The Design Process: From Concept to Heat and Mass Balance

Process
29th August 2024

Article by Tom Baxter CEng FIChemE

Tom Baxter kicks off a four-part series giving new graduates a better understanding of how the design process works in industry, from concept to execution

CHEMICAL engineers have a critical role to play in the journey to net zero. We must identify, design, and operate process plants that address the triple bottom line – people, profit, plant – with a clear emphasis on minimising greenhouse gas emissions.

IChemE is aligned with the United Nations Sustainability Development Goal 12: “responsible consumption and production”. Of particular relevance to the chemical engineer is the caveat “doing more with less”.

Graduating chemical engineers will be integral to achieving the goal and, although all will have conducted a design project, my experience indicates that many do not appreciate the full design and construct process.

This series of short articles aims to provide new graduates with a better understanding of the process, with the hope that fresh eyes can develop facilities that deliver on the UN’s sustainability aims.

Typical stages to first production are shown in Figure 1. The development team needs to produce information at the concept/select stages that will allow for a decision to proceed to Front End Engineering Design (FEED). That decision will need consideration and quantification of capital cost, operating cost, safety, environment, schedule, operability, technical risk, return on investment, and perhaps company reputation.

Figure 1: Design process stages

The development team will generate options that will deliver on the remit they have been given. The options might include differing feedstocks and reaction routes.

Following option generation, a coarse screen is undertaken where, often based upon experience, some options will be culled.

Let’s consider a hydrogen production facility for instance. A coarse screen has been undertaken and the development team have settled on two options – steam methane reforming with carbon capture and storage, or water electrolysis using renewable electricity. The reforming process is shown to be more cost effective. The water electrolysis route is more expensive but provides a much smaller greenhouse gas footprint. From an environment standpoint, electrolysis is a clear winner, but how do the development team make a decision?

In my opinion, we have to put a value on harm to the environment to make that decision; put a price per tonne on emissions of CO2, NOx, SOx, particulates etc. That would drive the concept to deliver on UN Goal 12. It is important to note that the concept selection process is where value can be added or eroded. Should the wrong concept be selected, no matter the quality of the follow-on works, value has been lost.

Front-end loading (FEL) is extremely important. FEL is carried out at a time when the ability to influence changes to the equipment configuration and design is relatively high and the cost to make those changes is relatively low.

The UK government states: “It has been proven that projects that have focused on front-end loading take time and cost out of a schedule. They are typically 20% lower cost and 10-15% faster than average projects and considerably faster and lower cost than poorly defined projects.” In other words, time spent at the front end of a project is time well spent.

Concept selection aims to identify if there are clear differences between the discriminators. Hence, option definition requires provision of a level of engineering detail to enable choices to be made.

Selection will also involve preliminary hazard and environmental analysis together with a review of the planning and consents process.

A first pass at process and utility flow diagrams, a preliminary heat and mass balance, mechanical equipment list, and coarse layout will be undertaken.

A key discriminator will be capital cost. Capital costs are classified as a function of the detail available and accuracy range. The more detail, the better the accuracy. The accuracy of cost estimates is classified as shown in Table 1.

Table 1: Project cost classification and cost ranges

Using proprietary cost estimation software, Class 4/5 norms, Lang Factors, in-house metrics and historical norms for similar systems, CAPEX and OPEX can be estimated together with the implementation schedule. This will allow for commercial analysis of the options.

The commercial analysis should be risked to understand outcome upsides and downsides. As illustrated in Figure 2, it is very important to understand risk. Here we see Option A producing the better 50% probable net present value (NPV) outcome. But it might not be chosen, as Option B protects the downside more effectively.

Figure 2: NPV risked spread

The concept option definition analysis will hopefully identify a preferred option that can be recommended for front-end engineering design.

Sometimes there is no clear option preference. In this instance, two or more options might be taken to FEED to provide more detail and a higher confidence in the cost estimates and other discriminators.

The FEED will seek to provide more definition and thereby more confidence in the estimates. This will cover operations and maintenance philosophy, process, mechanical, materials, piping, control, civils, structures, electrical, safety systems, environmental protection, layout, procurement, construction, schedule, project execution plan, Class 2 cost estimate, and end of life decommissioning.

The heat and mass balance (H&MB) is a hugely important piece of work and many FEED activities cannot begin until there is an approved heat and mass balance. If the H&MB is flawed, the deliverables from the other disciplines using the H&MB will be compromised. The FEED may be a house built upon sand.

For example, if compositions are incorrect, the material selection may be wrong, and safety compromised. If the process details for the mechanical engineers to provide an appropriate compressor is flawed then, irrespective of the thoroughness of the mechanical engineering work, the compressor design will also be flawed.

The H&MB will be prepared to deliver products to the buyer/delivery specifications. Use of appropriate thermodynamics and, accurate chemical reaction kinetics and physical properties is paramount. If an inappropriate equation of state is selected, then the consequences are obvious.

Some designs may require different modelling thermodynamics for differing unit operations.

A key skill is not modelling, but the knowledge to apply the correct thermodynamics and testing the robustness of the assumptions that are made for the H&MB preparation.

The H&MB also provides the information for the plant utilities, for example power and cooling/heating requirements. To reiterate, the H&MB is a critical piece of work.

My experience is that H&MBs are often based upon copies of previous similar projects.

With a hydrogen production facility the development team will have to choose between the more cost effective steam methane reforming, or the more expensive, but more environmentally friendly water electrolysis with renewable electricity

When we are seeking to provide chemical plant designs that seek to deliver against UN Goal 12, Xerox engineering is unlikely to be an appropriate basis.

Consider a new distillery. Xerox engineering would develop a H&MB that may not account for the capture and recycling of heat. Xerox engineering would not consider the adoption of vapour recompression where the system is designed for heat recovery. Scotch whisky producer Chivas Brothers1 deployed vapour recompression at one of their sites and claimed an energy saving equivalent to powering 5,000 homes.

As IChemE states in its explanation of SDG 12:2 “This goal therefore sits right at the core of all activities involved in the chemicals and process industry. It places huge responsibility on the shoulders of all engineers to find and implement solutions that break the traditional ‘Take – Make – Use – Dispose’ economic model found in most traditional material conversion processing plants and transform the industry based on a circular economic model.”

In other words, we need a new way of designing chemical plants. A monumental ask.

As stated previously, a project’s greenhouse gas footprint should be of prime consideration. Hence there should be a high focus on the energy use and emissions profile from the plant.

IChemE provides the following examples of how process engineering can be used to deliver sustainable solutions:

waste materials recovery and recycling processes where end-of-life products are collected, dismantled and reprocessed to make new metals and plastics for low-carbon sources of raw materials
utilising natural based feedstock to reduce the dependency of fossil-based feedstock
maximising energy efficiency and recovery to reduce the energy footprint of products
renewable energy sources used to reduce carbon emissions

I would encourage new graduates to have no fear in challenging the grey hairs to justify the approach being used for chemical plant design. And, given the parallel with Inherent Safety in Design (ISD), perhaps it is time to develop guidance for another ISD; Inherent Sustainability in Design.