(723e) Modeling and Control of An Industrial Hydrocracker Using the Discrete and Continuous Lumping Methods and Model Predictive Control

Modeling and Control of an Industrial Hydrocracker
Using the Dicrete and Continuous Lumping Methods and Model Predictive Control

Hasan Sildir, Ummuhan Canan,Yaman Arkun.
Department of Chemical and Biological Engineering, Koc University, Istanbul
Turkey.

Berna Cakal, Dila Gokce, Emre Kuzu,TUPRAS
Refineries, Korfez-Kocaeli,Turkey

Hydrocracking is a catalytic chemical process
which converts high-boiling  heavy petroleum fractions such as vacuum gas oil
into lighter and more valuable products like naphta, diesel, kerosene,
gasoline, and LPG. Hydrocracking takes place in the presence of rich hydrogen
at elevated temperatures and pressures. The products are free of sulphur and
nitrogen compunds which are hydrogenated into hydrogen sulfide and ammonia and
which are subsequently removed. The aim of this work is to develop a model for
an industrial hydrocracking reactor for optimization and control purposes.  Our
research is centered around the hydrocraking unit (HCU) of TUPRAS refineries
which consists of four catalytic beds with interstage cooling by hydrogen
quench. The overall reaction is exothermic and tight control of bed
temperatures is crucial for achieving the optimal product distribution.

For modeling purposes the methods of discrete
and continuous lumping [1,2] are used. In continuous lumping the reaction
mixture is treated as a continuum in which the reaction rate constant k
is a continuous function of the true boiling point of the mixture. A yield
distribution function p(k,K) is introduced to formulate the amount of
species with reactivity k formed from cracking the species with
reactivity K. The existing HCU models of this type are steady state
models and they do not explicitly include the heat effects. Our continuous
lumping model is a pseudohomogeneous non-steady-state plug flow reactor model
which includes both the material and energy balances. As such the model is
original. In the case of discrete lumping the reaction mixture is characterized
in terms of pseudocomponents that are defined for the lumped species boiling in
a particular temperature range (i.e.cut) [1]. The two sets of lumping methods
are fundamentally different approaches, and their joint development provides
additional insight into uderstanding the behavior of HCU and arriving at a
final reactor model for optimization and control. Model parameters were
estimated using parameter estimation methods.  With optimally tuned parameter
values, the predicted reactor bed temperatures, hydrogen consumption,
conversion and product distributions match TUPRAS` actual plant data very
closely. This is demonstrated for different feedstosks in our training and
validation data sets.

The developed HCU model is used for two
purposes: 1) To compute the optimal product distribution and the optimal
reactor inlet temperature setpoint values. This economic optimization is
performed under steady state conditions. 2) The dynamic model is used by a
Model Predictive Controller (MPC) to control the product amounts at their
optimal set points.  When the product amounts (light naphta, heavy naphta,
diesel, kerosene and bottoms) deviate from their optimal values due to
feedstock changes, catalyst deactivation and other disturbances, MPC makes the
necessary adjustments in the reactor beds inlet temperature setpoints.
Temperature increase in each bed and the weighted average bed temperatures
(WABT) are the addional variables which are kept within limits by MPC to
maintain a desired level of conversion and uniform catalyst deactivation across
the beds.  Reactor inlet temperatures are changed to their new setpoints by the
regulatory PID loops which adjust the hydrogen quenches between beds. This
cascade arrangement of MPC and PIDs provides both optimizing and regulatory
actions as shown in the figure below.

In the presentation, modeling, optimization and
control simulations will be given and compared with the present status in the
plant.

1.
Mohanty, S.; Saraf, D. N.; Kunzru, D. Modeling of a Hydrocracking

Reactor.
Fuel Process. Technol. 29, 1, 1991.

2.
Chou M. Y.; Ho, T. C. Continuum Theory for Lumping Nonlinear

Reactions. AIChE J. 34, 1519, 1988.