Problem Formulation

The equations used to model a dynamic process form a set of differential algebraic equations (DAEs). The numerical solution of these equations requires them to be reduced to a matrix of algebraic equations (AEs) and ordinary differential equations (ODEs). The number of differentiation steps required to complete this task is referred to as the index of the problem. Numerical solution methods are now available for systems of equations with an index greater than one (i.e. high index problems) but...

Acknowledgements

The continuous support and generosity of my supervisors, Assoc. Prof. Moses O. Tad , Prof. Terry N. Smith and Prof. Ravi Datta, was received with gratitude at all times. The exceptional technical skills of Mr. Colin Glassen and the assistance of many members of the technical staff at Curtin were crucial during the construction of the reactive distillation pilot plant and were greatly appreciated. The contribution of numerous and unknown reviewers was also significant, and their detailed...

Optimisation Procedure and Techniques

The objective function should reflect the net value added or profitability of the process. Table 6.9 indicates the typical value of each component in the feed, distillate and bottoms product, and also the value of energy. The values shown (in dollars per tonne) were assumed for this study and do not necessarily reflect current prices although care was taken to make the scenario realistic. With respect to the optimisation, the units and magnitudes are arbitrary (only the relative values affect...

Operating Modes of the Integrated Controller

Boiler Distillation Still

The ETBE purity and the isobutene conversion must both be controlled to specified targets in this mode. Ideally, this would be best achieved using direct measurements of the two control objectives in order to avoid the uncertainty that is associated with an inferential indication. However, this approach is impractical with linear controllers. The limitations of using a direct measurement of the ETBE purity or the conversion have been discussed previously for both one-point and two-point...

Overhead Condenser

The overhead condenser was constructed as a double pipe exchanger with condensation in the inner pipe. Chilled water from the shared laboratory network was used as the cooling medium. The duty required at maximum reboiler duty 7.5 kW was estimated to be 7.3 kW via simulations. This is attained at a cooling water flow of approximately 10 L min with a chilled water temperature rise of 10 C. The condenser configuration and dimensions are indicated in Figure 11.5. This results in a heat transfer...

Control System

An automatic control system was installed on the pilot plant. The basic system consists of three regulatory loops that stabilise the process and permit manual or one-point composition control. A fourth loop can be closed to provide two-point composition control or alternative one-point configurations. Unlike the industrial columns that were discussed in Chapters 8 and 10, the pilot plant has only four degrees of freedom since there is no reflux accumulator. The variables pairings are,...

Column Configuration Modelling Assumptions and Simulation Package

The ETBE column described in Chapter 3 a total of 10 ideal stages was used as the basis for the dynamic model presented here although the individual models as described in Sections 7.2.2-7.2.6 can be combined in other ways to simulate any other column configuration. The steady state model of the reactive distillation process used a modified set of MESH equations. This technique was retained in the dynamic model although several simplifying assumptions were added to offset the additional...

Output Multiplicity due to the Influence of the Energy Balance

This cause of multiplicity was also originally described by Jacobsen and Skogestad 1991 with respect to an ideal, binary, non-reactive distillation column. In certain cases, multiple steady states were not observed when the energy balances around each distillation stage were neglected i.e. the constant molar overflow, CMO, solution was considered . Although they only considered non-reactive distillation, this concept is equally valid in other areas, including reactive distillation and hybrid...

Output Multiplicity due to Azeotropes

Azeotropes

Azeotropes in mixtures of three or more components can result in a completely different type of distillation multiplicity. In this case, the multiplicity will be observable in any type of unit i.e. molar, volumetric, mass, duty, etc. and can be detected with a rigorous model or with a CMO model. Bekiaris and Morari 1993 were the first to elucidate the link between azeotropes and multiplicity in non-reactive distillation and were able to develop a geometrical tool to detect multiplicities...

Internal Reboiler

The predicted bottoms temperature at 950 kPag is around 160 C. This precludes the use of steam as a heating medium since a steam pressure of 1500-2000 kPa would be required to provide an adequate temperature difference. A lower operating pressure would allow steam to be used with a reduced pressure but the attainable performance would be compromised and additional catalyst would be required to compensate for the lower temperatures and, therefore, lower reaction rates. An internal, direct...

Chapter Six Design Issues III Integrated Plants for ETBE

6.1.2 ETBE Production in a Conventional MTBE Unit 6.1.3 ETBE Production in a MTBE Unit Reactive Distillation 6.1.4 Conventional ETBE Process Without Ethanol Recycle 6.1.5 Reactive Distillation ETBE Process Without Ethanol Recycle 6.2.1 Applicability of Reactive Distillation to ETBE Synthesis 6.2.2 Operating Constraints Within a Refinery Environment 6.2.3 Optimal Process for an Isobutene-Rich Feed 6.2.3.2 Reactive Distillation Process 6.2.3.3 Advantages of Reactive Distillation 6.2.3.4...

Effectiveness of the Integrated Control Scheme

Dynamic simulations suggest that the three control modes are each stable for a wide range of process disturbances despite the cogent process non-linearity and bidirectionality. Thus, the proposed integrated control scheme allows for the column operation to adequately reflect a changing economic environment that could require the process objectives to be regularly revised. The control objectives were derived from the process objectives in a manner which maximised the process linearity and...

Separation Section Packings

Reflux Column Design

A novel structured packing design was developed for this column. Sheets of expanded metal with appropriate openings 6 mm x 3.5 mm were cut into strips 300 mm wide and rolled into cylinders that fitted snugly inside the column. The rolled cylinders provided a substantial contact area and a complicated flow path, which suggests an adequate HETP. Air-water tests indicated that these packings also provide adequate capacity, holdup and radial dispersion and a low pressure drop. It was not possible...

Controller Performance Tests

Distillation Tray Downcomer

The integrated controller was implemented on the ETBE column within the SpeedUp dynamic simulation environment. The control loops were all tuned using recommended recommended settings for approximate first-order-plus-dead-time FOPDT models which were fitted to step responses Ogunnaike and Ray, 1994 . Although these models were a poor fit in some cases and there was a high degree of non-linearity in most cases as judged by the differences in the responses to positive and negative perturbations ,...

Open Loop Transitions in a MTBE Column

Column Tray Design Definition

The effects of various perturbations on the 17 stage MTBE column described in Table 8.1 were investigated via a series of dynamic simulations. The column was assumed to be operating in open-loop mode with the reflux rate and reboiler duty constant. Figure 8.9 indicates that there are three steady state solutions i.e. high, low and medium conversions with this set of input conditions, and the responses of the column were found separately from each steady state. The reboiler temperature was used...

Operability Issues

The presence of input multiplicity in hybrid reactive distillation influences the optimal design and selection of equipment, particularly the reboiler. The performance of both the ETBE and MTBE systems is sensitive to the principal operating variables e.g. Figure 4.3 . There is only a narrow range of conditions where phase and chemical equilibrium intersect favourably to promote effective reaction and separation concurrently. Therefore, precise control of all the manipulated variables is...

Degrees of Freedom Analysis

A simple two-product distillation column with a single feed and a total condenser has five degrees of freedom. These correspond to control valves that vary the following quantities the distillate product draw rate D the bottoms product draw rate B the reboiler duty QR or V to denote the internal vapour rate the reflux rate L for the internal liquid rate and the condenser duty Qc . The condenser and reboiler duties usually cannot be manipulated directly but the designations, QR and Qc, are used...

Effectiveness of Inferential Conversion Control

One-point control of the ETBE reactive distillation column using the reboiler duty or the bottoms draw rate to control the bottoms composition inferred via a stripping section temperature e.g. stage 7 was found to be closed-loop stable, unlike similar control schemes that used other temperatures e.g. the reboiler temperature to infer the ether purity. This control scheme has good disturbance rejection properties and yields fast responses but provides only implicit control of the reactant...

Thesis Outline

Reactive distillation is a very broad topic and, although a wide range of issues are discussed in this thesis, the focus is exclusively on hybrid reactive distillation i.e. reactive columns which include both reactive and non-reactive column sections with application for the synthesis of fuel ethers. Specifically, the example of ETBE production is used to illustrate many of the salient points, and was the basis for much of the simulation study and the experimental work. The thesis can be...

Combined Composition and Conversion Control

Distillation Control Degrees Freedom

The operation of reactive distillation processes requires careful optimisation increasing the product purity too far by manipulating the reboiler duty or by some other means can significantly inhibit the reaction. Satisfactory operation is certainly possible with a one-point composition only control scheme but satisfying the primary operating objective e.g. the bottoms purity specification does not necessarily guarantee adequate process performance. A very low reactant conversion can still...

Applicability of One Point Control

One-point control schemes have been the backbone of industrial distillation control for many years, although the advent of multivariate predictive controllers e.g. Dynamic Matrix Control, DMC has recently seen a shift towards more complex strategies. However, one-point control is still widely practised and has some inherent advantages compared with open-loop and two-point control. One-point control is relatively easy to implement, is not subject to interactions between opposing composition...

Primary Model Validation

To validate the reactive distillation model without experimental data, Smith's MTBE column was simulated for the case described in his patent application Smith, 1980 using both Pro II and SpeedUp. The numbers of rectification, reaction and stripping stages were based on previous estimates Abufares and Douglas, 1995 . The model described by equations 3.12 - 3.27 and 3.29-3.52 has three degrees of freedom excluding the feed definition and the mass of catalyst on each reactive stage the column...

Chapter Four Design Issues I Understanding Reactive Distillation

4.1 Operating Variables and Interactions 4.1.3 Stoichiometric Excess of Ethanol 4.1.7 Other Operating and Design Variables 4.2.2.1 Interactions Between Fractionation and Feed Composition 4.2.2.2 Optimising Fractionation 4.2.2.3 The Detrimental Effect of Excessive Fractionation

Number of Theoretical Stages excluding condenser and reboiler

Figure 4.8 - Effect of Fractionation on ETBE Purity and Conversion 4.2.2.4 MTBE Columns The effect of fractionation on MTBE columns was considered by repeating the above analyses with methanol in the feed. Interestingly, somewhat different results were observed. The benefit of increasing the reflux ratio was still evident the predominant effect of this is to recycle reactive components but no advantage was found from reducing the fractionation from the base case of 30 ideal stages The different...

Production Alternatives

The history of ETBE is intimately linked with MTBE. It has only ever been promoted as an alternative to MTBE and, because of the inherent similarities between the two agents, it is natural to consider MTBE production methods for ETBE. Indeed, a superficial examination of the problem would suggest that simply processing ethanol instead of methanol through the same equipment would be sufficient to produce ETBE, albeit with a yield penalty due to the less favourable reaction equilibrium. However,...

Control Objective

Operation Reboiler Duty

The controlled variable should always be selected in order to reflect the process objectives as reliably as possible. Before this can be done, it is necessary to determine the priority of the process objectives since there is only one available degree of freedom in a one-point control scheme. It is also necessary to find a means of determining whether the process objectives are being met. The most important process objective is likely to be the control of the bottoms product composition as the...

Control Configuration

Stripper With Reboiler

The control configuration designations refer to the two manipulated variables that are not used for inventory control. In one-point control, one of these variables is used to control a product composition and one is fixed or adjusted only intermittently normally to manage an equipment constraint . The distillate and bottoms product draw rates directly affect the feed split i.e. the column material balance while the reflux rate and reboiler duty only affect fractionation. The relative magnitudes...

ETBE Simulations

Fractionation Column Reboiler

The MTBE laboratory column that was used to validate the reactive distillation model was also used as the basis for the first simulations of an ETBE reactive distillation column. Smith's original column was assumed to contain ten ideal stages a total condenser, two rectifying stages, one reactive stage, five stripping stages and a partial reboiler. Three reactive stages were specified for the ETBE column so that a higher loading of catalyst was possible to accommodate the lower reaction rate of...

ETBE Production in a MTBE Reactive Distillation Unit

The reactive distillation process for MTBE differs from the conventional process in the combination of the second reaction stage adiabatic with the purification column and the elimination of some peripheral equipment. The modified process is depicted in Figure 6.2. Figure 6 2 - Ether Synthesis Via Reactive Distillation The pretreatment stage is unaffected. The operation of the first reaction stage isothermal can also remain unchanged. Although it is technically possible to perform all of the...

Causes of Input Multiplicity

The non-reactive distillation residue curve diagrams in Chapter 5 Section 5.1.1 show that the ETBE and the MTBE system each comprise two distillation regions separated by a distillation boundary. The phase behaviour of each system is governed by the low boiling azeotrope, ETBE-ethanol or MTBE-methanol, and the distillation boundary extends from this azeotrope to the unstable node in each system i.e. pure C4 or the low boiling azeotrope between C4 and methanol . The distillation boundary cannot...

Parallels Between the Energy Balance and the Unit Singularities Causes

Distillation Composition Profile

The explanations for multiplicity caused by unit singularities and by energy balance effects are fundamentally different but there are similarities between the two mechanisms which suggest that a unified method of analysis might be available. In fact, equations 8.4 and 8.5 the conditions for multiplicity due to unit singularities can be combined with inequalities 8.6 and 8.7 the conditions for multiplicity due to the influence of the energy balance according to equations 8.10 and 8.11 . These...

Column Topography

The function of the reactive section of the column is simply to provide a site for the main reaction to proceed and, as such, there is no particular requirement for separation. This suggests that only one equilibrium stage of a column needs to be packed with catalyst although the physical size of this stage could be quite large to meet the catalyst requirement. However, simulations show that higher conversions are possible where more than one equilibrium stage is reactive. Figure 4.4 shows the...

Distillation Residue Curves

Fractionation Column Reboiler

Residue curves descnbe the composition profiles that result when a mixture is subject to simple batch distillation as shown in Figure 5.1. The liquid composition changes as the more volatile components of the mixture are removed preferentially. Eventually, the composition of the residue approaches a stable node that is either a non-volatile pure component or a high-boiling azeotrope. The equations governing this process are the material balances equations 5.1 and 5.2 , the energy balance...

Definition and Description

Column Tray Design Definition

The concept of pseudo-multiplicity is proposed here to describe the difference between output multiplicity that is physically realisable and that which is not. Essentially, pseudo-multiplicity arises where multiple steady states are observed for a system which is described in molar units which have no practical significance as a molar flow controller cannot be implemented in practice but not in mass or volumetric units. Several examples of pseudo-multiplicity in reactive distillation have been...

Operating Variables and Interactions

Operation Reboiler Duty

Reactive distillation columns behave substantially differently to conventional distillation columns due to interactions between the chemical reaction s and vapour-liquid equilibrium. The effects of key design and operating variables are discussed below with reference to two ETBE columns a column with 10 theoretical stages and the configuration described in Chapter 3 and a column with 30 theoretical stages, based on a commercial MTBE column Simulation Sciences, 1995 . The salient characteristics...

Other Transient Responses

Catalyst Ageing

The basic dynamic model was modified to simulate two other disturbances to the reactive distillation system. Firstly, catalyst ageing was simulated by re-extending the model to incorporate reaction kinetics and decreasing the amount of active catalyst present according to a simple ramp function. Secondly, changing ambient temperature was simulated by modifying the reflux rate according to a pattern that might result if an air cooler operated at its duty limit with a control configuration that...

Reactive Distillation

Reactive distillation represents the conjunction of conventional mixed-reactor technology and fractionation processes. The combination of these unit operations in a single vessel was first proposed in the 1920s Keyes, 1932 but remained an obscurity until the 1980s. Since the 1930s, reactive distillation has been used in several specific applications but only with a homogenous catalyst. The use of heterogenous catalysts was first considered by Sennewald et al. 1971 but the potential of this...

General Definitions

Price Etbe Mtbe

Multiplicity refers to the condition where a one-to-one relationship does not exist between the inputs and outputs of a process. Input multiplicity is present when two or more values of an input variable produce the same output condition with all other members of the set of input variables the same. That is, the process inputs are not unique for a known output condition. Output multiplicity occurs when one complete set i.e. a set with sufficient members to fully satisfy the degrees of freedom...

Proposed Design Strategy for Hybrid Reactive Distillation

Isolation Flow Chart

There is an absence of satisfactory rigorous or empirical design methods for hybrid reactive distillation in the current literature. A practical and effective approach to reactive distillation design has not yet been established. Design tools such as residue curves and reactive residue curves are applicable for preliminary design proposals and screening studies i.e. determining the feasibility of a given reaction-separation but are not suitable for detailed design. In particular, these methods...

Hybrid Residue Curves

Fractionation Column Reboiler

A composition transformation has been widely recommended to enhance the applicability of reactive residue curves for reactive distillation design e.g. Ung and Doherty, 1995 Perez-Cisneros et al., 1996 . The transformed co-ordinates allow reactive tie-lines to be drawn and interpreted in a manner which parallels conventional distillation design methods. Essentially, the technique relies on producing a set of co-ordinates that are independent of the reaction. Once this is done, standard...

Optimal Process for an Isobutene Lean Feed

This section examines the implications of processing C4 hydrocarbon from a catalytic cracking unit CCU and, again, the effect on process selection. A typical feed composition of 15 isobutene, 35 n-butenes, 40 isobutane and 15 n-butane was assumed. Process designs were again completed for 5000 kg hr 1000 bbl day etherification units using a conventional technology, b a reactive distillation column that was designed for MTBE synthesis, and c two designs which were specifically tailored for ETBE...

Conventional ETBE Process Without Ethanol Recycle

The absence of any significant azeotrope between ethanol and butenes results in unreacted ethanol being recovered primarily with the ETBE product. As indicated above, the presence of an azeotrope between ethanol and water also results in some water being recycled to the reaction stage if is attempted to recover the unreacted ethanol from the distillate. There is much lower load on the recovery equipment due to the lower concentration of ethanol in the distillate. In fact, it is feasible, under...

ETBE Production in a Conventional MTBE Unit

Production Etbe

The four main process steps in the conventional MTBE process are a pretreatment to remove potential catalyst poisons from the feed streams b reaction usually two stages c purification of the ether product and d recovery of the unreacted alcohol for recycling. This configuration is shown in Figure 6.1 and is directly applicable to ETBE production. The pretreatment stage is common to all ether synthesis processes where a catalyst is used. The design of the pretreatment equipment and its operation...

Process Comparison for ETBE Production

Reactive distillation offers substantial process benefits lor both MTBE and ETBE production. The maximum isobutene conversion and ether purity for ETBE production are limited by the unfavourable reaction equilibrium, kinetics and azeotropes, when using the conventional technology. However, in a specially designed reactive column, these limitations can be overcome and conversions and purities above 99 can be achieved, matching the results attainable for MTBE production. The optimum design for...

Optimal Process for an Isobutene Rich Feed

As noted previously, isobutene can be sourced from several process units within a large refinery complex. The two most common sources of isobutene for etherification are steam cracking units 35-50 wt isobutene and catalytic cracking units 10-25 wt isobutene . This section examines the implications of processing C4 hydrocarbon from a steam cracking unit SCU and the effect it has on process selection. A typical SCU product composition of 40 isobutene, 30 n-butenes and 30 butadiene, was assumed as...

Reactive Distillation ETBE Process Without Ethanol Recycle

A reactive distillation process for ETBE synthesis would utilise the same principles as the MTBE process and should yield the same benefits of increased conversion, increased energy efficiency and reduced capital cost. The majority of the reaction say, 80 would be performed in an isothermal, tubular reactor operating at moderate conditions of temperature around 90 C and pressure 1500-2000 kPa to ensure all components remain in the liquid phase . The feed to the reactive distillation column...

Brief Biography of the Author

Sneesby completed a Bachelor of Engineering degree with first class honours at Curtin University of Technology between 1988 and 1991. As an undergraduate, he received several awards, including the Digby Leach Medal for the Most Promising Graduate from an Engineering Faculty 1991 and the CHEMECA Prize for the Best Chemical Engineering Design Project from an Australian University 1992 . He obtained three years of industrial experience with Mobil Adelaide Refinery where he worked as a...

Reboiler Duty Equation

The right hand side of equation 6.1 should be minimised for a selected value of x2 to find T, and x,, where T is evaluated at each incremental value of x in the adiabatic reactor. If T2 is relatively close to T,, equation 6.8b is a good approximation to equation 6.8a and can be used to calculate T more easily. The relationship between a, and x, will vary and should be calculated at each incremental value of x using the UNIFAC equations or a similar activity coefficient model. The results of the...

Reactive Distillation Model

Distillation Sections

Reactive distillation columns for fuel ether synthesis are actually hybrid columns consisting of three distinct sections. The reactive section is located between two non-reactive column sections that separate the reactants from the product. This configuration is shown in Figure 3.1. The stripping section purifies the ether product and recycles unreacted reactants to the reactive section. The rectifying section is required to prevent the loss of ether in the distillate. The combined action of...

Summary

Reactive distillation has enormous potential for the economical synthesis of tertiary ethers. Methyl tert-butyl ether MTBE has been commercially produced with this technology since the early 1980s and it appears that the process also has application for Ethyl fer -butyl ether ETBE and other ethers. However, the combination of reaction and distillation in a single unit operation produces a process complexity that inhibits expeditious design and tight control, and presents a technology risk for...

Thesis Contents

Brief Biography of the Author vii 1.2 Motivation and Significance of this Work 3 2.2 Reactive Distillation 7 2.4 Modelling Techniques 11 2.5 Operability and Controllability Issues 13 3 Steady State Simulation 18 3.2 Reactive Distillation Model 22 3.3 Primary Model Validation 29 4 Design Issues I - Understanding Reactive Distillation 34 4.1 Operating Variables and Interactions 34 4.2 Column Topography 43 Design Issues II - Design Methods 54 5.1 Feasibility of a Reaction-Separation 54 5.2...