General Definitions

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 in the system) of input variables maps to two or more distinct and independent sets of output variables. That is, the outputs are not unique for a given set of inputs. Figure 8.1 provides a graphical distinction between input and output multiplicity for arbitrary variables. Input multiplicity is present in the first chart as input variable values of both a and b result in the same value of the output variable. Output multiplicity is present in the second case as an input value of c could result in output values of either d or e.

The differentiation between input and output variables is best made with reference to control structures. Input variables are those which can be manipulated by controllers. The primary input variables for distillation include the reflux rate and the reboiler duty (or boilup), and the distillate and the bottoms rates, and combinations of these (e.g. the reflux ratio). The product draw rates can be considered as inputs because the inventory (level) controllers on the reflux accumulator and the reboiler sump will transmit changes in the product rates directly to the column. Output variables are those which are either controlled or used to describe the conditions of the process (e.g. product or stage temperatures, concentrations and yields). By convention, input variables are plotted on the *-axis while output variables are plotted on the_y-axis.

Ether Accumulator

fitiure 8.1 - Input Multiplicity (left) and Output Multiplicity (right) 8.1.2 Examples of Input Multiplicity

The effects of various operating variables on two hybrid reactive distillation columns were described in Chapter 4, Section 4.1. The reboiler duty (Section 4.1.7) was shown to have a bidirectional effect on the principal performance parameters (i.e. the ether purity and the isobutene conversion): depending on the magnitude of the reboiler duty, an increase in the duty could either increase or decrease the purity or the conversion, as indicated in Figure 4.3 which was constructed using simulation results for the 10 stage ETBE column. Input multiplicity exists in this case since some output conditions (e.g. the bottoms temperature, Tb) map to more than one value of a specific input variable (e.g. the reboiler duty, Qr) while all other input conditions are the same, as shown in Table 8.1.

Table 8.1- Input Multiplicity in an ETBE Column

Steady State A

Steady State B

Output Condition

Bottoms temperature (°C )

150

150

Input Conditions

Hydrocarbon feed composition

40% isobutene,

40% isobutene,

60% n-butene

60% n-butene

Methanol excess (mol%)

5%

5%

Total feed rate (L/min)

0.76

0.76

Pressure (kPa-g)

950

950

Reflux rate (L/min)

2.50

2.50

Reboiler duty (kW)

8.20

8.97

A similar, bidirectional dependence of the column performance on the reboiler duty is seen with the MTBE system. Table 8.2 describes the configuration of a 17 stage MTBE column (Nijhuis et al., 1993). Figure 8.2 shows the bidirectional effect of the reflux rate (L) on the bottoms temperature of this column for a constant bottoms yield of 35.0% by volume. Input multiplicity is present since, for example, a bottoms temperature of 145°C results with reflux rates of 130 and 535 m3/hr.

Table 8.2 - MTBE Column Characteristics

Design Parameter

Value

Rectifying stages (including total condenser)

3

Reactive stages

8

Stripping stages (including partial reboiler)

6

Feed stage

lowest reactive stage

Hydrocarbon feed composition (mol %)

36% isobutene, 64% n-butane

Stoichiometric methanol excess (mol %)

10%

Total feed rate

2752 kmol/hr

Overhead pressure

1000 kPa-g

Reflux ratio

—7.0

Price Etbe Mtbe
Figure 8.2 - Input Multiplicity in a MTBE Reactive Distillation Column

The two examples of input multiplicity indicated above can also be described analytically. The ETBE column multiplicity satisfies the condition given by equation (8.1) while the MTBE column multiplicity satisfies equation (8.2). A more general expression of this condition is given by equation (8.3), which describes any stationary point between an input variable (x,) and an output input variable (y) with other inputs (x2) constant.

5Tb

\

àQ,

L

sV

dLj

Qr

dx}J

Xi

+1 0

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