Maximum allowable tube diameter

If we use this rule of thumb, we here obtain a value for Ar ax of 40 K, which also confirms that a diameter of 4.6 cm is small enough to avoid a runaway as we then have a value of ATted.max of only 26 K (Figure 6.11.21). An alternative, although very rough estimation of the maximum allowable tube diameter to avoid a temperature... 

The maximum allowable tube stress will need to be calculated in order to calculate the actual reformer tube life. This is accomplished by using the Mean Diameter Formula given below. 

Vapor tube diameter 150mm from reactor to condenser Gas tube diameter following the condenser 80 mm Set pressure of bursting disk 1 bar g Maximum allowed working pressure 3.2 bar g Test pressure 6 bar g... 

Estimate the heat transfer area, using the maximum allowable heat flux. Take as 39,700 W/m for vertical and 47,300 W/m for horizontal reboilers. Choose the tube diameters and length. Calculate the number of tubes required. Estimate the recirculation ratio, not less than 3. 

A second criterion for the safe operation of a cooled fixed-bed reactor is obtained from Equ.(4-136). If the demand is fulfilled that the true tube diameter must not exceed the critical diameter, then this allows a distinct maximal temperature difference between maximum and wall temperature only. Mathematically this is obtained by setting d equal to dc resulting in Ci = 1 and inserting this into Equ.(4-136). 

Figure 6.13.4 shows the strong influence of the diameter of the tubes on the maximum allowable cooling (= inlet) temperature. An internal diameter of more than 2.5 cm (standard value used in industrial reactors) would lead to a strong decrease of the maximum allowable gas inlet temperature with the result of a strong decrease in the o-xylene conversion for a tube length of 3 m (Figure 6.13.1). Conversely, smaller tubes (<2.5 cm) would also allow a safe operation for inlet temperatures of more than 400 °C but then the selectivity to phthalic anhydride would strongly decrease (Figure 6.13.1) and so also would the number of tubes needed (investment costs).

The calculated and experimental temperature distribution is shown in Fig. 18. The fit of the axial concentration profile was even better. A perfect fit cannot be expected because of the simplified reactor model and reaction model and the use of constant average parameter values throughout the whole reactor. On the other hand, the agreement between simulation and experiment seems to be sufficient for a study of the behavior of industrial multitube reactors with larger tube diameters. It turned out that for a maximum allowable entrance temperature of 640 K and a conversion larger than 95 %, as demanded by economic considerations, the tube diameter has to be limited to 10 cm, a result that is in excellent agreement with reports on technical units. 

Coils now exist in many sizes and maximum allowable depth ratings, from 1" to 3Vi" in diameter. Most common coiled-tubing strings used in acidizing and wellbore cleanouts are in the range of l%"-2" in diameter. 

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