Optimizing steam equipment

Rotating equipment used in the process industry includes pumps, compressors, motors and turbines. For pumps and compressors with fixed speed motors, minimizing spill backs could result in significant power savings. Optimizing steam turbine operation and maintenance could save steam. The selection of process drivers, namely, motors versus steam turbines, could save energy costs. 

Although the details of establishing steam balances are discussed in Chapter 16 while steam equipment modeling equations are given in Chapter 15, the intention here is to combine all the related equations to develop a complete simulation and optimization model for the steam system. For specific applications, users can revise the model by providing specific configuration and conditions for the steam system. 

The efficiency of the Rankine cycle itself can be increased by higher motive steam pressures and superheat temperatures, and lower surface condenser pressures in addition to rotating equipment selection. These parameters are generally optimized on the basis of materials of constmction as well as equipment sizes. Typical high pressure steam system conditions are in excess of 10,350 kPa (1500 psi) and 510 °C. 

While process design and equipment specification are usually performed prior to the implementation of the process, optimization of operating conditions is carried out monthly, weekly, daily, hourly, or even eveiy minute. Optimization of plant operations determines the set points for each unit at the temperatures, pressures, and flow rates that are the best in some sense. For example, the selection of the percentage of excess air in a process heater is quite critical and involves a balance on the fuel-air ratio to assure complete combustion and at the same time make the maximum use of the Heating potential of the fuel. Typical day-to-day optimization in a plant minimizes steam consumption or cooling water consumption, optimizes the reflux ratio in a distillation column, or allocates raw materials on an economic basis [Latour, Hydro Proc., 58(6), 73, 1979, and Hydro. Proc., 58(7), 219, 1979]. 

Once the highest steam level is set, then intermediate levels must be established. This involves having certain turbines exhaust at intermediate pressures required of lower pressure steam users. These decisions and balances should be done by in-house or contractor personnel having extensive utility experience. People experienced in this work can perform the balances more expeditiously than people with primarily process experience. Utility specialists are experienced in working with boiler manufacturers on the one hand and turbine manufacturers on the other. They have the contacts as well as knowledge of standard procedures and equipment size plateaus to provide commercially workable and optimum systems. At least one company uses a linear program as an aid in steam system optimization. 

It is important to have the correct set of variables specified as independent and dependent to meet the modeling objectives. For monitoring objectives observed conditions, including the aforementioned independent variables (FICs, TICs, etc.) and many of the "normally" (for simulation and optimization cases) dependent variables (FIs, TIs, etc.) are specified as independent, while numerous equipment performance parameters are specified as dependent. These equipment performance parameters include heat exchanger heat transfer coefficients, heterogeneous catalyst "activities" (representing the relative number of active sites), distillation column efficiencies, and similar parameters for compressors, gas and steam turbines, resistance-to-flow parameters (indicated by pressure drops), as well as many others. These equipment performance parameters are independent in simulation and optimization model executions. 

Besides two different hydrolysis methods (i.e., acid hydrolysis and pretreatment without the addition of acid), two different pieces of pretreatment equipment were used to perform the experiments (Fig. 4). Acid hydrolysis was conducted in a microwave oven, while pretreatment was performed in a steam pretreatment unit. The microwave oven provides a closed system where the amount of water added is fixed and there is no loss of material during the process (17-18). On the other hand, the sampleshave to be rather diluted for the microwave oven to be efficient. Another disadvantage is that the microwaves penetrate the material only a few center-meters, and therefore this method is not feasible on a large scale. The microwave oven may, however, still be of interest in the laboratory as a screening method to analyze the composition of feedstock as well as to determine a range of optimal conditions for steam pretreatment. 

Application Production of ammonia from natural gas, LNG, LPG or naphtha. The process uses conventional steam reforming synthesis gas generation in the front-end, while the synthesis section comprises a once-through section followed by a synthesis loop. It is thus optimized with respect to enable ammonia plants to produce very large capacities with proven equipment. The first plant based on this process will be the SAFCO IV ammonia plant in Al-Jubail, Saudi Arabia, which is currently under construction. This concept provides the basis for even larger plants (4,000-5,000 mtpd). 

The furnaces obviously constitute the essential equipment of die hot section of the steam-cracking process, and they condition the satisfactory running of the overall installation. However, their optimal operating conditions and the netibility of the process to operating parameters can often only be evaluated on the ccmpletion of full-scale experiments on a pilot furnace. 

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