How do I determine the 'other stuff' in DE mixing problems?

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Discussion Overview

The discussion revolves around understanding differential equations related to mixing problems in a tank, specifically focusing on how to determine the components of the equations, particularly the "other stuff" that affects the outflow of chemicals when mixed with water. The scope includes theoretical aspects of differential equations, mathematical reasoning, and conceptual clarification of mixing processes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about determining the components of the differential equation for mixing problems, specifically what to include as "the other stuff" in the outflow rate.
  • Another participant suggests that "the other stuff" is represented by the ratio of the amount of chemical to the volume of water (A/V), and provides a method for calculating the volume over time.
  • A participant raises a question about how to express the situation when pure water is poured into a tank already containing a chemical mixture.
  • Another participant reiterates the concern regarding the expression of the rate of change when pure water is added to a pre-existing mixture.
  • One participant clarifies that if the outflow rate is based on the concentration of the chemical, the outflow can be expressed as a function of the amount of chemical and the volume of water, leading to a specific form of the differential equation.
  • A participant notes that the scenario involves both water flowing in and a mixture flowing out, indicating a more complex interaction.
  • Another participant describes a mass balance approach for the chemical in the tank, introducing variables for volumetric flow rates and concentrations, and references a specific equation for cases where pure water is introduced.

Areas of Agreement / Disagreement

Participants express various viewpoints on how to set up the differential equations for mixing problems, with some agreement on the basic form of the equations but differing interpretations on specific scenarios, such as the introduction of pure water. The discussion remains unresolved with multiple competing views on how to approach these problems.

Contextual Notes

Participants have not fully resolved assumptions regarding the mixing process, the definitions of variables, and the implications of different flow rates. The discussion reflects a range of interpretations and approaches to the problem without a consensus on the best method.

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I am having major problems understanding these types of questions, where you will have water and in will be chemicals mixing at a certain rate, and coming out of another tube at a rate, and then the question is to find out certain things, concentration at a time or whatever.

My question, the equation takes the form

dx/dt = in - out

x is the amount of chemical

The in is the rate coming in, however it seems the out part is usually the number multiplied by some other stuff. The question is, how do I know what to put in ''the other stuff''?
 
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the other stuff will be A/V where A is the amount of chemical and V is the volume of water in the tank
when setting up the differential equation you leave A alone since that is what were trying to solve for
for V, you take initial amount of volume+(rate in-rate out)*t
where rate in and rate out are in liters/min or whatever units you are using
 
how about when a chemical and water mix is already in the tank, and you are now pouring in pure water? How do I express that?
 
Woopydalan said:
how about when a chemical and water mix is already in the tank, and you are now pouring in pure water? How do I express that?

Then if your rate of change depends on that it will be some function of what's already in the tank which is some function of x.
 
If the amount of chemical is A and the volume is V, then the amount of chemical per unit volume is A/V as miglo said. If the water is flowing out at rate "r" then the chemical is flowing out at rate (A/V)r= (Ar/V). Since that is flowing out the rate is "out"= -(Ar/V). Saying that there is only water flowing out means that "in= 0". So the differential equation is
\frac{dA}{dt}= -Ar/V.
 
I was saying water is flowing in and a mixture is flowing out
 
If the volumetric flow rate out is equal to the volumetric flow rate in, and, if the tank is well-mixed so the concentration of the chemical coming out is equal to the concentration within the tank, then the mass balance for the chemical in the tank goes:

Vdx/dt = F (xin -x)

where F is the volumetric flow rate, V is the volume of fluid in the tank, x is the species concentration within the tank (and in the exit stream), and xin is the species concentration in the feed. This reduces to HallsofIvy's equation for the case in which the concentration in the feed is zero (pure water). I think HallsofIvy meant to say that only water is flowing in.
 

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