Operating Variables and Interactions

4.1.1 Introduction

Reactive distillation columns behave substantially differently to conventional distillation columns due to interactions between the chemical reaction(s) and vapour-liquid equilibrium. The effects of key design and operating variables are discussed below with reference to two ETBE columns: a column with 10 theoretical stages and the configuration described in Chapter 3; and a column with 30 theoretical stages, based on a commercial MTBE column (Simulation Sciences, 1995). The salient characteristics of these columns are indicated in Table 4.1. Both columns are capable of producing an ETBE product purity and an isobutene conversion in the industrially significant range, although the designs are quite different. Thus, a wide range of operating conditions can be investigated without considered imprudent designs.

In addition to the differences between reactive and non-reactive distillation, there are also substantial differences between ETBE columns and the more common MTBE columns and comparisons are made where relevant. The effects described below should be considered during design and operation of the column to ensure optimal performance.

Table 4.1 - Column Charjitenstics

10 Stage Column

30 Stage Column

Rectifying stages (inc. condenser)

2

8

Reactive stages

3

7

Stripping stages (inc. reboiler)

5

15

Feed stage

top stripping stage

top stripping stage

Hydrocarbon feed composition (mol %)

40% isobutene, 60% n-butene

15% isobutene, 85% n-butene

Stoichiometric ethanol excess

5.0 mol%

5.0 mol%

Overhead pressure

950 kPa

600 kPa

Reflux ratio

5.0

1.1

4.1.2 Feed Composition

The hydrocarbon feed composition to an etherification unit is usually fixed by upstream plant operations and vanes between 15% and 55% reactive isobutene depending on the type of unit and type of catalyst in use upstream. Standard FCC units produce C4 streams with 15-20% isobutene while FCC units equipped with new generation catalysts produce C4 streams with up to 35% isobutene. The C4 streams from steam cracking and isobutane dehydrogenation units are even richer in isobutene (up to 55%) (Miracca et al., 1996). In each case, the other components are predominantly n-butenes although other C4s (mainly isobutane and n-butane) are sometimes also present. Although the operation of the various catalytic and steam cracking units can be varied to increase or decrease isobutene production, other factors (e.g. maximising gasoline production) usually have a more significant economic impact and govern the operation of the unit.

Increasing the concentration of reactive isobutene in the hydrocarbon feed has four main effects on the operation of the reactive distillation column: (a) the energy cost, per kg of ether, decreases as less energy is used in heating and cooling the inert components; (b) the reactant concentrations in the reaction zone increase, with a favourable effect on the reaction equilibrium; (c) the reaction zone temperatures and the reaction zone temperature gradient increase as the stabilising effect of inert components is lessened, with a detrimental effect on the reaction equilibrium; and, (d) the specific reboiler duty must be decreased to maintain optimal reaction conditions, with a detrimental effect on product purity. The overall effect of increasing the isobutene concentration in the feed is usually to decrease the maximum conversion and corresponding ether purity.

The effect of the isobutene feed concentration in ETBE reactive distillation contrasts with MTBE processes where an isobutene concentration of around 60% was found to be optimal for conversion and energy efficiency. A consequence of this result is that, when the isobutene concentration in the hydrocarbon feed is low, ETBE synthesis may be more favourable than MTBE synthesis for some column configurations. However, the presence of significant azeotropes in the MTBE system, between methanol and various butenes, means that isobutene conversion and MTBE product purity can essentially be increased much further by adding stages and/or increasing reflux. This does not necessarily apply to ETBE systems.

The maximum conversion in both the 10 stage and 30 stage columns were determined for varying isobutene concentrations in the hydrocarbon feed to the primary reactor. For all cases, 80% conversion in the reactor was assumed and the reactive distillation reboiler duty was optimised with respect to the final, overall conversion. Table 4.2 shows the maximum final conversions and the corresponding ETBE product purities.

Table 4.2 - Effect of Hydrocarbon Feed Composition on Two ETBE Columns

10 Stage Column

30 Stage Column

Isobutene

Maximum

Ether

Maximum

Ether

Concentration

Conversion

Purity

Conversion

Purity

(mol%)

(mol%)

(wt%)

(mol%)

(wt%)

15

98.5

95.0

98.7

97.2

20

98.8

96.2

98.6

96.9

30

98.7

96.7

94.5

94.8

40

98.3

96.1

91.7

93.6

50

97.5

95.4

88.5

91.6

60

95.7

94.1

86.6

91.1

ETBE synthesis from pure isobutene and pure ethanol (three component system only) is not feasible as the intersection of phase and chemical equilibrium is not favourable. Too little isobutene is available in the liquid phase for reaction and, at pressures that result in favourable reaction equilibrium, the separation of ethanol and ETBE is difficult. The maximum isobutene conversion and ether purity attainable with a ternary system using the 10 stage column are around 83 mol% and 91 wt%, respectively.

4.1.3 Stoichiometric Excess of Ethanol

The stoichiometric ratio of reactants significantly affects the reaction conversion and the loads on the downstream product purification and reactant recovery equipment. If the reactant excess is too low, product conversion is adversely limited while if it is too high, purification costs are increased and/or product purity is decreased. The choice of percent excess essentially becomes a compromise between operating costs and the value added by the process (a function of market conditions). The control of the stoichiometric ratio is often complicated by the inability to accurately measure feed composition and the need to ensure that global constraints (due to the catalyst, reaction kinetics or other sources) are not violated.

In an MTBE column, the majority of unreacted methanol is recovered in the distillate product via the methanol-butene azeotropes. This places an upper limit on the methanol excess as sufficient butenes must be present in the distillate product to maintain the unreacted methanol below the azeotropic composition (7-12 mol% methanol, depending on the pressure). For example, if the hydrocarbon feed contained 60% isobutene and 40% n-butenes and the azeotropic composition at the operating pressure was 10 mol% methanol, then the maximum methanol excess would be approximately given by:

Unreacted methanol <10% * (n-butenes + unreacted isobutene)

< 10% >' approximately 41% total hydrocarbon feed

< 4.1 % x total hydrocarbon feed

Reacted methanol = reacted iBut = approximately 59% total hydrocarbon feed

Another consequence of recovering methanol overhead via azeotropes is that, below the maximum methanol excess as determined by the feed and azeotropic compositions, increasing the methanol excess has only a limited effect on the bottoms product purity.

However, in an ETBE column, unreacted ethanol is recovered directly with the ether product. This removes any restriction on the ethanol excess but increases the effect the stoichiometric excess has on the ether purity. Therefore, a compromise must be determined between isobutene conversion (which rises as the ethanol excess increases) and ether purity (which falls as the ethanol excess increases). The column configuration influences this decision by changing the relative magnitudes of the two effects but an ethanol excess of 4-7 mol% is considered sufficient to produce a high isobutene conversion without overly diluting the ether product with ethanol (thereby adding to downstream recovery costs) whilst providing a satisfactory driving force for the reaction. A very high purity ether product can be produced with a lower ethanol excess but some excess should always be used to suppress side reactions involving isobutene. The reaction zone conditions and distillate composition are essentially independent of the stoichiometric excess. Using the 10 stage column configuration as the basis for simulations, Figure 4.1 shows the effect of increasing the ethanol excess on isobutene conversion and ETBE purity.

100%

Conversion

"i

Stoichiometric Ethanol Excess (mol0/^

Fieure4.1 - Effects of the Stoichiometric 1-xcess of Ethanol on the 10 Stage ETBE Column

4.1.4 Column Pressure

In conventional distillation, the operating pressure of a column is normally set through an economic rationalisation of heat transfer costs, pumping costs and the value of improved separation. A lower pressure favours separation (due to increasing relative volatility) and reduces the cost of fluid handling and the reboiler while a higher pressure permits cheaper cooling media to be used in the condenser and reduces the heat transfer area required. The optimum design pressure is often the minimum that allows a satisfactory condenser design with either cooling water or air.

The choice of operating pressure in reactive distillation is complicated by the indirect etfect of pressure on the reaction equilibrium via changing phase equilibrium tempeiatures -increasing the pressure raises the reaction zone temperature and decreases the reaction equilibrium constant of exothermic reactions such as ETBE synthesis, thereby lowering conversion. The effect of pressure on the rate constant, via VLE temperature changes, must also be considered, if the reaction is kinetically controlled.

For both ETBE and MTBE systems, the non-ideality of the liquid phase adds two further restrictions to column operations. Firstly, below a certain pressure (about 130 kPa for ETBE and 300 kPa for MTBE columns) the highest boiling component is the alcohol rather than the ether. Operating below this pressure will drive the ether product back to the reaction zone and will result in low conversion and low ether purity. Secondly, the overhead pressure affects the composition (and presence) of the alcohol-butene azeotropes.

The range of effects that need to be considered in selecting the column operating pressure suggest that an accurate simulation is almost essential for reactive distillation column design. Table 4.3 shows the maximum conversion attainable (and the corresponding reaction zone temperatures and ether purity) in the two ETBE columns that are described in Table 4.1 for various values of the overhead pressure. The column pressure profile (i.e. the total pressure drop across the trays or packings) was fixed for each case and chemical equilibrium was assumed on each reactive stage at all pressures. With these assumptions, the optimum overhead pressure with respect to conversion was found to be 400-500 kPa. However, a higher pressure might be required in practice as the reaction equilibrium becomes less likely (or requires more catalyst) at lower pressures due to kinetic limitations. The optimum operating pressure with respect to conversion was found to be slightly higher in the 30 stage column. Interestingly, the effect of pressure on the ether purity (at the maximum conversion) was directionally opposite in two columns. Indeed, a much higher operating pressure could be preferred in a taller column in order to achieve a very high ETBE product purity (e.g. greater than 99.9 wt%).

This data shows that the choice of an operating pressure depends on many factors and the recommendation of a single 'optimum' operating pressure for ETBE columns is not appropriate. However, the variability in the data suggests that it is appropriate to undertake an optimisation of the operating pressure for a given set of site factors that include the feed composition, local production constraints and product specifications. Operating pressures of commercial columns could (and probably should) vary substantially from site to site. However, as a rule-of-thumb, it is considered that a column pressure such that the reaction zone temperatures are 5-15°C lower than those used in the pre-reactor should guarantee high conversion, high ether product purity and manageable reaction rates.

Table 4.3 - Effect of Column Overhead Pressure on the ETBE Columns

10 Stage Column

30 Stage Column

Overhead

Reaction

Maximum

Ether

Reaction

Maximum

Ether

Pressure

Zone

Conversion

Purity

Zone

Conversion

Purity

(kPa)

Temp (°C)

(mol%)

(wt%)

Temp (°C)

(mol%)

(wt%)

400

44-53

99.0

97.0

44-50

99.1

97.1

500

51-59

99.2

97.1

51-56

98.9

97.1

600

57-65

99.1

97.1

57-62

98.7

97.3

700

63-70

99.0

97.0

62-67

98.6

97.6

800

68-74

98.8

96.6

67-72

98.6

98.1

900

72-79

98.5

96.4

72-77

98.2

98.3

1000

77-82

98.2

95.9

77-81

98.0

99.1

1100

81-86

97.8

95.3

81-85

97.7

99.9

4.1.6 Reflux Ratio

Reflux has a dual purpose in reactive distillation. Increasing reflux rate enhances separation and recycles unreacted reactants to the reaction zone and, thereby, increases conversion. Several effects occur as a result of increasing the reflux ratio: (a) the concentrations of reactants in the distillate are reduced; (b) the reaction zone temperatures are reduced; and (c) the concentration of ether in the reaction zone is reduced. Each of these effects contributes to the increased conversion. Figure 4.2 indicates the relationship between reflux ratio and isobutene conversion for the 10 stage column only. Although the conversion increases monotonically with reflux ratio, the ether purity remains approximately constant for all ratios greater than 6 due to the increasing load on the stripping section as the amount of ETBE in the column increases.

High reflux ratios add to energy requirements and increase the minimum size of all components in the distillation system. Consequently, reflux and stages are often traded against each other in conventional distillation design in order to minimise the total cost of the process. However, it is not always possible to duplicate the effects of reflux with additional separation stages in reactive distillation as only the separation effects are recreated (i.e. the benefits of recycling components to the reaction zone are lost). This is particularly true for ETBE columns and a relatively high reflux ratio is favoured.

Vapor Pressure Etbe

4.1.7 Reboiler Duty

The reboiler duty is one of the principal points of control in a distillation column. In normal distillation, there is a monotonie, albeit sometimes highly non-linear, relationship between the reboiler duty and the principal operating objective (Kister, 1992). In reactive distillation, the reboiler duty must be set to ensure sufficient recycle of unreacted, heavy reactant to the reaction zone without excluding the light reactant from the reaction zone. If the reboiler duty is too high or too low, conversion, and subsequently product purity, is reduced. Figure 4.3 indicates the effect of reboiler duty on conversion and ether purity in the 10 stage ETBE column and clearly shows the presence of an optimum duty. There is only a narrow range of duties that produce an ether purity above 90%. Consequently, tight control is required to achieve acceptable operation.

Operation Reboiler Duty
Figure 4.3 - Effect of Rcboiler Duty on the 10 Stage ETBE Column

4.1.8 Other Operating and Design Variables

The extent of reaction performed prior to the reactive distillation column should simply be an economic optimisation between the relative costs of the two operations. Initially, it is easy to produce ether in a simple tubular reactor with some form of temperature control to prevent the exothermicity of the reaction from heating the reactor contents and, eventually, stopping the reaction due to equilibrium considerations. However, at higher conversions, very low reaction temperatures are required with a subsequent high demand for catalyst due to the low reaction rates. Under these conditions, reactive distillation becomes more economical.

The optimum feed point to the reactive distillation column is just below the reactive section to avoid ETBE decomposition which can result with the relatively ETBE rich product from the reactor. Feeding too far below the reactive section reduces the stripping potential of the column and increases the energy required for separation. Split feeding to each of the reactive stages is possible but creates a high concentration of ETBE at the top of the reactive section (leading to decomposition towards the bottom of the reactive section) and, again, increases the energy required for separation.

The feed temperature has only a very slight effect on the operation of either an ETBE or MTBE reactive distillation column. A cooler feed has a mildly beneficial effect on the reaction zone temperature but this can be offset by a shift in phase equilibrium. To minimise equipment requirements the feed should be supplied at its process temperature, which is likely to be close to the reactor temperature (70-90°C). If intermediate storage is required for any reason, an ambient feed is also acceptable. Neither feed heaters nor feed coolers are required for a satisfactory process design.

0 0

Responses

  • kaisa
    How does stages of distillation affect purity of mtbe?
    6 years ago
  • juuso
    Does increase reboiler duty force more molecules to the top?
    6 years ago
  • michelina
    How does feed stage affect condenser and reboiler duty?
    3 years ago
  • mollie
    What happens to composition when increase reboiler duty?
    3 years ago
  • Christopher
    How reflux ratio effects on reboiler duty?
    2 years ago
  • dieter
    How does reboiler duty effect distillation column?
    11 months ago

Post a comment