This work is dedicated to Karen Pearson and the FIT President’s Sustainability Council
for their support.
Keywords sustainability - circularity - material efficiency - metrics - intensive and extensive
properties
Background
To assess the sustainability of a process, one must evaluate both its material efficiency
and its circularity [1 ], [2 ]. A process is considered materially efficient if it generates a valuable product
with little to no waste ([Figure 1 ]); [3 ] on the other hand, a process is considered circular if it uses recycled inputs and
recycles outputs as part of a closed loop [4 ]. Therefore, a materially efficient process may be productive, but not circular.
Likewise, a closed-loop process may still be inefficient [5 ]. Herein, we will review and relate common metrics that are used to evaluate whether
a general process is both materially efficient and circular ([Figure 1 ]). Within this article, for a general process, we will consider the input and output
materials and their transformation, but not external considerations and factors such
as the related processes including up- and downstream efforts, or sourcing, use, and
disposal.
Figure 1 Process flow diagram comparing general, circular, and efficient processes.
The relationship between changes to inputs and outputs for a general process in a
closed system is governed by the law of conservation of mass ([Eq. (1) ]).
Δ
Inputs
kg
=
Δ
Outputs
kg
The mass loss of the inputs is equal to the mass gained by the outputs. The amounts
of inputs and outputs are the sum of their parts. Each part’s mass is an extensive
property unique to the system [6 ]. Inputs include chemicals and materials such as reactants, reagents, solvents, and
catalysts. Inputs are considered to be either fresh raw materials or content recycled
from a related process ([Eq. (2) ]).
Inputs
kg
=
Mass of Fresh Feedstocks
kg
+
Mass of Recycled Content
kg
Outputs include substances such as the main products, coproducts, byproducts, side
products, and wastes ([Figure 1 ] and [Eq. (3) ]). Coproducts are expected valuable outputs from a transformation, while byproducts
are other lower-value or undesired expected outputs [7 ]. Side products are outputs formed from competing or side reactions and may be expected
but are considered impurities. Byproducts and side products may be processed and recycled
and used as input feedstocks for further reactions. Wastes here are defined as anything
that is neither a product nor recirculated or recycled.
Output
kg
=
Mass of Products
+
Mass of Waste
To evaluate a process, its extensive properties are measured and related. Two extensive
properties taken as a ratio form an intensive property, which can be considered independent
of a system’s scale [8 ]. Intensive properties describe the state of the system or a transformation ([Eq. (4) ]).
Extensive Property
1
Extensive Property
2
=
Intensive Property
Standard intensive properties are used as metrics to evaluate systems, their states,
and their changes, and to compare systems across time and space. Metrics to evaluate
material efficiency relate measures before and after a process. Measures to evaluate
output efficiency relate to the amounts of desired and undesired outputs. The percent
yield of a process is a common intensive metric used to assess reaction efficiency
and an intensive property calculated as a ratio of two extensive properties, the amount
of obtained product, and the theoretical yield. However, while the percent yield does
relate inputs and outputs, it typically only addresses products and does not consider
competing factors including competing reactions or side products ([Eq. (5) ]).
Yield
%
=
Yield of Product
kg
Theoretical Yield of Product
kg
×
100
%
Therefore, to evaluate a process’s efficiency, we need a metric that holistically
relates the inputs and outputs of a process. Furthermore, to evaluate the relative
circularity we need two metrics, one for assessing the inputs and the other for assessing
the outputs.
To relate products to wastes, inputs, and outputs, distinctive intensive metrics known
as E-factor (EF) and process mass intensity (PMI) are used [9 ], [10 ]. PMI is determined as the ratio of the masses of the inputs to the masses of the
products explicitly relating the inputs and outputs, and the efficiency of a process
([Eq. (6) ]).
PMI
=
Mass of Inputs
kg
Mass of Products
kg
Focused on the product, compared to yield, PMI includes information about reagents
in addition to reactants. EF is determined as the ratio of the masses of the wastes
to the masses of the products, and it only addresses the outputs of a reaction ([Eq. (7) ]) [11 ].
EF
=
Mass of Waste
kg
Mass of Products
kg
EF and PMI are complementary and related, yet even when used together they do not
provide meaningful information about the nature of input materials ([Eq. (8) ]). EF also provides specific information about wastes, while PMI does not explicitly
isolate the waste components.
1
=
PMI
−
EF
To address the circularity and efficiency of a process and provide a comprehensive
understanding of a process’s material sustainability, a third metric should be used
alongside PMI and EF to address feedstock sourcing. An intensive metric frequently
reported for consumer products is the recycled content (RC), often reported as a percentage.
RC is determined as the ratio of the masses of the recycled inputs to the total mass
of the inputs ([Eq. (9) ]) [12 ].
RC
=
Mass of Recycled Content
kg
Mass of Inputs
kg
RC exclusively addresses inputs, yet alongside PMI and EF, this set of metrics provides
the information we need to assess the circularity and material efficiency of the inputs
of a process, their transformation, and outputs for a given system ([Figure 2 ]). While RC, PMI, and EF address neither energy efficiency nor the hazards of chemical
and material components, this set of metrics provides scientists and engineers a meaningful
way to evaluate the material sustainability of a process from mass balance data [13 ], [14 ].
Figure 2 Relating metrics for exploring circularity and sustainability of chemical transformations
and material processes.
Case Study: Craft Papermaking
To explore the utility and relationships of the different metrics, three different
papermaking processes were evaluated. Craft papermaking can be relatively simple,
where a fibrous feedstock is combined with water to generate a pulp, which is then
strained, pressed, and dried to make paper that is used for writing, printing, or
packaging [16 ]. Various processes may use fresh or recycled feedstocks, including fibers from agricultural
and municipal wastes. These impact the circularity of a process and product. The amounts
of paper products and disposed water ultimately determine the efficiency of a process.
Together, these considerations contribute to the sustainability of a process. Here,
we compare three related yet distinctive papermaking processes to highlight how RC,
PMI, and EF may be used to suggest improvements to reduce waste or increase the recycled
content.
Three craft paper-making processes were chosen to highlight how lignocellulosic feedstocks
may be processed to leverage food and office wastes, in the form of corn husks and
recycled paper. The first process upcycles waste corn husks treated with soda ash
to create recycled corn husk paper (RCH), the second makes paper from pure cotton
fiber (PCF), and the third uses a blend of recycled paper and untreated fresh corn
husks for the product paper (PAC) [15 ]
[16 ]
[17 ]. Though the three procedures produce different amounts of paper product, the RC,
PMI, and EF are calculated as ratios that account for the different scales. These
metrics are summarized for the three processes in [Table 1 ]. Raw data and the equations used to generate these metrics are included in the supporting
information.
Table 1
Recycled content, process mass intensity, and e-factor for the three papermaking procedures.
Both the RCH and PCF papers with either recycled or pure fibers have contrasting RCs
while the PMI and EF metrics are similar. This shows us the two processes are similarly
efficient, as the processes afford the same ratios of product to waste, despite being
made from different feedstocks. Comparing the RCH and PAC paper, we find high RC values,
denoting circular processes, and similar PMIs, yet the PAC procedure has a much lower
and desirable EF, coming from the 90% of the water being recycled after straining.
Thus, the PAC process is more efficient. Though the PMI is lower for the RCH process
compared to the PAC process, indicating more product per the same amount of inputs,
the EF is considerably lower for the PAC process. The RCH process may be improved
by including a water recycling step used in the PAC process, which serves to lower
the EF.
Similar to a Life Cycle Assessment, these three metrics used together do not serve
to identify one best procedure for all scenarios but rather to highlight hotspots
or areas of interest that may be adopted across similar procedures [18 ]. A given context may necessitate more or less recycled content, yet all procedures
benefit from a low PMI and low EF. As these metrics are related yet impacted by specific
aspects of a procedure (e.g., reagents, reactants, recycling…), they must be used
together to assess fundamentally different parts of a chemical or material transformation.
The best process is one that combines the circularity of using upcycled materials,
and the material efficiency of a high ratio of product to waste to create a more sustainable
process. While this broad insight is not novel, by using these metrics together one
can target specific inputs or outputs to determine how to reach these goals. Though
the best process does not yet exist, it can be fashioned with conclusions drawn when
evaluating the metrics of e-factor, recycled content, and process mass efficiency.
Conclusion
Sustainable transformations may be evaluated by identifying low process mass intensity
and e-factor and high recycled content processes. To best evaluate materials and processes
as efficient and part of a closed loop one must assess, evaluate, and report on the
sourcing, transformation, and isolation of products using intensive metrics [19 ]. Common, accessible, and complementary metrics including recycled content and e-factor
address circularity, while process mass intensity addresses efficiency. These may
be used collectively to address sustainability from mass balance data. Using these
intensive metrics together provides a systematic snapshot of a process that is easily
compared and related across boundaries and scales. By collecting these metrics focused
on materiality with other complementary metrics focused on energy and toxicity, we
can holistically evaluate chemicals, materials, and their transformations.
Bibliographical Record
