# SEPARATION COLUMNS

OVERVIEW

The course is focused on separations columns, and in particular columns for absorption and distillation. Thermodynamics is recalled briefly, and then the main equations are derived on a theoretical basis. Graphical tools are illustrated as well, with attention tothe underlying simplifying hypotheses. The theory is developed in parallel with exercises. In particular, a distillation problem is solved with UniSim (process simulation software) in the computer classroom.

## AIMS AND CONTENT

LEARNING OUTCOMES

Separation columns: design capability through the use of one the most modern software (UniSim). Capacity of management of pre-existing columns. Optimization of operating parameters of pre-existing columns.

AIMS AND LEARNING OUTCOMES

At the end of the module the student will be able to:

- design separation columns using graphical methods and by using design softwares (UniSim);
- manage and optimize the operating parameters of pre-existing columns;

Furthermore, the student will improve transversal skills such as communication skills and the ability to work in teams through the group work in the PC laboratory.

Teaching methods

The module consists of lectures (48 hours) and classes in the computer lab (2 hours).

SYLLABUS/CONTENT

The module deals with the columns for separation processes, i.e.e absorption and distillation. In each case, after some preliminary references to thermodynamics, the equations are developed and the semi-graphical methods that constitute the conceptual basis of modern numerical calculation methods are illustrated.

**Absorption.**

Introduction: Overview of the chemical-physical principles. Simple absorption. Stripping.

Study of local and global exchange coefficients: The absorption mechanism: the double film theory (Whitman) and the more recent theories (Higbie, Dankwertz). The double film theory: derivation from the theory of diffusion through a stagnant film. Correlations between local and global trasfer coefficients. Regimes: Gas Film Limiting, Liquid Film Limiting and Mixed.

Absorption equipment: Comparison between packed and plate columns. Packed columns: general characteristics. The various types of packings.

Fluid dynamic effects in packed columns: pressure losses in packed columns. Loading, Flooding and Minimum Wetting Rate.

Thermal effects in absorption columns: methods for evaluating temperature profiles in packed and plate columns. Typical temperature profiles. Consequences of thermal increases on the minimum (L / G) ratio, and on the performance of the column. Cooling systems.

Packed absorption columns: Calculation of the height of the column in the case of an isothermal column. Number of exchange units and height of exchange unit. Calculation of the minimum L / G ratio. Considerations on the evaluation of the minimum L / G in non-isothermal packed columns.

Plate absorption columns: Characteristics of the plates used for absorption columns: geometries and efficiencies. Calculation of the number of plates: graphical method based on the use of the operating line and the equilibrium curve. Graphical method based on the absorption factor.

Absorption associated with chemical reaction: Regimes: slow reaction, intermediate regime, fast reaction, instantaneous reaction.

**Distillation**

Continuous distillation of binary mixtures: Thermodynamics. Definition of volatility and relative volatility. McCabe-Thiele method: minimum reflow ratio (demonstration of the Underwood equation) and maximum (infinite). Corresponding values of the number of theoretical plates: maximum (infinite) and minimum (demonstration of the Fenske equation). Modifications necessary to apply the McCabe-Thiele method to the calculation of the number of theoretical plates in the case of columns with multiple feeds and side streams. Stripping and rectification columns.

Batch distillation of binary mixtures: Case of operation at constant product composition: calculation of the required heat and total process time. Case of operation at constant reflux ratio R: calculation of the required heat and total process time.

Continuous distillation of multi-component mixtures: Short-cut calculation methods: Erbar-Maddox method for the calculation of the number of plates. Fenske equation for the calculation of the minimum number of plates. Underwood method for the calculation of the minimum reflux ratio. Short-cut methods for the choice of the feeding plate. Rigorous calculation methods: the MESH equation system and the numerical methods of resolution.

Distillation column design: types of plates. Efficiency. Choice of the diameter of the column. Column fluid dynamics: weeping, coning, entrainment and flooding regimes. Packed distillation columns.

For each topic, the theoretical part is supported by practical exercises related to the calculation (design and verification) of the columns. The exercises are carried out either through analytical or graphical methods, or at the computer (in the computer lab) by using one of the most modern specific softwares (UniSim).

RECOMMENDED READING/BIBLIOGRAPHY

All the slides projected during the lessons are available in aul@web. The books listed below are suggested as supporting texts:

- R. Sinnott & G. Towler, Chemical Engineering Design, Fifth edition, Elsevier Science (2009).
- J.F. Coulson, J.H. Harker, Chemical Engineering, Vol. 2, Fifth Edition, Elsevier Science (2002).

## TEACHERS AND EXAM BOARD

**Ricevimento:** Please contact via e-mail:
paola.costamagna@unige.it

## LESSONS

Teaching methods

The module consists of lectures (48 hours) and classes in the computer lab (2 hours).

LESSONS START

Lessons start on February 22nd, 2021.

## EXAMS

Exam description

The exam is written-oral: the teacher assigns some questions, typical of an oral exam, to be answered and discussed in writing.

Assessment methods

The exam is written-oral: the teacher assigns some questions, typical of an oral exam, to be answered and discussed in writing. The questions are related to the various types of separation columns. The student must demonstrate that she/he has acquired design skills using traditional graphic methods, and that she/he is familiar with the calculation methods implemented by modern simulation softwares. The student must also demonstrate that she/he has acquired management skills for pre-existing columns.