Kristian M. Lien

Separation process design and synthesis based on thermodynamic insights

Abstract This paper presents a new methodology which employs physicochemical properties and their relationships to separation techniques for design and synthesis of separation processes. The methodology covers a wide range of separation problems and consists of selection and identification of separation techniques (and the corresponding separation tasks), the sequencing of the separation tasks and the determination of appropriate and consistent conditions of operation. For a specified multicomponent separation problem, subsets of feasible separation techniques are first identified for every binary and user-specified separation tasks. The number of alternatives for each separation task is reduced by systematically analyzing the relationships between properties, separation technique and conditions of operation. After the final step, an estimate of the final process flowsheet is produced with a list of possible alternatives for the separation tasks. Application of the methodology is demonstrated through examples highlighting the different steps of the methodology.

Complex distillation arrangements : Extending the Petlyuk ideas

Abstract The task of separating a multicomponent mixture into streams enriched in the respective constituents is commonly carried out in vonventional distillation columns arranged in series. However, due to the scrutiny of tighter requirements for energy and cost efficiency, current research aims at alternative column arrangements that offer savings in both operational (energy) and capital costs. Among these are the Petlyuk or dividing wall column, in which three components are separated in a single shell using only one reboiler and one condenser. In this paper we extend the Petlyuk ideas to separations of four components, although extensions to more components is straightforward. We provide a general definition of Petlyuk arrangements and discuss alternative structures from the literature. Following this overview we consider the arrangements which allows for implementation in a single shell using dividing walls or vertical partitions.

A new robust design model for gas separating membrane modules, based on analogy with counter-current heat exchangers

Abstract This paper presents a simple and robust algebraic design model for hollow fibre membrane modules used in gas separation. A major rationale for making such a model is that membrane separation is an area of increasing importance, but still there does not yet seem to exist any multicomponent model which is simple and robust enough to be useful in synthesis applications. The presented model has been verified against numerical and experimental results from the literature. The results presented here show that product purity is typically predicted within 2%. Module capacity prediction compares favourably with other models from the literature. It has been shown how the model can be extended to take into account pressure drop on the bore side of the hollow fibers. To our knowledge, this is the first reliable algebraic model for gas membrane separation systems able to handle multicomponent mixtures. The algebraic nature of the model and its simplicity should hereafter make it easier to optimize membrane system structures and operating conditions.

Difference points in extractive and reactive cascades. I – Basic properties and analysis

The interactions between reaction and separation in extractive and reactive cascades are addressed through the use of difference points. It is shown how the structural elements in a generic cascade such as (side) feeds, products and chemical reactions combine to a normalized linear combination where the effect of each individual contribution is readily visualized. We derive and analyze the fundamental mathematical and geometric properties of these difference points for individual and aggregated units. Finally, it will be shown how to draw straight sectional and overall material balance lines in composition space for reacting systems through the use of pseudo-compositions.

Diabatic column optimization compared to isoforce columns

Abstract This paper analyzes and compares optimization results for simulated diabetic columns to theoretically derived optimization results from the isoforce method for the same system. The separation of ethanol and water is used to illustrate the two methods and bring out their common and special features. The results, which are presented in McCabe-Thiele diagrams, show partial compatibility between the methods. The isoforce method is found to require total reflux ratios close to the azeotrope, and a reformulation of the method is suggested. It is also shown that the separation work obtained in an adiabatic stripping column can not be achieved in a diabatic column without adding a (distributed) condenser in the upper section of the column, or more plates.

Design of hybrid distillation and vapor permeation processes

Abstract An explicit algebraic design model for vapor permeation systems is presented. The model is valid for binary mixtures, and is based on a black-box representation of the transport properties across the membrane. Based on this design model, some of the trade-offs in a hybrid distillation and vapor permeation process are illustrated through parametric studies.

Multiplicity in reactive distillation of MTBE

This paper presents an explanation of why methyl tert-butyl ether (MTBE) production by reactive distillation may yield multiple solutions. Widely different composition profiles and conversions may, as already reported by Krishna and others, result with identical column specifications, depending on the initial estimates provided and the VLE models being used. A hypothesis yielding a qualitative understanding of this phenomenon has been developed. The inert n-butene plays a key role in the proposed explanation. As the reaction mixture is diluted with n-butene, the activity coefficient of methanol increases substantially and the temperature decreases. This dilution has a profound effect on the equilibrium conversion, enabling MTBE to escape from the reactive zone without decomposition. The minimum boiling point azeotrope between MTBE and methanol plays an important role in the internal transport mechanism for the heavy methanol, in particular when fed below or in the lower part of the reactive section.

The role of expert systems technology in design

Using a scenario format, this paper first reviews the nature of chemical process design, showing that designers quickly make major decisions with minimal information and constantly revise their strategy to solve a problem. To automate this activity on a computer will require models of the process being created at several levels of abstraction as well as models that capture the beliefs of the modeler about the abilities of himself, others and the aids available and models of strategies for complex problem solving. The second section of the paper extensively reviews current expert system concepts, illustrating each of them with design examples. It is argued that expert systems are knowledge based. The authors describe many of the control strategies used in todays systems, and also consider different problem representations - rules, logic and frames - and indicate when each might be preferred. The last section presents the authors views on what will be involved in creating a future expert system for design

Difference points in extractive and reactive cascades. II: Generating design alternatives by the lever rule for reactive systems

Abstract Using linear combinations of vectors in composition space, we formulate a lever rule for reactive distillation columns. This lever rule facilitates the proposal of alternative sequences of reactive distillation systems by allowing us to visualize how material balance constraints move as a function of reaction “turnover”. Our approach uses the concept of a pseudo-feed, which is the composition that results from mixing column distillate and bottom products. Our lever rule for reactive distillation columns uses linear combinations of so-called reaction difference points, stoichiometric coefficient vectors of reactants and products, and composition vectors. When a reaction causes no change in the total number of moles, the reaction difference point moves to infinity (Hauan, Omtveit & Lien (1996). Paper 5f, A.I.Ch.E. Annual Meeting, Chicago, Hauan, Westerberg & Lien (1999a). Chemical Engineering Science, 55(6), 1053–1075. Hauan, Ciric, Westerberg & Lien (1999b). Chemical Engineering Science, in press). We show how to carry out all geometric constructions entirely within a finite composition domain by decomposing the total stoichiometric coefficient vector into product and reactant stoichiometric coefficient vectors. In this case the lever rule compares distances along two parallel vectors. For an infinite extractive difference point (Westerberg & Wahnschafft (1996). Advances in Chemical Engineering, 23, 63–170) we can confine all geometric manipulations when proposing alternative reactive distillation processes to a finite composition domain by combining the vectors in a different order.

A graphical method for designing reactive distillation columns. I. The Ponchon-Savarit method

We show how to construct a Ponchon–Savarit diagram for a binary reactive distillation column, illustrating it specifically for isomerization and decomposition reactions. We first show the properties needed for points to lie on a straight line in composition/enthalpy space. Then, for the isomerization reaction, we show how to step off the stages using a reactive cascade difference point. In the Ponchon–Savarit diagram, the reactive cascade difference point has two elements. One is the composition coordinate formed as a linear combination of stoichiometric coefficient vectors and the top product composition. The other is the enthalpy coordinate formed by combining the top product molar enthalpy, the condenser molar duty and the molar heat of reaction. Finally, and in a similar manner, we construct the Ponchon–Savarit diagram for a decomposition reaction.