Analysis of Fired-Heater Fouling in a Refinery Coking Unit

Delayed Coking is a principal thermal cracking process utilized by petroleum refineries to decrease residual output. Fired heater performance is critical to coker unit profitability. Adversely affecting heater performance, coke deposits inside heater tubes must be removed periodically (decoked). There are three main methods of decoking fired heaters: online spalling, steam-air decoke, and pigging. For each decoking technique, the most widely-used procedures are described. Also, the merits and drawbacks of each method are discussed. A survey of industry decoking practice has been attempted as part of this study. Fire-side tube metal temperature (TMT) and heater inlet pressure (PIN) are the critical parameters used to analyze heater tube fouling. However, changes in flow rate, density, process temperature, and steam injection can cause a pass to have higher TMT or PIN than one with greater fouling. This study suggests approximation of coke deposit thickness to obtain a parameter normalized against changes in process variables. When used in conjunction with TMT and PIN, such a parameter can help in evaluating decoke effectiveness and identify the pass with least economically efficient operation. Economic analysis of fired-heater fouling considers direct costs and opportunity costs. Direct costs, such as plant utilities, fuel, in-house manpower, and outside contractor expense are assessed for each decoking method. This study also evaluates opportunity costs associated with reduction of throughput during decoking procedures. An example economic case is presented to illustrate the potential benefit of using estimated coke deposit thickness to schedule decokes.