Acetylene is produced by pyrolyzing—decomposing at high temperature—natural gas (predominantly methane):

The heat required to sustain this endothermic reaction is provided by feeding oxygen to the reactor and burning a portion of the methane to form primarily CO and some CO₂.

A simplified version of the process is as follows. A stream of natural gas, which for the purposes of this problem may be considered pure methane, and a stream containing 96.0 mole% oxygen and the balance nitrogen are each preheated from 25°C to 650°C. The streams are combined and fed into an adiabatic converter, in which most of the methane and all of the oxygen are consumed, and the product gas is rapidly quenched to 38°C as soon as it emerges from the converter. The residence time in the converter is less than 0.01 s, low enough to prevent most but not all of the methane from decomposing to form hydrogen and solid carbon particles (soot). Of the carbon in the feed, 5.67% emerges as soot.

The cooled effluent passes through a carbon filter in which the soot is removed. The clean gas is then compressed and fed to an absorption column, where it is contacted with a recycled liquid solvent, dimethylformamide, or DMF (MW = 73:09). The off-gas leaving the absorber contains all of the hydrogen and nitrogen, 98.8% of the CO, and 95%of the methane in the gas fed to the column. The “lean” solvent fed to the absorber is essentially pure DMF; the “rich” solvent leaving the column contains all of the water and CO₂ and 99.4% of the acetylene in the gas feed. This solvent is analyzed and found to contain 1.55 mole% C₂H₂, 0.68% CO₂, 0.055% CO, 0.055% CH₄, 5.96% H₂O, and 91.7% DMF.

The rich solvent goes to a multiple-unit separation process from which three streams emerge.

One—the product gas—contains 99.1 mole% C₂H₂, 0.059% H₂O, and the balance CO₂; the second— the stripper off-gas—contains methane, carbon monoxide, carbon dioxide, and water; and the third— the regenerated solvent—is the liquid DMF fed to the absorber.

A plant is designed to produce 5.00 metric tons per day of product gas. Your assignment is to calculate (i) the required flow rates (SCMH) of the methane and oxygen feed streams; (ii) the molar flow rates (kmol/h) and compositions of the gas fed to the absorber, the absorber off-gas, and the stripper off-gas; (iii) the DMF circulation rate: (iv) the overall product yield (mol C₂H₂ in the product gas/mol CH₄ in feed to reactor) and the fraction that this quantity represents of its theoretical maximum value; (v) the total heating requirements (kW) for the methane and oxygen feed preheaters; (vi) the temperature attained in the converter.

(a) Draw and label a flowchart of the process. Determine the degrees of freedom for the overall system, each individual process unit, and the feed-stream mixing point.

(b) Write and number a full set of equations for the quantities specified as (i)–(iv), identifying each one (e.g., C balance on converter, CH4 balance on absorber, ideal-gas equation of state for feed streams, etc.). You should end with as many equations as unknown variables.

(c) Solve the equations of Part (b).

(d) Calculate quantities (v) and (vi).

(e) Speculate on what additional processing step(s) the absorber and stripper off-gases might be subjected to, and state your reasoning.

Transcribed Image Text:

2CH4(g) → C,H2(g) + 3H,