Part 1: P&ID Diagrams
Process and instrumentation diagram for the simplified milk pasteurization plant. The process variable to be controlled here is the pasteurized milk temperature.
Solution
The initial P&ID diagram for the milk pasteurization plant is demonstrated as, ![]()
The process line design has some product-related, process-related, economic, and legal considerations. The raw material is treated and a quality end product is obtained. The raw milk is fed to the balance tank, also referred to as the constant heat tank. It passes to the milk pump and flows further to the controller and the result is fed into the plate heat exchanger. The plate heat exchanger has the heating, holding, and cooling sections.
The plant has a constant head tank, a flow controller, the milk entry pump, a filter and clarifier, a homogenizer, and the plate heat exchange. When the milk is processed it is extracted through the flow diversion valve, recording and indicating instruments, cooling systems as well as the piping systems that link to various components [1].
The block chart that illustrates the milk pasteurization process is ![]()
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Ot=19;
Of=80;
temp=[19 57 69 73 75 77 78 79 79.6 79.8 80];
time=0:1:10;
% The first order equation of the temperature sensor
a=temp-80;
b=Ot-Of;
gs=log(a/b);
figure(1)
plot(time,gs,'g-*');
grid on
vals=diff(gs)./diff(time);
slope=vals(1); % finds the gradient of the line
t_const=-1/slope;
temp_sensor=Of.*(1-exp(-time/t_const));
figure(2)
plot(time,temp,'b-*')
grid on
hold on
plot(time,temp_sensor,'r-*')
xlabel('Time (sec)')
ylabel('temperature (deg.celsius)')
title('Plot of Temperature Rise against Time')
legend('Measured','Estimated')
It is observed that there is a slight difference between the temperature sensor values in the measured and estimated values based on the equation,
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