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 synthesis. As with the previous case study, Pro/II was used for all simulations. A similar set of assumptions (chemical equilibrium, reaction model of Jensen and Datta, 1995) and property packages (UN1FAC and Soave-Redlich-Kwong), were also made in modelling the various systems described here.

6.2.4.1 Con ventional Process

The process schematic shown in Figure 6.1 was again used as the basis for simulations of the conventional etherification process. The MTBE case with 5% excess methanol and two ETBE cases (with and without a stoichiometric excess of ethanol) were initially modelled. The results have been summarised in Table 6.5. Although the composition of the column feed vanes significantly from the first case study, the same column (i.e. 30 stages) was successfully used for this case study. The main difference arises in the flow rate of the distillate product which increase substantially due to the increased presence of non-reactive C4 hydrocarbons. Consequently, a much lower reflux rate of 0.35 was specified. This results in an approximately equivalent reflux rate when compared with the isobutene-rich feed cases.

With equivalent equipment, the isobutene conversion to ETBE and the ETBE product purity are both significantly lower than found for the MTBE process when using an isobutene-lean feed. However, the differences between MTBE production and ETBE production are less than was seen for feeds that are isobutene-rich. The higher ether product purity has the unfortunate disadvantage of reducing the oxygen production rate as less ethanol is recovered with the ether product. The remaining unreacted ethanol is recovered with the distillate product (at concentrations of 2-3%) so that further processing of the distillate is required. A lower reboiler duty is required for ETBE production but there appear to be no other advantages and MTBE production would clearly be favoured on the results presented in Table 6.5, even if a cheap source of ethanol was available.

Table 6.5 - Conventional Prouves tor ETBE and MTBE ( Isobutene-Lean Feed)

MTBE with 5% Excess Methanol

ETBE with 5% Excess Ethanol

ETBE with Stoichiometric Ethanol

Reactor Feed Rate (kg/hr)

22,660

20,380

20,750

Second Stage Reactor Outlet Temperature (°C)

45

48

48

iBut Conversion: 1st Reaction Stage

84.0%

75.5%

73.9%

2nd Reaction Stage

64.2%

60.8%

56.3%

Overall

93.6%

89.8%

88.2%

Bottoms Composition (wt%)

2.1% EtOH, 0.8% DIB

Distillate Composition (wt%)

0 9% iBut, 98.0% other C<s, 1.1% MeOH

1.5% iBut, 97.3% other C4s, 1.2% EtOH

1.8% iBut, 97.1% other C4s, 1.1% EtOH

Oxygen Production Rate (kg/hr)

915

841

823

Net Alcohol Consumption (kg/kg ether)

0.365

0.527

0.511

Net Isobutene Consumption (kg/kg ether)

0.680

0.611

0.623

Reboiler Duty (MW)

3.07

2.80

2.84

Column Diameter (mm)

1650

1350

1350

6.2.4.2 Reactive Distillation Process

As was done for the case study with an isobutene-rich feed, a reactive distillation system was designed and optimised for MTBE production. The operating conditions and key results are shown in Table 6.6. This technology again outperforms the conventional process with respect to the isobutene conversion obtained but only by around 2% whereas an increase of around 5% was seen with the isobutene-rich feed. When the same equipment was used for ETBE synthesis, the isobutene conversion was around 5% lower and the ether purity was around 7% lower. However, compared with the conventional process for ETBE synthesis, the conversion was 2-3% higher. Unfortunately, the increase in conversion was at the expense of the purity.

The reactive distillation process again offers better energy efficiency (heat of reaction liberated 22°C higher than the conventional process) and reduced capital cost, especially if the ethanol recovery equipment can be dispensed with due to the reduction in the ethanol content of the distillate product. However, reactive distillation technology still appears insufficient to make the ETBE process competitive.

Table 6.6 - Reactive Distillation Process for ETBE and MTBE (Isobutene-Lean Feed)

MTBE With 5% Excess Methanol

ETBE With 5% Excess Ethanol

ETBE With Stoichiometric Ethanol

Reactor Feed Rate (kg/hr)

21,850

19,780

20,360

Overall Isobutene Conversion

97.1%

92.5%

89.9%

Bottoms Composition (wt%)

5.1% EtOH, 1.1% DIB

Distillate Composition (wt%)

0.5% iBut, 98.7% other C4s, 0.9% MeOH

1.2% iBut, 98.8% other CAs, 0.03% EtOH

1.6% iBut, 98.4% other C4s, 0.01% EtOH

Oxygen Production Rate (kg/hr)

915

899

879

Net Alcohol Consumption (kg/kg ether)

0.365

0.512

0.502

Net Isobutene Consumption (kg/kg ether)

0.656

0.593

0.611

Reflux Ratio

0.72

0.77

0.76

Reboiler Duty (MW)

3.07

2.80

2.84

Increase in Flooding (%)

0%

-2%

-7%

6.2.4.3 ETBE-Specifîc Reactive Distillation Designs

As with the isobutene-rich feed cases, a reactive distillation column can again be designed with fewer separation stages to increase the isobutene conversion. Similarly, a moderate number of separation stages can be combined with a high reflux ratio to produce both a high isobutene conversion and a high ether product purity. These cases were simulated here and the results are presented in Table 6.7.

The low capital cost process increases the isobutene conversion to 95.4% (an increase of around 3% compared with ETBE production in a reactive distillation column designed for MTBE synthesis). The corresponding ether purity is also higher than the cases presented in Table 6.6. However, the most significant results are shown in the third column of Table 6.7 which shows results for a high conversion, high purity case. The isobutene conversion and ether product purity for this case are not only significantly higher than all other ETBE cases, but are higher than the those for MTBE production with this type of feed. The ethanol concentration in the distillate is high enough to require careful consideration of the need for additional recovery equipment but this process appears competitive with MTBE production.

Table 6.7 - ETBE-Specific Rcaitive Distillation Desikms ( Ibobutene-Lean Feed)

Low Capital Cost Process

High Conversion, High Purity Process

Reactor Feed Rate (kg/hr)

19,180

18,390

Overall Isobutene Conversion

95.4%

99.5%

Bottoms Composition (wt%)

2.2% EtOH, 0.7% DIB, 0.22% C4s

0.18% EtOH, 0.6% DIB, < 0.01% C4s

Distillate Composition (wt%)

0.66% iBut, 98.5% other C4s, 0.76% EtOH, 0.11% ETBE

0.08% iBut, 99.0% other C4s, 0.87% EtOH, 1 ppm ETBE

Oxygen Production Rate (kg/hr)

824

788

Net Alcohol Consumption (kg/kg eiher)

0.496

0.476

Net Isobutene Consumption (kg/kg ether)

0.575

0.552

Number of Ideal Stages

12

18

Reflux Ratio

0.81

1.50

Reboiler Duty (MW)

2.80

3.80

Column Diameter (mm)

1500

2000

Table 6.8 compares the ETBE production rates and conditions for the three systems discussed above. The benefits of a specially designed ETBE process are up to 10% higher conversion and 4% higher ether purity. This translates into 10% lower consumption of raw materials. The downside is again increased operating expenses due to the higher reflux rate, reboiler duty and internal column vapour-liquid traffic.

Interestingly, for ETBE production, the best results were obtained from a feed that was relatively lean in isobutene: 99.5% isobutene conversion and 99.2% ether purity with an 18-stage column and a high reflux ratio. For MTBE production, the reverse was applicable. The best results for MTBE were obtained with a feed that was rich in isobutene: 99.8% conversion and 99.3% purity in a 30-stage column with moderate reflux ratio. For a feed that was lean in isobutene, the conversion to MTBE was at least 2% lower than was achieved with ETBE. High MTBE conversions could have been achieved by increasing the reflux ratio in the 30 stage column, but the shorter column ETBE would still produce around 2% higher yields at the same reflux ratio.

Table 6.8 - Comparison of the ETBE Procédés With Isobutene-Lean Feeds

(Dual Reactors)

Modified MTBE Reactive Distillation Process

Ideal ETBE Reactive Distillation Process

Reactor Feed Rate (kg/hr)

23,020

22,340

20 770

Overall Isobutene Conversion

89.8%

92.5%

99.5%

Bottoms Composition (wt%)

3.1% EtOH, 0.7% DIB, 0.01% C4s

6.1% EtOH, 0.9% DIB, 0.01% C4s

0.18% EtOH, 0.6% DIB, < 0.01% C4s

Distillate Composition (wt%)

1.5% iBut, 97.3% other C4s, 1.2% EtOH, nil ETBE

1.2% iBut, 98.8% other C4s, 0.03% EtOH, nil ETBE

0.08% iBut, 99.0% other C4s, 0.87% EtOH, 1 ppm ETBE

Oxygen Production Rate (kg/hr)

841

899

788

Net Alcohol Consumption (kg/kg ether)

0.527

0.512

0.476

Net Isobutene Consumption (kg/kg ether)

0.611

0.593

0.552

Number of Ideal Stages

30

30

18

Reflux Ratio

0.35

0.77

1.50

Reboiler Duty (MW)

2.80

2.80

3.80

Column Diameter (mm)

1350

1350

2000

This result attests to the range of interactions present between chemical and phase equilibria, and the complexity of the reactive distillation design problem. For MTBE reactive columns the dilution effect of reducing the isobutene concentration in the feed predominates over the cooling effect. For ETBE columns the reverse is true. The ether manufacturer should consider such effects when deciding the type of ether unit to be built (MTBE, ETBE or another ether) and optimising its design.

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