Eva Sørensen

Detailed mathematical modelling of membrane modules

Abstract Membrane technology is used for a wide range of separations from particle-liquid separations to gaseous and liquid–liquid separations. In this paper, we introduce a detailed model that describes a general membrane separation. The model disregards many common assumptions such as plug flow; constant temperature; constant pressure; binary mixture; steady-state conditions; and constant physical properties. Our approach is applicable to any membrane separation and in this paper we demonstrate its application to both liquid mixture separation (pervaporation) and gas separation in hollow-fibre modules. Both cases are seen to exemplify the need for a detailed model.

Total reflux operation of multivessel batch distillation

The multivessel batch column presented in this paper provides a generalization of previously proposed batch distillation schemes. A simple feedback control strategy for total reflux operation of a multivessel column is proposed. The feasibility of this strategy is demonstrated by simulations. An experimental column based on the proposed control scheme has been built and the experiments verify the simulations.

Simultaneous optimal design and operation of multipurpose batch distillation columns

This work presents a robust method for the integrated design and operation of batch distillation whereby optimal column sizing, process flexibility and operating policies are obtained simultaneously based on the complex economic trade-offs between capital investment, production revenue and utility costs. The proposed stochastic framework, which utilises a Genetic Algorithm and a penalty function strategy, is found to be successful in obtaining profitable and feasible column designs for many design scenarios including binary and multicomponent mixtures, single duty and multipurpose columns, as well as for regular and complex column configurations. The method can also be used with column models of different complexity. Given a set of design specifications and separation requirements, the optimal number of stages, reboiler duty, reflux profiles, product recoveries, time interval of each distillation tasks, process allocation and number of batches can be obtained. Several design case studies are presented and a comparison of optimal designs for various design scenarios, such as different production time, capital costs, process allocation and mixture characteristics, are discussed.

Control strategies for reactive batch distillation

A batch reactor may be combined directly with a distillation column by distilling off the light component product in order to increase the reactor temperature or to improve the product yield of an equilibrium reaction. The same amount of the light product should be removed as the amount being formed by the reaction at any time. A linearized model has been developed which describes the process behaviour satisfactorily for control analysis purposes. The controllability of a combined batch reactor/batch distillation column is found to depend strongly on the operating conditions and on the time during the run. In general, controlling only the reactor temperature (one-point bottom control) is difficult since the set-point has to be specified below a maximum value in order to avoid break-through of an intermediate component in the distillate. This maximum value may be difficult to know a priori. For the example considered in this study, control of both reactor temperature and distillate composition (two-point control) is also found to be difficult due to large interactions in the column. As with one-point bottom control, the reactor temperature has to be specified below a maximum value. However, energy can be saved since the heat duty can be decreased with time. Controlling the temperature on a tray in the column (one-point column control) is found to give good performance for the given process with no loss of reactant and a high reactor temperature, although no direct control of the reactor temperature is obtained.

A cyclic operating policy for batch distillation–theory and practice

This paper sets out a cyclic operation policy for batch distillation with repeated filling and dumping of the reflux drum. The policy has several advantages compared to conventional schemes; it achieves the maximum attainable separation in the column; there is a minimal need for control and it is less sensitive to disturbances and therefore safer to operate. In this study, the optimal operation of the policy is considered for different column configurations and it is found to have a considerable lower operating time than conventional policies for some separations. The practical implementation of the cyclic policy is discussed and results from a laboratory batch column with 8 sieve trays are presented. It is found that the cyclic policy is indeed easy to operate and yields satisfactory separation.

Design of high productivity antibody capture by protein A chromatography using an integrated experimental and modeling approach

An integrated experimental and modeling approach for the design of high productivity protein A chromatography is presented to maximize productivity in bioproduct manufacture. The approach consists of four steps: (1) small-scale experimentation, (2) model parameter estimation, (3) productivity optimization and (4) model validation with process verification. The integrated use of process experimentation and modeling enables fewer experiments to be performed, and thus minimizes the time and materials required in order to gain process understanding, which is of key importance during process development. The application of the approach is demonstrated for the capture of antibody by a novel silica-based high performance protein A adsorbent named AbSolute. In the example, a series of pulse injections and breakthrough experiments were performed to develop a lumped parameter model, which was then used to find the best design that optimizes the productivity of a batch protein A chromatographic process for human IgG capture. An optimum productivity of 2.9 kg L⁻¹ day⁻¹ for a column of 5mm diameter and 8.5 cm length was predicted, and subsequently verified experimentally, completing the whole process design approach in only 75 person-hours (or approximately 2 weeks).

Experimental verification and optimisation of a detailed dynamic high performance liquid chromatography column model

A detailed mathematical model of chromatographic separation processes is of crucial importance due to the increased application of this separation technique in the downstream processes of the biotechnological, pharmaceutical and fine chemical industries. In this paper, the development of a detailed dynamic model of an HPLC unit based on the equilibrium-dispersive approach and its verification by comparison with real experimental data, are described. Good agreement between the simulated elution profiles and the experimental data was obtained. Using this model, optimisation of chromatographic separation was conducted on a closed-loop recycling preparative chromatographic unit. It was found that the effective column length can be increased by recycling the feed sample back to the column for further purification which may improved purity and yield of the process.

Multi-objective optimisation of batch separation processes

Abstract This paper considers for the first time the simultaneous multi-objective optimisation of design and operation of batch distillation as well as of batch hybrid distillation/pervaporation processes. The overall problem is formulated as a multi-objective mixed integer dynamic optimisation (MO-MIDO) problem. The optimisation strategy comprises of different ranking procedures that allow the determination of the Pareto optimal set. A case study for the separation of a homogeneous tangent-pinch (acetone–water) mixture is presented for a dual-criteria optimisation case of minimising capital investment while at the same time minimising the energy consumption rate during the batch. It is found that the proposed distance ranking procedure yields the best Pareto optimal set when compared to other non-dominated sorting procedures. Furthermore, the distance ranking procedure was found to be further improved when used with an elitism operator.

Modelling of Direct Contact Membrane Distillation for Desalination

Direct Contact Membrane Distillation is a new process with exciting opportunities for use in desalination. With 60% of the worlds population expected to be experiencing severe water shortages by 2025, and unhealthy water estimated to be responsible for 90% of all disease in developing countries, the production of clean, affordable water is becoming increasingly important in order to sustain life on Earth. In this work, a mathematical model of membrane distillation was developed, taking account of all aspects of the process, more so than anything previously published. Its use can enable membrane distillation to fulfill its potential, and provide clean water worldwide.

Implementation and Student Perceptions of e-Assessment in a Chemical Engineering Module.

This paper describes work carried out at the Department of Chemical Engineering at UCL into the use of e-assessment in a second year module and, in particular, the student perceptions of this mode of assessment. Three quizzes were implemented in Moodle, the first two as formative assessment and the final quiz as summative assessment. The results were very encouraging and practically all students engaged with the process. An online survey was delivered to all students after the module, which showed that the students felt that e-assessment added value to their learning and they would like to see it implemented in other modules. The quizzes were intended to be mainly beneficial to the weaker students as it gave them an opportunity to go over key aspects of the material in their own time. Interestingly, the stronger students were even more in favour of e-learning than the weaker students, for whom the quizzes were originally designed.