PROCESS PRINCIPLES CHEMISTRY PROJECT

Instructions:

Project report to include:

1) Assignment Coversheet

2) A title page including the names of individuals in the group

3) A signed declaration about individual contributions, equally or otherwise

4) Contents page

5) Introduction to the project

6) Results for each of the questions

7) A summary page for the project

8) Conclusions and recommendations

9) References (if any)

10) Appendices

Only ONE report is to be submitted by each group to the Assignment Office on Level 2 of Building 204

Project Title: Manufacture of Synthesis Gas

Synthesis gas or Syngas, a mixture of hydrogen, carbon monoxide and carbon dioxide is produced from carbon-containing fuels like natural gas. Syngas is utilized in the production  of ammonia or methanol. Catalytic steam reforming of natural gas is a mature technology for the production of syngas.

Process Description

The process essentially consists of the steam-reforming reaction:

CH₄ + H₂O ↔ CO + 3 H₂                  ΔH = +206.16 kJ/mol CH₄                                           (1)

Reaction (1) is accompanied by the water-gas shift reaction

CO + H₂O ↔  CO₂ + H₂                    ΔH = - 41.15 kJ/mol CO                                              (2)

These reactions together generate a mixture of carbon monoxide, hydrogen, and carbon dioxide. The steam reforming reaction is endothermic and requires external heat input. Economics favour reactor operation at elevated pressures and temperatures, while the external heat needed to drive the reaction is often provided by the combustion of a fraction of the incoming natural gas feedstock or from burning waste gases. Heat transfer to the reactants is accomplished indirectly through a series of heat exchangers. Methane and steam react in catalyst filled tubes. Typically, the mass ratio of steam-to-carbon is about 3 or more to avoid "coking" or carbon build-up on the catalysts. At lower steam-to-carbon ratios, solid carbon can be produced via side reactions.

For the present case, natural gas may be assumed to consist entirely of methane (CH₄), although higher hydrocarbon compounds are almost always present in small concentrations. Methane and steam, at 27°C and 180°C, respectively are introduced to the process in a ratio of 3.0 moles of steam per mole of methane. The mixture is preheated to 480°C by exhaust gas from the firebox before it is introduced to the reformer.

The product gas leaves the reformer at 865°C and 1.7 MPa. Energy efficiency in steam reforming is improved by recovering heat from the burner exhaust gas, which leaves the firebox at 970°C. The exhaust gas is cooled in a series of heat-exchange operations that preheat the reformer feed streams to 480°C and pre-heat the combustion air to 310°C. The burner exhaust gas leaves the heat-recovery units and enters an exhaust flue at 140°C before release to the atmosphere.

Project Brief

Your company is considering the purchase of a plant that uses the previously described technology. The plant may be assumed to operate 350 days per year. The company from which the purchase may be made has indicated that the plant will produce 4.6 x 10⁵ metric tons per year of dry synthesis gas, when operated under the conditions specified above.

You have been asked to perform an analysis that will be a key part of determining the price that will be offered for the plant.

The following problems have been formulated in strategy sessions by the team negotiating the purchase and should be helpful in completing your task.

(1) Draw the flowchart and label all streams with all relevant information from the project description and from your calculations below.

(2) Five percent excess air is used in burning the reformer fuel; it is drawn into the system at 27°C and 65% relative humidity.

(i) Estimate the average molecular weight of the air.

(ii) Determine the flow rate of this stream (kmol, m³) per kmol of natural gas burned.

(3) (a) What are the compositions (mole and mass fractions) and volumetric flow rates (m3/kmol CH4 fed to burners) of

(i) the effluent gas from the reformer burners

(ii) the gas entering the exhaust flue?

(b) What is the specific gravity, relative to ambient air (27°C, 1 atm, 65% RH), of the stack gas as it enters the flue?

(c) Why is this quantity important in designing the flue?

(d) Why might there be a lower limit on the temperature to which the gas can be cooled prior to introducing it to the exhaust flue?

(4) The primary purpose of the reformer is to convert methane and water to carbon monoxide and hydrogen (Equation 1). The extent of this reaction is limited by chemical equilibrium.

 K_P1 = (y_CO y³_{H_2}) P²/(y_{CH_4} y_{H_2O})                      (3)

where,

log₁₀K_P1 = - {11,769 / T(K)} + 13.1927                 (4)

Subscript 1 in KP refers to the steam reforming reaction (Equation 1), yi is the mole fraction of species i, P is the system pressure (atm), and T is the temperature (K).

(a) If Equation 1 were the only reaction occurring in the reformer, estimate the composition of the product gas that would be leaving the reformer and the conversion of CH₄, assuming the product stream has achieved chemical equilibrium at 865°C and 1.7 MPa. It is specified that the molar ratio of steam to methane fed to the reformer is 3.0, whereas the stoichiometric ratio for the reforming reaction (Equation 1) is 1 mole of water per mole of methane.

(b) What would be the required flow rate of the natural gas and steam fed as reactants (kmol/h, kg/h)?

(c) Estimate the conversion of methane for steam-to-methane feed ratios of 1:1 and 2:1, and compare these to the conversion with a ratio of 3:1.

(d) Based on your results, explain in your own words why you think the ratio of 3 moles of steam per mole of methane was chosen for the process.

(5) As pointed out in the Process Description, the water-gas shift reaction (Equation 2) occurs in the reformer along with the reforming reaction (Equation 1). It too is controlled by chemical equilibrium.

 K_P2 = (y_{CO_2} y_{H_2}) / (y_CO y_{H_2O})                           (5)

where,

log₁₀K_P2 = {1197.8 / ??(??)} - 1.6485                         (6)

where the nomenclature is analogous to that in the preceding problem.

(a) Taking into account the occurrence of reactions given by both Equations 1 and 2, estimate the composition of the product gas leaving the reformer and the conversion of CH₄, assuming the product stream leaving the reformer has achieved chemical equilibrium at 865°C and 1.7 MPa, with the molar ratio of steam to methane fed to the reformer equal to 3.0.

(b) What would be the required flow rate of the natural gas and steam fed as reactants (kmol/h, kg/h)?

(c) What effect does the water-gas shift reaction have on the production of CO at the reformer conditions?

(d) The ratio of CO to H₂ can be an important variable in efficient use of  raw materials. In this case study a 3:1 steam-to-methane molar ratio of feed streams was specified. Determine how this feed ratio affects the ratio of CO to H₂ in the product from the reformer assuming the reaction products are in chemical equilibrium at 865°C and 1.7 MPa.

(6) The reformer product gas leaves the reformer at 865°C. Using the composition calculated in 5(a):