Riser temperature control

RISER TEMPERATURE CONTROL WITH STRIPPING STEAM... 

Fig. 35.13. Response of controlled reactor riser temperature, controlled stack gas oxygen concentration and uncontrolled regenerator temperature to step change in feed.

An example of a basic environmental test bed would be a temperature controlled tank of a liquid, perhaps sea water. This basic structure is applicable to a considerable range of products and can involve acceleration by using more severe conditions than in service. In one use of such a rig, the insulation on oil riser pipes is tested by circulating hot oil through the pipe whilst it is immersed in the tank. 

In current industrial practice, reactor (or riser) temperature is usually controlled by the flowrate of hot catalyst fed to the reactor from the regenerator. A slide valve in the... 

Until recently only a few papers were available on moving beds in cross flow [11-18]. This type of reactor is sometimes a favorable process solution for a selective catalytic process with a moderate catalyst rcsidence time and with a short gas residence time, especially when the process is accompanied by a continuous catalyst regeneration. The use of conventional short-contact-time reactors like fluidized-bed reactors, risers, and fixed-bed reactors does not always yield satisfactory results. This may be explained by problems connected with gas back-mixing, channeling of gas, low catalyst holdup, attrition of the solid catalyst, or difficulties in temperature control. 

One of the primary variables that an FCCU operator can use is riser temperature. Increasing riser temperature usually results in increased conversion, though not necessarily greater yields of gasoline. For units not on automatic reactor temperature control, riser temperature can be increased by ... 

Left to his own, the shift worker operating the FCCU control panel often chooses the simplest method to hold riser temperature constant. On many units the control panel operator has discovered that changing the... 

Most FCC units only have a few independent variables. Typically, these independent variables are the feed rate, feed preheat temperature, reactor/riser temperature, air flow rate to the regenerator, and catalyst activity. The feed rate and air flow rate to the regenerator are set by flow controllers. The feed temperature is set by the feed temperature controller. Catalyst activity is set by catalyst selection and fresh catalyst addition rate. Reactor temperature is controlled by the regenerator slide valve that regulates the catalyst circulation rate. The catalyst circulation rate is not directly measured or controlled. Instead, the unit relies on the heat balance to estimate the catalyst circulation rate. Except for these independent variables, other variables, such as regenerator temperature, degree of conversion, and carbon-on-catalyst, etc., will vary accordingly to keep the FCC unit in heat balance. These variables are dependent variables. 

Before storing the molds, they should cool down, so that no condensation occurs on the mold surface and corrosion is thus inhibited. The temperature control channels are to be kept closed in order to prevent exposure to oxygen. Another possibility is that the temperature control bore holes are completely removed from water before the deposition. Corrosion is not only a problem for the temperature control channels. Oil, as a temperature control medium, forms decomposition products that usually accumulate in the tank of the temperature control system. At some point, it gets into the mold and can clogg riser holes in the mold cores. It is therefore not only essential to replace the temperature control medium, but also to clean the heater tank. 

Figure 35.11 shows that the reactor riser temperature 7 can best be controlled by since the step response for a change in air flow and lSFrr,sp show a large response initially, followed by an inverse response. This is not dynamically favorable for control. 

Thus the ECCU always operates in complete heat balance at any desired hydrocarbon feed rate and reactor temperature this heat balance is achieved in units such as the one shown in Eigure 1 by varying the catalyst circulation rate. Catalyst flow is controlled by a sHde valve located in the catalyst transfer line from the regenerator to the reactor and in the catalyst return line from the reactor to the regenerator. In some older style units of the Exxon Model IV-type, where catalyst flow is controlled by pressure balance between the reactor and regenerator, the heat-balance control is more often achieved by changing the temperature of the hydrocarbon feed entering the riser. 

The flow rate of the regenerated catalyst to the riser is commonly regulated by either a slide or plug valve. The operation of a slide valve is similar to that of a variable orifice. Slide valve operation is often controlled by the reactor temperature. Its main function is to supply... 

Reaction control along riser axis for periodic operation or temperature profiling. 

After the secondary reformer of steam reforming plants the gas has to be brought down from around 1000 °C to about 350 °C for the HT shift. In earlier-generation plants two boilers were usually installed in series, with a bypass around the second to control the inlet temperature for the HTS. Common practice for a long time was to use a water-tube design. A famous example is the Kellogg bayonet-tube boiler, applied in more than 100 plants. Because of size limitations two parallel units were installed. For sufficient natural water circulation these boilers needed a steam drum at a rather high elevation and a considerable number of downcomers (feed water) and risers (steam/water mixture). 

The latest model is known as Exxon Flexicracker (Figure 22) (56). In this model, the U-tubes are replaced by a standpipe that is followed by upwardly sloped laterals, referred to as J-bends. This model takes advantage of riser cracking and spent catalyst in the J-tube is controlled by a slide valve. Reactor temperature is controlled by pressure differential between the regenerator and reactor. 

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