CHAPTER 8 BALANCES ON NONREACTIVE PROCESSES By Ms

CHAPTER 8 BALANCES ON NONREACTIVE PROCESSES By : Ms. Nor Helya Iman Bt Kamaludin Email: helya@unimap. edu. my 1 PTT 108: Mass and Energy Balances

Introduction �Non-reactive processes q Processes that undergo without chemical reaction. q Depends on the physical/environmental factors like temperature, volume, pressure, boiling and melting, as well as vaporization. �Normally in chemical process unit, Ws=0; ΔEp=0; ΔEk=0; Then energy balance equation become: Close System Open System 2 PTT 108: Mass and Energy Balances

Different symbols between close and open system CLOSE SYSTEM 3 OPEN SYSTEM n (amount) (molar flow rate) U (internal energy) (rate of enthalpy) Û (specific internal energy) (specific enthalpy) PTT 108: Mass and Energy Balances

Introduction (cont’d) �For this chapter, we will learn the procedure for evaluating ΔU and ΔH when table Ĥ and Û are not available for all process species. �Example enthalpy change (ΔĤ) for solid phenol at 25 ˚C and 1 atm converted to phenol vapor at 300 ˚C and 3 atm. 4 PTT 108: Mass and Energy Balances

Introduction (cont’d) 5 To evaluate changes in enthalpy or internal energy, we can make up any process path we want to simplify the calculations. They can often be evaluated for: 1. changes in P at constant T 2. changes in T at constant P 3. changes in T at constant V 4. changes in phase at constant T and P (e. g. , heats of vaporization) 5. mixing at constant T and P (heats of mixing) 6. chemical reactions at constant T and P (heats of reaction)PTT 108: Mass and Energy Balances Chapter 9

Hypothetical Process Path � State properties q Properties that depend on the state of the species (primarily on its temperature and state of aggregation, and also pressure). q Specific enthalpy (Ĥ) and specific internal energy (Û) are state properties species q 6 When a species passes from one state to another state, both ΔĤ and ΔÛ for the process are independent of the path taken from the first state to the second state. PTT 108: Mass and Energy Balances � We can construct a hypothetical process path which

Hypothetical Process Path (Examples) ΔĤ= (vapor, 300˚C, 3 atm) – (solid, 25˚C, 1 atm) �Cannot determine directly form enthalpy table – must use hypothetical process path consist of several step. �Check Table B. 1 for Phenol : P= 1 atm; Tm= 42. 5˚C and Tb= 181. 4˚C 7 PTT 108: Mass and Energy Balances

Procedure Energy Balance Calculations 8 1. Perform all required material balance calculations. 2. Write the appropriate form of the energy balance (closed or open system) and delete any of the terms that are either zero or negligible for the given process system. 3. Choose a reference state – phase, temperature, and pressure – for each species involved in the process. 4. Construct inlet-outlet table for specific internal energy (close system) or specific enthalpy (open system) q For closed system, construct a table with columns for initial and final amounts of each species (mi or ni) and specific internal energies relative to the chosen reference states (Ûi). q For an open system, construct a table with columns for PTTinlet 108: Mass and Energy Balances and outlet stream component flow rates ( ) and

Procedure Energy Balance Calculations (cont’d) 5. Calculate all required values of Ĥi or Ûi and insert the values in the appropriate places in the table. 6. Then calculate ΔĤ or ΔÛ for the system. Closed System: Open System: 9 7. Calculate any work, kinetic energy, or potential energy terms that you have not dropped from the energy balance. 8. Solve the energy balance for whichever variable is PTT 108: Mass and(often Energy Balances unknown Q or ).

Class Discussion EXAMPLE 8. 1 -1: Energy Balance on a Condenser The flowchart 10 PTT 108: Mass and Energy Balances

Solution STEP 1: Perform required material balance calculations. -None are required in this example because there are no material balance involved in this process. STEP 2: Write and simplify the energy balance. For open steady-state system: 11 PTT 108: Mass and Energy Balances

Ø Delete any of the terms that are either zero or negligible in process system. -No moving parts and no energy is transferred in the system: -No significant vertical distance separates the inlet and outlet ports: -Phase changes and nonnegligible temperature changes occur: 12 So, the energy balance become: PTT 108: Mass and Energy Balances

STEP 3: Choose reference states for acetone and nitrogen. N 2 (g, 25˚C, 1 atm) : convenient (Table B. 8) Ac (l, 20˚C, 5 atm) : choose from one of the process stream. STEP 4: Construct an inlet-outlet enthalpy table. References: Ac (l, 20˚C, 5 atm); N 2 (g, 25˚C, 1 atm) 13 PTT 108: Mass and Energy Balances

STEP 5: Calculate all unknown specific enthalpies In this step we need to find i. For changes in pressure (∆P) at constant T ii. For changes in temperature (∆T) at constant P iii. For changes in phases at constant P and T 14 Use heat of vaporization or heat of melting at normal boiling point and normal melting PTT 108: Mass and Energy Balances point,

�The process path involved for determination of = (0. 0297 + 4. 68 + 30. 2 + 0. 753) k. J/mol = 35. 7 k. J/mol *Use the same manner to obtain the values for 15 PTT 108: Mass and Energy Balances

-insert all specific enthalpies into inlet-outlet enthalpy table STEP 6: Calculate 16 PTT 108: Mass and Energy Balances

STEP 7: Calculate nonzero work, kinetic energy and potential energy terms -no need to be calculated since there are no shaft work and we are neglecting kinetic and potential energy. STEP 8: Solve the energy balance for 17 = -2320 k. J/s = -2320 k. W Heat must be transferred from the condenser at a rate of -2320 PTT 108: Mass and Energy Balances k. W to achieve the required cooling and

Changes in P at Constant T (no phase change or reactions) 18 Ideal gases: Independent of pressure ( unless undergo very large pressure changes) Real gases: must evaluate from: 1. enthalpy departure charts 2. an equation of state 3. tabulated data Liquids and solids: Nearly independent PTT 108: Mass and Energy Balances of pressure

Changes in T and Constant P (no phase change or reactions) Called sensible heat, heat that must be transferred to RAISE or LOWER the temperature of substance or mixture of substance and we usually find: Heat capacities help us calculate this change in enthalpy. The “heat capacity at constant pressure” is defined by: 19 PTT 108: Mass and Energy Balances �Thus, enthalpy changes at constant P are given

Changes in T and Constant P (no phase change or reactions) 20 PTT 108: Mass and Energy Balances

Changes in T at Constant V (no phase change or reactions) Here, we use the heat capacity at constant volume, defined by: Ideal gases: Liquids and solids: 21 PTT 108: Mass and Energy Balances

Heat Capacity Formulas Definition: Heat capacities, Cp and Cv are physical properties of materials and are tabulated in standard references. Units: J/(mol. K) and Btu/(lbm. ˚F) Relationships exist between Cp and Cv : where R = gas constant 22 PTT 108: Mass and Energy Balances

Heat capacity, Cp 23 Estimation of heat capacities, Cp q Kopp’s rule- simple empirical method for estimating Cp of solid or liquid at or near 20˚C based on the summation of atomic heat capacities (Table B. 10) of the molecular compound. (Cp)Ca(OH)2 = (Cpa)Ca + 2(Cpa)O + 2(Cpa)H = 26 + (2 x 17) + (2 x 9. 6) = 79 J/mol. ˚C q Polynomial expressions for Cp in Table B. 2 based on the experimental data for the listed PTT 108: Mass and Energy Balances compound.

Heat capacity, Cp (cont’d) �Table B. 2 demonstrated the heat capacities as a function of temperature (at low pressures) in the equation form for a number of solid, liquid, and gaseous substances. This is your source of ideal-gas heat capacities. �Tables of specific enthalpies eliminate the 24 need for use of heat capacities, i. e. , someone has already done the integrations for you. For "combustion gases" you should use the tables of molar enthalpies given in Table B. 8 and Table B. 9. PTT 108: Mass and Energy Balances

Heat capacity, Cp (cont’d) Mixtures: Where = heat capacity of the mixture = mass or mole fraction of the ith component = heat capacity of the ith component 25 PTT 108: Mass and Energy Balances

Class Discussion �EXAMPLE 8. 3 -1 �EXAMPLE 8. 3 -2 �EXAMPLE 8. 3 -3 �EXAMPLE 8. 3 -4 �EXAMPLE 8. 3 -5 26 PTT 108: Mass and Energy Balances

Phase Change Operations � Phase change such as melting and evaporation are usually 27 accompanied by large changes in internal energy and enthalpy � Latent heat q Specific enthalpy change associated with the transition of a substance from one phase to another at constant T & P. � Heat of fusion or heat of melting, ΔĤm (T, P) q Specific enthalpy different between solid and liquid forms of species at T & P q Heat of solidification (liquid to solid) is –ve value of heat of fusion. � Heat of vaporization, ΔĤv (T, P) q Specific enthalpy different between liquid and vapor forms of species at T & P q Heat of condensation (vapor to liquid) is –ve value of heat of PTT 108: Mass and Energy Balances vaporization.

Estimation of Heat of Vaporization 1. Trouton’s rule – accuracy between 30% 2. Chen’s equation – accuracy between 2% 3. Clausius-Clapeyron equation - plot In p* versus 1/T 28 PTT 108: Mass and Energy Balances

Estimation of Heat of Vaporization (cont’d) 4. Chaperon equation 5. Watson correlation – estimate ΔĤv at T 2 from known ΔĤv at T 1 �Estimation of Heat of Fusion 29 ΔĤm (k. J/mol) = 0. 0092 Tm (K) metallic elements = 0. 0025 Tm (K) inorganic compound PTT 108: Mass and Energy Balances = 0. 050 Tm (K) organic compound

Class Discussion �EXAMPLE 8. 4 -1 �EXAMPLE 8. 4 -2 �EXAMPLE 8. 4 -3 �EXAMPLE 8. 4 -4 30 PTT 108: Mass and Energy Balances

Psychrometric Charts �PSYCHROMETRIC chart (or HUMIDITY Chart) is 31 a compilation of a large quantity of physical property data in a single chart. The properties are: (a) Wet Bulb Temperature (b) Saturation Enthalpy (c) Moisture Content (d) Dry Bulb Temperature (e) Humid Volume �The Psychrometric Chart is particularly important for Air-Water system and normally is at Pressure of 1 atm. PTT 108: Mass and Energy Balances �Psychrometric Chart is very useful in the analysis

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Psychrometric Charts (cont’d) �To use Psychrometric Chart, you need to know TWO values to determine the values of the others on the chart. 33 IMPORTANT TERM: �Dry-bulb temperature, T – This is the air temperature as measured by thermometer, thermocouple, or other conventional temperature measuring device. �Absolute humidity, ha [kg H 2 O(v)/kg DA] – Called moisture content placed on the ordinate PTT 108: Mass and Energy Balances of the chart.

Psychrometric Charts (cont’d) �Relative humidity, hr Curves on the psychrometric chart correspond to specified values of hr (100%, 90%, 80%, etc. ). The curve that form the left boundary of the chart is 100% relative humidity (saturation curve). �Dew point, Tdp The temperature at which humid air becomes saturated if it is cooled at constant pressure. �Humid volume, (m 3/kg dry air) The humid air is the volume occupied by 1 kg of dry air plus the water vapor that accompanies it. 34 PTT 108: Mass and Energy Balances

Class Discussion �EXAMPLE 8. 4 -5 35 PTT 108: Mass and Energy Balances

THANK YOU…. 36 PTT 108: Mass and Energy Balances