(373au) Coping with Severe Feed Pressure Fluctuation in Ethylene Plant Using APC & RTO

Authors: 
Al-Khatib, A., SABIC
Butoyi, F., SABIC
Stable and continuous feed to a plant drive operational focus towards plant optimization. Yet, in this case study, the ethylene plant suffer from severe feed fluctuation, which causes plant operations to gear toward plant stabilization preventing feed loss upset. Relying on plant operators to stabilize the plant, feed maximize, and plant optimize is quite problematic; especially with varying range of experience. Consequently, losses in terms of feed maximization and ethylene yield could occur. To ensure these losses are captured, Advanced Process Control (APC) with Real Time Optimizer (RTO) were installed.

The ethylene plant in this case study encounter ethane feed fluctuation. The ethane feed from 18 barg to 6.5 barg, which is well below the plant design of nine barg. Once pressure reaches below nine barg, the plant capacity is reduced; accordingly, the operational strategy switches from plant optimization to maximize the feed in order to achieve the maximum plant capacity possible. Vice versa, once the pressure is recovered, the operational strategy is reversed. The dilemma is the reoccurrence of this scenario in frequent basis to the extent of reaching daily.

To minimize manual control, the ethylene plant has equipped with feed, furnaces, and cold sections APC applications, which on top of it is the RTO. The furnace APC is designed to push the feed to the limiting constraints in the furnace & feed system. Besides this, the furnace APC is included in the overall plant RTO program. When the plant-wide RTO runs, the furnace APC optimizes to the plant wide objective function. Using the dynamic models in RTO, the effects of changes in the furnace APC manipulated variables to cold sections downstream controlled variables can be predicted. Hence, if a downstream constraint is active, the RTO may make use of the furnace-manipulated variables to relieve the active downstream constraint.

With the operational strategy automated through APC and RTO, the plant demonstrated that it can maximize its feed intake ensuring plant optimizations are honored.