Evaluation of the minimum number of absorbers and the minimum solvent

solvent flow rate

The minimum solvent flow rate is determined using an infinite packing height. To do this, the packing height is set at 100 m. In this case, the section is characterized with the largest possible diameter due to the large stream of flue gases. This diameter, from the literature, must not be larger than 12 m. In this procedure the packing rating-design mode option must be activated to determine the diameter of the column. For this reason, it is necessary to specify two additional parameters which are:

• Base flood: represents the maximum percentage of flooding velocity allowed for the evaluation of the column diameter. In this case it is set at 80%.

• Base stage: the calculation of the diameter of the column is done in a specific point of the column. It usually corresponds to the point where there is a greater stress (gas/vapor flow rate is higher). Since the process is exothermic, this point will be between the bottom and the top of the column, to determine this I use the packing sizing tool which is usable for Radfrac columns.

Once the minimum solvent flow rate has been calculated, for each step it can be verified that the Equation 3.2 of Seader et al. is valid.

𝐿^𝑒𝑓𝑓₀ = (1 ÷ 2)𝐿^𝑚𝑖𝑛₀ (3.2)

3.6.2

Absorber design

In the absorber, 100 stages are set and rate-based is chosen. How to set up the absorber is shown in the Figure 3.12. The flue gases enter on plate 100 with on-stage convention while the solvent enters on plate 1. The pressure is 1 bar.

Now go to specification, reaction and set the reactions in the absorber from stage 1 to stage 100. You do not need to specify the holdup because it is calculated automatically by Aspen once you choose a packed column. In fact, holdup is related to packing and not to theoretical stages. Then I go to column internals and define a pack that occupies all the theoretical stages. In mode I choose rating. Mellapak Sulzer Standard 250Y is used as packing. To simulate the infinite height, I put 100 m as height. As diameter, I use an indicative value of 10 m which is used by Aspen to have a base to start with the calculations. In rate-based modelling, on the advice of Madeddu et al., a reaction condition factor equal to 0.9 and film discretization ratio equal to 10 is chosen. This means that the absorption reaction takes place in the bulk of the liquid and not at the interface between liquid and gas. In the film, therefore, only the penetration of CO2 into the aqueous solvent occurs, when the CO2 has penetrated and reached the equilibrium concentration in the liquid, here the reaction is triggered.

Always in rate-based modelling, in the sections menu, in the liquid phase, film resistance part, discretize film is selected, and 5 points are selected as suggested by Aspen Tech. In Vapor Phase, film resistance, consider film is selected (no reaction in the gas but only mass transport). In the design window I choose the design mode to calculate column diameter option. As base flood I choose 0.8 and as base stage, I choose 10. In case the stage is wrong, once the simulation works, you can see the column profile and change the stage.

The theoretical stage to calculate the minimum diameter is number 3 as shown in the Figure 3.13. In fact, the greatest flow of vapour is in correspondence with this tray. In fact, as expected, it is not number 10. Consequently, it will have to be changed.

Figure 3.12: Infinite height absorber configuration.

Chapter 3

Carbon capture efficiency is defined by Equation 3.3:

𝐶𝑂₂ 𝑐𝑎𝑝𝑡𝑢𝑟𝑒 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 % =𝑌_{𝐶𝑂2,𝑖𝑛}− 𝑌_{𝐶𝑂2,𝑜𝑢𝑡}

𝑌_{𝐶𝑂2,𝑖𝑛} (3.3)

In the Equation 3.3, the amount of CO2 moles entering absorption tower is shown with 𝑌_{𝐶𝑂2,𝑖𝑛} and the amount of CO2 moles from the absorption tower with 𝑌_{𝐶𝑂2,𝑜𝑢𝑡}. The simulation results are shown in the Table 3.12.

These data show a capture efficiency of 87%. Since the aim is to have an efficiency of 90%, the solvent flow rate must be increased. Furthermore, it will be necessary to change the diameter of the column which from the calculations carried out by Aspen is 2.83 m. This is an iterative process because increasing the solvent will also consequently increase the diameter of the column. In the next simulation the solvent flow rate, the theoretical stage for the calculation of the minimum column diameter will be changed and the column diameter.

This iterative process continues until I reach 90% CO2 capture efficiency. In this case it requires a few steps as the flows rate involved are small. A suggestion is to gradually increase the solvent flow rate because if it is increased a lot, Aspen will no longer find the convergence and you will have to start again.

Figure 3.13: Theoretical stage in which there is the highest flow of steam in the absorber.

Table 3.12: Simulation results where the solvent has the same flow rate as the flue gases.

Absorber

Stream Name Units FLUEGAS CLNGAS MEA RICHSOL

Mass Flows kg/hr 75764 75509 75764 76019

MEA kg/hr 0 36 18538 486

H2O kg/hr 1615 8005 51505 44953

CO2 kg/hr 7660 984 0 2

H3O+ kg/hr 0 0 0 0

OH- kg/hr 0 0 0 0

HCO3- kg/hr 0 0 17 525

CO3-2 kg/hr 0 0 29 74

MEAH+ kg/hr 0 0 2169 11631

MEACOO- kg/hr 0 0 3505 18343

N2 kg/hr 66489 66484 0 5

Mole Flows kmol/hr 2637 2841 3232 2877

MEA kmol/hr 0 1 303 8

H2O kmol/hr 90 444 2859 2495

CO2 kmol/hr 174 22 0 0

H3O+ kmol/hr 0 0 0 0

OH- kmol/hr 0 0 0 0

HCO3- kmol/hr 0 0 0 9

CO3-2 kmol/hr 0 0 0 1

MEAH+ kmol/hr 0 0 35 187

MEACOO- kmol/hr 0 0 34 176

N2 kmol/hr 2373 2373 0 0

At the second iteration, with a solvent flow rate equal to 80000 kg/h, a capture efficiency of 92% is found. So, I can stop with the iterations. The molar flow results for the second iteration are shown in the Table 3.13.

In the second simulation, the theoretical stage that requires the greatest diameter because it has the greatest flow rate is always number 3. As far as the calculated diameter is concerned, it is 2.98 m; as expected, this value has risen (it was 2.83 m). The stage with the greatest flow rate is number 3 as it has the highest temperature and therefore a greater evaporation of water. The temperature profile along the 100 stages is shown in the Figure 3.14.

Chapter 3

Table 3.13: Results of the second simulation with a solvent flow rate equal to 80000 kg/h.

Absorber

Stream Name Units FLUEGAS MEA CLNGAS RICHSOL

Mole Flows kmol/hr 2637 3413 2852 3038

MEA kmol/hr 0 320 1 8

H2O kmol/hr 90 3019 464 2635

CO2 kmol/hr 174 0 14 0

H3O+ kmol/hr 0 0 0 0

OH- kmol/hr 0 0 0 0

HCO3- kmol/hr 0 0 0 9

CO3-2 kmol/hr 0 1 0 1

MEAH+ kmol/hr 0 37 0 198

MEACOO- kmol/hr 0 36 0 186

N2 kmol/hr 2373 0 2373 0

The third simulation is the final one: only the diameter of the column is changed, and it is set at 2.97 m. To know in advance the minimum amount of solvent, a Radfrac column of plates in equilibrium can be built and the amount of solvent is quickly found.

Going then to set up a sensitivity analysis in which the solvent flow rate is varied for three cases, it is possible to see how the CO2 output from the absorber varies in clean gases.

Furthermore, in the Figure 3.15 it is possible to see the optimal diameter calculated by Aspen Plus.

The fact that as the solvent flow rate increases, the optimal diameter increases and the output CO2 flow rate decreases is correct because as the solvent increases, the CO2 captured will increase and as the total flow rate increases, the diameter must also increase for the large flow.

Figure 3.14: Temperature profile in the 100 theoretical stages of the absorber. The maximum temperature value corresponds to the tray with the greatest steam flow.

Now that I have found the minimum amount of solvent, I need to start decreasing the height of the column which will result in an increase in the column diameter and an increase in the solvent flow rate. With the decreasing of the height, it is necessary to check two parameters to understand the performance of the column which according to the literature are the temperature in the liquid and the flow rate of CO2. In practice, the absorber works well when the temperature profile is as linear as possible, i.e. without too flat areas because the absorption reaction being exothermic and if the temperature profile is linear, all the absorber stages make their contribution. To start, it is advisable to save the temperature of the liquid at this moment at an infinite height on an Excel sheet.

What emerges is that at 50 meters, to have an efficiency of around 90%, the flow rate must be between 81000 and 82000 kg/h. Using a flow rate of 81000 kg/h, an efficiency of 92% is obtained. As predicted earlier, the plate with more mass flow rate will also increase. In fact, from the Figure 3.16 this has moved from number 3 to number 4. The recommended diameter is 2.95 m

Using the same solvent flow rate as in the previous case, an efficiency of 91.6% and a diameter of 2.98 m is obtained. All cases are summarized in the Table 3.14.

Figure 3.15: Results of the sensitivity analysis.

Figure 3.16: Theoretical stage in which the largest diameter occurs.

Chapter 3

Table 3.14: Values obtained by varying the absorber height and solvent flow rate to obtain a capture efficiency of 90%.

Column

If you continue to decrease the column height, you are forced to increase the solvent and the diameter, and you will move towards a flat temperature condition. It can be guessed from the graph shown in the Figure 3.17. There will also be considerations about the economic point of view but in this work, they are not considered. From the temperature graph it is possible to see how by decreasing the height more and more, I arrive at the condition of 10 meters in which the profile is almost flat.

When a height of 10 meters is reached, the solvent flow rate becomes very large. This suggests that the optimum is placed between 10 and 15 meters. The Table 3.15 shows values for each meter of column between 10 and 15. Looking at the results shown in the Table 3.15, going below 15 meters in height leads to a very large increase in the solvent flow rate. For this work, the optimum dimension is the one calculated at 15 meters.

Figure 3.17: Temperature profile along the absorber for different heights.

Table 3.15: Results obtained for heights between 15 and 10 meters.

The Table 3.16 shows the apparent flow rates because I used a true components approach; it is useful to check that the work is correct. What emerges is that in the regenerated MEA there is always a quantity of CO2 present and this leads to a greater flow of CO2 in the solution leaving the absorber.

Table 3.16: Apparent flow rates obtained for an absorber height of 15 meters.

Apparent component mass delicate as there is the presence in the column of the reboiler and the condenser which are extra degrees of freedom to optimize. The procedure followed is like the one described for the absorber, however the different steps due to the presence of a condenser and the reboiler will be reported.

In the case of the stripper, the heat in the reboiler plays the part of the inlet solvent of the absorber case because I arrive with an exhausted amine to be cleaned which is an input data and I must decide the design parameter which is how much amine is regenerated. The flow