E2e4e3

For a given conversion efficiency, C, E4 is calculated from Equation (2). Then S2 is determined from Equation (1), and finally £3 is determined from Equation (3).

The solutions for 40, 50, 60, and 70% conversion efficiency are summarized in the table below.

Conversions

Stream Flowrates -

Conversions

Stream Flowrates -

Et

100

100

100

100

e2

150

100

66.7

42.9

E}

0

0

0

0

e4

150

100

66.7

42.9

100

100

100

100

s2

250

200

166.7

142.9

Si

100

100

100

100

s4

150

100

66.7

42.9

Note that P], P2, P3 and PA can be calculated from the equation

With the ethylene yield expression given in the problem statement, the net yield and ratio of total flow from the reactor, S2, to net ethylene yield are calculated below as a function of ethylene conversion efficiency.

Conversions

40%

50%

60%

70%

Net ethylene yield (1000 kg/h)

85.0

81.7

78.3

75.0

5'2/Net ethylene yield

2.94

2.45

2.13

1.91

Although the lowest conversion produces the least raw material waste (P2 = Net yield of ethylene), it also requires the highest flowrate of streams; this will result in higher utility costs and higher capital cost. These must be balanced as part of an economic analysis.

RCY.8 DEGREASER EMISSION REDUCTION

Metal surfaces are often cleaned using organic solvents in an open-top degreasing tank. One of the widely used solvents for such operations is 1,1,1-trichloroethane (TCE). TCE belongs to a group of highly stable chemicals known as ozone depleters. Figure 112 depicts a typical degreasing operation. The emission factor for the process shown is estimated to be 0.6 lb/lb of TCE entering the degreaser. The

t

1^13

Fn, „

Degreaser (1)

Solvent

Recovery (2)

I

^21

Figure 112. Schematic of a typical degreasing operation.

Figure 112. Schematic of a typical degreasing operation.

solvent from the degreaser is sent to a solvent recovery unit where 80% of the solvent is recovered and 20% of the solvent is disposed with the sludge.

1. To ascertain the feasibility of the installation of a vapor recovery system (see Figure 113), determine the amount of TCE vented to the atmosphere per pound of fresh TCE used.

2. If the vapor recovery system is 90% efficient, determine the fraction of trichloroethane lost to the atmosphere and the fraction going with the sludge.

Solution

Assume a basis of 1 kg for F0I. The mass balance equations around the two units for the process without the vapor recovery unit is

The equation for the amount of TCE emissions in terms of the amount of TCE entering the degreaser is

Figure 113. Schematic of degreasing operation with vapor recovery unit.

In addition, the amount of TCE recovered in terms of the amount entering solvent recovery is

To calculate the amount of solvent recovered, note that there are four equations [(1)-(4)] and four unknowns (F2\, F12, Fl3, and F24). First, rearrange Equation (4) in terms of F]2:

Substituting this result into Equation (1) leads to Equation (6):

Simultaneously solving Equations (3) and (6) gives the following result for F21:

F2i = 0.471 kg/kg fresh solvent

The amount of TCE emissions per kilogram of fresh TCE is then:

(0.25)(0.471)+ F13 -1=0 F13 = 0.882 kg/kg fresh solvent

The amount of TCE in the sludge is

Therefore,

= F12 ~ F2\ = 0.588 -0.471 = 0.118 kg/kg fresh solvent

Proceed to the vapor recovery unit calculation and write the mass balance equations around the three units for the process:

An equation for the amount of TCE emissions in terms of the amount of TCE entering the degreaser can also be written:

The equation for the amount of TCE recovered in terms of the amount entering solvent recovery is

Further, the equation for the amount of TCE lost to the atmosphere is

Equation (11) may be rearranged to give

This may be substituted into Equation (7):

Arrange Equations (10) and (14) in terms of F3l and set them equal to each other:

0.25F21 + Fl3 - 1 = Fx3/O.6 - 1 -F2X 1.25 F21 =Fl3 F2x = 0.533F13

Now, write the equation for the amount of TCE recovered in vapor recovery in terms of the amount of emissions from the degreaser. Use Equations (9) and (12):

To calculate the amount of TCE emissions from the degreaser, use Equation (10):

F13 = 0.6(1 + 0.533F13 + 0.9F13) = 4.286 kg/kg fresh solvent

To calculate the amount of TCE lost to the atmosphere, note that

Further, the amount of TCE lost in the sludge is

F21 = 1.6F13/3 = (1.6)(4.286)/3 = 2.286 kg/kg fresh solvent

The amount of solvent recovered can now be calculated:

= 1.25 F21 F24=F21( 1.25-1) = (0.25)(2.286) = 0.571 kg/kg fresh solvent

The amount of solvent lost to the atmosphere and the amount of fresh feed per solvent disposed of in the sludge for the system without vapor recovery is therefore

= 7.5 kg lost/kg waste solvent

= 8.5 kg fresh solvent/kg waste solvent

Next page rcy.8 degreaser emission reduction 693

The amount of solvent lost to the atmosphere and the amount of fresh feed per solvent disposed of in the sludge for the system with vapor recovery is

= 1.75 kg fresh solvent/kg waste solvent

By employing the vapor recovery system, the amount of solvent lost due to evaporation and fresh solvent requirements are considerably reduced.

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