Reactoare chimice cu curgere piston (ideala) cu amestecare completa de tip batch (autoclava)

Transcription

1 Reactoare chimice cu curgere piston (ideala) cu amestecare completa de tip batch (autoclava) Reactorul cu curgere ideala Toate particulele se deplaseaza intr-o directie de-a lungul reactorului, precum un piston, fara a se amesteca cu cele care le preceda sau care urmeaza in flux. Toate particulele se caracterizeaza prin acelasi timp de rezidenta: τ = V/Q V = H/w Concentratiile reactantilor si, deci, viteza de reactie scad de-a lungul reactorului. Gradul de conversie creste pe lungimea reactorului. Temperatura se modifica de-a lungul reactorului: opereaza fie in regim adiabatic fie in regim politermic. Poate fi integral sau diferential: The plug-flow reactor may be operated in the differential or the integral mode. Differential flow reactors are typically used for the determination of the kinetics of catalytic reactions. A reactor is operated as a differential reactor if it is a reactor with very small volume and low conversion. The differential reactor is assumed to be gradientless, that is, having a uniform concentration throughout the reactor, and hence the observed rate is that corresponding to the concentration C A0. In the differential mode (small conversion) the whole catalyst can be considered to be exposed to the same concentration of reactants. The influence of products is generally weak, except when the catalyst is extremely sensitive to one particular product. The plug-flow reactor operating in the differential mode is the simplest and most reliable way of determining the order of reaction with respect to reactants and products. This may be achieved by a systematic variation in the initial concentration of the reactants. In the differential mode further major advantages are: (i) it provides good kinetic data because of the short holdup (ii) the low conversion helps minimize heat and mass transfer problems (iii) the influence of parameters such as temperature, pressure and concentration can be studied separately. The major disadvantages of using a plug-flow reactor in the differential mode are: (i) at the small conversions required large errors in analysis can occur (ii) the technique is time-consuming in kinetic measurements (iii) high gas velocities may be required to keep conversions low. The integral plug-flow reactor has the advantage that large conversions facilitate analysis and provide more accurate kinetic data. Disadvantages of the integral mode are problems in maintaining isothermal operation, the existence of concentration gradients, and possible heat and mass transfer limitations. Furthermore, accurate kinetic analysis of integral data is generally more difficult than for differential ones.

2 In general in a laboratory reactor, in contrast to an industrial reactor, the depth of the catalyst bed is small, so that the reactor has a behavior intermediate between that of a plug-flow and a back-mixed flow reactor. Exemple: Reactoare catalitice tubulare similare schimbatoarelor de caldura de tip teava in teava, cu catalizatorul in tevi sau in spatiul intertubular, pentru reactii in faza gaz. Reactoare tubulare pentru reactii in faza lichida cu raport mare H/d, utilizate in sinteza organica industriala.

3 Coloane cu umplutura Sobe de sinteza in faza omogena (Ex. HCl) Reactorul cu amestecare completa Reactantii sunt supusi unei agitari intense. Particulele de reactanti introduse in reactor sunt imediat amestecate cu particulele deja existente => distributie uniforma in volumul aparatului. Variabilele procesului: concentratii, grad de conversie, viteza de reactie ating imediat valoarea finala, constanta de-a lungul reactorului. Opereaza intotdeauna in conditii izoterme. Timpul mediu de rezidenta al particulelor din amestecul de reactie este: τ = V/Q V = H/w Timpul de rezidenta pentru o particula individuala poate varia intre 0 si infinit.

4 Exemple: Reactoare cu sisteme de amestecare folosite in procese omogene lichide sau procese eterogene lichid-solid (suspensii), lichid-lichid (emulsii) si gaz-lichid.

5 Reactoare cu strat fluidizat. Reactoare catalitice cu strat fix, de dimensiuni mici. Reactoare ciclon (arderea sulfului sau prajirea piritelor). Pentru reactoarele cu amestecare completa precum si pentru reactoarele cu curgere ideala, dimensiunile reactorului, viteza de curgere a reactantilor, concentratiile initiale si finale sau gradul de conversie pot fi determinate doar daca cinetica reactiei este cunoscuta. Cascada de reactoare cu amestecare completa Viteza de reactie in reactorul cu amestecare completa este mica => pentru a obtine conversii mari trebuie utilizate reactoare de dimensiuni mari. => Se impune utilizarea unui lant de reactoare cu amestecare completa: compozitia amestecului de reactie se modifica prin trecerea de la un reactor la altul.

6 Un numar suficient de mare de reactoare in cascada => aceeasi conversie ca in reactorul cu curgere ideala. Reactorul de tip batch (autoclava) In reactor este incarcata o cantitate definita de reactanti. Reactantii raman in reactor pana se atinge gradul de conversie dorit. Modificarea concentratiei reactantilor in timp este similara cu cea din reactorul cu curgere ideala: Conditiile de regim se modifica in timp: independent de gradul de amestecare, nu doar concentratiile reactantilor, dar si temperatura, presiunea si, deci, constanta cinetica a reactiei se modifica in timp. Este politermic in functie de timp. Functionarea reactorului implica un ciclu de operatii: incarcarea conversia chimica descarcarea.

7 Utilizarea reactoarelor de tip batch Se recomanda in procese la scara mica: Fabricarea reactivilor chimici Fabricarea unor catalizatori Fabricarea produselor farmaceutice Fabricarea vopselurilor