Dynamics & Control of Distillation Columns
Consists of a review of column design followed by a detailed study
of the control and dynamics of columns. Topics include principles
of design & operation of packed bed and plate types of distillation
column; vapour liquid equilibria, tray design, operating lines,
number of theoretical stages, stage efficiency, significance of
reflux ratio: dynamic models of the distillation process; vapour & liquid
flow lags, composition lags, classical assumptions, effective XY
curve, mass balances in deviation form, state space model of simple
column: strategic control considerations; cut and separation, degrees
of freedom, economic incentives: conventional strategies for column
control; primary measurements, inferential measurement, mass and
energy balance schemes, dual composition control, Ryskamp scheme,
feedforward control, pressure control, internal reflux control,
override control, etc.: application of modern column control techniques;
steady state and dynamic decoupling, model based predictive control,
optimisation of column performance.
Coursework consists of demonstrations and simulation exercises
on the control and dynamics of distillation columns based upon
||CME 8384 (formerly ACS 684)
Control Schemes and Strategies (CME 8376)
||By report on assignment
By 1 x 2 hour examination
To provide a comprehensive, in-depth, understanding of the principal approaches to the control of distillation columns and their principles of operation, and to understand the dynamics of columns and the interactions involved.
To develop an understanding of the principles of the design of distillation columns.
To appreciate the objectives and operational constraints on the control of distillation columns.
To develop a feel for the dynamics of a distillation column, the interactions involved and the structure of dynamic models of such.
To become familiar with a range of conventional methods of controlling distillation columns.
To introduce modern techniques of column control and provide a basis for applying them in an industrial context.
It is essential that delegates have completed (or be familiar with the material covered in) the Control Schemes and Strategies (CME 8376) module before doing this one.
It is desirable, but not essential, that delegates have completed (or have some familiarity with the material covered in) the Modelling and Simulation (CME 8380) module before doing this one.
This module is of one week's full-time intensive study consisting of a variety of lectures, informal tutorials, problem solving, case studies and structured computer-based laboratory work. It is followed by an assignment to be carried out in the delegate's own time.
The time allocation for practical work provides for an exercise on column design based on the McCabe Thiel method and simulation exercises on the dynamic response and control of columns using the Matlab and Simulink packages. Pre-developed models of column dynamics will be provided as appropriate.
King M, Process Control: A Practical Approach, Wiley, 2011
Love J, Process Automation Handbook, Springer, 2007.
Luyben W, Practical Distillation Control, Van Nostrand, 1992.
Marlin T E, Process Control: Designing Processes and Control Systems for Dynamic Performance, 2nd Edition, McGraw Hill, 2000
Roffel B & Betlem B, Process Dynamics and Control, John Wiley & Sons, 2006.
Shinsky F, Process Control Systems: Application, Design and Tuning, 4th Edition, McGraw Hill, 1996.
Column design: Concept of binary mixtures and vapour-liquid equilibria. Ideal mixtures. Standard terminology. TXY and XY diagrams. Effect of pressure. Henry’s and Raoults laws. Relative volatility. Azeotropes. Concept of theoretical stage. Relationship between stages and plates. Murphree efficiency. Overview of layout of distillation plant: columns, stills, reboilers, condensers, reflux drums, receivers, pumps, etc. Alternative types of ancillary plant. Review of column internals: bubble cap and sieve plates, downcomers, weirs, etc. Classical assumptions such as constant molal overflow. Mass balances around top & bottom of column. Concept of operating lines. McCabe Theil method of design. Calculation of number of plates. Effect of reflux ratio on slope of operating lines. Total reflux. Minimum reflux ratio. Position of feed plate. Significance of condition of feed and slope of the q line. Effective XY curve. Batch distillation. Constant take-off rate versus product composition. Multicomponent distillation: extension of binary concepts to multicomponent mixtures. Key components. Sidestreams and multiproduct columns. Azeotropic and extractive distillation.
Control schemes: Economic incentives: over purification, minimum pressure, etc. Control objectives: quality, quantity and energy. Mass and energy balance control. Degrees of freedom: strategic and inventory variables, etc. Effect of sidestreams. Rijskamp scheme: pros and cons. Temperature profiles. Direct and inferential measurement of composition. Location of temperature measurements. Differential temperature. Pressure correction methods. Use of analysers. Control of drum level. Drum dynamics. Condenser duty control. Manipulation of reflux and take-off rates. Control of overhead composition. Flooding constraints and protection. Steady state and transient effects of varying reflux rate. Simultaneous control of top and bottom compositions. Still level control. Cascade control of reboiler. Feedforward control for feed disturbances: feedrate, composition and enthalpy. Pressure control by manipulation of cooling water flow rate, inerts venting and vapour take-off. Vacuum control by manipulation of air bleed. Internal reflux control. Constraint control. Control structure: interactions, sensitivity and relative gain array. Steady state and dynamic decoupling. Control of multiple columns. Advantages of model based predictive control. Optimising control.
Column dynamics: Objectives of modelling. Use of different model types. Tools for simulation: Matlab, Simulink & Speedup. Decomposition of vapour, liquid and concentration lags. Relative magnitudes. Vapour flow lags due to vapour and liquid holdup. Resistance-capacity model for single plate. Nature of liquid flow lags. Hydraulic model for single plate. Concentration lags: interactions between plates. Dynamic mass balance for single stage. Classical assumptions. Use of deviation variables and slope of effective XY curve. State space model for three stage column. Solution in terms of transfer functions. Structure of solutions. Dominant lag and holdup. Reflux control scheme. Explanation in terms of movement of stage corners on effective XY diagram. Superposition of dynamics of column and ancillary equipment.