Identifying the sulfur extraction unit system and controlling its processing cycle with the help of advanced controllers

Dissertation for Master's Degree in Electrical-Electronic Engineering (M.Sc)

Abstract

Claus method sulfur recovery process is one of the most common methods of separating elemental sulfur from acid gas resulting from the sweetening process in Migah gas and oil refineries. be But due to the complexity of this process and the multi-variable nature of the reaction furnace and the lack of suitable controllers, unfortunately, until now there has been no possibility of optimal control and proper efficiency for sulfur recycling in South Pars refineries, and the control of this important process unit has always faced many problems. In this research, two methods of system identification have been used, one method is mathematical modeling using least squares, and the other is based on artificial intelligence using artificial neural networks. By modeling this process, issues such as stability, controllability and visibility of the multivariable mathematical model have been examined and then the controller has been designed according to the obtained models. For the design of the controller, Durosh has also been used, a method of pole placement for multivariable mathematical model, and another method of using artificial neural network controller has been used. From the results of this research, we can mention the much higher efficiency of the sulfur recovery unit due to its optimal control and as a result of profitability and increased productivity and energy reduction, and its most important effect is the reduction of air pollution.

Key words: system identification, multivariable control, least squares, Adaline neural network, multi-layer perceptron, gas refinery, sulfur extraction unit.

1-1- Field development plan South Pars gas field

Field status: South Pars gas field is one of the largest independent gas sources in the world and is located on the common border line between Iran and Qatar in the Persian Gulf. This tank is located in a layer with a thickness of 450 meters at a depth of about 3000 meters below the sea floor. Its reserves amount to 464 trillion standard cubic feet of gas (13 trillion cubic meters) and 17,000 million barrels of gas condensate (equivalent to 8% of the world's total reserves and about 50% of the country's gas reserves). The area of ??the part located in Iran's maritime zone is about 3700 square kilometers. The gas in this tank is sour and in 4 layers, and the amount of hydrogen sulfide gas (H2S) in different layers is about [1] ppm 5000. Each phase of the South Pars gas field is designed to purify one billion cubic feet of sour gas per day of operation. The current program of this facility includes 28 phases, of which 10 phases have been put into operation so far.

1-1-2- The fifth refinery (phases 9 and 10)

Since the project was carried out in the fifth refinery (phases 9 and 10), therefore, a brief description of the operational processes of this refinery

The implementation of the ninth and tenth stages of the field development plan is designed to extract 2,000 million cubic feet and 80,000 barrels of gas condensate and 400 tons of sulfur per day.

The method of transferring gas and gas condensate to the refinery is in three phases. The marine facilities include 2 drilling platforms for drilling 2 descriptive wells and 20 development wells, 2 submarine pipelines of 32 and 4.5 inches each with a length of 105 km.

A gas refinery with a capacity of 2000 million cubic feet has been built on the coast, which includes units for receiving and separating gas and gas condensate, condensate stabilization. gasification, sweetening, dehumidification, dew point adjustment [2] and mercaptan removal [3] and gas compression for transfer, recycling and freezing of sulfur and monoethylene glycol regeneration unit for injection.

The gas product of the refinery will be sent to the third national gas transmission pipeline in Kangan region using the 56-inch pipeline of the first stage.

In the design of these two phases, like the other phases, the daily production of 50 million cubic meters of natural gas for export and domestic use and 80 thousand barrels of gas condensate and 400 tons of sulfur for export and the annual production of 1 million tons of ethane for consumption in petrochemical complexes and 1.05 million tons of liquid gas (LPG) have been considered..

Due to the nature of the gas in the South Pars reservoir, its H2S content is high, and due to the toxicity of this gas, it is extracted and sold after separating it by performing multiple sulfur reactions.

Therefore, the purpose of these two phases can be summarized in the preparation of four items:

Sweet gas with Dew point and calorific value[4] certain

Gas condensate with specific vapor pressure[5]

Sulfur with determined purity

Liquefied gas

Gas extracted from gas wells of a field after collection by separation centers with a pressure of about bar 100 is sent to the gas refinery for purification and sweetening.

1-1-3- Process units

The process units of the refinery include the following sections and the system identification and advanced controller design project for unit 108 (sulfur recycling) has been completed:

Slug unit 100 Catcher

Unit 101 Gas Sweetening

Unit 103 Condensate Stabilization

Unit 104 Gas Dehydration

Unit 105 Dew Point Adjustment Point

Propane Refrigeration Unit 107

Sulfur Recovery Unit 108

LPG Treatment Unit 114

With the advancement of technology in the oil, gas and petrochemical industries, the use of new methods and It is necessary to be up-to-date in order to reduce production costs and increase the efficiency and lifespan of equipment. In this regard, advanced and optimal system identification and control methods have a special place. These methods, in addition to considering the stability of the system despite disturbances, try to improve the performance of the process as much as possible.  Identifying systems and modeling is very important, especially in control science, and most of the famous controllers use the model structure in the control process, and in general, to check the behavior of the system and design the required tools, it is necessary to have a mathematical model of the system.

Mathematical model can be obtained in two ways: first through physical modeling, in which case it is necessary to fully understand the physics of the problem and the conditions governing it, which in many cases, especially in processes and Complex dynamic systems that have several input and output variables are not possible, and second, by identifying the systems, in this method, one can obtain the best mathematical model that can reproduce the desired data by only using a number of input-output data and sometimes knowing some properties of the system. The science of system identification has developed a lot and many methods have been proposed to identify systems. Methods based on output error and equation error structures are among these methods. Another method is to use orthogonal basis functions. These functions actually form an orthogonal series of basic transformation functions with one pole, two conjugate poles, or their generalization with a larger number of poles as a basis for expanding the identified transformation function, and during the identification process, the coefficients of these functions are obtained. 

Another system identification method, which is one of the main and central identification methods, is known as the least square method. In practical problems, classical methods are usually used first to identify the system. Then, with model evaluation methods, it will be checked whether the obtained model is acceptable or not. If the classical methods provide a suitable and acceptable model for the system, the rest of the methods are no longer used. But if the amount of noise was so much that the estimated model through classical methods did not meet the needs or did not have the necessary validity, usually the least squares method is used in the next step. Worldwide, extensive efforts have been made to increase the efficiency of this process and reduce investment costs.