Reaction section for the A7 example simulation

Overview

MTBE is manufactured by catalytically reacting isobutylene and methanol in a fixed-bed reactor at a moderate temperature and pressure. The reaction is exothermic and reversible and is carried out in the liquid phase over a fixed bed of ion-exchange resin-type catalyst. It is highly selective since methanol reacts preferentially with the tertiary olefin.

In the standard Hüls process, the reactor products are processed in the MTBE distillation column, where MTBE, t-butanol, dimerized isobutylene, and a trace amount of methanol are removed as the bottom liquid product. In the Ethermax process, further reaction of the isobutylene to MTBE takes place in a section of the distillation column containing the catalyst resin in tower packing. The MTBE is removed as the bottom product in a manner similar to the standard process. The MTBE product is greater than 99.5% pure and requires no further purification.

The key to operating the MTBE distillation column is to have a sufficient amount of C4s in the column feed to form azeotropes with the methanol in the feed. Conversely, if a proportionately large amount of methanol is present in the column feed, it may result in breakthrough of methanol with the MTBE bottoms product.

Therefore, suitable azeotrope formation is possible only when a limited excess of methanol is used in the reactor feed. In this manner, unreacted methanol, which has a higher boiling point than MTBE, is fractionated away from the MTBE column bottoms. The overhead product containing non-reactive linear butenes, iso-butane, normal butane, and unreacted methanol and isobutylene is sent to the methanol recovery section.

Process specifics

In the Ethermax MTBE process, which we model in this example, we mix an isobutylene-rich mixed C4 stream, a fresh methanol stream, and a recycled methanol stream. We then feed this mixture to the reactor section.

Table 1: Reactor feed stream conditions

* These values are calculated by the upstream unit. These values change when you make changes to the simulation.

** We calculate this value to specify the MEOH_IBTE_MOL_RATIO flowsheet variable instead. This value may change when you make changes to the simulation.

We want to determine and set the ratio of methanol to isobutylene that we feed to the reactor. We add the MEOH_IBTE_MOL_RATIO flowsheet variable to the simulation as well as a flowsheet equation (Eqn 3) to calculate its value. Typically, the methanol-to-isobutylene ratio in the reactor feed is kept low (close to 1:1) to minimize the costs of recovering unreacted methanol and to facilitate the operation of the MTBE column. Therefore, we set the value of the MEOH_IBTE_MOL_RATIO flowsheet variable to 1.

We can easily achieve an isobutylene conversion to MTBE of 90% to 93% in the reactor. We can achieve overall isobutylene conversions higher than those obtained in the standard process by either recycling a portion of the overhead product from the MTBE reactive distillation column or by providing a second reactor unit and reactive distillation column downstream of the first reactive distillation column. In this example, we use only one reactor (R1) and one reactive distillation column (T1).

We use the HX1 heat exchanger to heat the reactor feed stream (the combined mixed olefins, fresh methanol, and methanol recycle) to 43.5°C before it enters the R1 reactor.

Two side reactions can occur in the reactor to produce impurities in the product:

• Any water in the reactor feed (from the recycled methanol) instantly converts to t-butanol.

• Isobutylene can dimerize into di-isobutylene (2,4,4-Trimethyl-1-pentene).

While you should minimize the formation of di-isobutylene and t-butanol, their presence in small concentrations in the MTBE product is acceptable since these byproducts also have very high octane numbers. The reactors are cooled to under 200°F to prolong catalyst life and to minimize the undesirable side reactions.

The following table shows the three main reactions used in the stoichiometric reactor model as well as the base component and conversion fraction for each reaction.

Table 2: Primary reactions

The reactor uses the RxnConversion reaction submodel and the specified conversion levels in the preceding table. See Reaction submodels for the A7 example simulation for details on the RxnConversion reaction submodel.

Note: You can use the Reaction Generator to create the reaction submodel. You can open the Reaction Generator from the Advanced tab.

We carry out the reaction at 55°C. There is a pressure drop of 69 kPa across the reactor.

Figure 2: Conversion reactor configuration

We use the HX2 heat exchanger to further heat the reactor product to 72°C before we send it to the reactive distillation column (T1). We use the hot product coming from the T1 Column to reach the desired temperature in the column feed before it enters the reactive distillation.

The T1 Column has a total of 28 stages, a top pressure of 621 kPa, and an overall pressure drop of 76.5 kPa. For the reactive distillation process, the following figure shows the reaction zone (trays 7 through 12), the volume of catalyst used, and the kinetic reaction submodel used in the reaction zone.

Figure 3: Reactive distillation sections, volume of catalyst, and kinetic reaction submodel

In AVEVA Process Simulation, you can carry out multiple reactions at different sections of the Column. Therefore, you must specify the reaction submodel and the volume of catalyst for each tray in the Column.

The kinetic reaction submodel that we use for the reaction zone requires the density of the catalyst. This is not an expected value on other reactive distillation columns, and you will not see this variable in the main Tray data in the Mini Inspector. However, the Rxn submodel for each stage in the reaction zone includes the Rho_cat variable that you can use to specify the catalyst density for each stage. You can access this variable in the Simulation Manager or in the full Properties Inspector. The following figure shows this variable for Stage 7 in the full Properties Inspector.

Figure 4: Catalyst density at each stage

A suitable catalyst for this operation is the Amberlyst 15 polymeric catalyst developed by Rohm and Haas. With an apparent density of 770 g/L and a moisture content of 53%, the effective dry density is around 360 g/L, or 360 kg/m3.

There are a number of factors that affect the overall conversion rate of isobutylene. Some of these are:

• Methanol-to-isobutylene ratio

• Number of reaction trays

• Type of catalyst used

However, while the isobutylene conversion in the conversion reactor increases as you increase the methanol-to-isobutylene ratio, the overall isobutylene conversion reaches a maximum and then decreases as you increase the methanol-to-isobutylene ratio. This is due to the fact that more MTBE product is carried upward through the column stripping section into the reaction trays. This promotes the reverse reaction of MTBE to methanol and isobutylene, thus reducing the overall conversion of isobutylene.

The MTBE product (S7.z[MTBE]) is specified as 99.5% molar purity and is recovered at the bottom of the column. A reflux ratio of 1.1 is typically sufficient. The product cools in the HX2 heat exchanger as it heats the feed to the T1 Column.