Steam feed change

Table VI. Steam Feed Change,, Final Steady State Results ...

From Figure 4, M is independent of the nature of the feed. This is as expected — the maximum amount of coke that can be deposited would be expected to be a function of the nature of the catalyst alone, not of the type of feed. Earlier, we have shown [7] that increasing the acidity of the zeolite catalyst, or increasing the severity of steaming, can change the values of M appreciably. 

Table VII. Steam Feed Rate Change -2.5%, Limit Cycle Results...

Table XV provides a summary of the coal moisture, steam feed rate and exit gas pressure transient response runs showing the time required to reach the given condition. It provides a rough estimate for the values of the bifurcation points for these runs. Thus, the bifurcation point for the coal moisture step change runs lies between 30.08 and 31.05 wt % moisture. For the steam feed rate changes, it lies between -2.25% and -2.50%, and for the exit gas pressure, it is bracketed by the 3.14 MPa (31 atm) and 3.24 MPa (32 atm) values.

Changing the exit gas pressure also gave three multiple steady state responses in which the same set of operating parameters produced different reactor profiles. Finally, a rough estimate for the location of the bifurcation points was given for the coal moisture, steam feed rate, and exit gas pressure transient response runs. 

The model is used to compute the change of methane conversion at the exit from the reformer tube at different partial pressures of steam in feed to determine the optimum steam feed partial pressure as shown in Figure 3.10. This optimum value is found at a steam partial... 

Example 19.4 shows that the reboiler operation requires that the pressure of the boil-up stream be increased. This is not the only possible alternative. All that is necessary is that the temperature difference be 21.7°C. This could be accomplished by reducing the temperature of the steam to 163.4°C. In most chemical plants, steam is available at discrete pressures, and the steam used here is typical of medium-pressure steam. Low-pressure steam would be at too low a tenperature to work in the scaled-down column. However, the pressure and temperature of medium-pressure steam could be reduced if there were a throtding valve in the steam feed line to reduce the steam pressure. The resulting steam would be superheated, so desuperheating would also be necessary. This could be acconplished by spraying water into the superheated steam. A change in a utility stream is almost always preferred to a change in a process stream. The condenser is now analyzed in Examle 19.5. 

It is possible to analyze various situations in which disturbance variables change. We could, for example, analyze the change in bottom composition as a result of a change in feed flow. If it can be assumed that there are no changes in feed and steam temperature and bottom and distillate flow, how would the bottom composition xb respond to feed changes To analyze this, the model will be linearized with the condition that STsteam = = 0. 

For best operation, the feed rate to rotating equipment should be closely controlled and uniform in quantity ana quality. Because sohds temperatures are difficult to measure and changes slowly detected, most rotating-equipment operations are controlled by indirect means. Inlet and exit gas temperatures are measured and controlled on direct-heat units such as direct dryers and kilns, steam temperature and pressure and exit-gas temperature and humidity are controlled on steam-tube units, and direct shell temperature measurements are taken on indirect calciners. Product temperature measurements are taken for secondaiy control purposes only in most instances. 

Among the measures which have successfully prevented metal dusting are the use of additives (steam, and compounds of S, As, Sb, and P) in the feed, reduction of pressure, reduction of temperature, and material change. The most common additives are sulfur compounds and steam. Susceptibility can be reduced by using a material in which the total percent of Cr plus two times the percent of Si is in excess of 22 percent. In some environments, a. small amount of a sulfur compound will stop the dusting. When sulfur compounds cannot be tolerated in the process stream, a combination of steam and an alloy with a Cr equivalent of over 22 percent may be most desirable. 

The fractionator shell itself should often have some extra trays. Conventional instrumentation alone cannot always be expected to handle all the things that can happen to a fractionation system, such as changes in feed composition, reboiler steam pressure, or coolant temperature (especially for an air condenser during a sudden cold front). Experience for a given service is the best guide for extra trays. 

For larger boiler plants suddenly placed on high-fire and full-load conditions, the amine feed may need to be increased even where no increase in MU water is necessary. And for those steam generators using high-purity FW, particularly fine operational control may be required because maintenance of the correct pH within a very narrow range is usually a critical requirement, irrespective of temperature changes. 

It is decided to bleed off 0.25 kg/s of vapour from the vapour line to the second effect for use in another process. If the feed is still heated to the boiling-point of the first effect by external means, what will be the change in the steam consumption of the evaporator unit ... 

Most disturbances in chemical engineering systems are load disturbances, such as changes in throughput, feed composition, supply steam pressure, cooling water temperature, etc. The feedback controller s Auction when a load disturbance occurs is to return the controlled variable to its set-point by suitable changes in the manipulated variable. The dosedloop response to a load disturbance is called the regulator response or the closed-loop load response. 

We want to use a feedforward controller to make adjustments in steam flow to the reboiler, whenever the feed rate to the column changes, so that bottoms composition is held constant. The feedforward-controller design equation [Eq. (11.28)] gives... 

Figure 11.5h shows a combined feedforward-feedback system where the feedback signal is added to the feedforward signal in a summing device. Figure 11.Sc shows another combined system where the feedback signal is used to change the feedforward controller gain in the ratio device. Figure 11.6 shows a combined feedforward-feedback control system for a distiltetion column where feed-rate disturbances are detected and both steam flow and reflux flow arc changed to hold both overhead and bottoms compositions constant. Two feedforward controllers are required.

Modify your feedforward controller design of Prob. 11.10 so that it can handle both feed temperature and feed flow rale changes and uses a feedback temperature controller to trim up the steam flow. 

A plant operator makes changes from time to time in various input variables such as feed flow rate, steam flow rate, and reflux flow rate from one operating level to a new level. These changes can be either instantaneous or gradual, as sketched in Fig. 14,7. These step-response data are often easily obtained by merely recording the variables of interest for a few hours or days of plant operation. 

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