1. Process Status and Automation System Background of Nantong Acetate Fiber Co., Ltd. Nantong Acetate Fiber Co., Ltd. (hereinafter referred to as NCFC) is a large-scale Sino-US joint venture industrial enterprise mainly producing cellulose acetate tow for cigarette filters. It integrates thermal power, chemical industry, and textiles. The production area is divided into three functional areas: spinning, acetate sheet production, and utilities. Its main products are cellulose diacetate tow and its raw material, cellulose diacetate sheets. Acetic acid cracking and acetic acid distillation are completed in a single control unit. The entire production process has a high degree of automation. Due to increasing market demand, NCFC began its fourth phase expansion project in 2004, which significantly increased its scale and production capacity. The expansion of the acetic acid distillation production unit is a crucial part of the fourth phase project, mainly involving the expansion of seven production units. NCFC's process equipment adopted an RS3 (4-20mA output signal) control system in the second phase in 1993, and was upgraded to an RS3 (4-20mA + HART output signal) control system in the third phase in 1998. The RS3 control system is a classic DCS system, and its performance over the years has been well-established. However, from a developmental perspective, whether the RS3 system should continue to be used in the fourth-phase expansion project's automation system design requires comprehensive evaluation. 2. Selection of the Fourth-Phase Automation System 2.1 Control System Performance Evaluation When upgrading the system, we need to consider both direct and indirect economic benefits. According to reference data provided by Emerson, the lifespan of a DCS operator station is typically 5 years; the controller's lifespan is greater than 15 years; the I/O board's lifespan is greater than 20 years; and the field wiring (cables)'s lifespan is greater than 40 years. The maintenance cost of DCS boards increases over time, while the technical level of the boards decreases. To avoid wasting the original DCS investment, operator station upgrades are usually considered first, followed by controller upgrades and ultimately, I/O-level upgrades. Simply rebuilding the system from scratch would waste the hardware and software resources of the existing, still usable DCS system. The cost of rebuilding a system should not only consider the investment in purchasing a new system, but also the related engineering investments (such as manpower, material resources, and financial resources) and the direct loss of plant profits due to prolonged shutdown. The second and third phases of the acetic acid cracking and distillation unit both use Emerson's RS3 DCS system. Considering the structure and characteristics of the RS3 system, if it continues to be used in system expansion, although most control strategies and operating interfaces can be shared, a comprehensive evaluation of the system's overall performance is necessary. With the development of computer and automation technologies, the RS3 system, as a representative of older generation DCS systems, can no longer meet the increasingly sophisticated control and management requirements, and has some unavoidable problems: the operating environment, data bus, and network communication are closed systems, resulting in weak platform openness; the RS3 system is gradually being replaced by mainstream products, leading to difficulties in spare parts supply, high prices, and high maintenance costs; the RS3 system is limited by its internal functions and structure, resulting in poor scalability, etc. Currently, Emerson's flagship DCS product, the DeltaV system, inherits the advantages of the RS3 system while incorporating the latest automation technologies, such as support for various fieldbuses, industrial Ethernet, easy expansion, online upgrades, and embedded complex control algorithms and advanced control systems. Therefore, the DeltaV (4-20mA + HART output signal) system became the primary choice for the Phase IV system expansion, but it also brought several challenges: online upgrades, seamless integration of the old and new systems, and the redevelopment of control strategies and user interfaces. After careful technical communication and comprehensive analysis and evaluation, considering the long-term development interests of the factory, maintaining the system's advanced nature, scalability, and overall integrity, we ultimately chose a fully integrated solution: upgrading the original RS3 system, expanding with the new DeltaV system, and unifying the two operating interfaces. 2.2 Phase IV Control System Architecture The original Phase II and III systems included an RS3 system (7 pairs of redundant controllers) and two Tricon ESD emergency stop systems. The Phase IV system will add one DeltaV system and two Tricon ESD emergency stop systems to this foundation. Simultaneously, all existing RS3 operator stations were replaced with DOR and DeltaV operator stations based on the Windows platform. The DeltaV control system contains 8 pairs of redundant controllers, each controlling 8 relatively independent sub-areas to minimize communication between controllers. Two newly added Tricon ESD emergency shutdown systems control the safe shutdown of two pyrolysis furnace units respectively. [align=center] Figure 1: Phase IV Control System Architecture[/align] 2.3 Interface Design of New and Old Systems After the Phase IV system upgrade, the original RS3 system was required to have system openness and universality. The PEERRAY communication protocol was converted into a standard Ethernet communication protocol through a dedicated network interface (including hardware and driver software), achieving 100% compatibility with the newly expanded Phase IV DCS system in terms of control layer, operation layer, and communication. Under the premise of ensuring the integrity and high reliability of these data, complete sharing of I/O data, controller data, intermediate calculation data, and operation data was achieved, along with unlimited, fast, real-time data exchange, without any intermediate conversion interface services. Finally, the two different control systems were able to complete all production process control under the same operating system, thus achieving perfect integration. Since the Phase IV system is a collection of old and new systems, and heterogeneous systems, the interconnection between different systems became a key focus of the project implementation, and to some extent affected the normal operation of the system. Therefore, the following issues must be addressed in the interface design between systems: Communication between HMI and RS3/DeltaV: The Phase IV HMI (Human-Machine Interface) system requires replacing all existing RS3 operator stations with DeltaV operator stations on the Windows platform. However, the standard DeltaV Operator software cannot connect to RS3; DeltaV Operator for RS3 (DOR) software must be used in conjunction with RNI devices to communicate with RS3. Furthermore, it is recommended that each RNI connect no more than 3 DORs. Therefore, we divided the 10 operator stations into two groups: 6 stations with DOR installed, which can access both DeltaV and RS3 via RNI; and the remaining 4 stations with DeltaV installed, which can only access the DeltaV system. Although DOR, as the HMI software for the RS3 system, can view the DeltaV screen, its functionality differs significantly from that of DeltaV. For example, due to the differences between the toolbar and alarm bar, DOR cannot access the DeltaV alarm list and operation log, nor can it display the DeltaV real-time alarm bar. This is inconvenient for users who want to simultaneously view the DeltaV screen on DOR. Therefore, we modified the DOR system screen to allow users to freely switch between the DOR and DeltaV systems. Regarding communication between RS3 and DeltaV, since not all process units are being newly built during the production expansion, some equipment will become shared equipment in the third and fourth phase systems; some equipment originally controlled by the RS3 system will not be connected to the DeltaV system, but the control strategies need to be implemented in DeltaV. This necessitates establishing controller communication between the RS3 and DeltaV systems. We use two methods to achieve this: Modbus communication establishes a Modbus connection between the Modbus modules of RS3 and DeltaV. OPC Mirror data mirroring involves installing DeltaV OPC Server and Matrikon OPC Server for RS3 RNI on a single server, establishing an OPC Mirror relay to achieve data exchange between RS3 and DeltaV. Modbus communication is hardware-dependent, relatively reliable but with lower data transmission volumes, suitable for exchanging critical control data. OPC Mirror requires OPC software for data relay, suitable for less critical or large-volume data exchanges. Communication between DeltaV and Tricon ESD: In Phase IV of the project, two sets of Triconex Tricon triple-redundant fault-tolerant PLC control systems were used. These systems needed to include control of four cracking furnaces, safety protection, emergency shutdown, and fault memory functions. The Tricon system's I/O cards all employ a redundant structure, with each input/output point signal processed using a triple-selection circuit, resulting in extremely high safety and reliability. The Tricon ESD system needs to transmit alarm information and control flows from each measurement point to the DeltaV system's operating interface for monitoring; this information is crucial for shutdown or process diagnostics. As a simple and reliable communication method, Modbus communication has become the preferred method for communication between DeltaV and the Tricon ESD system: the DeltaV system acts as the Modbus master, and the Tricon ESD system acts as the slave, realizing communication between the two. Communication between DeltaV, RS3, and the control system mainly refers to providing process data to the control system for archiving or further statistical analysis. The system utilizes the DeltaV OPC Server and Matrikon OPC Server for RS3 RNI to provide a large amount of real-time OPC data services, realizing timely acquisition of process data, optimized control, and integrated control. 2.4 AMS Intelligent Device Management Almost all the instruments selected in the Phase IV project support the HART protocol, and DeltaV also uses I/O modules with integrated HART. Thus, through the integration of the AMS intelligent device management system with the DeltaV system, engineers can check equipment status, configure, adjust ranges, and perform diagnostics from a PC without going to the site. AMS also has the advantage of supporting more than 220 types of HART devices and 80 types of fieldbus devices. Supports online access to equipment diagnostic information, enabling continuous equipment monitoring and immediate fault detection. Timely measures are taken before a fault causes a major accident. Provides unique online performance diagnostics for optimal valve predictive diagnostics. Facilitates data transfer between the database and calibrator. Provides calibration results and all maintenance information for each device, aiding in compliance with various management agency standards. 2.5 Implementation Plan "A good start is half the battle," and a proper and meticulous implementation plan is crucial for the smooth progress of a project, especially for multi-system integration projects. We adhered to the commissioning principle of starting from individual components to the entire system, from each channel to each module to each subsystem, ultimately ensuring the accuracy of the entire integrated system and achieving successful start-up on the first attempt. 3. Control Room Design Scheme In the second phase design in 1993, a duty room and instrument maintenance room were set up in the control room, reserving space for future expansion needs. In the third phase design in 1998, the duty room was converted into a control cabinet room; in the fourth phase design in 2004, the instrument maintenance room was again converted into a control cabinet room. Therefore, Phases II, III, and IV share a central control room, which facilitates operation and maintenance, and also provides the infrastructure necessary for the upgrade and integration of the old and new systems. The plant was very satisfied with the ease of operation after the successful commissioning of Phase IV, a practice worthy of emulation in automation design. 4. Commissioning Results Currently, the two cracking furnaces in Phase IV have been successfully commissioned. From an economic perspective alone, upgrading the system within such a short construction period, based on the existing control system, saved significant DCS investment and related manpower, material, and financial resources, and greatly reduced direct plant profit losses caused by prolonged shutdowns. 5. Conclusion The expansion of the DCS system for the acetic acid cracking and distillation production unit adopted RS3, DeltaV, and Tricon systems, all world-class control systems. Integrating these three systems fully leverages their respective advantages, saving substantial engineering investment and shortening shutdown time. This expansion, from design to successful commissioning, represents an innovative attempt at seamless integration of the old and new DCS control systems. The key to the project's success lay in a comprehensive and in-depth study and understanding of each system, a keen insight into the processes, and a firm grasp of critical milestones. A meticulous and feasible implementation plan, coupled with rigorous and scientific project management, ensured that every stage of the project was under control until its successful completion.