In a wastewater treatment plant, wastewater sludge is dried in a dryer where the heat for the dryer is coming from saturated steam at 4 bar. The 4-bar steam is generated in a plant boiler supplied with boiler feedwater at 25C.

To generate the steam, 200 kg/h fuel oil consists of 87% C, 10% H, 2.3% S and the balance of nonvolatile ash in weight basis is burned in the plant boiler with 200% excess air and the heat is then transferred to the boiler feedwater.

The boiler is fed by the fuel oil and dry air at 60C and 100C, respectively producing combustion gas products at 200C. The fuel oil is completely burned, generating combustion gas products consists of CO2 and CO in a molar ratio of 6:1 and the balance of a mixture of SO2, H2O, N2 and O2.

The boiler has an efficiency of 65%, indicating that only 65% of the heat from the combustion is used to produce the 4-bar steam. The 4-bar steam is employed in the dryer to vaporize water from the sludge and then condensed as saturated liquid water at the same pressure.

Water vapor is removed from the sludge in the dryer, producing a concentrated sludge from the system. The process flow of the wastewater sludge drying system is given in Figure 1

(a) Draw and label the process flowchart of the system.

b) Determine the amounts of dry air and combustion gas products at the plant boiler (kmol/h).

(c) Determine the amount of 4-bar steam generated by the plant boiler (kg/h).

(d) Determine the energy involved in the dryer (kW).