Column Topography

4.2.1 Reactive Section

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 effect of varying the number of reactive stages in the 10 stage ETBE column described in Table 4.1. All other variables, including the number of separation stages, reflux ratio, reboiler duty and feed conditions were fixed.

The improved conversion with an increased number of reactive stages results from the benefits of the additional separation which are gained. Under most conditions, transferring all the catalyst in a column to a single stage would have a negligible effect on the overall conversion achieved. Note that this is different to the data presented in Figure 4.4 as it implies an increase in the number of separation stages at the expense of reactive stages.

Increasing the number of reactive stages above the optimum (four for this column) produced a detrimental interaction between the phase and chemical equilibrium which led to the decomposition of product on the lower reactive stages. This is to be expected as the increased fractionation that occurs with more stages concentrates the ether in the lower reactive stages and shifts the chemical equilibrium back to the reactants. An excessive number of reactive stages can also encourage unwanted side-reactions and increase the concentration of impurities in the ether product. By comparison, the MTBE simulations showed no optimum although the benefits of adding a reactive stage became progressively smaller when high numbers of reactive stages were already present, due to the increased likelihood of decomposition on the lower reactive stages.

Supplying catalyst on several reactive stages allows the total catalyst loading in the column to be increased and may, therefore, extend the time between catalyst changes or regenerations. However, during the life of the catalyst the main reaction site may shift within the column and change the effective number of rectification and stripping stages and, subsequently, change the conversion and purity attained.

Fractionation Column Reboiler

Number of Reactive Stages

Figure 4.4 - Effect of Additional Reactive Stages in the 10 Stage ETBE Column 4.2.2 Non-Reactive Sections

4.2.2.1 Interaction Between Fractionation and Feed Composition

The non-reactive sections of a hybrid reactive distillation column are vital to achieving the desired process performance. Ideally, the rectification section of a reactive distillation column for ether synthesis should: (a) remove light inerts from the reaction zone; (b) prevent loss of ether or alcohol in the distillate; and, (c) recycle unreacted reactants (olefin and alcohol) to the reaction zone. For an ETBE column, this would ideally require a separation between isobutene and the heaviest non-reactive hydrocarbon lighter than isobutene. In practice, this is almost impossible to achieve whilst maintaining acceptable reaction zone conditions. However, the loss of ether in the distillate can be minimised without rejecting isobutene from the column. More rectification stages are required to also prevent loss of ethanol with the distillate.

Number of Reactive Stages

Figure 4.4 - Effect of Additional Reactive Stages in the 10 Stage ETBE Column 4.2.2 Non-Reactive Sections

4.2.2.1 Interaction Between Fractionation and Feed Composition

The non-reactive sections of a hybrid reactive distillation column are vital to achieving the desired process performance. Ideally, the rectification section of a reactive distillation column for ether synthesis should: (a) remove light inerts from the reaction zone; (b) prevent loss of ether or alcohol in the distillate; and, (c) recycle unreacted reactants (olefin and alcohol) to the reaction zone. For an ETBE column, this would ideally require a separation between isobutene and the heaviest non-reactive hydrocarbon lighter than isobutene. In practice, this is almost impossible to achieve whilst maintaining acceptable reaction zone conditions. However, the loss of ether in the distillate can be minimised without rejecting isobutene from the column. More rectification stages are required to also prevent loss of ethanol with the distillate.

The stripping section should, ideally: (a) remove ether from the reaction zone to maintain favourable reaction conditions; (b) purify the ether product; (c) prevent loss of reactive olefin with the ether product; and (d), minimise ethanol loss with the ether product. In an ETBE column, this implies a separation between ethanol and ETBE, which is largely achievable at the conditions of temperature and pressure required for adequate reaction.

Manipulating the number of stripping stages provides a mechanism for controlling the volatility, flash point and composition of the ether product.

Although the separation objectives are clear, changes in the separation efficiency within a multicomponent system tend to adjust composition profiles rather than produce more clearly defined product splits when intermediate boiling components (e.g. ethanol) are involved. Consequently, an increase in rectification or stripping separation is not necessarily beneficial for reaction zone conditions. Too much rectification separation can result in isobutene loss via the distillate. Too much stripping separation can result in ethanol being drawn away from the reaction zone and concentrated just above the reboiler. For some processes, including ETBE, both the rectification and stripping separation must be optimised. Figures 4.5 and 4.6 present data based on the 10 stage ETBE column and both graphs include distinct optimums.

The ratio between the number of rectification and stripping stages and the feed composition are also important factors that should be considered. If the rectification separation is too great without light inerts present to stabilise the reaction conditions, then reboiler operation must be adjusted to compensate the reaction conditions resulting in a loss of ether purity. Tables 4.4 and 4.5 show this effect for two different feed compositions: a low isobutene feed with 15% isobutene in the hydrocarbon to the primary reactor; and, a high isobutene feed derived from a 50/50 mixture of isobutene and n-butenes. The number of reactive stages and the reflux ratio were kept constant for all simulations and the reboiler duty was optimised with respect to the conversion in each case. The combination of two rectification stage and 16 stripping stages produced the best results in each case. Adding rectification stages is beneficial with only a few stripping stages present but detrimental with many. With a high concentration of isobutene in the hydrocarbon feed, the magnitudes of all the effects are diminished and the need to optimise (rather than maximise) separation becomes apparent.

Reboiler Duty Distillation Column
Figure 4.5 - Effect of Rectification Separation

Number of Stripping Stages

Figure 4.6 - Effect of Stripping Separation

Number of Stripping Stages

Figure 4.6 - Effect of Stripping Separation

1 dhle 4.4 - Effect of Separation Stages ori ETBF Column^ with Lean Isobutene Feeds

Rectification Stages

4

Stripping Stages 8

16

98.6 mol%

99.4 mol%

99.4 mol%

2

conversion;

conversion; 99.7 wt%

conversion; 99.95 wt%

90.3 wt% purity (8.8 wt% butenes)

purity (0.21 wt% butenes)

purity (-0 wt% butenes)

98.8 mol%

99.5 mol%

99.5 mol%

4

conversion; 92.2 wt%

conversion; 99.2 wt%

conversion; 99.7 wt%

purity (6.8 wt% butenes)

purity (0.17 wt% butenes)

purity (~0 wt% butenes)

99.0 mol%

99.5 mol%

99.5 mol%

8

conversion; 98.6 wt%

conversion; 98.6 wt%

conversion; 98.9 wt%

purity (0.16 wt% butenes)

purity (0.16 wt% butenes)

purity (~0 wt% butenes)

Table 4.5 - Effect of Separation Stages on ETBE Columns with Rich Isobutene Feeds

Rectification

Stages

4

Stripping Stages 8

16

97.3 mol%

97.8 mol%

98.2 mol%

2

conversion;

conversion; 97.0 wt%

conversion; 97.2 wt%

93.1 wt% purity (6 6 wt% butenes)

purity (0.05 wt% butenes)

purity (~0 wt% butenes)

97.2 mol%

98.2 mol%

98.2 mol%

4

conversion; 93.2 wt%

conversion; 97.0 wt%

conversion; 97.1 wt%

purity (3.6 wt% butenes)

purity (0.08 wt% butenes)

purity (-0 wt% butenes)

97.1 mol%

98.0 mol%

98.2 mol%

8

conversion; 93.2 wt%

conversion; 96.7 wt%

conversion; 97.1 wt%

purity (3.5 wt% butenes)

purity (0.14 wt% butenes)

purity (-0 wt% butenes)

4.2.2.2 Optimising Fractionation

The optimal column topography depends on the operating objectives (e.g. maximum isobutene conversion, minimum ethanol in the ETBE product, etc.), the vapour-liquid loading and the feed composition. The simulation results presented above suggest that additional fractionation stages are sometimes detrimental but their effect is dependent on the column conditions. In order to investigate this phenomenon further, a series of column designs were completed for an isobutene-rich feed and an isobutene-lean feed.

The isobutene-rich feed was modelled on the product of a typical steam cracker and was specified to comprise 30 mol% isobutene, 40% n-butenes and 30% butadiene. The isobutene-lean feed was based on a typical catalytic cracker product containing 15 mol% isobutene, 35% n-butenes, 40% isobutane and 10% n-butane. Using a consistent design philosophy, the total number of stages and the reflux ratio were varied to determine the optimal column topography for each feed. The design basis was:

• equilibrium stages allocated in the ratio of one rectifying stage to one reactive stage to two stripping stages;

• feed to the uppermost stripping stage;

• overhead pressure fixed at 700 kPa;

• base case reflux ratio of 1.2 for the isobutene-rich feed and 0.8 for the isobutene-lean feed (selected to reflect the increased distillate rate which results from the presence of more C4 inerts in the isobutene-lean feed);

• a bottoms product composition specification of 0.1 wt% total C4s (selected to produce a low product RVP without overly constraining the column operation).

The total number of stages was varied from 6 (condenser, 1 rectifying stage, 1 reactive stage, two stripping stages and reboiler) to 42 (condenser, 10 rectifying stages, 10 reactive stages, 20 stripping stages and reboiler), and the reflux ratio was varied up to a maximum of 50% more than the base case.

Five designs for each feed composition are shown in Tables 4.6 and 4.7. The highest values of isobutene conversion and ETBE purity were achieved with relatively few theoretical stages and a high reflux ratio for both feed compositions (Design D). However, Design C would probably be preferred for isobutene-rich feed because it results in a distillate product that is relatively free of ethanol and ETBE. With an isobutene-lean feed, the benefit of restricting the number of stages and maximising the reflux ratio (i.e. Design D) is more pronounced and should easily compensate for the reduced distillate purity.

I able 4.6 - ETBE Reactive Distillation Column Designs (Isobutene-Rich fccJ)

Design A

Design B

Design C

Design D

Design E

Number of theoretical stages (rectifying/reactive/stripping)

7/7/14

7/7/14

4/4/8

3/3/6

2/2/4

Reflux ratio

1.2

1.8

1.8

1.8

1.8

Reboiler duty relative to Design A

1.00

1.25

1.23

1.25

1.77

Overall isobutene conversion

88.8%

89.5%

91.7%

92.6%

60.7%

Bottoms composition (wt%)

7.5% EtOH, 1.1% DIB, 0.1% C4s

7.1% EtOH, 1.6% DIB, 0.1% C4s

6.1% EtOH, 1.0% DIB, 0.1% C4s

5.4% EtOH, 0.5% DIB, 0.1% C4s

21.0% EtOH, 0.4% DIB, 0.1% C4s

Distillate composition (wt%)

6.0% iBut, 94.0% other C4s, 3 ppm EtOH

4.9% iBut, 95.1% other C4s, 1 ppm EtOH

4.2% iBut, 95.8% other C4s, 100 ppm EtOH, 20 ppm ETBE

0.4% EtOH, 100 ppm ETBE

19.5% iBut, 76.5% other C4s, 3.0% EtOH, 1.0% ETBE

Table 4.7 - ETBE Rcaciive Distillation Column Desmns (hobutene-Lean Feed)

Design A

Design B

Design C

Design D

Design E

Number of theoretical stages (recti lying/reactive/stripping)

7/7/14

7/7/14

4/4/8

3/3/6

2/2/4

Reflux ratio

0.8

1.2

1.2

1.2

1.2

Reboiler duty relative to Design A

1.00

1.25

1.25

1.27

1.41

Overall isobutene conversion

91.6%

92.8%

94.0%

97.1%

75.7%

Bottoms composition (wt%)

6.0% EtOH, 0.9% DIB, 0.1% C4s

5.5% EtOH, 1.0% DIB, 0.1% C4s

4.6% EtOH, 0.9% DIB, 0.1% C4s

1.3% EtOH, 0.7% DIB, 0.1%C4s

6.6% EtOH, 0.5% DIB, 0.1% Qs

Distillate composition (wt%)

1.1% iBut, 98.7% other C4s, 0.2% EtOH

0.9% iBut, 99.0% other C4s, 0.1% EtOH

0.7% iBut, 99.1% other C4s, 0.2% EtOH, 2 ppm ETBE

0.3% iBut, 99.0% other C<s, 0.7% EtOH, 20 ppm ETBE

3.7% iBut, 93.8% other C4s, 2.2% EtOH, 0.3% ETBE

The directional effects of both reflux and fractionation were consistent with earlier observations: increasing the reflux ratio was found to be universally beneficial but excessive fractionation was detrimental to the isobutene conversion and the ether purity. However, additional separation stages always produce a purer distillate product which contained less ethanol and ETBE. This is potentially significant if there is no ethanol recovery equipment downstream of the reactive distillation column (possible for ETBE production since the ethanol concentration in the distillate is typically very low) and ethanol is an unwanted contaminant or poison in a downstream process, and might be a sufficient justification for specifying a column with more than the optimal number of separation stages. Although the trade-off between reflux and stages can be employed to control the distillate purity, it is not possible to match the process performance (i.e. conversion and ether purity) from the optimal topography with a taller column regardless of the combination of reflux ratio and column specifications that might be used.

4.2.2.3 The Detrimental Effect of Excessive Fractionation in Reactive Distillation To confirm the apparent result that fractionation can be detrimental in the design of reactive distillation columns, a third series of designs were completed to compare reactive and non-reactive distillation. A hydrocarbon stream containing 25% isobutene and 75% n-butene was combined with a stoichiometric amount of ethanol and reacted to 75% isobutene conversion in order to create a suitable feed for a series of reactive and non-reactive columns with varying numbers of stages. The same distribution of stages (i.e. constant ratio of rectifying : reactive : stripping stages), product specification (i.e. 0.1% C4 in the bottoms) and reboiler duty were specified in each column. The non-reactive column was identical to the reactive column with the reactive stages exchanged for non-reactive stages.

The intuitive effect of increasing fractionation with a constant composition specification is to increase the yield of the product under control. In this case, the bottoms rate should increase with the number of stages in the column. This behaviour is seen exactly in the non-reactive column. This is shown in Figure 4.6 for two values of the reboiler duty (0.95 and 2.50 MW per 100 kmol/hr of C4 feed).

The reactive columns produced a completely different response and the bottoms product yield was maximised at an intermediate number of separation stages. This is shown in Figure 4.7 which describes the same cases as Figure 4.6. Figure 4.8 shows how the ETBE purity and the isobutene conversion vary with the total number of stages in the reactive column. As indicated previously, the interaction between the phase and reaction equilibrium is such that the isobutene conversion (and, hence, also the ETBE purity) is maximised when the column internal compositions are optimised. This occurs at an intermediate level of fractionation rather than an extreme.

2160

2080

2080

Fractionation Column Reboiler

12 16 20 24

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