In chemical plant engineering, it is often taken for granted that equipment should be optimized for the process. This sounds obvious to any plant engineer.
However, from the perspective of an owner’s engineer who has overseen multiple equipment installations within the same plant over a 20-year span, this assumption becomes questionable.
The initial design determines the long-term fate of a plant.
Failing to consider future use cases during the first design phase leads to decades of inefficiency, repeated modifications, and eventually structural limitations that no engineer originally intended.
In this article, I summarize the key points that should be considered when designing batch plants from a long-term, lifecycle-oriented perspective.
1. Designing a Single-Product, Single-Purpose Plant
When designing a plant for a single product, the process is often straightforward.
Corrosion may be moderate, temperatures low, and stainless steel or glass-lined reactors are typically sufficient.
In such cases, the “optimal” configuration is mostly predetermined and falls on the simpler end of plant engineering.
Example: Basic batch process
- Reaction
- Washing
- Concentration
- Discharge
A set of stainless-steel or GL reactors with jackets may be enough.
This level of design is sometimes labeled a “multi-purpose plant,” but in reality, it is far from truly flexible.
2. Making Equipment Multi-Purpose
If the intention is to build a true multi-purpose plant, each reactor must be capable of performing all steps—reaction, washing, concentration, and charging.
This inevitably requires:
- Additional condensers
- Additional columns
- More heat exchangers
- More complex piping
- Unified material specifications (often GL)
This raises the initial investment significantly and is becoming less realistic in today’s cost-sensitive environment.
3. Increasing Complexity in Modern Processes
3-1. Materials are becoming more complex
Simple GL-only design is often no longer viable.
More aggressive process conditions require:
- Hastelloy for reaction steps
- Stainless steel for high-temperature concentration
- GL for general-purpose steps
Attempting to optimize equipment around the process makes it difficult to maintain a unified material specification.
3-2. Processes themselves are becoming more complex
Even if the reaction is unchanged, the number of washing steps may increase, or additional separation steps may be introduced.
This leads to more:
- Reactors
- Receivers
- Pumps
- Columns
- Heat exchangers
The plant becomes equipment-heavy, even before considering future modifications.
4. Why Future-Proof Design Is Difficult
4-1. Modifications become inevitable
No plant continues producing the same product for 10–20 years.
When new products are introduced, modifications are always required—even if the plant was initially designed as multi-purpose.
For example:
- Additional concentration steps may be required
- A washing reactor may need to be repurposed
- Heat exchangers and piping materials must be upgraded
- Receiver tanks, pumps, and insulation must be added
4-2. Piping routes become increasingly complex
Each modification adds more fixed piping.
As more products with different reactor sequences are introduced, the plant becomes a maze of switching routes between reactors.
In commercial plants (unlike pilot plants), flexible hoses are unacceptable.
This is why modification after modification gradually fills the building with additional piping until physical limits are reached.
A plant with 10 reactors is already near the structural limit;
with 15–20 reactors, routing becomes nearly impossible due to building width constraints.
5. Modification vs Scrap-and-Build
5-1. Modifications
Economically reasonable in the short term.
Each investment remains minimal, allowing continued operation.
But:
- The limit of plant expansion is unclear
- Engineers rotate too frequently to recognize long-term saturation
- Inflation and rising construction costs make even small modifications harder
5-2. Scrap-and-Build
At some point, one may question the validity of multi-purpose design itself.
A dedicated single-product plant:
- Minimizes initial cost
- Allows stable long-term operation
- Can be dismantled after its lifecycle
However, in modern Japan, scrap-and-build has become extremely difficult due to:
- Escalating labor and material costs
- Strict building-code compliance
- High safety-management costs
- Complex control systems
- Lack of owner-side engineers
- Dependence on engineering contractors, which raises costs further
Conclusion
Optimizing plant design is not simply choosing materials or equipment for a given process.
It requires evaluating the long-term trajectory of the plant, the expanding complexity of processes, and the inevitability of modifications.
A plant built without considering future use will suffer decades of incremental changes, escalating costs, and routing saturation.
True optimization lies in balancing:
- Initial investment
- Long-term flexibility
- Equipment scalability
- Lifecycle cost
not only from a process viewpoint but through an integrated process–equipment perspective.
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