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Unformatted text preview: CE 561 Lecture Notes Fall 2009 p. 1 of 27 Day 30: A detailed reactor modeling example Before proceeding to the ‘special topics’ portion of the course, we will consider an example in more detail than we have in the past. We will look carefully at the modeling of the steam reformer presented by Froment and Bischoff, beginning on p. 482 of their book. The overall process that is to be carried out is conversion of natural gas (mostly CH 4 ) plus steam to CO and H 2 (synthesis gas). Synthesis gas is the feedstock for making ammonia (in which case we just want the H 2 ) and methanol (where both the CO and H 2 are needed – in a 1 to 2 ratio). Steam reforming is done in a furnace, in which there are many long, thin tubes packed with a nickel/alumina catalyst. The furnace is heated by burning some of the natural gas. A sketch of the furnace (very similar to the one on p. 399 of Froment and Bischoff) is shown below. Many more details about the kinetics of the reactions and how they were determined can be found in the original papers by Xu and Froment (AIChE Journal, volume 35, pp. 88-103 (1989)). In this example, we will attempt to model a single reactor tube. Data on the reactor geometry, feed composition and flow rates, etc. are as follows: Feed temperature: T o = 793 K Feed pressure: p to = 29 bar CE 561 Lecture Notes Fall 2009 p. 2 of 27 Feed flow rate per tube: 135.0 Nm 3 /h natural gas = 6.023 kgmol/h 399.2 Nm 3 /h steam = 17.81 kgmol/h Natural gas composition: 81.5% CH 4 by volume 14.1% N 2 2.8% C 2 H 6 1.0% CO 2 0.4% C 3 H 8 0.1% C 4 H 10 0.2% C 5 H 12 Tube internal diameter 10.16 cm Tube external diameter 13.22 cm Total tube length 12.0 m Heated tube length 11.12 m Rather than considering the detailed feed composition, we will use an equivalent (but imaginary) feed consisting only of methane, steam, CO 2 , N 2 , and H 2 . The total molar feed rates to the reactor are (for the real feed): Species H 2 O CH 4 N 2 C 2 H 6 CO 2 C 3 H 8 C 4 H 10 C 5 H 12 H 2 Feed rate (kgmol/h) 17.81 4.909 0.849 0.169 0.060 0.024 0.006 0.012 0.0 (lower) Heat of combustion (kcal/mol) 191.76 341.26 488.53 635.38 782.04 57.80 (higher) Heat of combustion (kcal/mol) 212.76 373.01 530.4 687.7 845.0 68.32 In the equivalent feed, the hydrocarbons will be replaced by CH 4 , CO 2 , H 2 O and H 2 such that the total heat of combustion of the feed is not changed. The equivalent feed will also have the same atomic composition and total mass flow rate as the actual feed. The atomic feed rates are Species H C N O Feed rate (kgmol/h) 56.7 5.43 1.70 17.9 The total heat of combustion of the feed is 1.024 × 10 6 kcal/hr (or 1.134 × 10 6 kcal/hr using higher heats of combustion). The flow rates of the equivalent feed that match the atomic composition, mass flow rate, and (higher) heat of combustion are given below. Using the higher heat of combustion gave results matching those in the Froment and Bishchoff textbook....
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- Fall '09
- Chemical reaction