MASS AND ENERGY BALANCES
9.6 MEASURING GREENNESS OF A PROCESS THROUGH ENERGY AND MASS BALANCES
At this point, we have reviewed the general concepts of mass and energy balance and through the examples in this chapter have seen how mass and energy balances are the first step to be able to:
. Understand the process
. Identify green chemistry/green engineering issues and potential improvements
. Quantify resource needs T Raw materials
T Utilities (heating, cooling, electricity)
T Ancillary materials such as nitrogen, solvents, and catalysts
. Identify and quantify recycling loops needed
. Identify and quantify hazardous materials
S N
CO2 O
CH3 O O
H3N
- N
CO2COCH2Cl O
CH3 O O
Cl N
Cl Cl
S N
CO2COCH2Cl O
CH3 O O
N ClCH2CO2CO
Cl N OMe
OMe
7-ACA methanol
water ammonia methanol
T By-products
T Unreacted raw materials T Ancillary materials
. Quantify process waste and emissions T Liquid, solid, and gaseous waste streams T Purges and potential leaks
T Potential fugitive emissions
. Estimate process efficiencies T Energy efficiency T Mass efficiency
. Identify potential energy optimization opportunities
. Quantify the impact of side and competing reactions
. Estimate emissions derived from energy and raw material production
In short, it wouldn’t be possible to evaluate processes for green engineering purposes (or in general) without performing a mass and energy balance in order to understand the process, measure efficiencies, and identify potential opportunities for improvement. One example of the direct application of mass and energy balance is the comparison of two routes for the production of 7-aminocephalosporanic acid. 7-ACA had been done traditionally with the chemical route shown above (the block process diagram is shown in Figure 9.9).
A new enzymatic route was developed and there was a question about which route was greener. The first step in making that comparison was to perform a mass and energy balance for both processes so that the green metrics for each process could be contrasted and evaluated. A sample of the calculations for one of the steps in the chemical synthesis is shown in Table 9.2.
Ceph C K+ salt Milling
Protection
PCl3reaction
PCl5reaction Quench
Precipitation
Harvest Ceph C K+ salt
Dichloromethane
Chloroacetyl chloride
Dimethyl- aniline
PCl3 Dimethyl- aniline
PCl5milling PCl5
DCM Methanol DCM
Water Ammonia
Methanol
Wet cake to dryer Filtrate & washes
Chemical Route
FIGURE 9.9 Block flow diagram for the chemical synthesis of 7-ACA.
Charge DCM 1064 20 20 20 20 Charge cephalosporin
C potassium
144 — 20 20 20 20 — 0
Milling, 15–25C; chilling if necessary, 15–60 min
— — 20 20 20 20 0.625 2,621 8,499 8,499
Transfer, line washing with DCM
93 — 20 20 20 20
Cool batch — — — — 20 3.5 — 0 — 50,657
Charge chloroacetyl chloride 161 — 20 3.5 20 3.5 0.08 423 — 3,442
Charge dimethylaniline 182 — 20 3.5 3.5 16 0.67 3,803 — 1,287
Chill to12C and stir — — — — 16 12 0.21 352 — 58,173
263
Once the mass and energy balances were completed, it was possible to estimate the green metrics for both the chemical and enzymatic routes, and to compare them. A sample of some of the metrics determined through the mass and energy balance is shown in Table 9.3.
Also, by performing complete mass and energy balances it is possible to identify EHS issues associated with the process, such as the amounts of hazardous materials that are either produced or utilized (e.g., dichloromethane, phosphorous chloride), and extreme operating conditions (very high or low temperatures and pressures). After the mass and energy balances are completed, we might be in a better position to answer the question of which process is greener. At this point we can say from the mass and energy balances that the enzymatic route is generally more resource efficient, requiring about half the mass of materials per kilogram of product and about 80% of the process energy. This is despite having a lower yield than that by the chemical route. In addition to that, we can say that the enzymatic route also avoids the use of hazardous materials and solvents (dichloromethane, phosphorous pentachloride, phosphorous trichloride, dimethyl aniline) required by the chemical route.
But can we really answer the question of which route is greener? In the 7ACA example, the enzymatic process uses considerable amounts of water and produces the associated aqueous waste, as can be seen by the aqueous waste produced. We can perhaps get an indication, but to answer the question fully, we will need to redraw the system boundaries to include the production of raw materials, the production of utilities (steam, refrigeration), and the treatment of waste, and that is done using a life cycle approach. That type of analysis is covered in (Chapters 16 through 20) in greater detail, but in a way, it can be explained as a mass and energy balance with expanded boundaries. After all, we have the ability to redraw the boundaries wherever they will render the appropriate answer to the right question.
Perhaps the greater difficulty is in defining the right question.
TABLE 9.3 Mass and Energy Balance Metrics Metric
Chemical Route
Enzymatic Route Mass
Yield (mol%) 75 67
Reaction mass efficiency (%) 14 46
Mass intensity [kg/kg 7-ACA (excluding water)] 81 44
Mass productivity [% (excluding water)] 1.2 2.3
Solvent intensity [kg/kg 7-ACA (excluding water)] 74 41
Organic phase waste (kg waste/kg 7-ACA) 80 43
E-factor including aqueous waste (kg waste/kg 7-ACA) 93 172 Energy
Electricity (MJ/kg 7-ACA) 4.2 1.8
Cooling above 20C (MJ/kg 7-ACA) 0.1 3.4
Cooling below 20C (MJ/kg 7-ACA) 6.0 2.8
Heating, steam or hot air (MJ/kg 7-ACA) 1.3 1.3
Electricity for refrigeration (cooling below 20C) (MJ/kg 7-ACA) 3.0 0.7
Total process energy Requirements (MJ/kg 7-ACA) 11.5 9.3
9.1 It is said that batch processes, by their nature, are always in a transient state. Why? A continuous process is usually run as close to steady state as possible. Give an example of when this is not the case for a continuous process.
9.2 The commercial production of acrolein by heterogeneously catalyzed gas-phase condensation of acetaldehyde and formaldehyde was established by Degussa in 1942. Today, acrolein is produced on a large commercial scale by heterogeneously catalyzed gas-phase oxidation of propene.15,16
(a) Draw a block diagram of the process according to the following description:
Propylene, air, and steam are compressed to 2 atm and then mixed at a molar ratio of 1 : 8 : 4. The gas mixture is fed to a multitubular fixed-bed reactor, which is operated at 350C and 2 atm. The conversion rate of propylene in this reactor is 95%. The effluent gas from the reactor is cooled to 250C and then fed into a gas washer. An aqueous stream and an organic liquid, 2-ethyl-1-hexanol, are used to wash the gas stream. The ratio of gas stream/aqueous stream/organic stream is 10.6 : 1.5 : 1. The residual gas leaves the gas washer at 70C and is introduced to the bottom of a gas cooler. The liquid stream from the gas washer is pumped into a series of distillation columns at 105C to recover the by-products, acrylic acid and acetic acid. From the gas cooler, the residual gas stream leaves at 19C and is fed into another gas washer to recover residual acrolein. The organic phase from the bottom of the cooler is recycled to the first gas washer at 45C. Part of the aqueous phase is combined with the organic phase of the second gas washer, cooled to 16C, and recycled to the gas cooler. Part of the aqueous phase from the gas cooler is recycled to the first gas washer. The second gas washer uses a water/
2-ethyl-1-hexanol mixture to wash the residual gas at 2C. The aqueous phase is then combined with part of the aqueous phase from the gas cooler to recover acrolein product. 2-Ethyl-1-hexanol is also recovered and combined with makeup 2-ethyl-1-hexanol and water. This stream is cooled to 2C and fed into the second gas washer. The chemistry involved is:
Primary reaction: