Modeling a transient plug flow reactor

John Kitchin

Contents

function main

clear all; clc; close all

    function dCdt = method_of_lines(t,C)
        % we use vectorized operations to define the odes at each node
        % point.
        dCdt = [0; % C1 does not change with time, it is the entrance of the pfr
               -vo*diff(C)./diff(V)-k*C(2:end).^2];
    end

Problem setup

Ca0 = 2; % Entering concentration
vo = 2; % volumetric flow rate
volume = 20;  % total volume of reactor, spacetime = 10
k = 1;  % reaction rate constant

N = 100; % number of points to discretize the reactor volume on

init = zeros(N,1); % Concentration in reactor at t = 0
init(1) = Ca0; % concentration at entrance

V = linspace(0,volume,N)'; % discretized volume elements, in column form

tspan = [0 20];
[t,C] = ode15s(@method_of_lines,tspan,init);

plotting the solution

The solution contains the time dependent behavior of each node in the discretized reactor. For example, we can plot the concentration of A at the exit vs. time as:

plot(t,C(:,end))
xlabel('time')
ylabel('Concentration at exit')

You can see that around the space time, species A breaks through the end of the reactor, and rapidly rises to a steady state value.

animating the solution

It isn't really illustrative to examine only the exit concentration. We can also create an animated gif to show how the concentration of A throughout the reactor varies with time.

filename = 'transient-pfr.gif'; % file to store image in
for i=1:5:length(t) % only look at every 5th time step
    plot(V,C(i,:));
    ylim([0 2])
    xlabel('Reactor volume')
    ylabel('C_A')
    text(0.1,1.9,sprintf('t = %1.2f',t(i))); % add text annotation of time step
    frame = getframe(1);
    im = frame2im(frame); %convert frame to an image
    [imind,cm] = rgb2ind(im,256);

    % now we write out the image to the animated gif.
    if i == 1;
        imwrite(imind,cm,filename,'gif', 'Loopcount',inf);
    else
        imwrite(imind,cm,filename,'gif','WriteMode','append');
    end
end
close all; % makes sure the plot from the loop above doesn't get published

here is the animation!

You can see that the steady state behavior is reached at approximately 10 time units, which is the space time of the reactor in this example.

Add steady state solution

It is good to double check that we get the right steady state behavior. We can solve the steady state plug flow reactor problem like this:

    function dCdV = pfr(V,C)
        dCdV = 1/vo*(-k*C^2);
    end

figure; hold all
vspan = [0 20];
[V_ss Ca_ss] = ode45(@pfr,vspan,2);
plot(V_ss,Ca_ss,'k--','linewidth',2)
plot(V,C(end,:),'r') % last solution of transient part
legend 'Steady state solution' 'transient solution at t=100'

there is some minor disagreement between the final transient solution and the steady state solution. That is due to the approximation in discretizing the reactor volume. In this example we used 100 nodes. You get better agreement with a larger number of nodes, say 200 or more. Of course, it takes slightly longer to compute then, since the number of coupled odes is equal to the number of nodes.

end

% categories: PDEs
% tags: reaction engineering

Published with MATLAB® 7.13