INTRODUCTION TO PROCESS DESIGN USING DWSIM? | Training Notes

Hi everyone,

As part of our activity at Motivated Academic , we have launched the Research Skills Series, a live training and webinar program designed to enhance the research capabilities of researchers, academics, and consultants.

The first session we ran this year was 'Introduction to Process Design using DWSIM', as it is a type of research method very close to my research. Given that the idea behind this newsletter is to provide support throughout the research project life cycle, I want to use it and share training notes from our Research Skills Series. I trust this will add value! Importantly, the bi-weekly newsletters will continue as normal, and I have already written a piece of advice on how to structure emails to prospective PhD supervisors. I expect to publish it here soon!

Before we dive into the case study, let's quickly recap the previous session. We encountered some technical issues. Even though LinkedIn allows the live stream up to 4 hours, it was suddenly cut off after 1:22 hrs. I felt really bad about letting you down and thank you for the kind words of encouragement from so many of you! I've now recorded the same case study and you can check it out below. Let's go through it again in a more structured manner.

Methanation case study - DWSIM

We will work through a detailed case study on producing synthetic natural gas (methane) from carbon dioxide and hydrogen. This is an important concept as companies look to utilise their CO2 emissions and offset carbon taxes/allowances. In this case study, we act as consultants and our task is to develop a conceptual process design for this methanation process.

The design intent is to produce 4 kmol/h of synthetic methane. We'll first design for a capacity of 1 kmol/h and then we can scale it up. The process must also be potentially viable and profitable.

Case Study Approach

Let's go through the typical conceptual process design methodology step-by-step for this case:

Step 1: Eliminate Differences in Molecular Type

The only feasible reaction pathway to convert CO2 and H2 into CH4 is methanation:

CO2 + 4H2 → CH4 + 2H2O

This exothermic reaction is reported to achieve around 98% conversion at 300°C and 50 bar pressure in literature. Stoichiometrically, to produce 1 kmol/hr CH4, we need 1 kmol/hr CO2 and 4 kmol/hr H2 as the feed rates. This will also generate 2 kmol/hr of water vapour as a byproduct.

Step 2: Check Gross Profit Margin

Before proceeding further, we need to check if this methanation pathway can potentially be profitable. This is done by calculating the gross profit margin, given by:

Gross Profit = Income from Product Sales - Cost of Raw Materials

For profitability, the income from methane sales must exceed the combined raw material costs of CO2 and H2.

Let's assume a methane selling price of ￡0.7/kg and a hydrogen cost of ￡2/kg. The CO2 cost is unknown and we'll determine the minimum viable value. Based on a mass balance, to produce 1 kg of CH4, we need 2.75 kg of CO2 and 0.5 kg of H2, while generating 2.25 kg of water.

Substituting these values, we get the gross profit equation: Gross Profit = $0.7 - 2.75x - $1 (where x is the CO2 cost in ￡/kg)

Setting this profit greater than zero, we find that x must be less than -￡0.11/kg for the process to be potentially viable.

In our case study, the company is paying ￡0.5-1.5/kg for CO2 emissions, which is higher than this minimum viable CO2 cost. So this methanation process could be cheaper than paying emission allowances.

Step 3: Develop Process Flow Diagram

Assuming 100% conversion initially, we can develop the basic process flow diagram:

1. Mix the CO2 and H2 feed streams
2. Send the mixed stream to the methanation reactor at 300°C, 50 bar to generate CH4 and H2O
3. Cool the reactor effluent to around 30°C for deep cooling and maximum H2O condensation
4. Use a gas-liquid separator to obtain the CH4-rich gas stream and the condensed H2O stream

This forms the core of the PFD, handling the distribution of components and their molecular transformation via methanation.

Step 4: Eliminate Temp/Pressure/Phase Differences

The feed streams need to be compressed and heated to the 300°C, 50 bar reactor conditions:

1. Compress the CO2 and H2 feeds from atmospheric pressure in multiple stages with intercoolers e.g. 3-stage compression with intercoolers. Calculate stage pressure ratios to avoid excessive temperature rise
2. After the reactor, cool the products to ~30°C for the phase separation of condensed water
3. Use a gas-liquid separator to obtain the CH4-rich gas and liquid H2O streams

The PFD now includes compression, heating/cooling, reaction and separation units.

Step 5: Task Integration

Certain tasks can be integrated into single unit operations for better process representation:

• Use multi-stage compressors with interstage coolers instead of individual compressors/coolers
• Rigorously select the property package for high pressure, non-polar/hydrocarbon systems like this Peng-Robinson is a common choice, validate with others if needed
• Consider any potential byproduct formation in the reactor Water-gas shift, CO2 reforming reactions at very high temperatures Include CO, CO2 byproduct components for accurate phase equilibrium

With these improvements, we can simulate the overall process more rigorously.

DWSIM Simulation Walkthrough

Let me now demonstrate how to implement and simulate this process in DWSIM:

First, in the component selection, specify all components - H2, CO2, CH4, H2O and also include CO as a potential byproduct. Then use the DWSIM recommendation for the property package - for high pressure hydrocarbon systems, Peng-Robinson is suitable.

For the CO2 compression section:

• Set the CO2 feed flow rate to 1 kmol/hr at 25°C, 1 bar
• Add a 3-stage compressor model with intercoolers after each stage
• Calculate and specify the stage pressure ratios (~3.79)
• Use coolers to bring the stream to 40°C after each stage
• The final stage takes CO2 to the 55 bar reactor pressure without cooling

Replicate a similar 3-stage compression section for the 4 kmol/hr H2 feed to 55 bar.

Next, use a mixer unit to combine the compressed CO2 and H2 streams into a single feed to the reactor.

For the methanation reactor:

• Use the Gibbs reactor model to find the equilibrium composition
• Specify 300°C temperature and 50 bar pressure (5 bar pressure drop)
• Allow all components we specified in the component matrix, including the CO byproduct
• May need to re-run the reactor for initialization

After the reactor, use a cooler unit to bring the effluent stream down to around 30°C for maximum water condensation.

Finally, add a gas-liquid separator block to obtain the CH4-rich gas stream and the liquid H2O stream.

In the simulation results:

• Check that the molar flow rates match the expected stoichiometry
• Analyse the gas and liquid phase compositions CH4 should be in the gas phase, H2O in the liquid phase
• Visualise and examine the overall process flow diagram

We can then explore various sensitivity cases:

• Effect of temperature, pressure on the equilibrium conversion
• Formation of byproducts like CO at higher temperatures
• Heat integration options for energy optimisation

That covers the full process simulation of the synthetic methane production process using DWSIM.

Let me know if you need any clarification or have additional questions!

We can discuss this case study in more detail.

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About the author

Dawid Hanak is a Professor of Decarbonisation of Industrial Clusters at the Net Zero Industry Innovation Centre , Teesside University . He brings the world-leading expertise in process design, techno-economic, and life-cycle assessment to drive innovation in industrial decarbonisation. He led the successful delivery of research and commercial projects in industrial decarbonisation, attracting over ￡4m of external funding. As a trusted advisor to businesses, think tanks, and public bodies, Dawid is passionate about sharing his knowledge and empowering others.

He also founded Motivated Academic , a platform where researchers, engineers, and consultants can access resources and training to advance their research and business skills.

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