A batch control system automates processes that make a fixed quantity of product through a defined series of steps, such as charging, heating, reacting, holding, cooling and discharging, and records what happened to every batch. It combines the regulating loops found in any process control system with recipes that define what to make and procedures that define how. The ISA-88 (IEC 61512) standard provides the common structure most batch systems follow.
This article supports our guide to industrial process control, which compares batch, continuous and semi-batch processes.
Batch vs continuous control
| Batch | Continuous | |
|---|---|---|
| Product flow | Fixed quantities processed in steps | Material flows in and out without stopping |
| Control focus | Sequences, recipes and step transitions | Holding steady state against disturbances |
| Setpoints | Change by step and by recipe | Mostly constant |
| Records | A record for every batch | Trends and production totals |
| Typical examples | Specialty chemicals, polymers, pharmaceuticals, food, coatings | Refining, bulk chemicals, power, continuous reactors |
The ISA-88 models in plain terms
Physical model: what equipment you have
ISA-88 describes equipment from the top down: a process cell contains units (for example a reactor with its jacket and agitator), which contain equipment modules (a charging system, a heating system), which contain control modules (a single valve, pump or PID loop). Defining this structure first makes the software modular.
Procedural model: what the equipment does
A procedure is broken into unit procedures, then operations, then phases. Phases are the reusable building blocks, such as Charge, Heat, Agitate, Hold and Transfer, each written once and parameterized by the recipe.
Recipes: what to make
Recipes hold the product-specific information: the sequence of phases, setpoints, quantities and durations. Separating recipes from equipment logic is the main idea of ISA-88: a new product becomes a new recipe, not new code.
Example: a simple reactor recipe
| Phase | Parameters | Ends when |
|---|---|---|
| Charge solvent | 500 kg, agitator at 60 rpm | Load cell reaches 500 kg ± 2 kg |
| Heat | Ramp 20 → 60 °C at 1 °C/min | Reactor at 60 °C (40 min at this ramp) |
| Charge reactant | Feed 120 kg over 60 min, hold 60 °C | Total fed = 120 kg |
| React and hold | 60 °C for 3 h | Timer complete and analyzer in spec |
| Cool | Cool to 30 °C | Reactor below 30 °C |
| Transfer | Pump to storage | Reactor weight below 10 kg |
The heat phase is a ramp: the temperature setpoint moves at 1 °C per minute and the PID loop follows it, often as a cascade to jacket temperature. If the reactor cannot keep up with the ramp, the phase should hold the ramp rather than let the gap grow.
Phase states and exception handling
ISA-88 phases follow a state model: Idle, Running, Complete, plus Holding/Held, Restarting, Stopping/Stopped and Aborting/Aborted. Most of the engineering effort, and most of the value, is in the abnormal paths:
- What happens if cooling is lost during the react phase?
- Can the batch be held and safely resumed after a power failure?
- Which steps can an operator skip or repeat, and who must approve it?
- What must be in a safe state before a phase can start?
Interlocks that protect people and equipment stay independent of the batch logic; the batch system should never be the only thing preventing an unsafe state.
Batch records
A batch record captures, for every batch, the recipe and version used, actual quantities, phase start and end times, setpoint and process trends, alarms, operator actions and sample results. In regulated industries it supports release decisions and must meet data integrity requirements; in a pilot plant it is the dataset that engineers analyze.
Batch control in pilot plants
Pilot plants use batch structures even when recipes change every run. Building phases once and editing recipes as data lets engineers change the experiment without reprogramming, and the same phases can carry into production. See pilot plant control systems.
Common batch control mistakes
- Hard-coding each product as its own sequence instead of recipes and phases.
- Writing only the normal path and leaving holds, aborts and restarts for later.
- Mixing safety interlocks into phase logic.
- Recording only final results, not the trends and events needed to investigate a bad batch.
Automating a batch process?
Sciotex designs batch and recipe control for reactors, blending and coating processes, from pilot scale to production, designed, programmed and supported in the US since 1997.
Frequently asked questions
A batch control system automates processes that make fixed quantities of product in defined steps, using recipes for product data and phases for equipment actions, and records what happened to each batch.
ISA-88, also published as IEC 61512, is the standard for batch control. It defines physical, procedural and recipe models so batch systems are modular and recipes can change without rewriting equipment logic.
A phase is the smallest procedural element, such as Charge, Heat or Transfer. Phases are written once and reused by many recipes with different parameters.
A master recipe is the approved template for making a product on a type of equipment. A control recipe is the copy executed for one specific batch, with that batch’s actual values.
Many do. Using phases and editable recipes lets engineers change experiments quickly while keeping sequences tested and safe.
