Autoclave Calculations - Aqueous Process Simulations (AQSim) .Autoclave Calculations Introduction

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Autoclave Calculations

Introduction

An autoclave is a high pressure, high temperature hydrometallurgy unit with carefully controlled conditions. From an OLI Software perspective, it is an isochoric (constant volume) calculation in which mass, pressure and temperature are allowed to vary.

In this chapter, you will simulate autoclaves using three general approaches. The first is to use Custom calculation. This calculation type allows you to select variables to fix and to free. The second type is the Autoclave calculation type. This fixes temperature and partial pressures, and calculations initial/final pressures and gas inflow amounts. The third option is using the Mixer set to isochoric. This option calculates temperature or pressure, and the user manually adjust the inflow rate of a given stream.

Contents Autoclave Calculations ................................................................................................................. 1

9.1 Basic Autoclave #1 Using Inert Gas .................................................................................................. 2

9.2 Basic Autoclave #2 Using Reactive Gas ............................................................................................ 9

9.3 Basic Autoclave #3 - Using Air ........................................................................................................... 12

9.4 Basic Autoclave #4 CO2/H2S/CH4 in seawater ................................................................................ 16

9.5 Basic Autoclave #5 - Calculating the Material Needed for a Sour Gas Experiment .......................... 20

9.6 Basic Autoclave #6 - Dense-phase liquid in an autoclave ................................................................. 25

9.7 Basic Autoclave #7 - CO2 Sequestration ............................................................................................ 28

9.8 Advanced Autoclave #1 - CO2 and H2S in headspace of a 5% NaCl-H2O solution ............................. 31

9.9 Advanced Autoclave #2 - Manual Autoclave setup using stepwise gas addition ............................. 40

9-2 Chapter 9 Autoclave Calculations Tricks of the Trade

9.1 Basic Autoclave #1 Using Inert Gas In this case, you will add 1 kg water to a 2 L autoclave and then fill the void with N2. The conditions are

ambient, 25 C and 1 atm. The N2 is presumed to be the gas that remains in the autoclave headspace after sparging (O2 removal) is completed.

The amount of N2 added is the the combination of N2 vapor filling the headspace, and N2 that dissolves in the water (N2 aqueous). Since N2 has a low water solubility, this second amount is negligible. H2O also evaporates, so the headspace will be a mixed gas, N2 and H2O.

We can use the Ideal Gas Law to estimate the amount of N2 needed to fill the headspace:

=

1 1 = 0.082057

298.15

= 0.040874

Although this estimate does not consider the H2O that is in the 1 L headspace or the amount of N2 dissolved in water, it is still an easy approach to get a reasonable estimate. You will compare this equation to the results at several points in this case. You will also review the results in greater detail in this case, because part of the goal of this first case it to show where the important variables and properties are that impact an autoclave calculation.

Getting Started

Start a new file and give it a name such as Chapter 9 Autoclaves

Create a new stream and rename it Basic Autoclave Inert Gas

Change the framework to MSE

Make sure the units are set to Metric-Batch-Moles

Add N2 as an inflow

Add a Single Point Calculation then select Autoclave as the calculation type

The basic grid should look like the one below

Tricks of the Trade Chapter 9 Autoclave Calculations 9-3

If you do not see the Autoclave then follow the instructions in the insert below.

Click the Specs button

Highlight N2 in the gas field then press OK

Change the Vessel volume to 2 L

Change the final temperature to 25 C and Final Pressure to 1 atm

At this point the system contains, 1 kg H2O (55.508 moles), the initial temperature is 25 C and the final temperature and pressure will be 25 C and 1 atm.

Calculate

The software will perform two calculations, one at ambient (initial) conditions and one at final conditions. It will compute the amount (in moles) of N2 needed to create a 1 atm pressure at final conditions. It will then use this N2 to compute the pressure at ambient conditions. The calculation pathway image below shows this.

Select Tools > Options Select the Enable Features box

Press OK.

If Autoclave option does not appear in the single point Calculation Type list, then restart software

9-4 Chapter 9 Autoclave Calculations Tricks of the Trade

View the Summary section

The calculated ambient pressure is 1 atm. It is identical to the final pressure, which is not surprising, since the ambient and final temperatures are also identical, and by design, the autoclave calculation uses the identical inflows for ambient and final conditions.

The final N2 partial pressure is computed to be 0.9687 atm.

Click on the Output tab -

The amount of N2 added is 0.04007 moles. This produces the 0.9686 atm partial pressure.

Review some of the remaining data

The size of the liquid phase, in moles, is 55.508 and the vapor is 0.407, or a 1364:1 ratio. The volumes are nearly identical: 1 L liquid and 1 L vapor.

Tricks of the Trade Chapter 9 Autoclave Calculations 9-5

Click on the Report tab

Scroll down to the last table, Element Distribution

The last row is N (molecular nitrogen). The distribution is 98.5% in the vapor and 1.5% in the liquid

phase. Conversely, 0.0023% of the water has evaporated (note the H(+1) and O(-2) rows).

These results infer two effects: as the final pressure increases, additional N2 will dissolve in the water, shifting the fraction of nitrogen to the liquid; As the total autoclave volume increases relative to the input liquid, additional water will evaporate, shifting the water to the vapor. This will be shown next.

Setting the Vessel volume to 5 L and 10,000 L

You will compute this case using larger autoclave volumes. The case above is a 2 L autoclave containing

1 kg (~1 L) water. The moles of liquid is computed to be 55.5076.

Click on the Input tab and change the Vessel volume to 5 L

Calculate

Click on the Output tab and view the Phase amounts

The moles of liquid is now 55.5037. The reduction of 0.0039 moles is because this amount of H2O evaporated into the larger headspace.

Scroll down to the Inflow grid and view the N2 inflow

The grid shows the amount of N2 required to fill the 4-L headspace. The amount is is 4-times greater

than the first case, which is expected for gas with a low water solubility.

Return to the Input tab and change the volume to 10000 L.

This creates a 9999 L headspace. The Liquid moles is now 42.69, and the 12.818 mole reduction is evaporation.

9-6 Chapter 9 Autoclave Calculations Tricks of the Trade

This phase distribution is important to modeling the autoclave system properly, especially at elevated conditions as will be seen in the next example.

Testing the 5 L case at 1 atm and 100 C final temperature

The purpose of this example is to raise the H2O partial pressure relative to N2. In the 5 L case above, the N2 inflow is 0.1634 moles. As temperature increases, the H2O vapor pressure increases, and the amount of N2 required to fill the vapor void will decrease.

Change the Final Temperature to 100 C

Change the vessel volume to 5 L

Calculate

View the Output tab and the Summary section

The amount of N2 added is zero (no value is shown). This is to be expected, because the vapor pressure of pure water at 100 C is 1 atm. Therefore, no N2 is needed to raise the pressure. Notice also that the ambient pressure is 0.0313 atm. This is the vapor pressure of pure water at 25 C. Therefore, the autoclave will have a vacuum pressure at ambient temperatures, and when heated to 100 C will reach atmospheric pressure. This, by the way is the principle of the double-expresso pot in reverse.

The water is added into the lower chamber. The pot is sealed at ambient pressures and is heated. The water boils and condenses in the upper chamber. When taken off the stove, the lower chamber cools,

Tricks of the Trade Chapter 9 Autoclave Calculations 9-7

producing a vacuum, this vacuum pulls the condensed water through the coffee. The system has two temperatures, two pressures, and one composition.

Testing the 20 L case at 50 atm final pressure and 100 C final temperature

This scenario contains two different sets of T, P conditions. The calculation approach however, is the same: the final T and P are known, and the N2 inflow amount is computed. This N2 amount is added to the Ambient calculation, and PT,A is calculated.

Change the Final temperature and Pressure to 100 C and 50 atm

Calculate

View the Summary section

The computed ambient pressure PT,A is 38.437 atm. Of this total, 38.404 atm is N2 (PN2=38.404) and 0.033 atm is H2O (PH2O=0.033 atm). Thus, to create a 50 atm final pressure, the autoclave would need to be charged with N2 at a regulator pressur