Question 2 of 5: Green Engineering, Aluminum Mass Balance and LCA

Question 2: Green Engineering, Aluminum Mass Balance and LCA (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Targeted Durability

Design a product to last its intended service life, and no longer, in a form that then degrades or is recovered harmlessly—e.g. compostable food-service containers for a single-use application instead of polystyrene foam that persists for centuries as litter and marine debris. Matching durability to service life avoids both premature failure (more production) and an "immortal" waste legacy.

(b) Unnecessary Capacity as a Design Flaw

Over-sizing wastes materials and energy for the whole life of the equipment—e.g. a pump or motor chosen "one size fits all" runs far below its best-efficiency point most of the time; right-sizing it (or fitting a variable-speed drive) cuts the electricity used, and so the upstream emissions from generation, every hour it runs, and uses less material to build.

(c) Material Unification

Minimizing the number of different materials makes a product easy to take apart and recycle—e.g. a computer case made from a single grade of aluminum with snap fits instead of glued mixed plastics and metals, or a mono-material (all-polyethylene) package, can be recycled as one clean stream instead of being landfilled or incinerated as an inseparable composite.

(d) Aluminum Mass Balance

Current situation (input − product = pollution → 100 − 91 = 9 t/yr lost):

                 ┌──────────────────────┐
 100 t/yr Al ───►│      FACTORY         │───► 91 t/yr in computer bodies
   (purchased)   │                      │
                 └──────────┬───────────┘
                            └──────────────► 9 t/yr pollution
                                  (stack particulate + turnings + wastewater)

With pollution prevention (pollution halved to 4.5 t/yr, product held at 91 t/yr, so input = 91 + 4.5 = 95.5 t/yr):

                 ┌──────────────────────┐
 95.5 t/yr Al ──►│      FACTORY         │───► 91 t/yr in computer bodies
   (purchased)   │   (P2 measures)      │
                 └──────────┬───────────┘
                            └──────────────► 4.5 t/yr pollution

Because production is unchanged, halving the pollution lets the factory buy 4.5 t/yr less aluminum (95.5 instead of 100)—cutting both waste and material cost: "pollution prevention pays."

(e) LCA: Paper Towels versus Electric Hand Driers

(i) Functional unit: drying one pair of hands (scaled to, e.g., all hand-dryings in the school's washrooms over one school year), so that many towels are compared fairly with a small share of one drier's life.

(ii) Stages/phases: raw-material extraction; manufacturing (materials processing and product fabrication); distribution/transport; use (including maintenance); end-of-life (disposal or recycling).

(iii) Higher-impact alternative at each stage:

• Raw-material extraction: paper towels—a continuous supply of wood fibre (forestry) is consumed every use, whereas the drier's steel, copper and plastic are extracted once.
• Manufacturing: paper towels—pulping, bleaching and papermaking are energy-, water- and chemical-intensive and repeated for every towel; the drier is manufactured once and amortized over thousands of uses.
• Distribution: paper towels—bulky cartons are trucked to the school repeatedly for years; the drier is delivered once.
• Use: electric drier—it draws roughly 1–2 kW for every drying, so its electricity (and the emissions of the generating grid) is its dominant burden; a paper towel needs no energy in use.
• End-of-life: paper towels—a continuous stream of used towels goes to landfill (producing methane) or compost; the drier is disposed of once, and its metals are recyclable.

(iv) Stage of greatest impact: for paper towels, the manufacturing stage (pulp and paper production, with its raw-material harvesting); for the electric drier, the use stage (electricity consumption). Which alternative is greener overall depends on usage volume and on the grid: on a low-carbon grid such as British Columbia's or Quebec's hydro-electric supply the drier generally wins over its service life.