THE CONTROL OF A FLUID CATALYTIC CRACKING UNIT

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ABSTRACT

The performance of the FCC units plays a major role on the overall economics of refinery plants. Any improvement in operation or control of FCC units will result in dramatic economic benefits. Present studies are concerned with the general behaviour of the industrial FCC plant, and have dealt with the modelling of the FCC units, which are very useful in elucidating the main characteristics of these systems for better design, operation, and control. Due to the large economic benefits, these trends make the process control more challenging. There is now strong demand for advanced control strategies with higher quality to meet the challenges imposed by the growing technological and market competition.

Usually, the optimum operating conditions are met where constraints are also active. As a consequence, it is important to control FCCU as close as possible to the operating and equipment constraints but without violating them.

This study reviews differentcontrol approaches of the fluid catalytic cracking unitaimed to control the main variables of the unit and to demonstrate the benefits of the control for the FCC unit.

 

 

 

 

 

TABLE OF CONTENTS

Title Page                                                                                                                                i

Certification                                                                                                                            ii

Letter of Transmittal                                                                                                               iii

Dedication                                                                                                                              iv

Acknowledgement                                                                                                                  v

Abstract                                                                                                                                  vi

Table of Contents                                                                                                             vii-viii

List of Tables                                                                                                                          ix

List of Figures                                                                                                                   x-xi     

List of Symbols                                                                                                                xii-xiii


CHAPTER ONE                              INTRODUCTION

1.0  BACKGROUND OF STUDY                                                                                   1-5

1.1 AIM AND OBJECTIVES OF RESEARCH                                                                  5

1.2 SCOPE OF RESEARCH                                                                                             5         

1.3 SIGNIFICANCE OF RESEARCH                                                                      5-6                                                                                          

CHAPTER TWO                             LITERATURE REVIEW

2.0. THEORETICAL PRINCIPLES                                                                                      7-8      

2.1. HISTORY OF FLUID CATALYTIC CRACKING PROCESS                                  8-10

2.2. PROCESS DESCRIPTION                                                                                  10-14

            2.2.1 PREHEAT SYSTEMS                                                                             11

            2.2.2 RISERS                                                                                                  11-12

            2.2.3 REACTORS                                                                                            11-13

            2.2.4 REGENERATORS                                                                                  13-14

            2.2.5 FLUE GAS SYSTEM                                                                              14

2.3. FCC FEED CHARACTERISATION                                                                 14-16               2.3.1 Paraffin                                                                                                           15

2.3.2 Olefins                                                                                                15

2.3.3 Naphthalene                                                                                  15-16

2.3.4 Aromatics                                                                                      16

 

CHAPTER THREE                         MODEL DEVELOPMENT

3.0 MODELLING OF FCCU                                                                          17-18

3.1 THE MODEL PREDICTIVE CONTROL SYSTEM FOR THE FCCU                 18-20

            3.1.1 THE MATHEMATICAL MODEL                                                         20-21

            3.1.2 MODEL BASED PREDICTIVE CONTROL SYSTEM                      21-22

3.2 DECOUPLING CONTROL OF FCCU                                                              22-24

            3.2.1 THE DYNAMIC MODEL                                                                 24

3.2.2 SELECTION OF LOOPS INTERACTION                                   24-27

3.2.3 DECOUPLING OF INTERACTING LOOPS                                27-29

3.3 MULTIVARIABLE CONTROL                                                                                                29-31

            3.3.1 MULTIVARIABLE CONTROL CONFIGURATION DESIGN                  31-35

 

CHAPTER FOUR                           

RESULTS AND DISCUSSION                                                                                           36-47


CHAPTER FIVE

CONCLUSIONS AND RECOMMENDATIONS                                                               48-50

 




 

LIST OF TABLES

Table                                                              Title                                                             Page

3.1     The transfer functions of the simplified model used by implementation of the MBPC.  21

                                               

 

 

 

 

 

 

 

 

LIST OF FIGURES

Figure No.                                          Title                                                                Page

1.1.           Products from typical barrel of crude oil                     2

2.1.           Schematic of the Fluid Catalytic Cracking Unit                                       10

3.1.           The identification scheme                                                  18

3.2.           The input-output informational characterization                                  19

3.3.           The predictive control structure of the catalytic cracking process             22

3.4       Schematic of Interaction for Controlled And Manipulated Variables          26

3.5       Schematic of a fluid catalytic cracking unit with one closed loop                  26

3.6     Block Diagram of the control of a fluid catalytic cracking unit with decoupler 28

4.1 The dynamic evolution of the riser outlet temperature and regenerated catalyst flow when the controller set-point - Tr increases

4.2. The dynamic evolution of the regenerator temperature and air flow rate when controller set point -Treg1increases

4.3. The dynamic evolution of the riser outlet temperature and regenerated catalyst flow    38

4.4.  The evolution in time of the regenerated catalysts temperature and air flow  when the feedstock increases

4.5 The dynamic evolution of the riser outlet temperature and regenerated catalyst flow when the temperature riser controller set point increases

4.6. The dynamic evolution of the regenerator temperature and air flow trend when the Temperature regenerator controller set point increases

4.7Simulated response of riser exit temperature under PI controller without decoupler       42

4.8     Simulated response of the regenerator temperature under PI controller without decoupler

4.9   Simulated response of riser exit temperature and under PI controller with decoupler    43

4.10 Simulated response of the regenerator temperature under PI controller with decoupler    44

4.11   Step identification for a FCC unit for control design purposes              46

4.12 Closed-loop performance of three decentralized control configurations for a FCC unit 47

 

 

 

 

 

 

 

 

LIST OF SYMBOLS

 

Cca: Catalytic coke on the catalyst (mass%).

Cpa: Air specific heat (kcal/kg.oC).

Cpc: Catalyst Specific heat (kcal/kg.oC).

Cpf: Gasoil Specific heat (kcal/kg.oC).

Crc: Coke on the catalyst of the regenerator (mass %).

Csc: Coke on the spent catalyst (mass %).

D12&D21: Decoupler transfer function.

Fa: Air flow rate to regenerator (kg/sec).

Frc: Catalyst flow rate to riser (kg/sec).

Ftf: Total feed flow rate to riser(kg/sec).

G(s): Transfer function.

ΔHcr: Heat of cracking reaction (kcal/kg).

ΔHfv: Heat of vaporization of gasoil feed (kcal/kg).

ΔHg: Heat of combustion of coke on regenerator (kcal/kg).

K: Steady state gain.

K': closed-loop gain.

Kc: Controller gain.

Mg: Catalyst hold-up in the regenerator (kg).

Mr: Catalyst hold-up in the reactor vessel and riser (kg).

Qmp:  feedstock flow (kg/sec)

Rcb: Rate of coke burned in regenerator (mass %/min.).

Rcf: Rate of coke formation (kg/min.).

Roc: Cracking reaction rate (kg/min.).

s: Laplace domain (1/sec).

Tfp: Feed temperature at riser entrance (oC).

Tg: Regenerator bed temperature (oC).

Tfp: Reactor bed temperature (oC).

Treg1: Regenerated catalyst temperature (oC)

Tr: Riser outlet temperature (oC)

t: Time domain (sec).

Greek letters

ρtf : Feed density (kg/m3).

τI : Time constant of integral controller (min.)

λ : Relative gain.

 

 



 

 


CHAPTER ONE

1.0.INTRODUCTION

 BACKGROUND OF STUDY

Public enterprises were established, to enhance Nigeria’s socio economic development, especially after independence in 1960. The major concern in this regard had been to accelerate development and economic self-reliance through ‘’economic nationalism.’’ Public enterprises thus reflect one of those instruments by which government intervenes in economic development rather than allow market forces to dictate the pace of development. According to Ayodele (2004), Nigeria relied heavily upon public enterprises, up to the mid-1980s, for the development, management and allocation of utilities and social services. They were seen as major instruments not only for the mobilization and allocation of public investment resources, employment generation and income redistribution, but also for determining government finances and the acceleration of overall economic development.

 

Adeyemo (2005), reflecting on Turkey, Mexico, India and Nigeria, noted that the establishment of public enterprises was premised on what he considered as obstacles to economic development in the post-independence states. It is also instructive to note that in Nigeria like many developing countries, public enterprises are used as employers of last resort. According to Hemming and Mansor (1988), state owned enterprises enable governments to pursue goals of social equity that the market ordinarily ignores. Similarly, Ugorji (1995) observed that public enterprises had been established for political reasons. Many government undertakings were used to provide jobs for constituents, political allies, and friends. The location of public enterprises and the distribution of government employment have further been defended on the need to maintain .federal character and promote national integration.

 

Other factors that accelerated the growth of Nigeria’s public sector were the indigenization policy of 1972 as enacted by the Nigerian Enterprises Promotion Decree. It was designed to control the commanding heights of the economy. The policy further provided the much needed legal basis for extensive government participation in the ownership and control of significant sectors of the economy. According to Adeyemo (2005), Nigerian public enterprises have come under gross criticism in spite of the impetus given to them. Their problems were so enormous that many Nigerians became greatly disillusioned. These criticisms vary from the lack of productivity/profitability to reliance on large government subsidies. Ogundipe (1986) once argued that between 1975 and 1985, government capital investments in public enterprises totalled about 23billion Naira. In addition to equity investments, government gave subsidies of N11.5 billion to various government enterprises. All these expenditures contributed in no small measure to increase government expenditures and deficits.

 

Generally, public expectations from these enterprises were largely unmet, despite the sizable proportion of public budgetary investible funds which were being allocated to them. In addition, public enterprises suffered from gross mismanagement and consequently resulted to inefficiency in the use of productive capital, corruption and nepotism, which in turn weakened the ability of government to carry out its functions efficiently. (World Bank 1991). However, given the financial impacts of the global economic crisis on the Nigerian economy, the public sector- led development strategy became unsustainable. This in turn propelled radical economic adjustments and reforms, one of which is the emphasis on less of government in the production, management and the allocation of resources in Nigeria. Consequently, Nwoye (2010) stated that Privatization in Nigeria was formally introduced by the Privatization and Commercialization Act of 1988, which later set up the Technical Committee on Privatization and Commercialization (TCPC), chaired by Dr. Hamza Zayyad, with a mandate to privatize 111 public enterprises and commercialize 34 others. The Federal Military Government promulgated the Bureau for Public Enterprises Act of 1993, which repealed the 1988 Act and set up the Bureau for Public Enterprises (BPE) to implement the privatization program in Nigeria. In 1999, the Federal Government enacted the Public Enterprise (Privatization and Commercialization) Act, which created the National Council on Privatization (NCP) chaired by the Vice President.

 

 STATEMENT OF PROBLEMS

The concept of privatization poses its own challenges. In this context, it is apposite to examine the objectives of privatization. In the words of Guislain, defining privatization objectives is an important exercise that should be undertaken as early as possible. Many privatization programs have foundered when clear objectives were lacking or where conflicting objectives were simultaneously pursued. The definition of objectives is not an easy task, however, and it is made no easier by the multiplicity of possible objectives and actors with different, often conflicting interests.

 

According to Adesanmi (2011), the government, set up the Bureau of Public Enterprise (BPE) to privatise and commercialise, as the case may be, public enterprises with the objective of reducing or eliminate the drain on public treasury. It also seek to reducing corruption, modernise technology, strengthen domestic capital markets, promote efficiency and better management, reduce debt burden and fiscal deficit, resolve massive pension funding problems, broaden the base of ownership of business. Others include generating funds for the treasury, promoting governance, attracting foreign involvement and attract back flight capital. Whether the BPE has met and realised these objectives is a matter that is open for debate. This paper attempted to assess the operation of the privatization scheme in Nigeria, determined its level of performance/productivity. It also proffered objective solutions for the amelioration of gaps.

 

Microeconomic theory predicts that incentive and contracting problems create inefficiencies stemming from public ownership, given that managers of state-owned enterprises pursue objectives that differ from those of private firms and face less monitoring. Not only are the managers’ objectives distorted, but the budget constraints they face are also softened. Empirical evidence shows a robust corroboration of this theoretical implication in several countries. How true is this for Africa? The study will also appraise the nature of the contracts between these firms and government in the pre and post-reform period and show how the contracts address three interrelated problems: information asymmetry, incentives and commitment.

 

OBJECTIVES OF THE STUDY

a)      To understand the extent and pattern of privatization

b)      To establish the results of privatization in Nigeria.

c)      To establish whether privatization has improved the performance of enterprises as anticipated.

d)     To outline policy lessons that can be learned from the privatization exercise.

 

RESEARCH QUESTIONS

To have an in-depth knowledge of this study, the following research questions will be considered:

a)      What is the extent and pattern of privatization in Nigeria?

b)      What has been the result of privatization over-time in Nigeria?

c)      Has privatization improved the performance of enterprises as anticipated?

d)     What are the policy lessons that can be learned from the privatization program?

 

RESEARCH HYPOTHESIS(S)

H1:       There is a relationship between privatization and productivity of formerly state owned companies

H0:       There is no relationship between privatization and productivity of formerly state owned companies

 

H2:       There is a relationship between government management of companies and the performance of such companies

H0:       There is no relationship between government management of companies and the performance of such companies

 

DATA SOURCES AND SCOPE OF WORK

For this study, we will be taking a very close look as formerly public enterprises that have been transferred to private individuals or corporations. The study will engage the assessment of former employees as well as the public to evaluate the performance of the firms now and before. Previous studies have shown that the measure to understand the impact of privatization is to use the return on shares, equity and asset approach. This method will also be implored, but more to it will be the perception and profitability of these new organizations. The study will also focus on the decrease in government expenditure in businesses and how these funds have been redirected to providing basic amenities for the populace. As a basis of scoping, we will be taking a look into organizations like NITEL, PHCN, among others.

 

RESEARCH DESIGN

Data for this thesis will be collected using a quantitative, survey-based methodology. This approach is important when causal relationships among the underlying theoretical constructs need to be examined. Self-administered questionnaires are considered to be the most appropriate tool as well as interviews. Most importantly, this method is quick, inexpensive, efficient, and can be administered to a large sample (McCelland, 1994; Churchill, 1995, Sekaran, 2000; Zikmund, 2003). To ensure that the questions are clearly understood and there is no ambiguity among them, a pre-test will be conducted.

 

Respondents will be selected to conduct the study. Descriptive analysis for the entire sample will be performed using SPSS (Statistical Package for the Social Sciences).

 

In this study, where relationships are being established, two statistical methods will be used to analyze, interpret, and test data related to the study. Sample percentage (SP) arranged in tables will be used to analyze respondent’s bio-data and hypothesis of the study will be tested by the use of Pearson's Coefficient of correlation. The analysis of data and testing of hypothesis will be based on the responses obtained through interview and questionnaire administered.

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