Discrete, Batch and Continuous Processes

What is the difference between discrete, batch and continuous processes?

Discrete, batch and continuous processes are the three main types of industrial production, classified by the form in which the product is made: as separate, countable units, as defined quantities produced according to a recipe, or as an uninterrupted flow from raw material to finished product. The process type largely determines which control systems, which standards and which data a plant needs. Anyone designing SCADA, PLCs, a DCS or an MES, or choosing Sensors and data storage, therefore starts by asking what kind of process is running.

Each process type has its own dynamics, control strategy and automation approach.

🧠 Why does the distinction between process types matter?

The process type determines how you count product, how you monitor quality and how you record provenance. A car plant follows every vehicle by its serial number, a pharmaceutical manufacturer follows a lot by its batch number, and a refinery can only say which stream passed through which unit at which moment.

International standards follow the same split. ISA-95 (internationally IEC 62264), the standard for integrating ERP with production, is explicitly intended for discrete, batch and continuous manufacturing and gives each type its own place in the equipment hierarchy:

The terms process cell and unit come from ISA-88, the standard for batch control. The same hierarchy is often used as the topic structure of a Unified Namespace, so that data from all three process types lands in a recognisable place.

🔹 What characterises a discrete process?

A discrete process produces separate, countable units: a car, a phone, a box or a bottle. Every step can be distinguished on its own, and the process has a stop-start character.

• Unit of production - pieces, often with an individual serial number per product
• Examples - assembly of cars, appliances or furniture; packaging of boxes or bottles; printing and labelling of products; machine building and electronics
• Control - PLCs running logic sequences and ladder logic, with a scan cycle of typically 1 to 100 milliseconds, plus motion control and robots
• Signals - many digital IO signals from proximity switches, photo-eyes and actuators such as cylinders and motors
• Operation - an HMI per line or station

Discrete manufacturing revolves around takt and throughput time. The cycle time per station sets the capacity of the line, and OEE measures how much of the theoretical capacity actually turns into good products. For packaging machines, PackML (ISA-TR88.00.02) defines a single state model, so that machines from different builders behave the same way within a line.

🔸 What characterises a batch process?

A batch process produces a defined quantity of product according to a recipe. Raw materials go into a vessel, undergo a sequence of operations such as dosing, mixing, heating, reacting and cooling, and the result leaves the plant as a single lot.

• Unit of production - the batch, with a batch number that links all raw materials, process conditions and quality checks
• Examples - sauces, chocolate and yoghurt; tablets and serums in pharmaceuticals; beer; mixing of chemicals or paint
• Control - sequential control, often written in SFC, with batch control systems based on ISA-88 (S88)
• Flexibility - you can switch recipes between batches, so the same equipment makes several products
• Hygiene - CIP procedures clean piping and tanks between batches or product changeovers

ISA-88 separates the recipe (what has to happen) from the equipment (what it happens with). ISA approved part 1 in 1995, IEC adopted it in 1997 as IEC 61512-1, and a revision followed in 2010. A master recipe describes the product for a given process cell; for each batch, a control recipe with the actual quantities is derived from it.

Batch processes are highly traceable. In regulated sectors this is mandatory: GMP requires a complete production record for every batch, and the US regulation 21 CFR Part 11 sets requirements for the trustworthiness of electronic records and signatures. That record is built up through the MES or the historian. An intermediate form is the semi-batch or fed-batch process, in which raw material is added gradually during the batch, as in fermentation.

🔻 What characterises a continuous process?

A continuous process feeds raw materials in without interruption and at the same time delivers a constant flow of finished product. The plant spends most of its time in a stable steady state; start-up and shutdown are short, high-risk transitions.

• Unit of production - a flow rate, such as tonnes per hour or cubic metres per day
• Examples - oil refining, petrochemicals, fertiliser, papermaking, drinking water and wastewater treatment, power stations
• Control - continuous closed control loops with PID controllers, often in a DCS, complemented by cascade control and advanced process control such as MPC
• Signals - mainly analogue measurements of temperature, pressure, level, flow and composition, monitored in real time
• Safety - a SIS intervenes on dangerous deviations; alarm management according to ISA-18.2 prevents alarm floods

Shutting down is expensive. A restart can take hours to days and often produces off-spec product. Refineries therefore schedule major maintenance in a turnaround: traditionally once every four to five years, and increasingly only after five or six years when equipment condition allows. A historian is indispensable in a continuous process, because quality and provenance can only be reconstructed from time-series data.

🔄 How do the three process types differ in practice?

🧩 Which hybrid forms exist?

In practice many plants are hybrid, with a different process type per area:

• Brewery - brewing happens in batches, filling on a discrete bottling or canning line
• Dairy - milk is pasteurised continuously (high-temperature short-time pasteurisation at a minimum of 72 °C for 15 seconds), cheese is made in batches and then packed as individual pieces
• Steel - melting in a converter or electric arc furnace happens per heat, while casting and rolling run continuously
• Pharmaceuticals - traditionally batch, but continuous manufacturing is gaining ground. The US FDA approved the first drug produced by continuous manufacturing in July 2015, and the international guideline ICH Q13 on continuous manufacturing was adopted on 16 November 2022 and has applied in the EU since 10 July 2023

In hybrid plants the transition between process types is the hardest part. That is where traceability switches from time window to batch number, or from batch to serial number, and where the MES, tracking and tracing and control systems must fit together precisely.

🔧 How do you choose the right automation for each process type?

A practical approach for a new project or a modernisation:

1. Determine the process type per area - divide the plant according to the ISA-95 hierarchy and state for each area whether it is discrete, batch or continuous.
2. Choose the control platform - PLCs for fast discrete logic, a DCS for large numbers of control loops, a batch system based on ISA-88 when recipes and product changes are central. For modular plants, Module Type Package offers a middle way.
3. Define traceability - serial numbers, batch numbers or time-stamped process data, with a historian as the backbone for batch and continuous processes.
4. Pick fitting KPIs - OEE works well for discrete lines but says little about a distillation column; there you measure yield, energy per tonne and stability.
5. Align maintenance and security - schedule patches and changes in the windows the process allows: between batches, during a product change or during a turnaround.

🔐 What does the process type mean for OT security?

The process type determines where the biggest risks lie and when you can intervene:

• Continuous processes - availability comes first. Patches often wait for a planned stop, so compensating measures such as segmentation in line with IEC 62443 and network monitoring are essential. A disruption can trigger a costly emergency shutdown through the SIS.
• Batch processes - the integrity of recipes and batch records is central. A tampered recipe produces a rejected or even dangerous lot, and every change has to be validated under GMP.
• Discrete processes - many small PLCs, robots and machines from different builders, often with their own remote access. The sheer number of devices and suppliers makes management and access control difficult.

❓ Frequently asked questions

Is water treatment a continuous process?

Yes, water treatment is a typical continuous process: water flows without interruption through the treatment stages and is controlled by loops on flow, level and quality. Some sub-steps, such as backwashing filters, do run in cycles. Control is usually done with PLCs and SCADA, because the installations are spread over many locations.

What is the difference between a batch process and a discrete process?

A batch process makes a quantity of product, such as 5,000 litres of sauce, that is only finished as one lot when the recipe completes. A discrete process makes separate, countable pieces that each pass through the steps individually. Batch production is tracked per batch number, discrete production usually per serial number or per production order.

Is a DCS only used for continuous processes?

A DCS is mainly used for continuous processes, because it manages thousands of control loops redundantly with one shared database. Many DCS vendors also offer a batch module based on ISA-88, which makes a DCS suitable for large batch plants as well. For fast discrete machines you would usually choose a PLC.

Can you use OEE for batch and continuous processes?

OEE originated in discrete manufacturing and works best there, with world class at around 85%. In batch processes you can calculate OEE per unit, but you have to deal deliberately with cleaning time and waiting time between batches. In continuous processes, yield, throughput and energy per tonne often give a better picture.

Why is the pharmaceutical industry moving towards continuous manufacturing?

Continuous manufacturing needs less floor space, shortens lead times and enables real-time quality monitoring. Since the international guideline ICH Q13 of November 2022 it is clear how regulators assess continuous manufacturing of medicines. A “batch” is then defined as a quantity or a time period of the continuous production.

Which process type is the hardest to automate?

No process type is inherently the hardest, because each has its own challenge. Continuous processes demand a lot of control engineering and high availability, batch processes demand strict recipe management and validation, and discrete processes demand fast logic and the coordination of many machines. Hybrid plants are often the most complex, because several worlds meet there.

📌 In summary

Discrete processes make separate, countable pieces, batch processes make defined lots according to a recipe, and continuous processes deliver an uninterrupted flow of product. The process type determines the control system (PLC, batch system or DCS), the standard (PackML, ISA-88 or ISA-95), the way you trace product, your approach to monitoring and safety, and the windows in which you can carry out maintenance and security updates.