Download Reactor Design Midterm.docx...Reactor Design Midterm.docx. and more Exams Nursing in PDF only on Docsity!
Reactor Design Midterm
Reactor design - correct answer solving mole/molecular species balances how are generation and consumption terms linked - correct answer via the reaction stoichiometry rate of generation of a reactant is - correct answer negative rate of consumption of a reactant is - correct answer positive rate of generation of a product is - correct answer positive rate of consumption of a product is - correct answer negative 3 main types of reactors - correct answer Batch, CSTR, PFR difference between a PFR and a PBR - correct answer a PBR always has a catalyst does a PFR have a catalyst - correct answer sometimes Concentration varies in what type of reactors - correct answer batch and PFR reactors how does concentration vary in a batch reactor - correct answer with time but tank is well-mixed how does concentration vary in a PFR - correct answer with position along the reactor
but properties at any given position are constant with time CSTR operates at the... - correct answer exit concentration concentration in the CSTR is constant with time and position when can mole balances be solved in terms of conversion - correct answer one reaction the only way that the molar flowrate or number of moles is changing is from chemical reaction conversion must be written in terms of the... - correct answer limiting reactant we want to ____ the reactor volume needed to achieve a desired conversion - correct answer minimize why do we want to minimize the reactor volume - correct answer easier to control heat/energy balance in a smaller reactor easier to achieve perfect mixing uses fewer resources (e.g. solvents, catalyst) easier to drive fluids through a smaller reactor Levenspiel plot are useful for - correct answer visualizing kinetics of the system as a function of conversion to get a "first pass" of how to design the reactor can be used to decide which type of reactor to use or how to sequence reactors to maximize conversion while minimizing reactor volume usually not used for the final design on a levenspiel plot use a ____ in an upward sloping region - correct answer PFR on a levenspiel plot use a ____ in the downward sloping region - correct answer CSTR
F can also change due to diffusion in/out What causes V and v to change - correct answer For gases, they can change due to changes in N and F via the ideal gas law, V=NRT/P for example as N increases, V increases For gases, they also change due to T when reaction is non-isothermal P in a flow reactor where there is pressure drop V in a constant pressure batch reactor where a change in moles is occurring for liquids, we can always assume that changes in F,T, and P do not affect v look over the equations equilibrium constant for a reaction indicates the extent to which... - correct answer products are favored over reactants large values ( over 1000) of equilibrium constant mean what is significantly favored? - correct answer products small values (less than 0.001) of equilibrium constant mean what is significantly favored? - correct answer reactants when is reaction reversible, in terms of equilibrium constant - correct answer when values are between 1000 and 0. what does equilibrium constant influence - correct answer it influences rate law and hence rate and hence mole balance and hence decisions about reactor design equilibrium constant also indicates the extent that the forward and reverse reactions are contributing to the rate Kp is the - correct answer equilibrium constant in terms of pressure, use partial pressures instead of concentrations for each species kp=_____*(RT)^delta - correct answer kc
delta is the change in the number of moles going from reactants to product is Kp used for liquids - correct answer no keq equals - correct answer kp for gases keq for liquids batch reactors have no... - correct answer inflows or outflows are batch reactors constant volume and constant pressure - correct answer constant volume if have rigid walls, which is common sometimes constant pressure if is a piston bench top reactors - correct answer batch reactors used in chemistry experiments helps to learn about rate constant and rate law for reaction if reaction is studied using bench top, does not mean reaction must be carried out in a batch reactor Practice Problem 8 is an example of this time to run a batch includes... - correct answer time needed to fill the reactor, time to mix the contents, heat the contents to reaction temperature, run reaction, empty reactor, and possibly clean the reactor in batch process, this time is significant compared to the time required for the reaction Estimates for these times are provided in Table 5- CSTRS are - correct answer continuous do CSTRs require maintenance - correct answer yes, like batch they require maintenance however time spent on maintenance is small compared to time of reaction
perfect mixing means - correct answer no gradient in velocity, concentration, temperature, or reaction rate tubular reactors have continuous variation of _____ in the axial dimension - correct answer velocity, concentration, temperature, reaction rate why do PFR mole balances require integral over the length of the reactor - correct answer since reaction rate varies down the length of the reactor how is reactor length in a PFR/PBR related to reactor volume - correct answer V=Z*A how is the performance for multiple PFRs in series related to the performance of a single PFR with equal volume - correct answer they are identical in performance do laminar flow reactors exist - correct answer yes, you just can't assume perfect mixing in these. PFRs are good estimates for LFRs can tubular reactors with catalyst packing experience pressure drop - correct answer yes what causes pressure drop - correct answer when fluid flows past packing in the bed, which causes friction (drag), which decreases the pressure of the fluid what causes drag - correct answer fluid flowing over the surfaces of the pellets fluid also experiences drag due to walls of the reactor. This surface area is much less than the pellets. Drag due to walls has insignificant influence on pressure of the fluid. when do we need to worry about pressure drop - correct answer when it influences volumetric flowrate, and hence concentration, of the fluid do we need to worry about pressure drop with liquids - correct answer no because liquids are incompressible so pressure drop does not significantly influence
volumetric flowrate of liquids and thus we do not need to worry about the pressure drop how do we account for the affect of pressure drop on volumetric flowrate in gases - correct answer we use the ideal gas equation how do we account for pressure drop itself - correct answer using the Ergun equation. we simplify the Ergun equation significantly so we can focus on influence that the pressure drop has on reactor design, rather than how fluid properties influence pressure drop Ergun equation is an ODE, similar to tubular reactor mole balance. coupling these two equations creates what we call the "tubular reactor design equations". The two ODES must be solved simultaneously using mathematical software what class of reactors do membrane reactors fall into - correct answer tubular reactors specifically, a membrane reactor is a "shell and tube" type reactor, where the reactor is the tube what are the walls of a membrane reactor comprised of - correct answer a porous membrane that is permeable to some species. The shell is used either to supply species gradually by diffusion or collect species that have diffused out of the reactor what controls the rate of diffusion through the membrane - correct answer the difference in concentration between the shell and tube and the mass transfer rate constant Kc what do small values of Kc mean - correct answer slow rate of diffusion and less change to the composition in the reactor what do large values of Kc mean - correct answer high rate of diffusion and more change to the composition of the reactor
what is the difference between a packed bed reactor and a fluidized bed reactor - correct answer in a packed bed reactor, pellets remain in place, whereas in a fluidized bed reactor, they are mobile Which reactors do not exhibit any gradients in concentration, temperature, or any other process variables at any given instant in time? - correct answer batch, semi-batch, CSTR all of these are tanks with agitators, so we assume that the contents are perfectly mixed. In PFRs and PBRs, they are perfectly mixed in the radial dimension due to plug flow but the concentration and other process variables such as temperature vary down the length of the reactor. In which reactor can the flow terms be neglected in the mole balance equations - correct answer batch general balance equation - correct answer accumulation=in-out+generation- consumption for mass and energy, the general balance equation is - correct answer accumulation=in-out when dealing with moles, the general balance equation is - correct answer accumulation =in-out+generation-consumption generation and consumption terms are valid when dealing with moles general balance equation when dealing with a steady-state system - correct answer accumulation= general balance equation when dealing with a bath system - correct answer in- out= what system variables are relevant to net consumption, Gj - correct answer concentration, temperature, catalytic activity
what is the system volume for a CSTR - correct answer liquid volume, NOT the tank volume CSTR key assumptions - correct answer well mixed steady state CSTR operates at the exit conditions What is a batch reactor and how does a batch reactor work - correct answer fill the batch reactor mix it up so it is well mixed carry out reaction drain clean Batch reactor key assumptions - correct answer well mixed no inflow or outflow Can we cancel out the accumulation term in a batch reactor? - correct answer no, but there is no inflow or outflow what is the mole balance for a batch reactor - correct answer dNA/dt=rA*V how does the mole balance for a batch reactor change when is is a constant volume batch reactor - correct answer dCA/dt=rA what variable do we solve for in the batch reactor? - correct answer time/reaction time rate of reaction in the batch reactor is a function of - correct answer time What is a PFR and how does a PFR work? - correct answer tubular reactor, turbulent flow aka plug flow
what must X be in terms of when we solve a conversion mole balance - correct answer the limiting reactant why does X need to be in terms of the limiting reactant - correct answer because the limiting reactant sets the bonds for how far the reaction can proceed when can constant fluid density be assumed - correct answer when the reaction occurs in liquid phase. This means that the volumetric flowrate entering the system is equal to the volumetric flowrate exiting the system how to find PFR volume on a levenspiel plot - correct answer it is the area under the levenspiel curve how to find CSTR volume on a levenspiel plot - correct answer it is the area of the rectangle, drawn from the point on the curve on the right if you have a PFR and a CSTR which are both the same volume, which one will give the higher conversion for this reaction - correct answer PFR because it maintains a higher concentration and thus a faster rate. The CSTR operates at the lowest possible concentration what does a rate law depend on - correct answer concentration and temperature do rate law exponents need to be positive - correct answer no, they also don't have to be integers two important things to remember about elementary reactions - correct answer if you are told a reaction is elementary, assume that means elementary as written so do not alter the stoichiometry if you are working a problem and not given information about the rate law, you can assume that it is elementary, just write this assumption down what is kc - correct answer the equilibrium constant k(forward reaction)/k(reverse reaction)
what is the difference between a reversible reaction and a reaction in equilibrium - correct answer in a reversible reaction, both the forward and the reverse rates are appreciable an equilibrium reaction represents the special case where the forward and reverse rates are equal to each other (and hence the net rate is 0) what are the general units for -r - correct answer mol/(volmetime) how to calculate rate constant - correct answer k=Ae^(-E/RT) units of activation energy - correct answer kJ/mol units of frequency factor for a 1st order reaction - correct answer s^(-1) units of frequency factor for a second order reaction - correct answer Lmol^(- 1)s^(-1) A, the frequency factor, quantifies what - correct answer the frequency of occurrences that could lead to a chemical reaction exp(-E/RT) in the arrhenius equation represents what - correct answer the probability that such an occurrence indeed will result in a chemical reaction, specifically it is the Boltzmann probability, which is derived from statistical mechanics what is a transition state - correct answer a compound with chemical structure "in between" that of the reactants and products transition state is at the highest energy, aka the activation energy, because it is "contorted" in shape, which costs energy. why might FA0/-rA decrease with conversion - correct answer when temperature dependence is matters, so initially when something is temperature dependent, like if it is adiabatic, the conversion will decrease as the temperature increases then
what to do if given k at wrong temperature - correct answer use arrhenius equation to get k what is definition of residence time - correct answer volume/volumetric flowrate what happens when we place PFRs in series - correct answer nothing, it is the same as a single PFR with the same volume this is different from CSTRs think about how PFR is just area under the levenspiel plot how to increase conversion in a PFR - correct answer increase the volume are pellets in a PBR for the most part perfect spheres - correct answer no, the pellets themselves are highly porous materials. The flow pattern around them can be approximated as flow around a sphere By what mechanism do reactants and products move through catalyst pellets in a PBR - correct answer diffusion the region immediately outside of the pellet is stagnant, meaning that it is not flowing. This region is also called the boundary layer. Reactants and products travel through the boundary layer and into/out of the catalyst via diffusion What causes pressure drop in a PBR - correct answer drag catalyst pellets create a resistance to flow. As the fluid contacts the pellets, friction between the fluid molecules and the pellet surfaces causes a pressure drop. do large or small particles diffuse better - correct answer small what does pressure drop cause in a PBR - correct answer decreased pressure causes concentration to decrease decreased concentration influences the rate law account for pressure drop using the___ - correct answer Ergun Equation
as diameter of pellet in PBR increases... - correct answer alpha decreases so less pressure drop as diameter of pellet in PBR decreases... - correct answer alpha increases so more pressure drop how can larger catalyst pellets impede reaction rate - correct answer larger catalyst pellets mean larger paths for diffusion of molecules in the catalyst pores, which can impede the reaction rate what is different about solving PBR vs PFR - correct answer 1. PFR, need to include Ergun Equation
- expressions for partial pressure, equation 4-27. the lowercase p accounts for effect of pressure drop on volumetric flowrate pressure drop influences... - correct answer volumetric flowrate and hence concentration and this is true even in cases when delta=0 aka number of moles doesn't change if we want to maximize production of a product in a membrane reactor what should we do - correct answer choose a membrane that is highly selective for that product this will remove the product from the reactor, thus lowering the concentration of B and pushing the equilibrium to the right What is the driving force for diffusion in a membrane reactor - correct answer concentration difference What is the equation for the flux of a molecule through a membrane - correct answer R=km*Cx where km is the transport coefficient
what would a batch reactor look like on a levenspiel - correct answer same as pfr
