Process plant performance

A process unit in a chemical process plant performs some operation on the inlet material streams to produce the desired outlet streams. In the design of such a unit the design calculations model the operation of the unit. A process unit and the design equations... 

Madron, F. (1992). Process Plant Performance. Measurement and Data Processing for Optimisation and Retrofits. Ellis Horwood, Chichester, England. 

The economic treatment discussed so far is hmited to analytical laboratories where samples are received from an outside source it will not apply to laboratories attached to processing plants performing quahty-control analyses. The cost of the automatic equipment, in these cases, wiU be small in relation to the plant cost, and it wiU be the improved precision of analysis and speed of response that wiU have the greatest economic significance. Automatic analysers in production hnes are ideal for quality control, and there is ample scope for additional automation. However, this is an area where the... 

Madron, F. Process plant performance. Measurement and data processing for optimisation and retrofits , Ellis Horwood (1992) (Section 3.2.4)... 

For the required desorption of CO2 from the carbonated sorbent, an additional process unit is necessary for optimal overall SE-SMR process plant performance. The main purpose with the SE-SMR process development which was to reduce the emission of CO2 to the atmosphere, was achieved. However, the desorption apparatus increase the total costs and the energy consumption of the process plant. On the other hand, the heat released by the adsorption of CO2 may compensate for some of the energy consumed in the desorption process. 

The UOP Sarex process has been used since 1978 for the separation of high purity fmctose from a mixture of fmctose, glucose, and polysaccharides (87,88). The pilot-plant performance of fmctose—glucose separation is given in Table 6. 

The basic seed processing plant design is based on 70% removal of the sulfur contained in the coal used (Montana Rosebud), which satisfies NSPS requirements. Virtually complete sulfur removal appears to be feasible and can be considered as a design alternative to minimize potential corrosion problems related to sulfur in the gas. The estimated reduction in plant performance for complete removal is on the order of 1/4 percentage point. The size of the seed processing plant would have to be increased by roughly 40% but the corresponding additional cost appears tolerable. The constmction time for the 500 MW plant is estimated to be ca five years. 

Defining the requirements for a pilot-plant control system is often difficult because process plant experience for comparison and evaluation is commonly lacking and the design is frequentiy performed by personnel inexperienced in either instmmentation systems or pilot-plant operations. The isolated and often intermittent nature of pilot-plant operations also inhibits evolution and promotes individual unique installations. This compHcates the selection process. 

Adaptive Control. An adaptive control strategy is one in which the controller characteristics, ie, the algorithm or the control parameters within it, are automatically adjusted for changes in the dynamic characteristics of the process itself (34). The incentives for an adaptive control strategy generally arise from two factors common in many process plants (/) the process and portions thereof are really nonlinear and (2) the process state, environment, and equipment s performance all vary over time. Because of these factors, the process gain and process time constants vary with process conditions, eg, flow rates and temperatures, and over time. Often such variations do not cause an unacceptable problem. In some instances, however, these variations do cause deterioration in control performance, and the controllers need to be retuned for the different conditions. 

Flow-sheet models are used at all stages in the life cycle of a process plant during process development, for process design and retrofits, and for plant operations. Input to the model consists of information normally contained in the process flow sheet. Output from the model is a complete representation of the performance of the plant, including the composition, flow, and properties of all intermediate and product streams and the performance of the process units. 

More microprocessor-based process equipment, such as smart instruments and single-loop controllers, with digital communications capability are now becoming available and are used extensively in process plants. A fieldbus, which is a low-cost protocol, is necessary to perform efficient communication between the DCS and these devices. So-called mini-MAP architec ture was developed to satisfy process control and instrumentation requirements while incorporating existing ISA standards. It is intended to improve access time while... 

Some plants have been using computer control for 20 years. Control systems in industrial use typically consist of individual feedback and feedforward loops. Horst and Enochs [Engineering h- Mining]., 181(6), 69-171 (1980)] reported that installation of single-variable automatic controls improved performance of 20 mineral processing plants by 2 to 10 percent. But interactions among the processes make it difficult for independent controllers to control the circuit optimally. 

The design and layout of the physical facihties that make up the processing-plant flow sheet are an important aspect in the implementation and successful operation of such systems. Important factors that must be considered in the design and layout of such systems include (1) process performance efficiency, (2) rehability and flexibihty, (3) ease and economy of operation, (4) aesthetics, and (5) environmental controls. 

Process Familiarization The analysts first step in preparation for analyzing plant performance is to become completely familiar with the process. Analysts should review ... 

Unit layout as installed is the next step of preparation. This may take some effort if analysts have not been involvea with the unit prior to the plant-performance analysis. The equipment in the plant should correspond to that shown on the PFDs and P IDs. Wmere differences are found, analysts must seek explanations. While a hne-by-line trace is not required, details of the equipment installation and condition must be understood. It is particularly useful to correlate the sample and measurement locations and the bypasses shown on the P IDs to those ac tuaUy piped in the unit. Gas vents and liquid (particularly water-phase) discharges may have been added to the unit based on operating experience out not shown on the P IDs. While these flows may ultimately be small within the context of plant-performance an ysis, they may have sufficient impact to alter conclusions regarding trace component flows, particularly those that have a tendency to build in a process. 

It is not unusual for 30 0 percent of the process design effort to be spent in developing a new database. The amount of time required at this stage in the analysis of plant performance for analysis of the unit... 

Intended Use The intended use of the model sets the sophistication required. Relational models are adequate for control within narrow bands of setpoints. Physical models are reqiiired for fault detection and design. Even when relational models are used, they are frequently developed bv repeated simulations using physical models. Further, artificial neural-network models used in analysis of plant performance including gross error detection are in their infancy. Readers are referred to the work of Himmelblau for these developments. [For example, see Terry and Himmelblau (1993) cited in the reference list.] Process simulators are in wide use and readily available to engineers. Consequently, the emphasis of this section is to develop a pre-liminaiy physical model representing the unit. 

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