Once a reaction is completed, the valuable products and perhaps unreacted species will need to be separated so that they can be sold and recycled, respectively. In addition, sometimes we will need to employ other types of unit operations to transform the materials to the right form so that they can be formulated, better managed, or sold (e.g., granulation, milling). In a typical chemical plant, most of the plant footprint and operating costs are related to separations and other unit operations, such as distillation towers, crystal- lizations, and liquid extractions. Given this, there are many opportunities to apply green engineering principles integrally in the separations and purifications to provide the desired product.
Separations and unit operations themselves represent a very wide topic that can occupy several volumes, let alone trying to cover unit operations that integrate green engineering principles. One common way is to visualize the variety of unit operations and classify them according to separation, transport, size enlargement, and size reduction operations, as shown in Figure 11.3. It is also common to classify separations according to the number of phases involved (e.g., homogeneous, heterogeneous). As can be seen, many types of unit operations are available in the process design toolkit. However, the level of technical understanding varies between operations. For example, in the United States, about 90% of chemical process separations are distillations,10 so it is not a surprise that considerable effort has been dedicated to optimizing this unit operation. However, this lack of separations diversity has to
a certain point stifled the implementation of new technologies that might contribute to greener, more sustainable manufacturing.
One of the challenges of the engineer and process chemist is to look beyond the status quo toward more innovative chemical processing options that might render benefits from a green engineering viewpoint. The best known option is not always the best alternative.
For example, the total energy requirements for concentrating mother liquors using some membrane separation processes (e.g., solvent resistant nanofiltration) are about 20 times smaller than that required for atmospheric distillation. This also translates into energy- related CO2emissions that are almost 6000-fold smaller for nanofiltration systems. This is also true when there is a driving force or equilibrium condition that will hinder the separation in the normal setting, as where there are azeotropes or eutectic mixtures. The following example illustrates what options might be available in the case of an azeotropic mixture.
Example 11.4 There is the need to separate an aqueous mixture of isopropyl alcohol.
However, isopropyl alcohol forms an azeotrope with water, so normal atmospheric distillation will not provide the purity required. What other options could be used? What are their disadvantages from a green engineering perspective?
Solution There are several ways to “break” the azeotrope and dehydrate alcohols.
Examples include:
. Azeotropic distillation: will require a mass separating agent
. Extractive distillation: will require a mass separating agent
FIGURE 11.3 Examples of separations and size reduction/augmentation unit operations.
and might not withstand certain solvents or impurities in the solvent mixture Additional Points to Ponder Are there any advantages from a green engineering perspective? How would you know which option above is best?
As we did with reactors, Table 11.6 provides examples of questions to consider during the design and selection of unit operations. It can be seen that some of the questions for selecting separations unit operations are very aligned with questions for selecting reactors. This alignment occurs because, as with reactors, we are faced with the challenge of selecting the best separation or unit operation that will:
. Maximize resource utilization
. Minimize hazards
. Account for life cycle implications
Example 11.5 Your boss (who really likes to look at data) has asked you to compare two technology options for dehydrating isopropanol so that it may be recycled back into a process. The feed stream is composed of 59 wt% isopropanol and 41% water. Isopropanol with at least 99.5% purity is desired. The options presented to you are:
. Azeotropic distillation using benzene as a mass separating agent
. Extractive distillation using propylene glycol as a mass separating agent
What recommendation would you give to your boss and how would you justify your decision?
Solution Since you know that the green engineering principles would exclude the use of benzene as a mass separating agent given its toxicity, the extractive distillation seems a better alternative. However, you also know that the green engineering principles would advise you to avoid the use of a mass separating agent altogether. You know that pervaporation might be suitable for this task, so you decide to use mass and energy indicators to compare the two proposed options along with your idea of using pervapora- tion, as follows:
Option 1: Azeotropic distillation. For the design and system proposed, the ratio between the upper and lower layers of the azeotrope leaving the azeotropic column is 93.6/6.4ẳ14.6.11 This ratio was used in calculations for the material balance. The chemical losses considered for this option are the fugitive emissions of isopropanol and benzene (estimated at 1% according to their boiling points)12and the wastewater produced as a waste stream, containing isopropanol and benzene. The composition of the upper and lower layers of the decanter were given in the literature. The overall mass balance is shown in Figure 11.4.
The main energy requirements are the heating requirements to operate the columns, the cooling requirements for condensing, and the electricity for pumping. To calculate the energy requirements, the following assumptions can be made:
TABLE 11.6 Examples of Questions to Consider During Unit Operation Design and Selection to Integrate Green Engineering Principles
Green Engineering Principles
Examples of Questions to Consider During Separation Selection and Design 1. Engineer processes and products holistically,
use systems analysis, and integrate environmental impact assessment tools.
How is this separation going to fit in the overall process design?
Can mass integration be used?
Design of products, processes, and systems must include integration and interconnectivity with available energy and materials flows.
What will be the effects if I change the separation sequence (e.g., removing B first instead of A)?
Can we use hot streams coming out of this separator to heat cold streams?
Are there any fugitive emissions from the separator that need to be accounted for in an impact assessment?
Is this separation generating additional waste that I need to treat elsewhere?
Can I combine this separation with another, or combine it with a reactor?
Do I even need to separate/purify?
2. Conserve and improve natural ecosystems while protecting human health and well- being.
If additional materials are needed, can renewable materials be used for this separation?
Material and energy inputs should be renewable rather than depleting
Can renewable energy be used to operate the separation?
3. Use life cycle thinking in all engineering activities.
Products, processes, and systems should be designed for performance in a commercial “afterlife.”
How would this separation affect energy consumption for the process and for the plant?
Is this separation maximizing the overall mass efficiency of the process?
Material diversity in multicomponent products should be minimized to promote disassembly and value retention.
Is this unit operation configuration contributing to additional separations? Do we really need it?
Targeted durability, not immortality, should be a design goal.
What type of energy is being used in this process?
Embedded entropy and complexity must be viewed as an investment when making design choices on recycle, reuse, or beneficial disposition.
Can mass separating agents be avoided?
If a mass separating agent cannot be avoided, can it be recovered?
4. Ensure that all material and energy inputs and outputs are as inherently safe and benign as possible. Designers need to strive to ensure that all material and energy inputs and outputs are as inherently nonhazardous as possible.
Are the materials used in this separation benign? If not, how can we eliminate or avoid them?
Are we integrating inherent safety principles?
Can we eliminate occupational exposures by design with this unit operation?
Are the operating conditions extreme for this separation?
Can any other aspects of this separation pose a hazard to the facility?
Green Engineering Principles
Examples of Questions to Consider During Separation Selection and Design 5. Minimize depletion of natural resources.
Separation and purification operations should be designed to minimize energy consumption and materials use.
Is this separation maximizing mass efficiency?
Is it really needed?
Is this separation minimizing energy use? Are we purifying beyond the point required?
Products, processes, and systems should be designed to maximize mass, energy, space, and time efficiency.
Products, processes, and systems should be “output pulled” rather than “input pushed” through the use of energy and materials.
Is heat transfer being optimized to maximize energy efficiency?
Is mass transfer being optimized?
Can mass separating agents be avoided, minimized, or recycled?
Can we apply process intensification?
6. Strive to prevent waste. It is better to prevent waste than to treat or clean up waste after it is formed.
Are we generating additional waste with this separation that could otherwise be avoided?
Are we using the easiest separation technique for this system?
Can we apply some heuristics for separations?
Can mass separating agents be avoided, minimized, or recycled?
Is heat transfer being optimized to maximize energy efficiency?
Is mass transfer being optimized?
7. Develop and apply engineering solutions while being cognizant of local geography, aspirations, and cultures.
Do we have in-house expertise for this technology, or do we need to develop the expertise?
If we implement the new separation, would training be available in the local language?
Are human factors being integrated into the design?
8. Create engineering solutions beyond current or dominant technologies; improve, innovate, and invent (technologies) to achieve sustainability.
Are there fewer traditional separations that would provide better mass efficiency?
Can this separation be combined with the reactor?
Design for unnecessary capacity or capability (e.g., “one size fits all”) solutions should be considered a design flaw.
Can two different separation techniques be combined?
9. Actively engage communities and stakeholders in the development of engineering solutions.
Can we work with our vendors and suppliers to design a separation system that would:
Eliminate the need for a mass separating agent?
Reduce energy consumption?
Have fewer fugitive emissions?
Be inherently safer?
. Heat losses: 15%
. All distillates and bottoms cooled to 25C (including the distillate of the azeotropic column before being fed into the decanter)
. Reflux ratio: 1.3
. Operating temperature of the preconcentration column: 80C; for the azeotropic column, 67C, and for the recovery column, is 80C;
Data used13:
. Basis: 1 kg of feed
. Cplof isopropanol (I)ẳ154 J/mol
. DHnof isopropanolẳ45.5 kJ/mol (b.p. 83C)
. m.w. isopropanolẳ60 g/mol
. Cplof water (W)ẳ75.37 J/mol
. DHnof waterẳ43.99 kJ/mol (b.p. 100C)
. m.w. waterẳ18 g/mol
. Cplof benzene (B)ẳ136 J/mol
. DHnof benzeneẳ33.9 kJ/mol
. m.w. benzeneẳ78 g/mol
Feed at 25ºC:
Isopropanol 59% [0.59 kg]
Water 41% [0.41 kg]
Azeotropic D. Column Recovery Column
Isopropanol recovered 0.5824 kg, 99.5% purity [0.579 kg isopropanol 0.0033 kg water
0.0001 kg benzene] Water to WWTP
0.4129 kg [0.006 kg I 0.0002 kg B 0.4067 kg W]
Decanter Fugitive Emissions:
Isopropanol: 0.005 kg Benzene: 0.01 kg
0.0921 kg [0.0133 kg I 0.0004 kg B 0.0784 kg W]
[0.0123 kg I 0.0003 kg B 0.0017 kg W]
[0.001 kg I 0.0002 kg B 0.0767 kg W]
Pre-concentration col.
[0.58 kg I 0.08 kg W]
[0.005 kg I 0.33 kg W]
0.2716 kg I 1.0421 kg B 0.0310 kg W
0.2849 kg I 1.0425 kg B 0.1094 kg W
Benzene makeup:
0.0101 kg
FIGURE 11.4 Mass balance for azeoptropic distillation of Example 11.5.
DH ẳ sensible heatỵvaporization heatẳ1 h
Xn
i
miCpiDTỵ ð1ỵRịmvDHv
" #
ẳ 1 0:85
0:6258 kg I
60g=mol ð154 J=molCị ỵ0:4875 kg W
18 g=mol ð75:37 J=molCị
ð8025ịC 8<
: þ2:3
615 g I
46 g=molð42:6 kJ=molị ỵ 85 gW
18 g=molð44 kJ=molị )
ẳ1928 kJẳ1:93 MJ=kg feedẳ3:3 MJ=kg isopropanol recovered
Heating requirements of azeotropic column:
DH ẳ sensible heatỵvaporization heatẳ1 h
Xn
i
miCpiDTỵ ð1ỵRịmvDHv
" #
ẳ 1 0:85
"
0:8673 kg I
60 g=mol ð154 J=molCị ỵ1:0426 kg B
78 g=mol ð136 J=molCị 8<
: þ0:1127 kgW
18 g=mol ð75:37 J=molCị
#
ð6725ịCỵ2:3
"
284:9 gI
60 g=molð45:5 kJ=molị þ1042:5 gB
78 g=molð42:6 kJ=molị ỵ109:4 gW
18 g=molð44 kJ=molị
#)
ẳ2757 kJẳ2:76 MJ=kg feedẳ4:73MJ=kg isopropanol recovered
Heating requirements of recovery column:
DH ẳ sensible heatỵvaporization heatẳ1 h
Xn
i
miCpiDTỵ ð1ỵRịmvDHv
" #
ẳ 1 0:85
"
0:0133 kg I
60g=mol ð154 J=molCị ỵ0:0004 kgB
78 g=mol ð136 J=molCị 8<
: þ0:0784 kgW
18 g=mol ð75:37 J=molCị
#
ð8025ịCỵ2:3
"
12:3 g I
60 g=molð45:5 kJ=molị þ 0:3 gB
78g=molð33:92 kJ=molị ỵ 1:7 gW
18 g=molð44 kJ=molị
#)
ẳ60 kJẳ0:06 MJ=kg feedẳ0:10 MJ=kg isopropanol recovered
Total heating requirements:
8:13 MJ=kg isopropanol recovered Cooling requirements:
Since all the bottoms and distillates are assumed to be cooled to 25C (the temperature of the feed), the energy required for cooling is
8:13 MJ=kg isopropanol recovered Electricity requirements:
Electricity requirements are negligible in comparison to heating and cooling require- ments. To simplify the calculations, a rule of thumb using the average standard pumping electricity requirements for the economical pipe size (15 m long) was used, with an average pumping energy of 0.003 kJ/kg pumped:
ð0:003 kJ=kg pumpedịð1ỵ0:66ỵ0:335ỵ0:5824ỵ0:0921ỵ0:0778 ỵ0:0139ỵ1:4368ỵ1:3447ịkg ẳ1:7102kJ=kg feed
ẳ2:9105MJ=kg IPA recovered
Option 2: Extractive distillation.For the extractive distillation option, the solvent/feed ratio (or simply the feed ratio, on a molar basis) is normally between 1 and 4.14,15Two feed ratios were used in the calculations:
(a) S/Fẳ1 (minimum in the normal range) (b) S/Fẳ2.5 (middle point in the normal range)
The overall mass balance for these two cases is shown in Figure 11.5.
Extractive D. Column Propylene G. Stripper
Pre-concentration Col.
80 ºC
80 ºC
120 ºC
F=1 [0.772 kg PG]
F= 2.5 [1.929 kg PG]
Feed at 25 ºC:
IPA 59%
Water 41%
[0.59 kg I 0.41 kg W]
Isopropanol recovered 0.5824 kg, 99.5% purity [0.579 kg isopropanol 0.0033 kg water 0.0001 kg propylene glycol]
Fugitive Emissions:
0.005 kg isopropanol
Makeup propylene glycol 0.0002 kg PG
[0.0059 kg I 0.3303 kg W]
Water to W 0.4128 kg [0.006 kg I 0.0001 kg 0.4067 kg W [0.0001 kg I
0.0001 kg PG 0.0764 kg W]
FIGURE 11.5 Mass balance for extractive distillation of Example 11.5.
columns operate at roughly 80C) and the stripper (operating at 120C), the cooling requirements for condensing and cooling, and the electricity for pumping. To calculate the energy requirements, the following assumptions can be made:
. Heat losses: 15%
. Stripper bottoms not cooled before being recycled to the extractive distillation column (remaining distillates and bottoms cooled to 25C)
. Reflux ratio used for all the columns: 1.3
. Pumping energy: 0.003 kJ/kg pumped
Data used:
. Basis: 1 kg of feed
. Cplof isopropanolẳ154 J/mol
. DHnof isopropanolẳ45.5 kJ/mol (b.p. 83C)
. m.w. isopropanolẳ60 g/mol
. Cplof waterẳ75.37 J/mol
. DHnof waterẳ43.99 kJ/mol (b.p. 100C)
. m.w. waterẳ18 g/mol
. Cplof propylene glycol (PG)ẳ180 J/mol
. DHnof propylene glycolẳ64.4 kJ/mol (b.p. 187C)
. m.w. propylene glycolẳ76 g/mol
Heating requirements for preconcentration column:
DH ẳ sensible heatỵvaporization heatẳ1 h
Xn
i
miCpiDTỵ ð1ỵRịmvDHv
" #
ẳ 1 0:85
0:59 kg I
60 g=molð154 J=molCị ỵ0:41 kg W
18 g=molð75:37J=molCị
ð8025ịC 8<
: þ2:3
579 g I
46 g=molð42:6 kJ=molị ỵ 79:7 gW
18g=molð44 kJ=molị )
ẳ1924 kJ ẳ1:92 MJ=kg feedẳ3:3 MJ=kg isopropanol recovered
Heating requirements for the extractive distillation column:
(a) S/Fẳ1
DH ẳ sensible heatỵvaporization heatẳ1 h
Xn
i
miCpiDTỵ ð1ỵRịmvDHv
" #
ẳ 1 0:85
"
0:5791 kg I
60 g=mol ð154 J=molCị ỵ0:0797 kg W
18 g=mol ð75:37 J=molCị
#
ð8025ịC 8<
: þ0:772 kg PG
76 g=mol ð180 J=molCịð80120ịC þ2:3
"
582:1 g I
60 g=molð45:5 kJ=molị ỵ 3:3 g W
18 g=molð44 kJ=molị ỵ 0:1 g PG
76 g=molð64:4 kJ=molị
#)
ẳ1124 kJẳ1:24 MJ=kg feedẳ1:93 MJ=kg isopropanol recovered (b) S/Fẳ2.5
DH ẳ sensible heatỵvaporization heatẳ1 h
Xn
i
miCpiDTỵ ð1ỵRịmvDHv
" #
ẳ 1 0:85
0:5791 kg I
60 g=mol ð154 J=molCị ỵ0:0797 kg W
18 g=mol ð75:37 J=molCị 2
4
3
5ð8025ịC 8<
: þ1:929 kg PG
76 g=mol ð180 J=molCịð80120ịC þ2:3 582:1 g I
60 g=molð45:5 kJ=molịỵ 3:3 g W
18 g=molð44 kJ=molịỵ0:1 g PG
76 g=molð64:4 kJ=molị 2
4
3 5 )
ẳ995 kJẳ0:995 MJ=kg feedẳ1:71 MJ=kg isopropanol recovered Heating requirements for propylene glycol stripper:
(a) S/Fẳ1
DHẳsensible heatỵvaporization heatẳ1 h
Xn
i
miCpiDTỵ ð1ỵRịmvDHv
" #
ẳ 1 0:85
"
0:0001 kg I
60 g=mol ð154J=molCị ỵ0:0764 kg W
18 g=mol ð75:37 J=molCị 8<
: þ0:772 kg PG
76 g=mol ð180 J=molCị
#
ð12080ịCỵ2:3
"
0:1 g I
60 g=molð45:5 kJ=molị þ76:4 g W
18 g=mol;ð44 kJ=molị ỵ 0:1 g PG
76 g=molð64:4 kJ=molị )
ẳ607 kJẳ0:61 MJ=kg feedẳ1:04 MJ=kg isopropanol recovered
DH ẳ sensible heatỵvaporization heatẳ1 h
Xn
i
miCpiDTỵ ð1ỵRịmvDHv
" #
ẳ 1 0:85
0:0001 kg I
60 g=mol ð154 J=molCị ỵ0:0764 kg W
18 g=mol ð75:37 J=molCị 2
4 8<
: þ1:929 kg PG
76 g=mol ð80 J=molCịð12080ịC þ2:3 0:1 g I
60 g=molð45:5 kJ=molị ỵ 76:4 gW
18 g=molð44 kJ=molị ỵ 0:1 g PG
76 g=molð64:4 kJ=molị 2
4
3 5 )
ẳ735 kJẳ0:74 MJ=kg feedẳ1:26 MJ=kg isopopanol recovered Total heating requirements:
(a) S/Fẳ1: 6.27 MJ/kg isopropanol recovered (b) S/Fẳ2.5: 6.27 MJ/kg isopropanol recovered
This indicates that for each scenario shown, the total heating requirement is not sensitive to the feed ratio.
Cooling requirements:
The energy required for cooling in this option is that needed to condense and cool the product stream to 25C, condense and cool the overhead stream of the stripper from 120C to 80C, and condense the distillate of the pre-concentration column and cool the waste to 25C.
Therefore, it is independent of the S/F ratio.
Total cooling requirements:
3:49 MJ=kg feedẳ5:96 MJ=kg isopropanol recovered Electricity requirements:
The electricity requirements are negligible compared to the heating and cooling requirements. To simplify the calculations, a rule of thumb using the average standard pumping electricity requirements for the economical pipe size (15 m long) was used, with an average pumping energy of 0.003 kJ/kg pumped:
(a) S/Fẳ1
ð0:003 kJ=kg pumpedịð1ỵ0:0002ỵ0:5824ỵ0:3362ỵ0:0766ỵ0:6588ỵ0:772ịkg
ẳ1:02102kJ=kg feed
ẳ1:8105MJ=kg isopropanol recovered
(b) S/Fẳ2.5
ð0:003 kJ=kg pumpedịð1ỵ0:0002ỵ0:5824ỵ0:3362ỵ0:0766ỵ0:6588ỵ1:929ịkg
ẳ1:4102kJ=kg feed
ẳ2:4105MJ=kg isopropanol recovered
After doing the calculations above, you notice that both options presented to you have green engineering challenges so you decide to add an additional option to the assessment
Option 3: Pervaporation. In this separation process, a multicomponent liquid stream is exposed to a membrane that preferentially permeates one or more of the components. As the feed liquid flows across the membrane surface, the preferentially permeated compo- nent passes through the membrane as a vapor. Transport through the membrane is induced by maintaining a lower vapor pressure on the permeate side of the membrane than that of the feed liquid. Since different species permeate the membrane at different rates, a substance at low concentrations in the feed stream can be highly enriched in the permeate.
Thus, separation occurs, with the efficacy of the separation effect being determined by the relative volatilities and the physicochemical structure of the membrane. For this compar- ison, the pervaporation system shown in Figure 11.6 was considered. The feed is equalized and heated to 70C. The entire process is maintained at 70C to maximize the vapor pressure difference across the membrane.11 After the pervaporation module, the resulting isopropanol–water mixture was assumed to have 20% isopropanol.16This stream is then distilled to recover and recycle the isopropanol. Therefore, with this option there is no need for external substances, although a distillation column is needed to complete the circle. The chemical losses considered for this option are the fugitive emissions of ethanol (estimated at 1% according to its boiling point) and the wastewater
Feed at 25 ºC:
IPA 0.59 kg Water 0.41 kg
Water removal column
Water to WWT 0.4127 kg [0.006 kg I 0.4067 kg W]
Pervaporation Module Equalizer
Tank
IPA recovered 0.5823 kg, 99.5% purity [0.579 kg I
0.0033 kg W]
Fugitive Emissions:
0.005 kg IPA
0.0135 kg W 0.0991 kg I
Permeate:
0.1051 kg I 0.4202 kg W 0.6891 kg I
0.4235 kg W
FIGURE 11.6 Mass balance for pervaporation of Example 11.5.
also shown in Figure 11.6.
Another issue to take into consideration is the membrane life. A membrane operating under these conditions has an average life of 3 years. A wide range of total fluxes have been reported in the literature, for different initial concentrations. The reported fluxes are mainly between 0.01 and 2.2 kg/m2h.17–20For the sake of comparison, three cases for membrane life can be considered: at 0.01, 1.1, and 2.2 kg/m2h. If the system operates 8 h/day during each day of the 3 years (8760 h), we have:
At 0:5 kg=m2h) ẵð0:01 kg=m2hịð8760 hị1 ẳ 1:1106m2=kg of feed:
ẳ 1:8106m2=kg of IPA recovered At 2:5 kg=m2h) ẵð1:1 kg=m2hịð8760 hị1 ẳ 1:26104m2=kg of feed:
ẳ 2:16104m2=kg of IPA recovered At 5:0 kg=m2h) ẵð2:2 kg=m2hịð8760 hị1 ẳ 2:5104m2=kg of feed:
ẳ 4:3104m2=kg of IPA recovered These fluxes are taken as an average for the purposes of this study, but in practice it is commonly observed that a decrease in the water content results in a progressive decrease in the flux.
The main energy requirements are the heating needed to operate the pervaporation unit at 70C and the heating required for the distillation column operating at about 80C.
In addition, there are energy requirements for condensing and cooling, and electricity for pumping. To calculate the energy requirements, the following assumptions were made:
. Heat losses: 15%
. Isopropanol stream recovered cooled to 25C
. Reflux ratio used for the distillation column: was 1.312
. Pumping energy: 0.003 kJ/kg pumped for all the pumps except the permeate pump
. Permeate pump assumed to be a vacuum pump operating at about 10 mbar pressure (1 kPa)
Data used:
. Basis: 1 kg of feed
. Cplof isopropanolẳ154 J/mol
. DHnof isopropanolẳ45.5 kJ/mol (b.p. 83C)
. m.w. isopropanolẳ60 g/mol
. Cplof waterẳ75.37 J/mol
. DHnof waterẳ43.99 kJ/mol
. m.w. waterẳ18 g/mol