Data processing: generic control systems or specific application – Specific application – apparatus or process – Chemical process control or monitoring system
Reexamination Certificate
1998-09-17
2001-07-31
Grant, William (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Chemical process control or monitoring system
C700S266000, C700S268000, C422S051000
Reexamination Certificate
active
06269286
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to gasification, and more particularly to a system and method for integrated gasification control.
II. Related Art
Gasification is among the cleanest and most efficient technologies for the production of power, chemicals and industrial gases from hydrocarbon feedstocks, such as coal, heavy oil, and petroleum coke. Simply stated, gasification converts hydrocarbon feedstocks into clean synthesis gas, or syngas, composed primarily of hydrogen (H
2
) and carbon monoxide (CO). In a gasification plant, the feedstock is mixed with oxygen (O
2
) and they are injected into a gasifier. Inside the gasifier, the feedstock and the O
2
are subjected to a high-temperature and a high-pressure. As a result, the feedstock and the O
2
break down into syngas.
In addition to H
2
and CO, the syngas contains other gases in small quantities, such as ammonia, methane and hydrogen sulfide (H
2
S). As much as 99% or more of the H
2
S present in the syngas can be recovered and converted to elemental sulfur form and used in the fertilizer or chemical industry. Ash and any metals are removed in a slag-like state, and the syngas is cleansed of particulates. The clean syngas is then used for generating electricity and producing industrial chemicals and gases.
Gasification allows refineries to self-generate power and produce additional products. Thus, gasification offers greater efficiencies, energy savings, and a cleaner environment. For example, a gasification plant at a refinery in El Dorado, Kans. converts petroleum coke and refinery wastes into electricity and steam, making the refinery entirely self-sufficient for its energy needs and significantly reducing waste and coke handling costs. For these reasons, gasification has increasingly become popular among refiners worldwide. Currently, there are several hundred gasification plants in operation worldwide.
The operation of the gasification plant requires various control systems to control the gasifier and other equipments connected thereto. Currently, gasification plants utilize independent controllers, for example, proportional integral derivative (PID) controllers, to independently control various processes in the gasification plant. The independent controllers operate separately and do not interact with each other. As a consequence, the desired setpoint at each controller must be entered separately. Unfortunately, independent controllers often provide poor response, which results in increased wear and tear of the gasifier and other associated equipments. Specifically, poor controller response can damage a gasifier refractory vessel (a layer of bricks in the gasifier designed to keep heat inside the gasifier) and thermocouple temperature sensors that measure the temperatures in the gasifier. Poor controller response also leads to gasifier shut downs and “off-spec” syngas that does not meet required specifications.
For these reasons, a need has been recognized for an integrated control system that will control various critical components of the gasification plant. An integrated control system should improve the reliability of the gasification plant by reducing gasifier shut downs and maximizing run-time. Also, an integrated control system should reduce wear and tear of the gasifier and other associated components.
SUMMARY OF THE INVENTION
The present invention is directed toward an integrated control system (ICS) for a gasification plant. The ICS controls the operation of a gasifier and other critical components of a gasification plant. The present invention increases the performance of a gasification plant by controlling the operation of a gasifier and other critical components by an integrated controller, rather than by several independent controllers.
The ICS is a sub-system of a larger distributed control system that controls the operation of the gasification plant. Briefly stated, the ICS controls the following:
(i) oxygen to carbon (O/C) ratio in a gasifier;
(ii) syngas demand or the desired output of a gasifier;
(iii) load constraints;
(iv) moderator flow into a gasifier;
(v) air separation unit (ASU);
(vi) oxygen header vent valves; and
(vii) syngas header pressure.
The ICS provides safer operation and increased equipment life of the gasifier and other critical components by controlling the O/C ratio. Optimum hydrocarbon conversion occurs when the O/C ratio is controlled. According to the present invention, the O/C ratio is controlled by controlling the oxygen and carbon flow rates into the gasifier.
The syngas demand is determined from a demand setpoint value and a demand signal. The demand signal is produced by macro conversion of a carbon flow rate.
The load constraints are determined from a feed pump setpoint value, a feed pump PV/SP, where PV/SP is the actual power to the desired power ratio, an oxygen valve position, and an oxygen vent/recycle value.
The flow of moderators (steam) into the gasifier is controlled by adjusting one or more oxygen line steam valves and carbon line steam valves. If recycled black-water is also used as a moderator, the black-water flow is controlled by adjusting the speed of a black-water pump.
The oxygen discharge from the ASU is controlled by adjusting an oxygen compressor inlet valve. The amount of oxygen vented through oxygen header vent valves is controlled by adjusting the position of the vent valves. The syngas header pressure is controlled by three methods: a high pressure control; a low pressure control; and a “low low” pressure control.
The present invention provides a method for controlling an oxygen to carbon (O/C) ratio in a gasification plant. The method comprises the steps of: determining a syngas demand based on load constraints, the syngas demand being representative of a desired output of a gasifier; determining oxygen and carbon setpoint values based on an oxygen to carbon (O/C) ratio setpoint value and the syngas demand, and adjusting oxygen and carbon valves in the gasification plant based on the oxygen and carbon setpoint values, respectively.
The present invention provides a method for determining an oxygen setpoint value in a gasification plant. The method comprises the steps of: multiplying an oxygen setpoint value by a carbon flow rate to generate an oxygen setpoint high limit; determining an oxygen demand constrained by a carbon flow rate at a low selector from a syngas demand and the oxygen setpoint high limit; multiplying the oxygen setpoint high limit by a predetermined factor to generate an oxygen setpoint low limit, and determining a constrained oxygen setpoint value at a high selector from the oxygen setpoint low limit and the oxygen demand constrained by the carbon flow rate.
The present invention provides a method for determining a carbon setpoint value in a gasification plant. The method comprises the steps of: determining a carbon setpoint low limit at a high selector from an oxygen flow rate and a syngas demand; multiplying the oxygen flow rate by a predetermined factor to generate a carbon setpoint high limit; determining a constrained carbon setpoint value at a low selector from the carbon setpoint high limit and the carbon setpoint low limit; and dividing the constrained carbon setpoint by a O/C ratio setpoint to generate the carbon control setpoint value.
The present invention provides a method for controlling an oxygen flow in a gasification plant. The method comprises the steps of: calculating a compensated oxygen flow from an oxygen flow rate and oxygen temperature at a flow compensator; converting the compensated oxygen flow to a molar oxygen flow at a molar converter; multiplying the molar oxygen flow by an oxygen purity value to generate an oxygen flow signal; receiving the oxygen flow signal and an oxygen control setpoint value at a PID controller and generating a PID controller output signal; velocity limiting the PID controller output signal at a velocity limiter; and adjusting an oxygen valve using the velocity limited PID controller output signal.
The present invention prov
Gulko George M.
Tse Daniel W.
Wallace Paul S.
Bahta Kidest
Grant William
Howrey Simon Arnold & White
Reinisch Morris N.
Texaco Inc.
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