Applied chemical processes Energy in chemicall processes. Types of energy. Overall energy balance

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1 Applied chemical processes Energy in chemicall processes types of energy overall energy balance heat balance cooling and heating in chemical proc. electrical energy Types of energy Energy in chemical processes occurring in various forms: heat, chemical energy, electrical energy, internal energy, potential energy, kinetic energy, etc. Each type of energy is usually connected to a specific activity, but also can be converted to other type as: when pumping media is heated Types of energy can be divided according to their relations to the flow of substances: Bound to the energy flowing agents: potential, kinetic, internal Energy passing through the system boundary: thermal energy, pv work Overall energy balance By analogy with the law of conservation of mass, the law of conservation of energy. For a closed system is: Σ energy input = Σ energy output + accumulation usual case of balance calculation steady state (i.e. accumulation = 0) Special case (start up, shut down) - unsettled state definitions: heat supplied to the system: plus sign (+) work provided by systém: plus sign (+) 3

2 Unsettled state Generally start up or shut down of technological systems For the energy balance calculation is necessary: - knowledge of energetic capacity (consumption of energy) - requested time (time for start up/shut down) For the lack of accurate data (eg heat exchanger capacity) the calculation generally based on estimates and experience from similar operations. The operating temperature is often associated with the need to install the heating elements into the reactor (usually electric heating elements). When oversizing -take place during steady operation, at undersizing approachto steady state takes too long time. 4 The overall energy balance II Mathematical expression of the conservation of energy in the steady state : Epot Ekin pv U Q Epot Ekin pv U W E E pot kin g m z mv m weight [kg] z,z height [m] v,v velocity [m s - ] g acceleration of gravity [m s - ] p,p pressure [Pa] V,V volume [m 3 ] U,U internal energy [J] Q heat [J] W work [J] Thermodynamics: pv+u=h -> related to weight unit kg: v v gz h q gz h w [J kg - ] 5 Energy comparison In the case of heat changes - E pot a E kin insignificant v v gz h q gz h w Transport to m: E pot = 9,8. = 9,8 J kg - [J kg Speed of water flow m s - : E kin = / = 0,5 J kg - Water heating by o C : Δh=c p ΔT = 4, = 480 J kg - - ] Units, Units, Units! c p (H O) = 4, J kg - K - specific heat capacity Δh [J kg - ] c p (H O) = 75,38 J mol - K - molar heat capacity Δh [J mol - ] 6

3 enthalpy heat Enthalpy - heat Internal energy - its change in a system is equal to the heat brought to the system at constant pressure. dependent on standard state standard state: temp. 73,5 K; pres. 0,35 kpa temp. 98,5 K; pres. 0,35 kpa only one standard state during whole calculation! main enthalpy changes during: - heating/cooling - state (phase) change - chemical reaction 7 Balance of Enthalpy without state (phase) change only substance heating/cooling: h h i t i, hi, c p, idt c p, i t t <c p > mean or average heat capacity hi hi, hi, c pl, idt hi, vyp( ) t ( t ) during state change - calculation of phase change included: t c pg, i dt liquid heating state change (evaporation enthalpy) gas heating for mixtures the sum of all components enthalpy + enthalpy of mixing (often neglected) 8 Balance of Enthalpy chemical reaction reaction enthalpy - enthalpy of formation or enthalpy of combustion of participating compounds h mh r h r aa+bb = mm+nn n i i M, sl v h i, sl nh N, sl A, sl B, sl Δh r [J mol - ] heat deliberated or consumed during one reaction turn over i.e. dependent on form of chemical reaction. i, sp Energy change is determined by equilibrium and real reaction progress n i i ah v h bh 9 3

4 Data 0 Data II e-tabulky Calculation Steam enters heat exchanger with flow rate 30 m/s at pressure 500 kpa and temperature 00 o C. From output flows condensate of temperature 50 o C and flow rate m/s. Height distance between input and output is 3m. How much heat is deliberated from kg of steam. Solution Data: Steam enthalpy 00 o C, 500kPa = 854,3 kj kg -, hot water enthalpy 50 o C is 63,78 kj kg - (std. state H=0 at 0 o C, 00kPa). ΔE ΔE v v Δh w gz h w - gz h pot kin q ΔE pot =g(z -z ) = 9,8. (-3) = -7,54 J kg - ΔE kin = ½ (v - v ) = 0,5. (30 - ) = -449,5 J kg - Δh= h -h = 63,78-854,3 = -,5 kj kg - q = -7,54-449,5-, = J kg - 4

5 z z z=0 Steam enters heat exchanger with flow rate 30 m/s at pressure 500 kpa and temperature 00 o C. From output flows condensate of temperature 50 o C and flow rate m/s. Height distance between input and output is 3m. How much heat is deliberated from kg of steam. Solution Data: ΔH f 98,5K (H O (g))= kj mol -, ΔH f 98,5K (H O (l))= -85,830 kj mol - Cp = A + B t + C t + D t 3 + E/t [t= K/000] ΔE pot = -7,54 J kg -, ΔE kin = -449,5 J kg - (viz. soln..) hi hi, hi, cpl, idt hi, vyp( ) t t c pg, i dt A B C D H O (l) H O (g) E Δh= [At + B*t / + C*t 3 /3 + D*t 4 /4 E/t] t t [kj mol - ] Δh (00C-5C) (H O (g)) = -5,98 kj mol -, Δh (5C-50C) (H O (l)) = +9,5 kj mol - Δh vyp, 98,5 = ΔH f 98,5K (H O (l))-δh f 98,5K (H O (g))= - 44,0 kj mol - Δh = ,0+9,5 =-40,48 kj mol - => -48,89 kj kg - q= , = J kg - t x = 5 o C (98,5K) 3 Calc. - chem. reaction SO + ½ O = SO 3 reactor heat exchanger reactor heat exchanger reactor reactor (H f0 ) SO = -300 kj/mol (T=300 K) (H f0 ) SO3 = -400 kj/mol (T=300 K) (C pm0 ) O= (C pm0 ) N = 9 J mol - K -, (C pm0 ) SO = 44 J mol - K - (C pm0 ) SO3 = ( T) J mol - K Transport of liquids Bernoulli equation energy balance for hydrodynamic calculation. v p v p gz w c gz e ρ ρ w c work of pump e dis energy loss for ideal liquids e dis =0 dis estimation of e dis usually based on exp. value n l v edis λ ζ j d j λ pipe friction factor, l length of piping, d pipe diameter, v flow rate a ζ coefficient of local resistance 5 5

6 Electric energy specific type of energy with universal applicability easy transport no weight changes simple regulation pollutant free - heating(in explosive environnment) ohmic, el. arc, inductive - mechanical operation actuation (pumps, stirrrers,...) performance/consumption, efficiency - electrochemical processes el. energy conversion to chemical and oposite R = U/I P = U. I R resistance [Ω], U- voltage [V], I- current [A], P-performance [W] 6 Calculation 3 Membrane electrolyser for Cl production operates at 3,3V and current density 4kA/m. Electrode area is m. How many electrolysers can be connected to teh power source of performance 50 kw. Solutions I = j. A I elz = = 4000 A x U celk U celk = n. U elektrolyzeru P zdroj = U celk. I n= P zdroj / (U elektrolyzeru.. I) n = / (3, ) =,36 tj. electrolysers 7 Heating and cooling the method of heating / cooling and the choice of heat transfer medium depends on the type and extent of the process temperature Heating exists in almost all chemical plants distillation, absorption, desorption, freeze drying, dissolution, crystalysation, drying, etc. -direct heat exchange medium is in direct contact with the heated substance. ideal heat transfer, low investment costs but leads to weight changes (eg, dilution) - indirect heat exchange medium circulates in a separate circuit. more difficult to heat, increased investment, there is no interaction between the medium and the heated substance 8 6

7 Combustion gases heating The primary heat source - from the combustion of solid, liquid or gaseous fuels. in technological processes is most commonly used gaseous fuels - natural gas, coal gas, reformate gas, petrochemical fumes. production, etc. basic advantage is the ease of adjustability, minimal production of solid waste. Possibility of direct contact with the heated substance in limited technologies (silicate industry, drying, annealing) In the production of steam is often used in so-called cogeneration, which is also producedby steam and electric power. 9 Heat carriers / Heat transfer media / Heat transfer fluids Basic requirements for the heat carriers transmits the highest amount of heat per unit mass or volume are cheap and readily available are chemically stable in the range of pressures and temperaturesused in non corrosive for pipes not flammable, toxic or explosive not too viscous allow the option to controlthe transmitted energy 0 Heat carrier - water most common, readily available, non-toxic, good thermal conductivity, high heat capacity, heat of condensation suitable for direct heating where it is desired dilution (diffusers) steam saturated steam or overheated water steam. Applicable to temperatures up to 300 o C. higher temperature - high pressure (higher investment costs) hot water for heating up to 00 C, higher temperatures require increased pressure (described application at 355 C, 8 MPa). 7

8 Demands on water quality Heat carrier - water Heat water to be demineralized circuits - high purity exchangers in cooling water flows through the pipes of small cross - sensitive to solid deposits (heat transfer deterioration, clogging pipes) The most common cause - equilibrium disruption of HCO 3- and CO in the presence of Ca + to form CaCO 3. The mineral content also supports the growth of microorganisms (bacteria, algae, fungi) Demands on water quality are increasing the demand for long-term continuous operation. organic substances Heat carriers Overheating leads to degradation to form the carbon slurry. When exceeding the limit values it must be replaced the entire load - expensive mineral oils Allow heat to higher temperatures without increasing pressure. Oils guarantee protection against corrosion and rusting, high oil film resistance and excellent lubricating properties, are resistant to water washout Special mixtures e.g. dowterm (difenile 6,5% and difenyloxide 73.5%) boiling point 58 o C for atm. pressure. Melting point - C. Non-toxic compound, forming deposits, non-corrosive. Using both in liquid and gaseous form. Heat of evaporation - less than 9x to water. 3 Molten salts Heat carriers Enables high temperatures - heating up to 500 o C. (e.g.: molten 40% NaNO, 7% NaNO 3 and 53% KNO 3, application range o C) At higher temperatures degradation occurs and increasing the freezing point of the mixture. Risk nitration respectively. oxidation org. substances (Not for org. processes) Molten metals Allow heat to high temperatures up to about 800 o C. (eg alloy Pb and Bi, Hg, or occasionally molten alkali metals) High heat transfer coefficient. Construction and materials - very demanding. Use only if no other option. 4 8

9 Heat carriers - cooling air Only for big temperature difference. Low coefficient of heat transfer. Low cost. water often enough raw water. Heated above 50 C - release of dissolved gases (reducing the heat transfer surface) and increases the risk of the formation of deposits. For higher temperatures are used circuits with treated water. 5 Heat carriers - cooling ammonia the most common cooling medium is used for compressor and absorption cooling applications up to temperatures of -70 o C, low pressure, high heat transfer coefficient, toxic, flammable to explosive. SO, freons, CO, metane, etane, etylene, propane freons CF Cl (R-), HCF Cl (R-) negativeinfluence to the ozone layer substutued by propane For temperatures below -00 o C adiabatic expansion of gases is used eg. for the air liquefaction 6 Energy in chem. technology Conventional ammonia production by steam reforming 7 9