Applied chemical process

Transcription

1 Applied chemical process s Chemical reactor The most important reaction-related factors for the design of a reactor are: 1) The activation principle selected, together with the states of aggregation of the reactants and the resulting number and types of phases involved 2) The concentration and temperature dependence of the chemical reactions 3) The heat of the reactions taking place The concentration and temperature dependences of a chemical reaction are described by the reaction rate In practice most reaction systems are complex and include parallel, sequential, and equilibrium reactions Basic types of reactors Tubular reactor Continously stirred-tank reactor Batch stirred tank reactor In practical operation, the ideal states are achieved only approximately Examples of typical nonidealities include : 1) The formation of real flow patterns, such as dead zones, short-circuit flows, and channeling 2) Transport processes in the individual phases, such as axial backmixing 3) The formation of concentration and temperature profiles as a result of transport resistances in and between phases 4) Segregation processes and incomplete mixing of reactants 1

2 Batch reactor Concentrations of substances not depends on place in the reactor Concentrations of substances depends on the time Advantages: 1) Quick production changeover possible; use for substances produced on a small scale 2) Process steps upstream or downstream of the reaction can also be performed in the reactor 3) Better process control than in continuous operation when solid or highly viscous phases form or are present 4) Well-defined residence time Batch reactor Principal applications: 1) Liquid-phase reactions 2) liquid-solid reactions Disadvantages: 1) Relatively high operating costs due to long downtimes and high manpower requirements 2) Quality differences between charges because reaction conditions are only partly reproducible 3) Limited temperature control capabilities, especially with highly endothermic or exothermic reactions Batch reactor balance INPUT + GENERATION OUTPUT + ACCUMULATION 0 + ν i r V s dt 0 + dn i V ν ir s dni dt dci ν i r dt x Vs r conversion (-reaction) dt 0 dηi ci ν ir Relative degree of dt conversion (i-component) 2

3 Continuous stirred-tank reactor There are not local differences in composition or temperature Steady state A step change in concentration at the inection T 0,n A0 T 1, n A1 T 1, n A1 Continously stirred-tank reactor Principal applications: 1) Liquid-phase reactions 2) gas-liquid reactions 3) Gas liquid reactions over suspended catalysts Advantages: 1) Low operating costs, especially at high throughputs 2) Consistent product quality due to reproducible process control 3) Wide range of throughput Disadvantages: 1) Final conversions lower than in other basic reactor types because of complete mixing (ie, unreacted starting materials can get into the product stream) 2) High investment costs to implement continuous operation 3) Changeover to other products generally complex and time-consuming because of reaction-specific design Continously stirred-tank reactor balance INPUT + GENERATION OUTPUT + ACCUMULATION n i0 + ν i r V n i1 + 0 V ν ir n n i0 i1 0 1 ci ci τ i τ - mean residence time 3

4 Tubular dv r S nai n A n A +dn A n Ae z0 z z+ zl The composition depends on the longitudinal coordinate of the reactor - Composition at a given location not depends on time - steady state at time balance in the form of differential equations Frequent use of heat exchange Tubular Principal applications: 1) Homogeneous gas-phase reactions 2) liquid-phase reactions 3) Gas liquid reactions 4) Gas- and liquid-phase reactions over solid catalysts Advantages: 1) Favorable conditions for temperature control by heat supply or removal 2) No moving mechanical parts, hence especially suitable for high-pressure service Disadvantages: 1) Very high degree of specialization, often with complicated design and high investment costs 2) Relatively large pressure drops Tubular balance INPUT + GENERATION OUTPUT + ACCUMULATION n i + ν i r S n i +dn i + 0 dxi dni S i S ν ir V c i dci S V i conversion (-reaction) 4

5 Trubkový reaktor látková bilance V r n e n i dn n o η e o dη Pokud e obemový průtok konstantní tvv i FS F e rychlost proudění zprůměrovaná podle průřezu dc F ν r ( c, T ) liquid phase gas phase endo exo 5

6 gasliquid solid phase - gas solid phase - liquid 6