Simulation And Optimization Of Absorption-Stabilization System Of Delayed Coking Unit

Coking process is a hot treating process using residual oil as raw material. It's the most important residual oil process which has the advantages of high adaptability to raw materials, well-developed technology and low investment. With the increasing popularity of heaviness and inferior quality of crude oil, the yield of residual oil is increasing year by year, but its quality is decreasing. Although all refineriy plants have carried out capacity expansion through transformation for coking equipments, most of them haven't upgraded correspongding absorption-stabilization system. That resulted in reduction of LPG (Liquefied petroleum gas) recovery, unstability of equipment operation, and inferior quality of products. The rich gas produced in coking equipment contained light hydrocarbons and 20 mol% C2, which can be recycled and used as raw material for ethylene pyrolysis. Maintaining absorption-stabilization system's stable operation and improving C2 and LPG recovery is significant to relieve the tension of naphtha and enhance absorption-stabilization system's competence.The original energy capacity of a refinery's coking equipment was 0.8 Mt·a-1. Due to the increase of crude oil thoughput and reduction of its qulality, the energy capacity was expanded to 1.6 Mt·a-1. This expansion resulted in more than 10 mol% light hydrocarbons in dry gas of absorption-stabilization system,1900 Nm3·h-1 non-condensable gas in stabilizer and flooding. Based on real data, this paper established the model of absorption-stabilization system by choosing approperiate property methods and unit operation blocks in UniSim Design. Some parameters'influences on energy consumption, products (dry gas, LPG, stabilized gasoline) and equipment operation were analyzed, such as feed composition of stabilizer, top and bottom temperatures of stripper, molar flow of supplementary absorbsent and temperature of absorber. The multivaribale and steady optimizer HoneywellSQP was adopted for overall optimization. While maintaining the equipment's steady operation, LPG recovery was increased from 51.0%to 90.2%, and annual economic profit was increased 87.5 million yuan.In order to recover C2 in rich gas, process flow using desorbed gas as raw material was designed. Due to small absorption selectivity, crude gasoline absorbed some C2, C1 and N2 while absorbing LPG. Thus, desorbed gas contained not only C2 and LPG but also some N2 and CH4. Since N2 and CH4 didn't react in ethylene pyrolysis furnace, in order to reduce energy consumption, their concentration shoule be lowered to below 5 mol%. Therefore, the process flows using condensed desorbed gas and non-condensable gas in stabilizer as raw materials were designed with recovery of C2 40.0% and 53.4% respectively. In order to further improve C2 recovery, process flow of absorption-stabilization system combined with organic steam membrane was designed which increased C2 recovery to 63.8%. Compared to original process flow, annual economic profits of these three cases were increased 153.54 million yuan,160.36million yuan and 163.18 million yuan respectively.