Steady-state operation

Volumetric heat generation increases with temperature as a single or multiple S-shaped curves, whereas surface heat removal increases linearly. The shapes of these heat-generation curves and the slopes of the heat-removal lines depend on reaction kinetics, activation energies, reactant concentrations, flow rates, and the initial temperatures of reactants and coolants (70). The intersections of the heat-generation curves and heat-removal lines represent possible steady-state operations called stationary states (Fig. 15). Multiple stationary states are possible. Control is introduced to estabHsh the desired steady-state operation, produce products at targeted rates, and provide safe start-up and shutdown. Control methods can affect overall performance by their way of adjusting temperature and concentration variations and upsets, and by the closeness to which critical variables are operated near their limits. 

The characteristic separation curve can be deterrnined for any size separation device by sampling the feed, and coarse and fine streams during steady-state operation. A protocol for determining such selectivity functions has been pubHshed (4). This type of testing, when properly conducted, provides the relationships among d K, and a at operating conditions. These three parameters completely describe a size separation device and can be used to predict the size distribution of the fine and coarse streams. 

The economics of most processes are determined by the steady-state operating conditions. Excursions from these steady-state conditions generally average out and have an insignificant effec t on the economics of the process, except when the excursions lead to off-specification produc ts. In order to enhance the economic performance of a process, the steady-state operating conditions must be altered in a manner that leads to more efficient process operation. 

Figiire 8-38 ihust rates the typical spread of values of the controhed variable that might be expected to occur under steady-state operating conditions. The mean and root mean square (BMS) deviation are identified in Fig. 8-38 and can be computed from a series of n observations Cl, C9,. . . c as fohows ... 

For laboratory purposes it is sometimes convenient to recycle the foamate directly to the pool in a manner analogous to an equihbrium still. This eliminates the feed and bottoms streams and makes for a more reliable approach to steady-state operation. However, this recycling may not be advisable for colligena measurements in the presence of slowly dissociating collector micelles. 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

Steady-state operation is considered. In this case to satisfy conservation laws it will be assumed that the stream of a component that crosses a boundary inward, and does not come out, has been converted by chemical reaction. 

The material balance for steady-state operation of a perfectly mixed reactor is ... 

In the steady-state operation of the OXITOX reactor, pelletized solid of catalytieally aetivated sodium earbonate slides down a Silo type reaetor. Counter-current to the solid flow, the polluted air rises through the sliding bed of solids. At reaction temperature the following reaetion oecurs ... 

The steady-state operation of any reactor requires that the heat removed should equal the heat generated ... 

In the last part of Chapter 7.4 (Transient Studies) the experimental work on ethylene oxidation was shown. There the interest was to investigate what occurs and how fast, after a thermal runaway started. The previous chapter discussed the criteria of how to design reactors for steady-state operation so that runaways can be avoided. One more subject that needs discussion is what transient changes can cause thermal runaways. 

This program helps calculate the rate of methanol formation in mol/m s at any specified temperature, and at different hydrogen, carbon monoxide and methanol concentrations. This simulates the working of a perfectly mixed CSTR specified at discharge condition, which is the same as these conditions are inside the reactor at steady-state operation. Corresponding feed compositions and volumetric rates can be calculated from simple material balances. 

The first objective of the antisurge control system is to protect the compressor. This can be accomplished for some disturbances by using the PI algorithm with a large value of bj. However, it is also necessary to maximize the region in which the compressor can operate with the recycle valve closed. This increases the efficiency of the compressor at lower throughputs. Steady-state operation with recycle is extremely inefficient. Therefore, from this perspective, small values of bj are highly desirable. 

Historical data management—This includes the data acquisition and storage capabilities. Present-day prices of storage mediums have been dropping rapidly, and systems with 80 gigabyte hard disks are available. These disks could store a minimum of five years of one-minute data for most plants. One-minute data is adequate for most steady state operation, while start-ups and shutdowns or other non steady state operation should be monitored and stored at an interval of one second. To achieve these time rates, data for steady state operation can be obtained from most plant-wide D-CS systems, and for unsteady state conditions, data can be obtained from control systems. 

There is a steady state operation (i.e., no ehange with time in the system)... 

The design equations for a CFSTR with perfeet mixing, eonstant fluid density, and steady state operation are as follows. If u is the volumetrie flowrate and K = kj/kj, relative reaetion rate eonstant, where kj, kj, and kj are the speeifie reaetion rate eonstants for reaetions 5-357, 5-358, and 5-359. The rate expressions of A, B, R, S, and T are... 

If heat ean be removed as fast as it is generated by the reaetion, the reaetion ean be kept under eontrol. Under steady state operating eonditions, the heat transfer rate will equal the generation rate (see Figure 6-26). If the heat removal rate Qj. is less than the heat generation rate Qg (e.g., a eondition that may oeeur beeause of a eooling water pump failure), a temperature rise in the reaetor is experieneed. The net rate of heating of the reaetor eontent is the differenee between Equations 12-44 and 12-45. 

Designers must include all the tasks in their designs. Designs for steady state operation have been reported to lack start-up or shutdown capability. Some plant designs lacked the vents and drains required to empty, flush, and clear the equipment. 

Many HVAC system engineering problems focus on the operation and the control of the system. In many cases, the optimization of the system s control and operation is the objective of the simulation. Therefore, the appropriate modeling of the controllers and the selected control strategies are of crucial importance in the simulation. Once the system is correctly set up, the use of simulation tools is very helpful when dealing with such problems. Dynamic system operation is often approximated by series of quasi-steady-state operating conditions, provided that the time step of the simulation is large compared to the dynamic response time of the HVAC equipment. However, for dynamic systems and plant simulation and, most important, for the realistic simulation... 

Ching C. B., Ruthven D. M. (1985) An Experimental Study of a Simulated Counter-Current Adsorption System - I. Isothermal Steady State Operation, Chem. Eng. Sci. 40 877-885. 

This system requires direct steam injection into the still with the liquid, all the steam leaves overhead with the boiled-up vapors (no internal condensation) in a steady-state operation, and system at its dew point. Steam is assumed immiscible with the organics. Steam distillation is usually applied in systems of high boiling organics, or heat sensitive materials which require separation at vacuum conditions. 

At the flood point, liquid continues to flow down the column, but builds up at a greater rate from tray to tray. Sutherland [69] demonstrated that flooding moves up the column from the point of origin. For this reason it is important to design perforated trays without downcomers with extra care, as changing internal rates are quickly reflected in performance if the proper hole requirements are not met. They are a usefiil tray for steady state operations. 

Screw compressors have two rotors with interlocking lobes and act as positive-displacement compressors (see Figure 44.11). This type of compressor is designed for baseload, or steady state, operation and is subject to extreme instability should either the inlet or discharge conditions change. Two helical gears mounted on the outboard ends of the male and female shafts synchronize the two rotor lobes. 

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