Life-Cycle Assessment Principles for the Integrated Product and Process Design of Polymers from CO2

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Abstract

To minimize environmental impacts, coupling integrated product and process design with life-cycle assessment (LCA) is a powerful yet challenging approach. A challenge in LCA is the proper accounting for all co-products occurring along the entire supply chain, known as allocation problem. In this paper, we provide a systematic analysis for LCA-based product and process design including alternative allocation methods. We apply the approach to polyol production from CO2. Here, alternative allocation methods lead to very different polyol products and processes. Finally, we derive general principles regarding co-product allocation in LCA-based product and process design.

Introduction

Today, most polymers are synthesized from fossil-based feedstocks. As alternative and renewable carbon feedstock, carbon dioxide (CO2) has recently been successfully utilized as building block for polyols, a direct precursor of polyurethanes (Langanke et al., 2014). In addition to the direct utilization of CO2 as feedstock for polyols, CO2 can also be used indirectly to produce further chemicals in the polyol supply chain.

Both direct and indirect utilization of CO2 provide novel degrees of freedom to design the polyol supply chain. These degrees of freedom can be exploited to minimize environmental impacts in the design of a polyol polymer with desired properties and the associated processes in the supply chain. For this purpose, we conduct an integrated design of both product and processes coupled with life-cycle assessment (LCA).

Integrated product and process design is a challenging problem by itself (Gani, 2004, Adjiman et al., 2014). Coupling the integrated design problem with LCA introduces further challenges; in particular the evaluation of co-products has to be addressed since many products are co-produced in the chemical industry, e.g., in (bio-) refineries, steam crackers and also for CO2 capture from CO2 point sources (von der Assen et al., 2013). In this case, the environmental impacts have to be allocated to the individual co-products. A set of allocation methods exists in LCA. The choice of the allocation method is known to often lead to strongly varying product footprints (Curran, 2007).

The goal of this work is to analyse the effect of alternative allocation methods in LCA-based product and process design of polyols. The design approach consists of the integrated optimization of the polyol supply chain and the polyol product with respect to minimal environmental impacts (Section 2). As representative environmental impact, the carbon footprint is chosen as single LCA metric to avoid further complexity by multiple environmental objectives. The polyol product has to meet target properties such as the molar mass, OH number, and glass transition temperature. The polyol supply chain includes all processes for production of feedstocks and energy from cradle to gate. We illustrate how alternative allocation methods change both the optimal monomer composition and the supply chain of the polyol (Section 3). We finally derive general principles regarding allocation approaches in LCA-based product and process design (Section 4).

Section snippets

Formulation of Optimization Problem

The aim of LCA-based product and process design is to identify a product and its supply chain with minimal environmental impact. The design problem can be generally formulated as the following non-linear program (NLP):mins,vh=CT⋅sminimalenvironmentalimpactss.t.gvs≤0Constraintsfromsupplychainprocesses1≤D⋅v≤uproductdesignconstraintss∈ℝknprocessvariablesv∈ℝlmproductvariables

The following sections specify the details of problem (1) for CO2-based polyols.

Polyol Supply Chain Superstructure

Polyether polyols are the most common type of polyols. They can be synthesized from the reaction of a multi-functional alcohol starter (glycerol in this work) with epoxides such as propylene oxide (PO). PO can partly be substituted by CO2 making the overall reaction more energy efficient since the production of PO is energy intensive (von der Assen and Bardow, 2014). Each molecule of CO2 reacts with one molecule of PO forming a poly-carbonate (PC) unit in the polymer chains. In addition to PC

Evaluation of Co-Products in Life-Cycle Assessment

In LCA, process systems are typically modelled as set of linear equations. The product under study is specified in the functional unit as vector f∈ℝm. LCA process data is given as technology matrix A and intervention matrix B. Each entry ai,j of A quantifies the flow of product i to process j. To supply only the product flow in f, matrix A is scaled with a scaling vector s (Heijungs and Suh, 2002):Av⋅s=f.

For the process of polyol synthesis modelled by Eq. (2), the corresponding entries in A

System expansion

If there are no constraints on the expanded functional unit, all products can be added to f˜ in Eq. (9). In this case, the optimal expanded functional unit includes the polyol and an infinite amount of CO2 from air capture. The air-capture process provides a net GHG sink for the cradle-to-gate scope. The optimal polyol contains only glycerol and CO2-based POM units, which are produced without any co-products. However, this solution is degenerate in the context of polyol design since the mono

Conclusions

In this work, we have presented an approach for coupling an integrated product and process design problem of polyols with LCA. In particular, we systematically integrated allocation methods from LCA to account for co-products in the supply chain. The influence of allocations methods on LCA results has received much attention in the LCA literature. This experience from LCA can be used for in product and process design for co-product evaluation. In the presented example of CO2-based polyols, we

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