Application of thermodynamics in the design of complex chemical processes

Introduction

Thermodynamics is the scientific basis for the design, optimization and operation of complex chemical processes. Its application allows analyzing the behavior of physical and chemical systems, facilitating the prediction of the feasibility of reactions, the efficiency in the use of energy and the selection of adequate equipment.

In the chemical industry, knowledge of thermodynamic principles is important to develop processes that maximize the conversion of raw materials into high-value products while simultaneously minimizing energy consumption and environmental impact. This discipline is integrated into all stages of process design, from initial conception to industrial implementation.

The purpose of this article is to discuss how the principles of thermodynamics apply to the design of complex chemical processes, highlighting their fundamentals, applications, modeling and simulation, as well as current challenges and trends in the industry.

Fundamentals of thermodynamics in chemical processes

The thermodynamics of chemical processes is the engineering science focused on the study of the interrelationship between heat and work with chemical reactions or physical changes of state within thermodynamic laws, involving measurements of thermodynamic properties and applying mathematical methods for the study of chemical reactions of processes.

The structure of “thermodynamic” chemistry is based on the first two laws of thermodynamics. Starting from the first and second laws of thermodynamics, four mathematical expressions called “fundamental Gibbs equations” can be obtained. From these four, a large number of equations related to thermodynamic properties of the thermodynamic system can be derived using relatively simple mathematics. This outlines the mathematical infrastructure of chemical thermodynamics.

This engineering science focuses on the analysis of the interactions of energy and matter in closed and open systems, establishing the basis for the evaluation of the behavior of chemical processes. Its fundamental laws delimit the operating margins and determine the feasibility of industrial processes. Understanding these laws is indispensable for optimizing operating conditions and ensuring the sustainability of operations.

Thermodynamic laws

• First Law (Conservation of Energy): It states that energy is neither created nor destroyed, only transformed. In chemical processes, this law applies to the energy balance, considering the inputs and outputs of heat, work and matter.
• Second Law (Entropy): Defines the direction of spontaneous processes and states that the entropy of an isolated system always increases. This law is key to evaluate the efficiency of processes and the possibility of energy recovery.
• Third Law: Describes the behavior of substances at temperatures near absolute zero, where the entropy of a pure crystal reaches a minimum value. It is fundamental for the design of processes operating at cryogenic temperatures.

Thermodynamic properties

• Enthalpy (H): Represents the amount of energy exchanged as heat at constant pressure, essential in the design of reactors and heat transfer systems.
• Entropy (S): Measures the degree of disorder of a system and determines the irreversibility of processes.
• Gibbs Free Energy (G): Indicates the spontaneity of chemical reactions, fundamental for predicting chemical equilibrium.
• Heat capacity (Cp, Cv): Allows to evaluate the amount of heat required to modify the temperature of a system, important in the thermal design of equipment.

Thermodynamic analysis of industrial processes

The thermodynamic analysis of industrial processes consists of applying the laws of thermodynamics to evaluate the efficiency and productivity of operations. This analysis begins with the identification of the system and the realization of energy balances to detect losses and to be able to establish improvements in the process. In addition, energy analysis allows quantifying inefficiencies, providing considerable information to optimize processes.

For example, in chemical processes such as distillation, thermodynamics defines the temperature and pressure conditions necessary to achieve efficient separations. Likewise, in complex chemical reactions, thermodynamic analysis is key to identify the reaction equilibrium and improve process performance.

As a complement, the following video presents the fundamental principles of thermodynamics and its role in the energy transformations that occur during chemical processes. Courtesy of: youtube.

Applications of thermodynamics in the design of chemical processes

Thermodynamics plays a fundamental role in the design, analysis and optimization of industrial chemical processes. By applying these laws, it is possible to establish the limits of energy and material efficiency, as well as to determine the optimum operating conditions to maximize the conversion of raw materials and minimize energy losses.

Chemical equilibrium analysis

Chemical equilibrium is critical in determining the optimum operating conditions that maximize product yield. Using Gibbs free energy and equilibrium constants, the appropriate pressure and temperature conditions are analyzed to shift the equilibrium towards the formation of desired products. This analysis is crucial in industrial processes such as ammonia synthesis and methanol production.

Energy efficiency evaluation

Thermodynamics allows the identification of opportunities to improve energy efficiency through the analysis of energy and energy balances. This approach facilitates waste heat recovery, process integration and reduction of energy losses, contributing to sustainability and reduction of operating costs.

Equipment and material selection

The design of chemical reactors, heat exchangers and separation systems is based on thermodynamic principles. The selection of materials resistant to extreme temperature and pressure conditions, as well as the sizing of equipment, is based on the analysis of thermodynamic properties to ensure safety and operational efficiency.

Chemical process modeling and simulation

Modeling and simulation of complex chemical processes are key tools for optimizing industrial design and operation. Specialized software such as Aspen Plus and HYSYS incorporates advanced thermodynamic models that allow predicting the behavior of complex mixtures, evaluating operating conditions and optimizing the use of resources. These tools integrate material and energy balances, phase equilibrium models and energy efficiency analysis, providing accurate solutions for decision making.

The use of optimization algorithms and dynamic simulations makes it possible to analyze operating scenarios, evaluate the impact of variations in process conditions and design advanced control strategies. This results in greater efficiency of industrial processes, reducing costs and increasing competitiveness.

Thermodynamic models in process simulation

The simulation of chemical processes in industrial plants depends to a large extent on the correct selection of thermodynamic models that allow accurate estimation of the physicochemical properties of the mixtures involved. These properties include density, enthalpy, heat capacity, among others, which vary according to composition, pressure and operating temperature. Since it is practically impossible to know all these properties experimentally for each operating condition, it is necessary to resort to reliable prediction methods.

The thermodynamic model choice must be the most appropriate, since an inadequate model can generate inaccurate results that affect the design and optimization of equipment and process streams. The following are the most commonly used thermodynamic methods in the simulation of chemical processes:

1. Equations of State (EOS)

• Peng-Robinson (PR): Used for systems with hydrocarbons and non-polar gases, suitable for high pressure conditions.
• Soave-Redlich-Kwong (SRK): Similar to PR, but with improvements in the prediction of liquid properties and vapor-liquid equilibrium (VLE).
• Redlich-Kwong (RK): Suitable for gases at moderate pressures and elevated temperatures.

2. Activity coefficient models

• NRTL (Non-Random Two-Liquid): Ideal for non-ideal liquid mixtures, especially in separation and distillation processes.
• UNIQUAC (Universal Quasi-Chemical): Applicable to complex liquid mixtures and highly non-ideal systems.
• Wilson: Suitable for fully miscible liquid systems.

3. Models based on excess energy

• Margules: Used for simple binary systems.
• Van Laar: Used in liquid mixtures with moderate deviations from ideality.

4. Empirical correlation methods

• Lee-Kesler: Suitable for calculating properties of light and moderate compounds.

5. Electrolytic system models

• Pitzer: Used in aqueous solutions of salts at high concentration.
• Electrolyte NRTL: Suitable for systems containing electrolytes, acids and bases.

Model selection criteria

To select the appropriate thermodynamic model, the following aspects should be considered:

• Nature of the system: Polarity, presence of electrolytes or complex compounds.
• Operating conditions: Pressure and temperature ranges.
• Phases present: Liquid-vapor, liquid-liquid or solid-liquid equilibrium.
• Experimental data availability: To calibrate or validate the model.

The correct selection and application of these models is essential to ensure accurate and reliable simulations, thus optimizing the design and operation of chemical plants.

Thermodynamic model application case studies

Ammonia synthesis (Haber-Bosch process)

The ammonia synthesis by the Haber-Bosch process requires an exhaustive analysis of the chemical equilibrium between nitrogen and hydrogen, catalysts, pressure and temperature. The optimization of these variables is based on thermodynamic models that maximize conversion and minimize energy consumption, ensuring the economic viability of the process.

Oil refining

Fractional distillation, catalytic cracking and reforming processes require precise control of temperatures, pressures and mixture compositions. The application of thermodynamic principles is essential to design distillation columns, select operating conditions and improve product recovery, optimizing overall refinery efficiency.

Current challenges and trends in the chemical industry

Thermodynamics represents an important engineering science within the chemical industry, as it directly affects both process efficiency and the optimization of chemical reactions and sustainability. One of the fundamental principles of thermodynamics is the conservation of energy, which implies that all chemical processes must carefully consider energy inputs and outputs to avoid waste and maximize efficiency. In this context, thermodynamics helps to identify optimal conditions for reactions, such as temperature and pressure, resulting in greater efficiency in the use of resources and energy.

In reactive systems, thermodynamics must effectively provide the basis for interpreting how and why reactions occur, and under what conditions they are most favorable. Optimization of reaction conditions, such as the choice of catalysts, manipulation of temperature and pressure, and control of energy, can maximize the conversion of reactants into useful products. Through enthalpy, entropy and Gibbs free energy changes, more efficient processes can be designed, reducing costs and improving reaction yields.

In addition, thermodynamics is essential for process simulation. Thermodynamic models make it possible to simulate the behavior of systems at the molecular and predictive level, which facilitates process optimization prior to large-scale implementation. This allows adjustments to operating conditions and improved efficiency.

In terms of sustainability, thermodynamics also plays a key role in energy and resource management. Thermodynamically efficient processes not only reduce energy consumption, but also minimize the generation of unwanted by-products and waste, aligning with efforts to reduce the environmental footprint. For example, systems that take advantage of waste thermal energy and efficient thermodynamic cycles, such as gas turbines or heat exchangers, contribute to sustainability by optimizing the use of available energy resources.

The integration of thermodynamics into current technological trends applied to chemical processes is expected to contribute to automation, adjusting and controlling process variables in real time to maintain optimal conditions and improve operational efficiency. In addition, this would allow faster and more accurate decisions to be made, continuously optimizing processes based on detected variations.

Conclusions

Thermodynamics is the basis for the design of complex chemical processes, providing the necessary tools to optimize the efficiency, safety and sustainability of industrial operations. Its rigorous application allows maximizing the conversion of raw materials, minimizing energy consumption and mitigating environmental impacts, contributing to the development of competitive and sustainable industrial processes.

Efficient simulation of chemical processes depends directly on the proper selection of thermodynamic models, since they allow accurate prediction of the behavior of systems under various operating conditions. Choosing the right model determines the accuracy of the calculations, and contributes to the optimal design and safe operation of industrial plants, fundamental aspects for the competitiveness and sustainability of the chemical sector.

Thermodynamics underpins the optimization of today’s chemical processes. Its understanding improves reaction and production efficiency and facilitates sustainability by ensuring the proper use of energy and resources. Integrating these principles with emerging technology trends, such as advanced simulation, automation and artificial intelligence, further enhances the transformation of the chemical industry towards a more efficient and environmentally friendly future.

References

1. Smith, J. M., & Van Ness, H. C. (2005). Introduction to Chemical Engineering Thermodynamics (7th ed.). McGraw-Hill.
2. Jou, F. Y., Yaw, D. C., & Foster, R. F. (2010). Chemical Engineering Thermodynamics (2nd ed.). McGraw-Hill.
3. Smith, R. (2005). Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design (2nd ed.). Butterworth-Heinemann.