Draft:Stripping Enhanced Distillation

Stripping Enhanced Distillation (SED) is an innovative method for distillation in which a stripping gas is used for separation in the column. This approach improves the separation performance of the columns and the stripping gas can be recycled afterwards through the vapor distillate and be used in the previous units of the plants.

Common applications

Stripping enhanced distillation (SED) applications include chemical purification, environmental engineering (wastewater treatment), food & beverage processing, petroleum refining, pharmaceutical manufacturing, biofuels production, chemical processing, and water treatment. Amongst these, one of the most important applications of Stripping Enhanced distillation is the purification of Dimethyl ether. Dimethyl ether (DME) is a non-corrosive, non-toxic color-less gas at room temperature which is environmentally friendly and easy to liquefy and is derived from dehydration of methanol. DME can be produced from various feedstocks, such as natural gas, coal, biomass, and municipal solid waste, through the process of methanol dehydration. DME has a lot of useful applications, for instance it can be used as a clean-burning alternative to diesel fuel due to its high cetane number, which means it ignites easily under compression, making it suitable for use in diesel engines with minor required modifications. In addition it can be blended with liquefied petroleum gas (LPG) to enhance the combustion properties and reduce emissions. Using SED for DME purification reduces the energy consumption for distillation by 20–30%, and minimizes the product losses at the same time.

DME production

The primary advantage in producing DME is the flexibility to use a wide range of feedstocks. This adaptability stems from its foundational component, syngas, which can be derived from biomass or various fossil-based raw materials like oil, naphtha, coal bed methane, shale gas, or natural gas through a wide range of processes such as

1. Steam Methane Reforming (SMR)
2. Partial oxidation (POX)
3. Autothermal Reforming (ATR)
4. Gasification
5. Dry Reforming

Sorption Enhanced DME Synthesis (SEDMES) Process Flow Diagram

The syngas produced from these processes can be used downstream for production of DME through a wide range of processes. One of the most common processes for DME production is called Sorption Enhanced DME Synthesis (SEDMES). The SEDMES process converts CO2 and green hydrogen into DME in a single reactor step which results in achieving high conversion rates and efficiency due to the in-situ removal of water and shifting the equilibrium towards product formation. This method has demonstrated the ability to extend catalyst life and enhance overall process efficiency, especially when dealing with diluted CO2-rich gas streams. It can increase the CO2 conversion rate from approximately 10% to about 85%, which significantly reduces the need for recycling and downstream processing. The SEDMES process integrates two steps within a single reactor. Initially, CO2 and hydrogen are converted into DME and water as products. Due to the equilibrium nature of this reaction, DME can revert back to carbon dioxide, preventing full conversion. To counter this, the second step involves adsorbing the produced water using an adsorbent material, thus hindering the reverse reaction. In this process, two commercial catalysts and an adsorbent material are applied: CZA for MeOH synthesis, and alumina for dehydration of MeOH and LTA 3A solid adsorbent for the in situ removal of water. This dual-process approach in one reactor enhances yield and significantly improves efficiency.

Reaction of direct DME synthesis from syngas: ${\ce {2CO2 + 6H2 <-> CH3OCH3 + 3H2O}}$

Stripping enhanced distillation (SED) process flow diagram

DME purification

The DME produced in the SEDMES process requires purification prior to be used as a final product, therefore subsequent further processing is necessary, typically involving multiple distillation stages. In general, two distillation columns are used, the initial column has the task of separating CO2 and other non-condensable gases from DME and the second one is used to further purify the product by removing methanol and water. The initial distillation step poses the greatest challenge, focusing on separating CO2 and residual gases from DME and heavier components such as H2O and MeOH. The complexity of separating CO2 from DME arises from their strong affinity and the proximity and similarity of their vapor-liquid equilibrium (VLE) curves across a wide pressure range, particularly in the Henry’s law region. In this case the Stripping Enhanced Distillation (SED) is a useful method in which the processes of distillation and stripping, are combined to improve the separation efficiency. In this case selection of the stripping gas is a critical matter and the stripping gas needs to have low affinity towards the heavy component or the bottom product of the distillation process. For this purpose, hydrogen and methane can be good options for separation of CO2 from DME as they have low affinity towards DME and methanol.

SEDMES process flowsheet including the SED column. DME synthesis and purification.

In the case of using hydrogen as the stripping gas, an electrolyzer unit is required to supply hydrogen for both the SEDMES unit feed and the stripping gas for the SED column. Figure 3 illustrates the conventional SEDMES process flowsheet, encompassing DME synthesis and purification units. Stripping enhanced distillation facilitates the single-step separation of DME from CO2 and other unconverted gases from the SEDMES multicolumn system. The column has two feed intakes: one for the SEDMES DME product and another for the hydrogen stripping gas. The DME product stream enters the column around the middle section, based on separation requirements, while the hydrogen feed enters below the bottom stage and can be supplied in various ways. the first distillation column is utilized to remove the CO2 and other non-condensable gases from the mixture of DME, water and methanol and the second distillation column is used to obtain pure DME as the distillate product by separating the methanol and water from the mixture .

Modelling approaches

Stripping Enhanced Distillation (SED) can be modeled using a variety of process simulation software packages, including:

• Aspen Plus
• HYSYS
• PRO/II
• CHEMCAD
• gPROMS
• UniSim Design
• Matlab with Simulink

These software packages offer advanced modeling and simulation capabilities to accurately represent the SED process and optimize its parameters.

Process simulation with Aspen Plus

Aspen Plus is a process simulation software developed by Aspen Technology which is used extensively in the chemical, petrochemical, and other process industries for designing, modeling, and optimizing various chemical processes. Aspen Plus enables the simulation of chemical processes to predict the behavior of chemical plants. This includes steady-state and dynamic simulations. Users can create detailed process flowsheets that represent the sequence of operations and the interconnections between different unit operations such as reactors, distillation columns, heat exchangers, and more and the software includes a wide range of thermodynamic models to accurately predict the properties and phase behavior of chemical mixtures.

Equation of state

For simulation of this process in aspen plus, the thermophysical properties of the components involved (e.g., DME, MeOH, CO2, CO, H2 and H2O) are required. These are available in the Aspen Plus database. Furthermore, the physical interactions inside the DME mixtures such as CO2–DME and CO2–H2O interactions should be assigned using the right thermodynamic model. In this case, using the predictive Soave Redlich Kwong equation of state (PSRK EoS) is suggested to be adopted as a default method for simulation and analysis of such process, as it allows the right prediction of the VLE data over a large range of pressure and temperature and is suitable for mixtures containing supercritical compounds.

Type of distillation columns

Aspen Plus offers several types of distillation columns, each designed for different applications and levels of complexity. The three main types of distillation columns in Aspen Plus are RADFRAC, DSTWU, and DISTL.

• RADFRAC is a comprehensive and rigorous distillation model that can handle a wide range of distillation problems, including complex configurations like azeotropic, extractive, and reactive distillations this model uses rigorous equilibrium stages, rate-based methods, or equilibrium stage models and it accounts for vapor-liquid equilibrium, mass transfer, heat transfer, and hydraulics. RADFRAC model is suitable for detailed design and analysis and is used when high accuracy is needed, especially for non-ideal systems or systems involving reactions. However, it is a computationally intensive model and requires detailed input data and more setup time.

An example of the process simulation of the SED in Aspen Plus software

• DSTWU is a shortcut distillation method based on the Underwood, Gilliland, and Winn methods. This model provides a quick estimation of the number of stages and reflux ratio required for a given separation and is a simplified and less rigorous model compared to RADFRAC. DSTWU uses empirical correlations for quick estimations and does not account for detailed stage-by-stage calculations. This model is suitable for quick estimations and scoping calculations when detailed input data is not available. However, it is less accurate than rigorous methods and is not suitable for final design or detailed analysis.

• DISTL is a basic distillation model used for simple binary separations that assumes constant relative volatility and uses the Fenske-Underwood-Gilliland method. This model is limited to simpler separations with fewer components for preliminary design for binary systems and is not suitable for multi-component or complex systems.

The choice of the distillation column model in Aspen Plus depends on the specific requirements of what is needed to be simulated. In this case the suggested method is the RADFRAC as the SED system is a multi-feed system that requires a more detailed and rigorous design. The system works in such way that the feed entering the SED column in the middle and hydrogen is injected at the bottom as the stripping gas. The CO2 and other non-condensable gases are separated as the vapor distillate from the top as offgases with the target of 99% separation of these gases from the DME feed. This is achievable by rigorously optimizing the column using RADFRAC model. The Liquid distillate is extracted at the bottom and is a mixture of DME, methanol and water which furtherly is going through another column in which the water and methanol are separated at the bottom and the DME with the mass purity of 98% can be delivered as the final product of this unit.

References

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