(668c) Using Process Simulators in Chemical Engineering Lectures: Case Study, Ethyl Lactate Production

The need for new products and a new culture of caring for the environment, saving and optimizing energy, the rationalized use of fuel, etc, makes necessary to Chemical Engineering look for feasible and economic alternatives to solve these new challenges. Process simulation is a process of physical or chemical transformation by a mathematical model involving the mass and energy balances coupled to phase equilibrium, transport equations and chemical kinetics. This is an efficient and effective tool for analysis, synthesis and optimization of the process.

The development of process simulators worldwide has unfolded rapidly. Mainly due to three aspects: a) Computers with faster processors, graphical interfaces that facilitate the handling of graphics, storage of large amounts of data. b) Structured programming languages that facilitate more efficiently  the solution of systems of equations and the same simulation. c) Determination of properties of mixtures and components with less deviation of the data obtained experimentally.

From real data (used to obtain a mathematical model and/or validate the results of the model or simulation) can gain information that allows analysis, synthesis and optimization. Furthermore, there is not required a lot of experiments that increase the cost and destabilize the operation of any process plant, because if the pattern used is appropriate (after validation), the reproducibility of results is excellent.

Therefore, the Department of Chemical and Environmental Engineering of the National University of Colombia has been working since 1992 with the simulator HYSIM (later renamed HYSYS), since 2001 with ASPEN ENGINEERING SUITE and created the Process Simulation course in 2003. This course aims to integrate, reinforce and consolidate the knowledge of the discipline courses, present the advantages for parameter estimation, fit and model validation, design, simulation, analysis and optimization of process simulators aspenONE ® V .7.3, SuperPro Designer ® and BR & E ProMax ®, and the interaction of these with Excel and Matlab.

This article aims to present a study conducted by a group of students that make up the subjects (Process Engineering, Plant and Equipment Design) and Modeling and Simulation of Chemical Processes, which select a compound of interest, evaluating different synthesis routes thereof and select one. Simulate the selected route in ASPEN PLUS and ASPEN HYSYS and propose an utility equation to selection and subsequent equipment sizing and optimization. The case study is ethyl lactate production from lactic acid and ethanol.

Ethyl lactate, also known as green solvent is an organic ester, biodegradable, used as a food additive, flavoring and cellulose, cellulose acetate, cellulose ethers, and various resins solvent. It is a particularly attractive solvent in the coating industry because of its high solvency, high boiling point, low vapor pressure and low surface tension. Likewise, ethyl lactate is a desirable coating for wood, polystyrene and metals, and also acts as an effective separator and paint remover. Also, ethyl lactate holds promise as an effective and nontoxic replacement for petroleum-based solvents.

One of the production routes of ethyl lactate (L₁E) is the esterification from aqueous lactic acid solution (88 wt %) in the presence of Amberlyst 15 cation-exchange resin as a catalyst. This heterogeneous catalyst has the advantage of be friendly with the environment, is not corrosive, easy to separate, and easy handling and storage in comparison with other homogenous catalysts like sulfuric acid.

Reactions are carried out in a chemical reactor or in a reactive separation column, in the first are necessary more unit operations for ethyl lactate separation and recovery. The use of reactive distillation allows an efficient production of organic acids esters, because the reaction and separation process integration in a unique equipment. Furthermore, the higher conversion gained is comparable with the equilibrium conversion in a batch simple reactor. These synthesis pathways have undesired reactions because of the bifunctional nature lactic acid, which undergoes intermolecular esterification in aqueous solutions above about 30 wt % to form linear dimer (L₂) and higher oligomer acids (mainly L₃, L₄). The extent of homoesterification increases with increasing acid concentration, thus complicating the use of lactic acid as a reactive substrate. When mixed with an alcohol, lactic acid and its oligomers undergo esterification.

The extent of oligomerization is inversely related to the water content of the solution.  In reactive distillation or any other lactic acid esterification scheme, oligomer formation and esterification adversely affect ethyl lactate yield and pose a considerable challenge in predicting process behavior. For accurate design, therefore, it is critical to include and characterize oligomer reactions in the process model. Ethyl lactate (L₁E), typically the desired product, can be recovered from the mixture, but its yield is reduced from the theoretical maximum by the presence of the oligomeric compounds.

The monomer (L₁) esterification and oligomer (L₂, L₃) formation and esterification, are completely described by the following set of reactions:

L₁+ EtOH ⇌ L₁E+H₂O

L₂+ EtOH ⇌ L₂E+H₂O

L₃+ EtOH ⇌ L₃E+H₂O

L₂+ H₂O ⇌ 2L₁

L₃+ H₂O ⇌ L₁+ L₂

In lactic acid esterification, where a series of relatively slow reactions occurs, a reliable knowledge of the kinetics is, thus, essential to simulate the process effectively. The kinetics of esterifying lactic acid and its oligomers with ethanol over Amberlyst 15 cation-exchange resin have been determined via batch reaction studies.

The kinetic model is useful in both batch and continuous process designs for lactic acid esterification. It uses nth-order, reversible rate expressions for esterification and oligomerization reactions. Because the reversible reactions, concentrated (88 wt %) lactic acid is the preferred feedstock because it contains relatively little water, also because less alcohol is required and is not necessary feed preheating.

Components such as diethyl lactate (L₂E) and triethyl lactate (L₃E) not found in ASPEN HYSYS and ASPEN PLUS databases, estimation of their properties (in both simulators) was performed using its structural formula and physical properties found in the literature. Also was performed the estimation of the thermodynamic model parameters and its validation using equilibrium data reported in the literature.

In this way a comparison was made between the process that use a single reactor another that use a reactive distillation column, from the operational, of safety, and economic point of view, presents the advantages and disadvantages of each. We performed the design, modeling, simulation, equipment sizing, optimization and energy integration of the process. To optimize the process, we performed a sensitivity analysis between the main independent variables of the process such as temperature, excess molar ratio, feed tray, Holdup time, among others, to see the behavior of the conversion, selectivity of lactate acetate and raised economic function for the two production routes.