Page 134 - Chemical process engineering design and economics

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Example 3.4 Packed-Bed, Catalytic Reactor___________________

In this problem, we will analyze a packed-bed catalytic reactor. Heat may be either
transferred into or out of a reactor, depending on whether the reaction is exother-
mic or endothermic. One design for transferring heat is to pack the catalyst into
tubes, approximately 5.0 cm (2 in) in diameter, and arrange them in parallel inside
a shell. A heat-transfer fluid flows into the shell surrounding the tubes, removing
or adding heat. We will consider the production of formaldehyde synthesized by
oxidizing methanol with air. Formaldehyde ranks 25th by volume among all
chemicals produced. Its major end uses are 60% for adhesives and 15% for plas-
tics [23].

Process Chemistry

Because formaldehyde synthesis is exothermic, the reactor requires a coolant to
remove the excess enthalpy of reaction. Thermodynamically, we should run the
reaction at as low a temperature as possible to increase conversion, but at low
temperatures, however, the rate of reaction decreases. At high reaction tempera-
tures unwanted side reactions occur. Commercially, the reaction occurs from 600
°C (1110 °F) to 650 °C (1200 °F), which results in a methanol conversion of 77 to
87 % when using a silver catalyst [24]. Because formaldehyde and methanol can
form flammable mixtures with oxygen, we should carry out the reaction with mix-
ture compositions outside of its flammability range. The oxygen used is less than
the stoichiometric amount.

Process Analysis

Methanol flows at the rate of 1000 kmol/h (22051b mol)into the reactor, shown in
Figure 3.4.1, where methanol is oxidized catalytically to formaldehyde under non-
adiabatic conditions. The reactants enter the reactor at 500 °C (932 °F), and the
products exit at 600 °C (1110 °F). The methanol in stream 1 and air in stream 2
are both at 500 °C, and the methanol conversion is 80 %. To minimize possible
combustion of methanol and formaldehyde, we set the molar flow rate of oxygen
at 80% of the stoichiometric quantity. The reaction is

CH 3OH(g) + 1/2 O 2(g) -> HCHO(g) + H 2O(g) (-37,280 cal, 298 K) (3.4.1)

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