Flowsheet ESP Training Manual Ver-1 Seaw ater Evaporation – Effects 2 and 1 16-1

Chapter 16 Seawater Evaporation – Effects 2 and 1

The Application This process is a continuation of the previous chapter. It is evaporating seawater in a three-effect system

plus a crystallizer. The first effect is already completed, and it will be used to create the two effects shown in this chapter. The objective in this effect is to evaporate 30% of the inlet flow.

Figure 16-1- Effects 2 and 3 of the overall seawater evaporation system

Creating the Chemistry Model

Open the existing Case file Open the existing file created in the last chapter, Seawater Multi-Effect Evaporation.

There is no change to the chemistry model in this chapter.

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Building the Process

Building the Second Effect

The second effect is nearly identical to the first effect, with the differences being the settings. Therefore, the simplest way to create it is by following the procedure in the previous chapter. The Main instructions will be included here.

Adding the five blocks in the second effect The following table contains the specifications for the five new blocks. Three of the blocks contain

Parameter specifications: Pump-2, HX-2, and Split-2.

Block name Pump-2 Pumparound Mix-2

HX-2 Flash-2 Split-2

Block type Mixer Mixer Heat Exchanger Separator Flow Splitter

Inlet Streams Effect-1 Out Feed to Effect 2

Pumparound-2

Mix-2 Out HX-2 Flash-2 l iquid

Outlet Stream Feed to Effect 2 Mix-2 Out HX-2 out Flash-2 Vapor Flash-2 Liquid

Pumparound-2 Effect-2 Out

Parameter Specification

Absolute pressure Vapor Fraction Flow basis = mass

Parameter Value

0.3 bar outlet pressure

7.5 mass% in vapor

Pumparound-2=0.8 Effect-2 out=0.2

Pump-2 (0.3 bar)

Add a Mixer block to the lower right of the Split-1 stream and label it Pump 2 (0.3 bar).

Link the Effect-1 out stream to it.

Add a new outlet stream and label it Feed to Effect-2.

Change the Pressure specifications to Absolute pressure and enter a value of 0.3bar.

Pumparound Mix-2

Add another Mixer block to the upper right of Pump-2 and label it Pumparound Mix-2.

Rotate the block counterclockwise.

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Link the Feed to Effect-2 stream to the inlet.

Create a new inlet stream and label it Pumparound-2.

Create a new outlet stream and label it Mix-2 Out.

HX-2, heat exchanger

Add a Heat exchanger above the Mix-2 block and label it HX-2.

Rotate the block counterclockwise.

Link the Mix-2 Out stream to it.

Add a new stream and label it HX-2 out.

Change the Process Stream Spec. type to Vapor Fraction.

Enter 7.5%.

Flash-2 flash drum

Add a separator to the upper right of HX-2 and label it Flash-2.

Link the HX-2 Out stream to it.

Create new Vapor and Liquid outlet streams and label them Flash-2 vapor and Flash-2 liquid.

Split-2 (flow splitter)

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Add a Flow splitter to the lower-right of the flash-2 block and label it Split-2.

Rotate it clockwise.

Link the Flash-2 Liquid to the inlet.

Link the Pumparound-2 stream to its outlet.

Create a new outlet stream and label it Effect-2 Out.

Open the Split-2, Outlet Split Edit window -

Change the Flow basis to Mass and the fraction as shown in the image below.

Setting the Pumparound-2 as the Tear stream Right-mouse-click on the Pumparound-2 stream and select Set Tear Stream.

Click on the Pumparound-2 stream again and select the Definition tab in the Properties panel.

Enter the following values:

Composition Value Temperature 80 C

Pressure 0.3 bar

Stream Amount 25000 kg/hr

H2O 95 kg

NaCl 5 kg

Table 16-1 – Pumparound-2 composition guess

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If you cannot access the Definition tab, click off the Pumparound-2 stream and then click back on. The panel may need to be refreshed, because you just signaled to the software that this stream is a tear stream.

Figure 16-2 - Initial guess to the Pumparound-2stream that will be used to converge the recycle

Calculate.

The calculation should converge quickly because a good initial guess was provided, and because the process is straightforward. If it does not converge quickly, check your work to see if something was missed.

Add the Callouts Add callout tables to the Feed to Effect-2, HX-2 out, Flash-2 vapor, Flash-2 liquid,

Pumparound, and Effect-2 out.

Use the following variables for each callout:

Feed to Effect-2 HX-2 Out Flash-2 Vapor Flash-2 Liquid Pumparound-2

Temperature Pressure Mass

Temperature Mass

Mass Mass Mass

Table 16-2 - The Variables to be in each Callout table

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Figure 16-3- The converged flowsheet with the Callout tables added to the second effect streams

The Vapor mass is 2283 kg/hr which is 28.8% of the feed to Effect-2 (7917 kg/hr). The target is 30% evaporation of the inlet feed. So you will increase the HX-2 vapor fraction value to 7.9%.

Click on the HX-2 block to activate the Properties panel.

Change the Vapor Fraction (mass %) value to 7.9.

Figure 16-4 - updating the HX-1 vapor fraction to 7.9%

Calculate.

Figure 16-5 - Updated vapor flow after setting the HX-2 vapor fraction to 7.9%

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The updated vapor rate is now 2376 kg/hr. This is 30% of the inlet rate and so the target specificati ons are met.

Save the file.

Review the HX-1 energy balance Click on the HX-2 block to access the Properties panel.

Click on the Watch tab at the bottom of the panel -

Figure 16-6 - The Heat Balance surrounding the HX-2 block

The required heat duty is 6.31e6 KJ/hr. This energy will be sourced from the vapor phase produced by the third effect. It will be set up in the next chapter.

Building the First Effect

The third effect is again nearly identical to the first and second effect. The difference is the settings. The third effect will evaporate 50% of the inlet feed. You will use the same procedure as above to create this section.

Adding the five blocks in the first effect The following table contains the specifications for the five new blocks. Three of the blocks contains

Parameter specifications, Pump-1, HX-1, and Split-1.

Block name Pump-1 Pumparound

Mix-1

HX-1 Flash-1 Split-1

Block type Mixer Mixer Heat Exchanger Separator Flow Splitter

Inlet Streams Effect-2 Out Feed to Effect-1 Pumparound-1

Mix-1 Out HX-1 Flash-1 l iquid

Outlet Stream Feed to Effect-1 Mix-1 Out HX-1 out Flash-1 Vapor Flash-1 Liquid

Pumparound-1 Effect-1 Out

Parameter Specification

Absolute pressure Vapor Fraction Flow basis = mass

Parameter

Value

1 bar outlet

pressure

12 mass% in

vapor

Pumparound-1=0.8

Effect-1 out=0.2

Pump-1 (1 bar)

Add a Mixer block to the lower right of the Split-2 stream and label it Pump 3 (1 bar).

Link the Effect-2 Out stream to it.

Add a new outlet stream and label it Feed to Effect-1.

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Change the Pressure specifications to Absolute pressure and enter a value of 1 bar.

Pumparound Mix-1

Add another Mixer block to the upper right of Pump-1 and label it Pumparound Mix-1.

Rotate the block counterclockwise.

Link the Feed to Effect-1 stream.

Create a new inlet stream and label it Pumparound-1.

Create a new outlet stream and label it Mix-1 out.

HX-1, heat exchanger

Add a Heat exchanger above the Mix-1 block and label it HX-1.

Rotate the block counterclockwise.

Link the Mix-1 Out stream to it.

Add a new stream and label it HX-1 out.

Change the Process Stream Spec. Type to Vapor Fraction.

Enter 12%.

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Flash-1 flash drum

Add a separator to the upper right of HX-1 and label it Flash-1.

Link the HX-1 Out stream to it.

Create new Vapor and Liquid outlet streams and label them Flash-1 vapor and Flash-1 liquid.

Split-1 (flow splitter)

Add a Flow splitter to the lower-right of the Flash-1 block and label it Split-1.

Rotate it clockwise.

Link the Flash-1 Liquid to the inlet.

Link the Pumparound-1 stream to its outlet.

Create a new outlet stream and label it Effect-1 Out.

Open the Split-1, Outlet Split Edit window -

Change the Flow basis to Mass and the fraction as shown in the image below:

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Setting the Pumparound-2 as the Tear stream Right-mouse-click on the Pumparound-1 stream and select Set Tear Stream.

Click on the Pumparound-1 stream again and select the Definition tab in the Properties panel.

Enter the following values:

Composition Value

Temperature 100 C

Pressure 1 bar

Stream Amount 15000 kg/hr

H2O 90 kg

NaCl 10 kg

Table 16-3 - Pumparound-1 composition guess

If you cannot access the Definition tab, click off the Pumparound-1 stream and then click back on. The panel may need to be refreshed, because you just signaled to the software that this stream is a tear stream.

Figure 16-7 - Initial guess to the Pumparound-1 stream that will be used to converge the recycle

Calculate.

The calculation should converge quickly, because a good initial guess was provided, and because the process is straightforward. If it does not converge quickly, check your work to see if something was missed.

Add the Callouts Add callout tables to the Feed to Effect-1, HX-1 out, Flash-1 vapor, Flash-1 liquid,

Pumparound, and Effect-1 out.

Use the following variables for each callout:

Feed to Effect-1 HX-1 Out Flash-1 Vapor Flash-1 Liquid Pumparound-1 Temperature

Pressure Mass

Temperature

Mass

Mass Mass Mass

Table 16-4 - The Variables to be in each Callout table

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Figure 16-8- The converged flowsheet with the Callout tables added to the third effect streams

The Vapor mass is 2246 kg/hr which is 40.5% of the Feed to Effect-1 flow (5541 kg/hr). The target is 30% evaporation of the inlet feed. So you will increase the HX-2 vapor fraction by 25% value to 15%.

Click on the HX-1 block to activate the Properties panel.

Change the Vapor Fraction (mass %) value to 15.

Figure 16-9 - updating the HX-1 vapor fraction to 15%

Calculate.

Figure 16-10 - Updated vapor flow after setting the HX-2 vapor fraction to 15%

The updated vapor rate is now 2597 kg/hr. This is 47% of the inlet rate and still not within the target value. You will change the value once again, 10% higher or 16.5%.

Click on the HX-1 block to activate the Properties panel.

Change the Vapor Fraction (mass %) value to 16.5.

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Figure 16-11 - updating the HX-1 vapor fraction to 16.5%

Calculate.

Figure 16-12 - Updated vapor flow after setting the HX-2 vapor fraction to 16.5%

The updated vapor rate is now 2754 kg/hr. This is 49.7% of the inlet rate and is within the target range. Save the file.

Review the HX-1 energy balance Click on the HX-1 block to access the Properties panel.

Click on the Watch tab at the bottom of the panel -

Figure 16-13 - The Heat Balance surrounding the HX-1 block

The required heat duty is 6.93e6 KJ/hr. This energy will be sourced from an inlet Steam that will be added in the next chapter.

Summary

This chapter includes instructions to add the second and third effect. All three effects are easy to converge because good initial guesses were used and because the three recycle streams are not interdependent. This will change in the next chapters where the counter-current steam and flash vapor is used in the HX-1 and HX-2.