Operability Issues

8.6.1 Equipment Selection

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 required. This is readily attainable where a flow is controlled but restrictive in designing the reboiler. For example, integrated exchanger networks are desirable to conserve energy and minimise utility consumption but might not provide sufficient control over the reboiler duty. A fired heater permits more precise control but is more costly to operate. Therefore, there is a trade-off between operability, performance and cost.

Clearly, output multiplicity also affects the operability of hybrid reactive distillation columns. Unwanted transitions between parallel steady states could result in substantial production losses. In an extreme case, regular transitions could make a reactive column unworkable and result in a return to more conventional technology. Steady state simulations can be used to determine the potential benefits of using a hybrid column but they do not indicate whether the potential is realisable. If a complex and costly advanced control system is required to permit a hybrid column to operate effectively, or additional operating staff are required, the steady state benefit of utilising reactive distillation technology could be offset by increased operating costs and reduced flexibility, operability and reliability

The need to reconcile operability and performance extends to other processes and many design modifications, particularly those directed at increasing integration between process units. The present example suggests that it is crucial to consider the difference between a realisable benefit and a potential benefit in preparing a project justification. 8.6.2 Start-Up

The presence of output multiplicity and, therefore, hysteresis in some hybrid columns has significant implications for the start-up of those columns. Since the operating conditions at any time are a function of the process inputs and the history of the column, the start-up procedure can influence the steady state process performance. Some start-up procedures will culminate with the column operating at or close to the design conditions while other procedures will confine the column conditions to the low conversion solution branch.

Referring again to the 17 stage MTBE column, Table 8.6 shows two sets of steady state conditions for process inputs that could be found during a start-up. The column temperatures are close to the design basis in both cases and only the reboiler duty needs further adjustment to match the targets. In the first case (A in Table 8.6), the reboiler duty must be decreased by approximately 3% while in the second case (B) the duty must be increased by the same amount to reach the design conditions. A comparison of the temperature profiles in cases A and B with the design targets suggests that case A would provide the fastest route to the design point since many of the temperatures are already close to the desired values.

The steady states given by cases A and B were disturbed by making the requisite changes in the reboiler duty to attain the design targets. The transient responses are shown in Figures 8.27 (case A) and 8.28 (case B). Although start-up A settles more quickly than start-up B, it converges to a low conversion steady state. Start-up B is considerably slower but is effective in attaining the design basis conditions. Figure 8.29 shows the various steady states on a bifurcation diagram of the column with a constant volumetric reflux rate (i.e. 1.20m3/hr) and clearly explains the observed behaviour. The output multiplicity creates restrictions on the start-up procedure that are not present for columns that do not exhibit this phenomenon.

Table 8.6 - Steady State Conditions Found During Two Start-Ups

Design Basis

Start-Up A

Start-Up B

Operating Conditions

Hydrocarbon feed composition

36% isobutene,

36% isobutene,

36% isobutene,

64% n-butane

64% n-butane

64% n-butane

Methanol excess (mol%)

10.0

10.0

10.0

Reflux rate (m3/hr)

1.20

1.20

1.20

Reboiler duty (MW)

50.6

52.2

49.0

Temperature Profile (°C)

Condenser

81

80

81

Mid-reactive section

95

99

84

Feed stage

112

112

86

Mid-stripping section

130

130

87

Lower stripping section

132

132

91

Reboiler

152

132

122

Time (hrs)

Figure 8.27 - Transient Response to a Decrease in Reboiler Duty to the Tarmi (Start-Up A)

Time (hrs)

Figure 8.27 - Transient Response to a Decrease in Reboiler Duty to the Tarmi (Start-Up A)

Figure 8.28 - Tranbient Response to an Increase in ReboilerDuty to the Targei (Start-Up B)
Butane Reboiler
Figure 8.29 - Start-Up Restrictions Predicted bv the Bifurcation Diagram

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