Consider the air over a city to be a box 100km on a side that reaches up to an altitude of 1.0 km. Clean air is blowing into the box along one of its sides with a speed of 4 m/s. Suppose an air pollutant with a reaction rate k=0.20/hr is emitted into the box at a total rate of 10.0kg/s. Find the steady-state concentration if the air is assumed to be completely mixed.

Respuesta :

Explanation:

It is known that equation for steady state concentration is as follows.

            [tex]C_{a} = \frac{QC}{Q + kV}[/tex]

where,   Q = flow rate

              k = rate constant

              V = volume

              C = concentration of the entering air

Formula for volume of the box is as follows.

                 V = [tex]a^{2}h[/tex]

                    = [tex]100 \times 100 \times 1[/tex]

                    = [tex]10000 km^{3}[/tex]

Now, expression to determine the discharge is as follows.

                  Q = Av

                      = [tex]100 \times 1 \times \frac{4 m}{s} \times \frac{km}{1000 m}[/tex]

                      = 0.4 [tex]km^{3}/s[/tex]

And,    m (loading) = 10kg/s,

           k = 0.20/hr

as   1 [tex]km^3 = 10^{12}[/tex] L (if u want kg/L as concentration)

Now, calculate the concentration present inside as follows.

     [tex]C_{in} = \frac{10kg/s}{0.4 km^3/s}[/tex]

                 = 25 [tex]kg/km^3[/tex]

Now, we will calculate the concentration present outwards as follows.

       [tex]C_{out} = {C_{in}}{(1 + k \times t)}[/tex],

and,      t = [tex]\frac{V}{Q}[/tex]

               = 25000 s or 6.94 hr

Hence,   [tex]C_{out} = \frac{25}{(1 + 0.20 \times 6.94)}[/tex]

                         = 10.47 [tex]kg/km^3[/tex]

Thus, we can conclude that the the steady-state concentration if the air is assumed to be completely mixed is [tex]C_{out} = 10.47 kg/km^3[/tex] and [tex]C_{in} = 25 kg/km^3[/tex] .

The steady-state concentration if the air is assumed to be completely mixed is;

C_in = 25 kg/km³

C_out = 10.47 kg/km³

We are given;

Side lengths of box; x = 100 km

Height of box; h = 1 km

speed of clean air; v = 4 m/s = 0.004 km/s

Reaction rate; k = 0.2 /hr

Emission rate; E' = 10 kg/s

Volume of box is; V = length * width * height

Thus;

V = 100 * 100 * 1

V = 10000 km³

Formula for the discharge of air;

Q = Av

where A is area and v is speed. Thus;

Q = 100 * 1 * 0.004

Q = 0.4 km³/s

Formula for concentration present inside;

C_in = E'/Q

C_in = 10/0.04

C_in = 25 kg/km³

Now, time is;

t = V/Q

t = 10000/0.4

t = 25000 s = 6.944 hours

Formula for concentration present outside;

C_out = C_in/(1 + kt)

Thus;

C_out = 25/(1 + (0.2 * 6.944))

C_out = 10.47 kg/km³

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