ENERGY BALANCESPROBLEMS ChE 31 Introduction to Chemical Engineering Main Reference: “Elementary Principles of Chemical Processes” by Felder and Rousseau 3rd Edition Prepared by: JACapunitan A1 pared by CG Alfafara 1. The contents of a flask are heated from 25oC to 80oC. The energy balance can be written as: (a) ΔU + ΔEK + Δ EP = Q –W (b) ΔU = Q – W (c) ΔU = Q (d) ΔU + ΔEK = Q Which is true? (a) Q = 0 (b) Q > 0 (c) Q < 0 2. A tray filled with water at 20oC is put into a freezer. The water turns into ice at -5oC. (a) ΔU = W (b) ΔU = Q – W (c) ΔU = Q (d) ΔU = 0 Which is true? (a) Q = 0 (b) Q > 0 (c) Q < 0 Which is true? (a) W = 0 (b) W > 0 (c) W< 0 3. A chemical reaction takes place in a closed adiabatic (properly insulated) rigid container. The energy balance can be written as: (a) ΔU = W (b) ΔU = Q – W (c) ΔU = Q (d) ΔU = 0 4. An exothermic chemical reaction takes place in a closed isothermal rigid container. The energy balance can be written as: (a) ΔU = W (b) ΔU = Q – W (c) ΔU = Q (d) ΔU = 0 Which is true? (a) Q > 0 (b) Q < 0 (c) Q = 0 5. Steam enters a rotary turbine and turns a shaft connected to a generator. The inlet and outlet ports are at the same height. Some energy is transferred to the surroundings as heat. For this system, (a) H E K E P Q W s (b) H E K Q W s (c) H E K Q (d) H E K W s Which is true? (a) Q > 0 (b) Q < 0 (c) Q = 0 Which is true? (a) WS = 0 (b) WS > 0 (c) WS < 0 6. A liquid stream flows through a heat exchanger in which it is heated from 25oC to 80oC . The inlet and outlet pipes have the same diameter, and there is no change in elevation between these points. For this system, H E K Q W s (b) H Q W s (c) H Q (d) H E W s (a) K Which is true? (a) Q > 0 (b) Q < 0 (c) Q = 0 Which is true? (a) WS = 0 (b) WS > 0 (c) WS < 0 7. Water passes through the sluice gate of a dam and falls on a turbine rotor, which turns a shaft connected to a generator. The fluid velocity on both sides of the dam is negligible, and the water undergoes insignificant pressure and temperature changes between the inlet and outlet. For this system, (a) H E P Q W s (b) H E P W s (c) E P W s (d) E Q W s P Which is true? (a) Q > 0 (b) Q < 0 (c) Q = 0 Which is true? (a) WS = 0 (b) WS > 0 (c) WS < 0 8. Crude oil is pumped through a cross-country pipeline. The pipe inlet is 200 m higher than the outlet, the pipe diameter is constant, and the pump is located near the midpoint of the pipeline. Energy dissipated by friction in the line is transferred as heat through the wall. For this system, (a) H E P Q W s (b) H E P W s (c) E P W s (d) E Q W s P Which is true? (a) Q > 0 (b) Q < 0 (c) Q = 0 Which is true? (a) WS = 0 (b) WS > 0 (c) WS < 0 9. An endothermic chemical reaction takes place in a continuous reactor that contains no moving parts. Kinetic and potential energy changes from inlet to outlet are negligible. For this system, H E K Q W s (b) H Q W s (c) H Q (d) H E W s (a) K Which is true? (a) Q > 0 (b) Q < 0 (c) Q = 0 Which is true? (a) WS = 0 (b) WS > 0 (c) WS < 0 10. A fuel oil is burned with air in a boiler furnace. The combustion produces 813 kW of thermal energy, of which, 65% is transferred as heat to boiler tubes that pass through the furnace. The combustion product pass from the furnace to a stack at 650oC. Water enters the boiler tubes as a liquid at 20oC and leaves the tubes as saturated steam at 20 bar absolute. Calculate the rate (kg/h) at which steam is produced. (From steam tables: Ĥ (H2O(l), 20oC) = 83.9 kJ/kg; Ĥ (steam, 20 bar, saturated) = 2797.2 kJ/kg) 10. Liquid water is fed to a boiler at 24oC and 10 bar and is converted at constant pressure to saturated steam. Calculate the heat input required to produce 15,000 m3/h of steam at the exiting conditions. Assume that the kinetic energy of the entering liquid is negligible and that the steam is discharged through a 15-cm ID pipe. (From steam tables: Ĥ (H2O(l), 24oC, 10 bar) = 100.6 kJ/kg; Ĥ (steam, 10 bar, saturated) = 2776.2 kJ/kg; V (steam, 10 bar, saturated) = 0.1943 m3/kg))