Chapter 15 Seawater Evaporation Effect 3

Transcription

1 Chapter 15 Seawater Evaporation Effect 3 The Application This process will evaporate seawater using a three-effect system and a crystallizer. The process will include two external steam feeds, one feeding the three effects and the other to the crystallizer. The seawater will flow at 10 mt/hr and the effective evaporation will be 95%. At 95%, NaCl salts should begin to form. Figure Effect 3 of the overall seawater evaporation process Creating the Chemistry Model Creating the Case file This is a new case, so create a new file. Save the new file with the name Seawater Multi-Effect Evaporation. Flowsheet ESP Training Manual Ver-1 Seaw ater Evaporation Effect

2 Adding Chemistry to the Model You will start with a simple salt-water composition. The Seawater chemistry contains multiple inflows, including Ca, Mg, SO4, HCO3, Br, and other components. However, the goal here is to keep the chemistry simple until the simulation is converged. Then the remaining components can be added. NaCl is by far the most significant salt in seawater, and so most of the mass required is obtained with H2O and NaCl. Click on the Chemistry tab and then the Inflows subtab. Enter NaCl to the list. Figure 15-2 Adding NaCl to the Chemistry Inflows List Return to the flowsheet view. Building the Process Building the Third Effect The Inflow seawater rate is 10,000 kg/hr. The third effect will evaporate 20% of this seawater and the pumparound recycle will have a 4:1 ratio. Those are the basic settings. Also, as explained above, the seawater contains NaCl only until the complete process is converged. Then the remaining components will be added. Specifications Value Seawater Feed rate 10,000 kg/hr Steam Rate 20% (+/- 1%) Recycle Ratio 80% (4:1) Figure Third Effect Specifications Setting the Units Setting the units for the flowsheet before adding any streams or blocks eliminate a significant amount of editing time once the simulation work begins. Setting the units before starting work is good practice for all work in Flowsheet ESP. Open the Units Manager window using the Units button on the Toolbar Change the process units to Metric-Flowing-Mass Seawater Evaporation Effect 3 Flowsheet ESP Training Manual Ver-1

3 Figure The Units Manager window Click on the Customize button - Make the following changes: to open the Units Editor. Variable Basis or units Aqueous Composition Concentration Solid Composition Concentration Mass kg/hr Volume m3/hr Pressure bar Energy kj/hr Table Changes to the global flowsheet units Building the process Close the windows when done. Figure Updated Units Editor windows Create the feed stream and the Vacuum unit Click on the Mixer object in the library, and drag it to the bottom-left of the screen. Label it Vacuum 0.1 bar. Flowsheet ESP Training Manual Ver-1 Seaw ater Evaporation Effect

4 Change the Pressure setting to Absolute Pressure and enter a value of 0.1 bar. Figure Updated parameters to the Vacuum 0.1 bar block Click on the Add Stream button - Create one stream to the inlet and one stream to the outlet. Press Esc to deactivate the Add Stream action. Label the inlet stream 10mT Seawater feed. Label the outlet stream Feed to Effect-3. Create the Pumparound Mix-3 and the Pumparound-3 stream Click on the Mixer block from the Library and drag it to the upper right of the Vacuum block. Label it Pumparound Mix-3. Rotate the block counter-clockwise Link the Feed to Effect-3 stream to the Mixer. Add a new stream to the inlet and outlet of the Pumparound Mix Seawater Evaporation Effect 3 Flowsheet ESP Training Manual Ver-1

5 Label the inlet stream Pumparound-3 and the outlet stream Mix-3 out. Enter the following composition for the Seawater feed and Pumparound-3 streams: Component Seawater Feed Pumparound-3 Temperature, C Pressure, bar Stream Amount, kg/hr 10,000 32,000 H2O NaCl Table Composition of the Seawater feed and the Pumparound stream Figure updated Parameters table for the Seawater and Pumparound-3 streams Calculate. Review the Mix-3 Out results. Figure Updated Parameters table for the Mix-3 Out block Adding the Heat Exchanger and Flash Drum Click on the Heat Exchanger object in the library and drag it above the Mix-3 Out stream. Flowsheet ESP Training Manual Ver-1 Seaw ater Evaporation Effect

6 Label it HX-3. Rotate the HX-3 block counterclockwise - Change the HX-3 Spec type to Vapor Fraction and enter a value of 5 mass%. Figure Updated Parameters for the HX-3 block Link the Mix-3 Out stream to the inlet of the HX-3. Add an outlet stream from the HX-3. Name it HX-3 Out. Click on the Separator block in the Library and drag it to the upper right of the HX-3. Label it Flash-3. Link the HX-3 stream to the inlet. Add two new streams, one to the Vent and one to the Liquid-3 outlet. Label these streams, Flash-3 Vapor and Flash-3 Liquid Seawater Evaporation Effect 3 Flowsheet ESP Training Manual Ver-1

7 Calculate. Adding Callouts to review the results Right-mouse-click on the Seawater feed stream and select Add Callout. Double-click on the Callout table and add Mass Aqueous from the Phase Flows section. Figure The Select Variables window showing the variable list and the selected variables fields Add Callout tables for the Pumparound, HX-3 Out, Flash-3 Vent, and Flash-3 Liquid streams. Use the following variables (delete all others). Table The Variables to be in each Callout table 10mT Seawater Feed Pumparound-3 HX-3 Out Flash-3 Vapor Flash-3 Liquid Temperature Pressure Mass-Aqueous Temperature Mass Aqueous Temperature Mass Mass-Vapor Mass-Aqueous Figure The Flowsheet diagram with the Callout tables added Adding the Flow Splitter and linking the Recycle stream Click on the Splitter block in the Library and drag it to the lower right of the Separator. Name it Split-3. Rotate the block clockwise - Link the Flash-3 Liquid stream to the inlet of the Split-3 block. Link the Pumparound-3 stream to the outlet of the Split block. Add a new stream to the Outlet of the Split block label this Effect-3 Out. Flowsheet ESP Training Manual Ver-1 Seaw ater Evaporation Effect

8 Click on the Split-3 block to access the Properties panel. Click on the Edit button in the Parameters section Enter the following specifications: Flow Basis Pumparound-3 Effect-3 out Mass 0.8 fraction 0.2 fraction Figure The updated flow fractions for the Split-3 block Right-mouse-click on the Pumparound-3 stream, and select Set Tear Stream. Figure Setting the Tear (recycle) stream using the right-mouse-click dropdown window Calculate The simulation should converge quickly, because the Pumparound-3 guess with which we started is close to the converged state Seawater Evaporation Effect 3 Flowsheet ESP Training Manual Ver-1

9 Figure The converged process using the initial settings The Flash-3 Vapor mass is 83 kg/hr above the 2000 kg/hr target value. This target can be tightened by changing the Vapor Fraction setting in HX-3. It is currently set to 5%. The flow exiting the heat exchanger is 41,669 kg/hr, and so to achieve 2000kg/hr the vapor fraction is 4.8%. Change the Vapor Fraction (mass %) value to 4.8. Calculate. Figure updating the HX-3 vapor fraction to 4.8% The updated vapor rate is now 2013 kg/hr. This is slightly high than desired, and is because the recycle rate increased by about 300 kg/hr. This is well within the 1% tolerance set in the specification at the beginning of the chapter. Flowsheet ESP Training Manual Ver-1 Seaw ater Evaporation Effect

10 Figure Updated vapor flow after setting the HX-3 vapor fraction to 4.8% Save the file. Review the HX-3 energy balance Click on the HX-3 block to access the Properties panel. Click on the Watch tab at the bottom of the panel - Figure The Heat Balance surrounding the HX-3 block The required heat duty is 5.67e 6 KJ/hr (5.67e 9 J/hr). At this point in the simulation procedure, this energy is simulated. There is no utility steam to provide the energy. Once the remaining effects are created, the counter-current steam will be linked. Summary This chapter presents the basic approach to set up an evaporator effect. It converged easily because a good initial guess was used in the Pumparound-3 stream. A good guess was possible because the flow and recycle specification are known. Lastly, this process does not include the counter-current steam streams to supply the heat to the HX-3 block. This will be done in later chapters once the system is completed Seawater Evaporation Effect 3 Flowsheet ESP Training Manual Ver-1