Proposed Design Strategy for Hybrid Reactive Distillation

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 do not reflect the influence of the column vapour-liquid loading which is a primary consideration in column design.

A conventional distillation design strategy relies on determining the minimum reflux ratio and the minimum number of ideal stages required for a specified separation. This approach cannot be used for reactive distillation as the separation cannot be specified independently of the reactant conversion. Similarly, conventional short-cut methods are not applicable to reactive distillation. Once the traditional design methods have been necessarily abandoned, rigorous column simulations must be employed. Fortunately, computational power and available modelling techniques are now sufficient to prevent this from being an overly onerous task.

Many simulation cases can be run within several hours but a directionless search still lengthens the design process and risks a non-optimal solution. A focussed design strategy is proposed in Table 5.1. It is important to note that the design process should be iterative and a successful design may require several revisions of the operating conditions. Figure 5.10 provides a diagrammatic representation of the proposed design process and indicates where the process may become iterative by showing recycle steps in dashed lines. Each box represents a task or a design decision that must be resolved before progressing further. The basic rationale for the various recycle loops is as follows:

• The effect of the operating pressure on the reaction rate and the product compositions is strongly dependent on external factors. The optimum will vary substantially between installations and rigorous simulations provide the only effective method of optimisation (see Section 4.1.4).

• The reflux ratio should be increased if the resulting hydraulic loading is insufficient to support a column diameter of at least 1.2 m (suggested practical minimum).

• The column diameter effectively determines the internal layout and imposes restrictions on the distributor design and packing arrangement. Standard configurations might be unsuitable for some column diameters thereby requiring novel designs or a revision of the reflux ratio.

• The principal variables that most affect the process design are the reactant excess, the overhead pressure and the reboiler duty. It is, therefore, important to establish the effects of each of these before finalising a process design.

Table 5.1 - Proposed Design Strategy for the Hybrid Reactive Distillation of Fuel Ethers

Stage

Items

Considerations

1. Design Basis

a) hydrocarbon feed composition b) target ether purity c) target hydrocarbon conversion

A isobutene rich feed improves energy efficiency but increases reaction zone temperatures. A isobutene lean feed increases the minimum column diameter but has a cooling effect on the reaction zone.

The purity target is dependent on blending requirements but there are usually no specific constraints on conversion.

2. Reaction

d) stoichiometric excess e) column pressure

Excess alcohol is required to prevent side-reactions. High excess favours conversion; low excess favours purity.

Increasing pressure raises temperatures in the column increasing the reaction rate but reducing the equilibrium constant. Relative volatility decreases with pressure. Pressure has an indeterminate effect on the internal composition profile.

3. Distillation

f) key components g) 'minimum'reflux ratio h) minimum hydraulic loading

The key components in both non-reactive column sections determine the effect of increasing fractionation.

The minimum reflux ratio cannot be determined using conventional techniques but performance declines rapidly if the reflux ratio is too low. Reflux promotes separation and reaction by recycling the light reactant to the reaction zone. A minimum column diameter of 1.2 m is suggested for catalyst installation and removal.

Table 5.1 (conO - Proposed Design Strategy for the Hybrid Rcailive Distillation of Fuel Ethers

Stage

Items

Considerations

4. Column Topography

i) reactive stages j) rectifying stages k) stripping stages 1) stage efficiencies

Should be sufficient to allow enough catalyst to be installed to provide a relatively long cycle time between replacements.

Optimise to eliminate ether from the distillate but allow light reactant to return to the reaction zone.

Optimise to provide required ether purity without starving the reaction zone of the heavy reactant.

High rectifying efficiencies and moderate stripping efficiencies can be expected. Testing required for novel reaction zone packings.

5. Heat Transfer

m) reboiler n) condenser

Optimised for maximum purity or conversion. Determine duty from rigorous simulation.

6. Column Dimensions

o) height p) diameter

0.6-L.2 m per actual stage. Redistributor required at top of reactive section. 70-80% flood in non-reactive sections. Testing required for novel reactive packings.

7. Optimisation

q) pressure, reflux, and reboiler duty

Some synergies may be realised between key design variables.

OS OO

Isolation Flow Chart
Figure 5.IQ - Flow Chart for Reactive Distillation Design
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Responses

  • sabrina
    How distillate flow rate effecting dynamics of reactive batch distillation?
    7 years ago

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