Jul-2020

Model predictive control in a lube oil complex

An application of model predictive control targeted optimum performance from a refinery’s propane deasphalting unit.

HANDAN ÇEVIK S¸ANLI and BERKAY ER
TupraÅŸ

Article Summary

Model predictive control (MPC) refers to a class of computer control algorithms that utilise an explicit process model to predict the future response of a plant. At each control interval, an MPC algorithm attempts to optimise future plant behaviour over a time interval known as the prediction horizon by computing a sequence of future manipulated variable adjustments. It was originally developed to meet the specialised control needs of petroleum refineries and power plants. MPC technology can now be found in a wide variety of application areas including chemicals, automotive, food processing, and aerospace applications.¹

Turkish Petroleum Refineries Corporation (Tupras) is the largest industrial corporation in Turkey with a refining capacity of 30 million t/y crude oil in four refineries. Ä°zmir refinery is the largest refinery in terms of annual processing capacity, with 11.9 million tonnes of crude oil.

Advanced process control (APC) projects began at Tupras in 2006 with a third generation MPC technology, the Shell Multivariable Optimizing Controller (SMOC) algorithm, to create a sustainable and developable process control approach in the refinery. APC applications were commissioned on multiple units at Ä°zmir refinery. With technological advances in MPC algorithms, Tupras decided to continue with fourth generation MPC technology, Robust Multivariable Predictive Control Technology (RMPCT), in 2017. RMPC was supplied by Honeywell, and PCT was supplied by Profimatics. Both are third generation MPC technology. When Profimatics was purchased by Honeywell, the two algorithms were merged and RMPCT was born. The commercial name for RMPCT is Profit Controller. In this article, an application of Profit Controller on a propane deasphalting unit in the lube oil complex of Ä°zmir refinery is presented. A process overview and base layer control strategies are presented, followed by discussion of controller scope, variables, inferentials, response testing, and dynamic modelling. The control algorithm, economic variables, commissioning and tuning, and results are discussed in the context of controller commissioning.  

Process overview
A propane deasphalting unit located in Ä°zmir refinery’s lube oil complex has the main task of producing high quality bright stock by removing asphaltic hydrocarbons from vacuum residue.

Extraction column section
Vacuum residue from the lubes vacuum unit is pumped to the propane deasphalting unit as feed (see Figure 1). The unit input is controlled by FC-1. Prior to entering the cooler exchanger, a small stream of propane is added to the feed. The propane is a viscosity cutter to permit effective heat transfer and avoid plugging of the exchanger. The mixed stream flows through the exchanger to the middle of the rotating disc contactor (RDC) column. The feed temperature is controlled by TC-1, which manipulates the opening of the exchanger bypass.

Propane solvent is pumped from the propane receiver drum. Some solvent is used to dilute the feed while the main part of it goes to the solvent cooler where it is brought to the desired temperature by TC-2. The flow controller FC-2 determines the amount of solvent for dilution, and solvent entering the bottom part of the RDC is determined by FC-3. Solvent entering the bottom part of the column flows through the bottom section of the RDC column. As this happens, the oil fractions of the residue are dissolved, leaving heavy asphalt to settle at the bottom while the oil is carried in solution through the top of the RDC column. Deasphalted oil leaves the top of the RDC column with the bulk of the solvent and flows to the deasphalted oil solvent recovery system. Asphalt mixture from the bottom of the RDC flows to the asphalt recovery section.

Deasphalted oil recovery section
The deasphalted oil mix firstly flows to the low pressure steam evaporator, where part of the solvent is vaporised before flowing to the flash tower T-1 (see Figure 2). The inlet temperature of T-1 is controlled by TC-4. Hot vapour from the flash is condensed in the heat exchanger. The condensed solvent flows to the high pressure propane receiver drum. The deasphalted oil mix from T-1 flows to the second effect deasphalted oil flash tower T-2, where the vaporised solvent separates from the liquid. This liquid then flows to the steam reboiler,  where still more solvent is evaporated. The deasphalted oil mix from T-2 flows to the deasphalted oil stripper T-3, where superheated steam removes the last traces of solvent from the oil. Deasphalted oil from the bottom of the stripper is pumped to the storage section.

Asphalt recovery section
Asphalt mix from the bottom of the RDC column flows to the asphalt mix furnace, where most of the solvent is vaporised. The furnace outlet temperature is controlled by TC-5, which resets fuel pressure controller PC-2. The partially vaporised asphalt mix then flows to asphalt tower T-4. Asphalt mix from the bottom of the flash flows to the asphalt stripper T-5, where it is stripped of residual solvent with superheated steam. The stripped asphalt is pumped from the bottom of the stripper to the storage section.
 

Base layer control issues
Before implementation of the profit controller, a base layer control study including instrumentation checks, PID tuning optimisations, and required process control configuration changes was completed.

The base layer control scheme of the RDC column is shown in Figure 4. Critical control issues for the RDC column are discussed in the following.

Dilution to feed ratio
A small stream of the propane solvent is added to the feed prior to entering feed exchanger 1. This propane is a viscosity cutter to permit effective heat transfer in the exchanger and to avoid plugging of the exchanger. If the feed throughput has been changed then the amount of this flow should proportionally change. Too small a value can lead to plugging and too large a value increases energy consumption. RIC-1 ratio controller is a solution for this control issue; it calculates the set point for dilution flow FC-2 such that desired dilution to feed ratio is achieved. The time filter constant for the ratio controller is configured in such a way that a change in the feed flow does not affect solvent flow too quickly. This also avoids disturbances in the propane vessel.

RDC bottom interface level
According to the initial design of level control for the RDC column, the operator adjusts the level by manipulating bottom flow. This could lead to unsafe operation if the operator does not act on a level alarm; it also disturbs the furnace outlet temperature. A solution to this problem is a base layer control change; the level controller is configured as a master controller, and the bottom flow controller is configured as a manipulated slave controller. Then the level controller is tuned slowly.