Coil outlet temperature

Instead of conversion, some producers prefer to use other identifications of severity, including coil outlet temperature, propylene to methane ratio, propylene to ethylene ratio, or cracking severity index (33). Of course, all these definitions are somewhat dependent on feed properties, and most also depend on the operating conditions. 

An ethylene plant contains more than 300 equipment items. Traditionally, operators were trained at the site alongside experienced co-workers. With the advent of modem computers, the plant operation can be simulated on a real-time basis, and the results displayed on monitors (107). Computers are used in a modem plant to control the entire operation, eg, they are used to control the heaters and the recovery section (108). A weU-controUed plant is much more profitable than a poorly controlled plant. For the heaters, a model-based control system is gaining importance (109). Instead of simply controlling the coil outlet temperature (COT), severity is actually controlled. The measurement of severity (either or C H /CH ratio) requires on-line effluent... 

Heating coil outlet temperature. An increase from 910° to 925° F. gave a 12% increase in the yield of lump coke. 

Commercial operations are typically once-through at coil-outlet temperatures of 1050-1100°F. and pressures of 350 to 450 p.s.i.g., using 2 to 3 lb. of catalyst per barrel of feed. When processing heavy stocks, naphtha may be included in the feed to ensure complete vaporization, said to be essential for best results. The catalyst employed at Sarnia consists of natural and activated clay that has been previously used for contact treatment of lubricating oils. The catalyst is filtered from the liquid fuel oil, or tar, produced in the process and is normally discarded. Use of the catalyst permits continuous operations to be carried out at much more severe cracking conditions than could be otherwise employed. Erosion is... 

Likewise, Table XII reports a comparison of delayed coking models with industrial plant data for five different feedstocks covering a large range of different gasoil recycle ratios. The coil outlet temperature is in the range of 490°C and coil outlet pressure is about 5 ata. The results obtained confirm the reliability of this model. 

Several recommendations were made, including (a) avoid starting up under full vacuum (6) pump the tower out whenever temperature falls below 300°F during an interruption (c) monitor coil outlet temperatures (d) take steps to prevent water accumulation in heater coils. 

Slowly raise the coil outlet temperature to FIOO F (about 200°F/hr). 

Rapidly drop the coil outlet temperature to 850 F and keep increasing the steam flow to keep the coil inlet pressure constant. 

The maximum allowable superheat coil outlet temperature is 700°F to protect the carbon steel tubes from failure. At low steam generation rates, I have seen furnace firing rates and thus heater charge rates reduced to keep from overheating the steam above 700°F. If the steam is not used eventually to drive turbines, or as a reactant in a catalytic process (steam-methane reformer for or NHj production), then the heater capacity is being limited for no logical reason. 

The closed, high-purity loop which rejects heat to river water is designed for a maximum heat load of (i Mw. One pump of 900-gpm capacity provides ( irculation for the closed water loop. Flow control valves proportion the flow to the various panels such that panel coil outlet temperatures arc... 

Example 19-5. Material Balance and Crack per Pass. A cracking plant such as that shown in Fig. 19-7 is to operate at a coil outlet temperature of 915 F ann. > 400 psig. The topped crude oil feed of 24 API has a Characterization Factor 01 I1., and it is assumed that 80 per cent of 27.0 API gas oil can be distilled from the topped... 

In order to adequately describe the size of a heater, the heat duty, the size of the fire tubes, the coil diameters and wall thicknesses, and the cor lengths must be specified. To determine the heat duty required, the maximum amounts of gas, water, and oil or condensate expected in the heater and the pressures and temperatures of the heater inlet and outlet must be known. Since the purpose of the heater is to prevent hydrates from forming downstream of the heater, the outlet temperature will depend on the hydrate formation temperature of the gas. The coil size of a heater depeiuLs on the volume of fluid flowing through the coil and the required heat duty. 

Water at 293 K is heated by passing through a 6.1 m coil of 25 mm internal diameter pipe. The thermal conductivity of the pipe wall is 20 W/m K and the wall thickness is 3.2 mm. The coil is heated by condensing steam at 373 K for which the film coefficient is 8 kW/m2 K. When the water velocity in the pipe is I tn/s, ils outlet temperature is 309 K. What will the outlet temperature be if the velocity is increased to 1.3 m/s, if the coefficient of heat transfer to the water in the tube is proportional to the velocity raised to the 0.8 power ... 

The temperature difference between inlet and outlet temperature at the coil(s) of the refrigerant should be smaller than 1 °C (AT < 1 °C), to ensure a uniform condensation on the total coil. On warmer areas no ice will condense until the temperature at the ice surface has increased to the warmer temperature on the coil. For large surfaces it is necessary to use several coils or plates in parallel, each of which must be separately temperature controlled. If the condenser is operated in an overflow mode, the weight of the liquid column should not change the boiling temperature of the liquid at the bottom of the column measurably. 

Location of air inlet Reactor outlet temperature Cross-sectional area Length of bed to cooling coils Diflfusivity of H2, temp. = 100°F... 

The design specification for the absorber requires that the tail gas leaves the absorber at 1 0°C, make-up water to be supplied at 7°C to the top plate, and inclusion of two independent sets of cooling coils in the column. The first set of coils runs in the top half of the column. The inlet temperature for these coils is 7°C and the design exit temperature is 20°C. The second set of coils occupy the bottom half of the column using cooling water from the normal cooling-water circuit. Therefore, the inlet temperature is 20°C, and the outlet temperature is 40°C. 

In the simulation, feedrate, steam ratio, and inlet temperature were maintained constant. The pressure and feed conversion at the coil outlet were specified. The inlet pressure and firebox temperature profile were adjusted to meet the two specifications. A uniform temperature profile... 

Feed conversion Coil outlet pressure Peak tubeskin temperature Residence time... 

The feedstock cannot be raised to the reaction temperature instantaneously in a furnace tube. The temperamre varies along the tube according to a certain profile. Figure 15 offers an illustration for three outlet temperatures (805, 815 and 825 Q The figures on the x-axis do not directly represent the length of the tube but the number of coils (vcr-... 

The jacket temperature, Tj, in Equation 7.4.8, equals the average of the jacket inlet and outlet temperatures. For a coil also use the average of the inlet and outlet temperatures. First, determine if there is sufficient heat-transfer area by assuming a simple jacket. The area of fire jacket is given in Table 7.3. The area will be about the same for simple, pipe coil, and dimple jackets. If the jacket area is insufficient, then determine if coils will provide the additional surface area. The reactor volume should be compensated for the volume occupied by the coils. 

Reactors and Catalysts. The reactor configuration consists of two reactor trains of two reactors in series each with two beds (8). Since the reactors are adiabatic, each bed temperature, except the first bed, is controlled by recycle hydrogen gas. However, as shown in Figure 1, the bed temperatures increase with reactor depth. A maximum reactor inlet temperature is set to avoid coking in the furnace coil and the first bed. A maximum reactor outlet temperature is limited by reactor shell metallurgy. The hydrodesulfurization catalyst, which occupies about 70% of a total catalyst volume, is packed in a lower half of the second bed and the whole third and fourth beds. The rest is the hydrodemetallation catalyst. We have used Orient Catalyst HOP-802 as the hydrodesulfurization catalyst, which contains about 2% Ni and 8% Mo on an alumina support. 

Example Results. A demonstration of the fit of the model to commercial data is shown in Table II. The last column compares recent commercial test data for ethane-propane against model predictions. The close prediction of ethylene, ethane, and propylene yields provides confidence in the cocracking synergism mechanism built into the model. Figure 3 compares predicted and observed process-operating profiles for the case of the n-butane reactor. The firing level in the reactor simulation was adjusted to meet the observed conversion at the coil outlet. Agreement in the process, tubeskin, and firebox temperatures is well within the precision of the data. 

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