Batch control for a chemical plant

An early NT-based S88 batch system is providing benefits for Yorkshire Chemicals in Leeds BY VIVIAN BECK

An automated S-88 compliant batch control system for a new production facility manufacturing aqueous and nitrosyl type dyes and intermediates has been installed at Yorkshire Chemicals in Leeds.

Process equipment, such as raw material storage containers, are used by multiple process units. Some units can produce different products whilst others are limited to making just one product.

Automating these new units, based on the new S-88 batch standards: allows multiple batches to be simultaneously processed; automates operator interaction, thus improving consistency and product tolerances/specifications; provides product and batch traceability; and reduces delays and the possibility of contamination.

In addition, the S-88 design provides a flexible upgrade path for future expansion and future integration to Yorkshire Chemicals’ IT system.

S88 system overview

In order to implement an S-88 compliant batching system, both the physical layout and the procedural control methodology were modelled according to the recommended S-88 design procedures.

The relationships between the different components of the models are shown in the diagram above. Each master recipe defines: the equipment needed; the specific and proportional parameters, such as quantities; and the actions required to produce a batch.

When the master recipe is activated, it combines batch quantities, parameters and current plant status with the master recipe – transforming it into a Control recipe.

The Control recipe is therefore a copy of the master recipe, but includes specific equipment, connections and process variables. To understand this, each recipe consists of four parts, namely:

{{* a recipe header – administration data for the recipe;* equipment requirements;* procedures – sequence of process actions needed;* and formulae – I/O and process variables transferred for each phase.}}

A definition of the master recipe involves selecting the required process classes, transfer classes and phases for the required recipes from the plant models already defined. Selecting process and transfer classes, as opposed to process and transfer units, also ensures that the master recipe remains both equipment and path (that is, connection) independent.

Batch management in the PC/PLC environment is handled primarily by the batch managing software in the PC.

Main tasks include: production scheduling; batch initialisation – verification of recipe; required materials and equipment; calculation of actual quantities required (based on formula % and batch size); and batch execution – a combination of the batch manager and PLC allocating equipment, configuring and enabling phases.

The strategy of the batch is determined by the operation and phases implemented. It’s also a factor of the order in which they are placed in the recipe and batch recording and tracking – for quality assurance and product traceability.

System design

This project provided Yorkshire Chemicals with a turnkey Batch Operating System (BOS), compliant with the ANSI/ISA SP88.01.1995 standard for batch control.

Starting from a set of process flow sheets and a basic knowledge of the plant, Thurnall and Yorkshire’s engineers worked together to develop detailed Physical, Procedural and Process models of the plant. From these, a full Functional Design Specification (FDS) for the upgrade was developed.

Implementation of a truly S-88 compatible batch management system requires the software to reside in two locations – the batch management host (PC) and the process connected devices (PLC).

For this project, the batch management software resides on an Intel-based PC within a Windows NT environment. This interface allows the operator to supervise the active batches by communicating with the programmable logic controllers.

Using a top-down programming philosophy, the operations phases are generated. This allows operation in one of three modes:

* Automatic mode – all equipment is controlled by the batch management software and PLC’s. The operator interaction has been reduced to a minimum.

* Manual mode – operations constituting a Unit Procedure can be stepped by means of a button at the control panel.

* Forced operations – operators can override any automatic control, and change the logic state of individual plant items or values.

The new automated system was implemented using Siemens PLCs, linked by an H1 Ethernet network. The batch manager (BOS/Server) is connected by means of a separate Ethernet network. The layout of the hardware allows the 12 SCADA view nodes to be distributed as a system on the plant floor, alongside the PLC remote I/O modules.

Batch management software was Intellution’s Fix BOS and SCADA, Fix DMACS, with an Oracle database. All aspects of production were investigated and engineered into the system. These included procedures for recovery in the event of a failure, as well as system and plant security.

Aspects specific to batch production were similarly incorporated, i.e. watchdogs for monitoring communications failure critical with the parallel processing of the PLC and batch management.

A further project called for the re-automation and integration of an existing processing unit. Re-use of the control software designed for the unit, combined with a few plant alterations, reduced overall engineering time and costs.

At project completion the new and existing units had identical functionality. This showed that the S-88 based design and implementation allowed for a simplified, structured and modular upgrading path.

Increased productivity has been directly credited to improved automation, reduction of operator interaction, multi-batch processing and reduction of losses.

This system is thought to be one of the first S-88 compliant batch operating systems, developed on Windows NT, to be commissioned in the UK. Based on time to complete a batch and the number of batches being processed over a fixed period, the project has been a success.

* The Author is with Thurnall.