective of preventing corrosion of the equipment. The deoxygenation (hereinafter referred to as deaeration) process includes (1) a vacuum deaeration method using a packed column, and (2) deaeration is simultaneously performed in the final stage flash chamber (hereinafter referred to as the "final stage evaporator"). There is a final stage degassing method. Comparing the performance of both methods, method (2) is generally superior.
This method is said to be inferior to method (1). This is because the theoretical basis of the rate-determining factor in method (2) is not fully understood, so at present both methods have no choice but to base their degassing theory on the double-layer theory. [Conventional Final Stage Degassing System] Here, a conventional final stage degassing system will be explained based on FIG. 1. The make-up seawater flowing into the evaporator is branched from the outlet 4' of the heat recovery part condenser 5 of the cooling seawater line 4 passing through the heat release part condenser 5 of the evaporation chamber 1, and flows into the make-up seawater line 6.
This make-up seawater is sprayed into the final stage of the evaporator by the deaeration nozzle 7, and mixed with the circulating brine 10 that has flowed in from the previous stage evaporation chamber. In the figure, 2 is a circulation pump that supplies water to the condenser 5 of the heat recovery section of the flash evaporator, and 3 is a discharge pump for maintaining a certain concentration. The rate-limiting factors for degassing according to the double boundary film theory are the gas-liquid contact area, film thickness, and zonation time in addition to the O ₂ partial pressure, but even with this flow, the temperature inside the final stage evaporation chamber is higher than the inflowing seawater, so There is a decrease in O ₂ concentration due to the increase in the temperature of the supplementary seawater, and an increase in the dissipation rate due to the formation of a spray. However, because the residence time is short, the performance of the final stage deaeration method is higher than that of a separate vacuum deaeration tower method, which increases the gas-liquid contact area and residence time due to the uniform spray inside the tower and the packing material. It is unavoidable that it is inferior. Adopting the final stage degassing method as described above does not require a separate vacuum degassing tower, which is a major cost reduction factor, but on the other hand, performance is inferior, and the material of the evaporator body is Upgrading becomes difficult to avoid, leading to a significant increase in costs. Recent MSF constituent materials are copper alloys, with some exceptions.
High-grade materials such as stainless steel and titanium have been selected to prevent corrosion. Corrosion problems that occur in MSF include general corrosion of carbon steel, electrolytic corrosion of aluminum bronze/titanium, stainless steel/carbon steel, stress corrosion and pitting corrosion of stainless steel, and inlet attack at the inlet of heat transfer tubes. , these can be practically prevented to a large extent by removing dissolved oxygen. Therefore, it is essential to develop a degassing system that is inexpensive and has sufficient performance in order to reduce the cost of MSF. [Objective of the present invention] The present inventor focused on the fact that the conventional final stage degassing method is not based on the double film theory, but that water evaporation is the main rate-determining factor in a flash evaporation environment.
As a result, the present invention has been completed. That is, an object of the present invention is to provide a deaeration method for a flash evaporation type seawater desalination apparatus in which the deaeration efficiency in the final stage deaeration method is improved. [Structure of the Present Invention] As a means for achieving the above object, the present invention consists in preheating the make-up seawater before it flows into the final stage evaporation chamber. That is, the present invention provides a flash evaporation type seawater desalination device that uses part of the heat release section cooling seawater as make-up seawater, and a make-up seawater preheater is installed in the make-up seawater line for flowing make-up seawater into the final stage evaporation chamber. This is a deaeration method for a flash evaporation type seawater desalination apparatus, which is characterized in that the temperature of the make-up seawater is raised to a temperature higher than the temperature inside the evaporator, and then the air is degassed within the evaporator. In the present invention, before the make-up seawater flows into the final stage evaporator, a make-up water preheater is provided, and the temperature is raised by the temperature at which flash evaporation is possible in the final stage evaporator (for example, 1 to 2°C). The goal is to change the rate-determining factors of gas from the gas-liquid contact area and residence time to flash evaporation. The present invention will be explained in detail based on FIG. FIG. 2 shows the flow of the present invention. The heat release part is the same as the conventional method shown in Figure 1, but the supply seawater line 6
The make-up seawater passing through exchanges heat with the heating fluid (steam or hot water) in the make-up water preheater 8, and is heated to a temperature 1 to 2 degrees Celsius higher than the temperature inside the final stage evaporator. This heated make-up water flows into the evaporator and is degassed while flashing and evaporating, and is lowered to the temperature of the brine that has flowed in from the previous stage. (1) Make-up water preheater This preheater 8 serves to raise the temperature of the seawater that has passed through the heat recovery section condenser 5 to a temperature high enough for make-up water to flash evaporate in the final stage evaporation chamber 1. change in balance,
There is no phase change. (2) Final stage flash chamber The preheated seawater flash evaporates here, lowering the temperature and reducing the dissolved oxygen concentration. These relational expressions are shown in (1) to (13). A Mass balance F _T =F _B +F _E (1) F _T C _T =F _B C _B +F _E C _E (2) However, F _T : Makeup water inlet flow rate [Kg/h] F _B : Mixed with brine Immediate flow rate [Kg/h] F _E : Evaporation amount from makeup water [Kg/h] C _T : Oxygen concentration before makeup water degassing [mol/Kg] C _B : Oxygen concentration after makeup water degassing [ mol/Kg] C _E : Oxygen concentration dissipated into the gas phase [mol/Kg] B Equilibrium relationship C _B ^* =P _gas・H (3) P _gas /π=X _gas =C _E /C (4) P _H2O /π=X _H2O =C _H2O /C=10 ³ /18/C(5) From equations (4) and (5), P _gas /P _H2O =X _gas /X _H2O =C _E /55.5 (6) Therefore, m =C _E /C _B ^* =55.5/P _H2O・H (7) However, C _B ^* : Equilibrium oxygen concentration after degassing of supplementary seawater [mol/Kg] C: Molar concentration of all gas components [mol/Kg] C _H2O : Molar concentration of 1 kg of water = 10 ³ / 18 = 55.5
[mol/Kg] X _gas : Mole fraction _{of oxygen} [ ₋ ] Total pressure in the phase [atm] m: Partition ratio [-] H: Henry constant [mol/Kg・atm] C Evaporation rate (R), non-equilibrium condition (NECR) and temperature drop (△T) R=F _E /F _T (8) (NECR)=C _B ^* /C _B (9) From the above, R=C _T −C _B ^* /C _B ^* (m−1)= C _T −C _B (NECR)/C _B (NECR) (m-1) (10) F _E = [C _T −C _B (NECR)]/C _B (NECR) [(55.5/P _H2O
・H)-1〕 (11) On the other hand, the temperature drop △T due to evaporation of F _E is △T=F _E H _V /F _T C _P (12) where H _V : Latent heat of vaporization of water [Kcal/Kg] C _P : Specific heat of water [Kcal/Kg・C] In equations (10) and (11), the partition ratio m is uniquely determined only by Henry's constant and temperature. Also, temperature:
40-50°C Pressure: saturation pressure, (NECR) during nozzle spraying was determined from actual scale equipment data (NECR) = 0.5. Based on these results, equations (10) and (11) show that the evaporation rate R and the concentration C _B after degassing have the following relationship. R∝C _T /C _B That is, when the inlet O ₂ temperature is constant, the deaeration rate in the case of the final stage deaeration method is controlled by the flash evaporation rate. Now, in order to evaporate the flash, it is necessary to make the temperature higher than the temperature of the final stage bulk by an amount corresponding to the required amount of flash evaporation. For this reason,
The key point of the present invention is the final stage degassing process, which is characterized by preheating the make-up seawater before it is supplied to the final flush chamber. 8 in FIG. 2 is the preheater, and the heating heat sources flowing through the heating heat source line 9 include an evaporator first stage brine, an intermediate stage brine,
Uses high temperature produced fresh water. (If heater evaporation is used,
Water production ratio decreases. ) Note that 11 is a heating fluid outlet. Examples Table 1 shows examples in which the present invention was applied to an evaporative seawater desalination apparatus with a water production capacity of 24 m ³ /d and a maximum brine temperature of 120°C.

【table】

[Effects of the present invention]

As detailed above, the present invention preheats the makeup seawater before it flows into the final stage evaporation chamber and then introduces it into the final stage evaporation chamber, so dissolved oxygen in the makeup seawater is reduced. is efficiently degassed,
As a result, a remarkable effect is produced in terms of preventing corrosion of the device.

[Brief explanation of drawings]

FIG. 1 is a flow diagram for explaining a conventional final stage degassing system, and FIG. 2 is a flow diagram of the present invention.

Claims (1)

[Claims] 1. In a flash evaporative seawater desalination system that uses part of the cooling seawater as make-up seawater, a make-up seawater preheater is installed in the make-up seawater line for flowing make-up seawater into the final stage evaporation chamber, and the make-up seawater is 1. A deaeration method for a flash evaporation type seawater desalination apparatus, characterized in that the temperature is raised to a temperature higher than the temperature inside the evaporator, and then the evaporator is degassed.