| OCR Text |
Show q2 = c m m 2 / m i ) (2) where the process fluid temperature T corresponds to the cumulative absorbed duty qj (q2) and process mass flow rate m j (m2). This expression reflects conservation of energy in that when the process mass flow rate is increased, (m2/mi > 1) a larger amount of energy, q2, must be absorbed to bring the process temperature up to the same level, T. Simulations to test the feasibility of using flow rate modulation had to abide by the constraints of the expected control system for the overall fuel firing rate and the process mass flow rates. The overall fuel firing rate is governed by the average of the 12 coil outlet temperatures, and the mass flow rates to adjacent coil pairs is varied depending on the pair-wise averages of the COTs. The objective of this system is to obtain: i + l 12 l-^COT(n) = l-^COT(n) (3) 2^ v ' 12 n = i n = 1 for i= 1,3,5,7,9, and 11 To demonstrate that the statement of Eq. 3 is achievable using the control system described above, a simulation was performed using model inputs for the normal firing condition. The level of flow rate modulation predicted prior to the simulation was based on the energy transferred to each pair of coils in the baseline simulation of normal firing conditions. Based on this information and application of Eq. 2, a prediction of the percentage decrease or increase in the mass flow rate was made and input to the model. The total process flow rate was not changed. Flow rate modulation to adjacent coil pairs did not exceed 1.3% of the baseline mass flow rates. The mass flow rate to coils 1 and 2 was decreased, that to coils 3 and 4 was increased, and that to coils 5 and 6 was decreased. Figure 9 shows the predicted pairwise average COTs from the simulation employing flow pass balancing compared with those obtained in the baseline simulation in which no flow pass balancing was used. This simulation clearly indicated that improved uniformity in C O T s would be achieved using small levels of flow pass balancing. 3.2 Case Study #2 - Decoking Operations Thermal cracking of hydrocarbons is always accompanied by coke formation. During normal operation, coke slowly deposits on the inside walls of the process coils, leading to reduced heat transfer to the process fluid and increased pressure drop, and eventually plugged coils in the most extreme case. Depending on the feedstock, the severity of cracking, and the type of furnace, the run length can vary from days to months. To bum off the coke from the tube walls, the furnace has to be periodically shut down to perform decoking operations. Typically, a combination of high temperature air and steam is passed through the process coils to both blast the coke off the walls and partially oxidize it inside the coils. The unburned coke and gaseous products can then either be collected for disposal or rerouted back into the firebox for subsequent burnout. Simulations discussed in this section relate to modeling conditions inside the firebox during the latter type of decoke operation. 9 |
