93 Example 2 Oxy Reactor For

Ethylene dichloride (EDC) is used to manufacture vinyl chloride monomer (VCM), which is one of the largest commodity chemicals produced in the world. EDC may be produced by the direct chlorination of ethylene or oxychlorination of ethylene in the presence of oxygen and hydrogen chloride. Pyrolysis of EDC produces VCM and an equal amount of hydrogen chloride as a co-product. This hydrogen chloride produced in the pyrolysis reactor is utilized by the oxychlorination process as one of the reactants. Therefore, the component processes of direct chlorination, EDC pyrolysis and oxychlorination are combined to develop a balanced process for the production of VCM with no net consumption or production of hydrogen chloride:

C2H4 + 2 HCl + 2 O2 Catalys> C2H4Cl2 + H2O (9.2)

The development of oxychlorination technology in the late 1950s encouraged new growth in the vinyl chloride industry. Here, we will be considering an oxychlorination (OXY) reactor to illustrate the application of computational flow modeling to reactor engineering.

In the oxychlorination process ethylene reacts with dry hydrogen chloride and oxygen to produce EDC and water. Though commercial processes for oxychlorina-tion differ somewhat, the reaction, in general, is carried out in the vapor phase in either a fixed bed or a fluidized bed reactor. The reaction is carried out at a temperature of about 200°C and at a pressure of about 500 kPa (Ullmann, 1986). At these operating conditions, the reactants are in gaseous form. Air, oxygen-enriched air or pure oxygen is used to supply the oxygen necessary for the reaction. Oxychlorination catalysts contain copper (II) chloride as the main active ingredient along with numerous additives. The catalyst used in the reaction is solid at the operating conditions. It is a highly exothermic reaction and, therefore, an efficient means of heat removal is essential for temperature control. Higher reactor temperatures result in increased by-product formation and catalyst deactivation. Most modern, large-capacity EDC plants therefore use fluidized bed reactors to carry out this process (Ullmann, 1986).

In fluidized bed reactors the gaseous reactants are introduced below the bed of solid catalyst particles. The upward flowing gas fluidizes the solid catalyst particles (under fluidized conditions the gravity force acting on the solid particles is compensated by the drag force exerted by the gas flow and particles behave like a fluid). The high mobility of the solids ensures excellent heat transfer characteristics, which makes fluidized bed reactors an appropriate choice for highly exothermic gassolid reactions, especially for large-capacity plants (Kunii and Levenspiel, 1991). A schematic of a fluidized bed OXY reactor is shown in Fig. 9.6. Manufacturing companies have developed different versions of fluidized bed reactor technology, and despite its widespread use in practice, the technology of fluidized bed OXY reactors is still very complicated and details are closely guarded. The complexity of the technology originates in the extremely complex fluid dynamics of these reactors. Depending on particle characteristics (size, shape, density, restitution coefficient, etc.), geometry of the equipment (diameter, height, gas distributor, etc.) and operating conditions (gas and solid flow rates, pressure, temperature), fluidized bed reactors may exhibit different regimes of gas-solid flows. Small changes in reactor configuration or any of the operating conditions may change the underlying fluid dynamics and, therefore, may change the performance significantly.

Products/ unconverted reactants

Internal Cyclone

Fluidized bed

Cooling coils

Grid

Air Ethylene HC1

Air Ethylene HC1

FIGURE 9.6 Schematic diagram of OXY reactor (from Ranade, 1999b).

Major reaction engineering issues in fluidized bed reactors are discussed in several excellent textbooks (see, for example, Kunii and Levenspiel, 1991). The conventional reaction engineering models (discussed in these textbooks) along with the knowledge of reaction chemistry, kinetics and thermodynamics may allow the reactor engineer to establish a relationship between reactor volume (amount of catalyst), feed flow rates and yield and selectivity obtainable under specific operating conditions. Several issues related to catalyst (activity, high temperature stability, ageing, etc.) need to be known. Of course, even at this stage, some assumptions about the underlying flow processes need to be invoked to estimate rates of backmixing, mass and heat transfer processes. Computational fluid dynamic models can help the reactor engineer to obtain the required information about the fluid dynamics. It is also possible to couple simulations of reactions within the CFD framework (see, for example, Samuelsberg, 1994). Details of the modeling of fluidized bed reactors are discussed in Chapter 12. In this section, use of a computational flow model to enhance the performance (capacity) of an existing industrial OXY reactor is discussed.

9.3.1. Capacity Enhancement of an Existing OXY Reactor

In order to explore the possibility of enhancing the capacity of an existing OXY reactor, several issues need to be carefully examined. Due to the sensitivity of fluidized bed reactors to operating flow rates, the strategy of forcing more feed through the reactor to enhance the capacity has a rather restricted applicability. The increased feed rate may cause such problems as increased catalyst carry-over etc. The other alternative to enhance capacity is to use oxygen-enriched air or pure oxygen as the oxidation medium instead of ordinary air. This will allow a higher feed rate of reactants without increasing the total gas flow rate through the reactor. Processes operated with oxygen-enriched air or pure oxygen may also lead to significant reductions in gas treatment problems. Before converting the existing process/reactor operating with air, either to an oxygen-enriched or pure oxygen process/reactor, it is essential to ensure that the existing reactor hardware is able to handle such a change. It must be ensured that the local concentrations of oxygen and flammable compounds (such as ethylene) are within safe limits. Considering the extreme corrosiveness of the system, likely scenarios such as malfunctioning of the gas distributor, and the effect on reactor performance and safety of operations need to be carefully evaluated. Computational flow models can be used to achieve this.

In an industrial OXY reactor, air is introduced in the bottom conical portion of the reactor (below the grid). The air stream is mixed with other reactants (ethylene and HCl) in specially designed mixing elements attached to the grid to ensure fast and adequate mixing. Various proprietary and elaborate designs are used to ensure proper mixing of ethylene- and oxygen-containing streams and to restrict the volume of fluid containing a flammable mixture. Without disclosing any proprietary information, one of the simplest and effective mixing elements, a so-called 'mixing cup' is schematically shown in Fig. 9.7. This mixing element is designed in such a way that the composition of mixture exiting these cups is outside the flammability envelope. Air enters mixing elements attached to the grid through the bottom orifice. Ethylene and HCl streams are supplied to each element via a suitable distributor. The grid of the OXY reactor may contain several mixing elements. The reactor engineer has to ensure that air is fed to these various mixing elements uniformly. Non-uniform distribution

Ethylene/HCl feed pipe (from the distributor)

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