A batch process loads a vessel, reacts, drains and cleans, then repeats; a continuous process feeds reagents in at one end and takes product out at the other without stopping. Batch is simple to build and flexible but loses time between runs; continuous gives steady output but is harder to build and the hardest to start up and shut down safely. Build everything as batch first, and go continuous only for products needed in large, ongoing quantities.
Part of Industrial Chemistry
Choosing between run-by-run and steady-flow production for a chemical process, and the arithmetic that decides it.
The Carrier’s Position
Batch processing, per the carrier, loads reagents into a vessel, runs the reaction, drains the product, cleans, repeats; it is simple to build and operate, flexible (the recipe can change between batches), and inefficient for large quantities. Continuous processing feeds reagents continuously in and product continuously out; it is more efficient and more consistent, harder to build, and harder to start and stop safely. The carrier names the Haber, contact and Solvay processes as continuous, and recommends batch first and continuous only for sulfuric acid, ammonia and soda ash in large ongoing quantities.
A separate article on feeding biogas digesters, Batch vs Continuous Feed, applies the same idea to a different process and has different figures; do not read across.
What You Need
- Demand estimate: how much product per day or week, and for how long.
- Vessel volume and cycle time for a batch, or flow rate and vessel volume for a continuous run.
- A start-up and shut-down procedure written out for any continuous unit, since these are the dangerous phases.
- Spare capacity: a second vessel is cheaper than a longer day.
Method: Choosing
Step 1: Compute daily output of a batch plant from vessel volume, batches per day and yield.
Step 2: Compare with demand. If one batch vessel meets it, stay batch.
Step 3: If not, add a second batch vessel first, staggered so one is reacting while the other is being cleaned.
Step 4: Go continuous only when the product and demand justify it, and only after the process is mastered as batch.
Worked Example: A 200-Litre Batch Vessel
All inputs below are illustrative; they are chosen to show the method.
- Working volume: 200 litres per batch (the carrier uses 200 litres as its production-scale example).
- Reaction time 4 hours, plus 1.5 hours to drain, clean and refill: cycle = 5.5 hours.
- Batches per 24-hour day if run around the clock: 24 ÷ 5.5 = 4.4, so 4 whole batches.
- Output at an assumed 80% of working volume as product: 4 × 200 × 0.80 = 640 litres per day.
- A continuous reactor holding the same 200 litres with a mean residence time of 4 hours takes a flow of 200 ÷ 4 = 50 litres per hour, or 50 × 24 = 1,200 litres per day running around the clock. At the same assumed 80% product fraction that is 1,200 × 0.80 = 960 litres per day.
The continuous unit of the same size makes 960 against 640 litres per day, a gain of 960 ÷ 640 = 1.5 times, because it never stops for cleaning and refilling. The advantage comes with a cost: it holds live reagents at all times, and shut-down means the vessel must be emptied safely. Mean residence time is vessel volume divided by flow.
Common Mistakes
- Going continuous for a product made twice a month.
- Treating start-up as an afterthought. The dangerous moments in a continuous plant are start and stop.
- Idling a batch vessel hot between runs.
- Skipping a second vessel and then blaming the process.
Safety
Continuous plants concentrate hazard in time
A continuous unit holds reactive inventory all the time. Write the emergency stop and the start-up sequence before the first run, practise them on water, and keep operators trained for the transitions. See Safety and Waste and Pressure and Temperature.
What’s Next
- Learn next: Heat Management — the limit on how large a vessel can be
- Combine with: Yield and Efficiency — what each run actually makes