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DEVELOPMENT AND PERFORMANCE EVALUATION OF A TRAY COLUMN UNDER VERTICAL AND TILT CONDITIONS
ABSTRACT
In this work, the operation and performance of a novel Separation Tray Column under Vertical and Tilt conditions is presented. The Tray Column used for the work is a Downcomerless liquid initiated and controlled Valve Tray with capacity to shut portions of the tray lacking liquid at any time. To evaluate the mass transfer performance of this column, an oxidation experiment was conducted at 300C and the percentage of Fe (II) oxidised to Fe (III) from contaminated water by air was chosen as the system response. The influence of the Gas flowrate (x1 in litres per minute), the Liquid flowrate(x2 in litres per minute) and the Angle of tilt (x3 in degrees) on the response was represented by polynomial models and their effects were studied using the Student’s t-test and Fischer’s F-test for the Analysis of Variance (ANOVA) for each model. Out of the 5 models, the experimental data were found to be best represented by a linear polynomial model with Coefficient of Determination R2 = 0.9635. The test of significance of the individual coefficients in the model based on the Student’s t-test showed that only parameters x1 and x2 (i.e. the gas and liquid flowrates) are significant at 95% confidence level. The angle of tilt (with p=0.18) had no significant influence on the amount of Iron (II) oxidised. The ANOVA of this model showed that the model was significant as the calculated F-value (F-model = 17.60) exceeded the tabulated F-value (6.16), and also from its probability value (p=0.008) which is less than 0.05. From this test also, the model parameters x1 and x2 had significant influence as their calculated F-values (164.57 and 45.38 respectively) were much higher than their tabulated F-values (F=7.71). The parameter x3 and their interaction effects were still not significant from this test as their calculated F-values (5.42 for x3, 0.06 for x1x3 and 0.2 for x2x3) are less than their tabulated F-values of 7.71. These findings were further validated by the x-y scatter plots which showed that for all the liquid flowrates, the % Fe (II) oxidised increased with increasing vapour flowrates but the impact of the angles of tilt was minimal. The implication of these is that the operational efficiency of this novel tray column will not be compromised by column tilt of up to 20 degrees from the vertical.
Keywords: Gas to Liquid Processes, Stripping Ships, Tray Column Tilt and Motion, Novel Separation Tray Column, Mobile and Floating Platforms.
TABLE OF CONTENTS
Cover Page………………………………………………………………… ……..i
Certification…………………………………………………… ……………………….ii
Dedication…………………………………………………………………………iii
Acknowledgement………………………………………………………………..iv
Abstract……………………………………………………………………………v
Table of Contents ………………………………………………………………..vi
Nomenclature ………………………………………………………… …………xi
List of Tables ……………………………………………………………………xii
List of Figures/Charts …………………………………………………………..xix
CHAPTER ONE (INTRODUCTION)……………………………….…………1
1.1 Background of study ….…………………………………………….1
1.2 Problem Statement…..………………………………………………2
1.3 Research Objectives…………………………………………………3
1.4 Research Hypothesis……………..……………………………………4
1.5 Justification of Study……………….…………………………………4
1.6 Scope of Study………..………………………………………………7
CHAPTER TWO (LITERATURE REVIEW)………………….………….9
2.1 Plate Columns with Downcomers ……………………………………9
2.2 Plate Columns without Downcomers ……………………………….11
2.2.1. Perforated Trays without Downcomers ………………………12
2.2.2. Baffle Tray Columns ………………………………………..13
2.3 Valve Tray Designs used on Crossflow Trays …………………….15
CHAPTER THREE (RESEARCH METHODOLOGY)……………………21
3.1 Conception of the Proposed Tray Configuration and its
Mode of Operation…………………………………………………21
3.2 Mechanical Design of Plunger-Cap Multi-Float Valve Tray ……..26
3.2.1 Formulation of a Weight-Volume Relationship Model for the Float
Valve……………………………………………………………….…26
3.2.2 Optimisation of the Cylindrical Float Dimensions…………..27
3.2.3 Fitting Models to the Empirical Data…………………………29
3.2.4 Design Calculations…………………………………………..31
3.2.4.1 Determination of Cylindrical Float Dimensions……………….31
3.2.4.2 Determination of Liquid Flow Rate………………..………36
3.3 Mechanical Details of Plunger-Cap Multi-Float Valve Tray and Test
Column…………………….………………………………………37
3.4 Multi-Float Plunger-cap Valve Tray Specifications…………..…44
3.5 Experimental Investigation………………….…………………..45
3.6 Factorial Design of Experiment ………………………………..48
3.6.1 Selection of System Response …………………………….48
3.6.2 Selection of Factors ………………………………………….49
3.6.3 Experimental Factor Space, Null level and Variation Interval….49
3.6.4 Sample Preparation and Analysis ……………………………52
CHAPTER FOUR (RESULTS AND DISCUSSION) …………………….53
4.1 Experimental Results…………………………………………..53
4.1.1 Analysis of Pure Linear Model Based on First Order Linear Experimental Design ………………………….55
4.1.2 Analysis of Linear Model with Interactions based on First Order Linear Experimental Design………………57
4.1.3 Analysis of Linear Model with Triple Factor Interactions Based on First Order Linear Experimental Design……60
4.1.4 Analysis of Pure Quadratic Model Fit Based on Box Wilsons Second Order Experimental Design…………61
4.1.5 Analysis of Quadratic Model with Interactions Based on Box
Wilsons Second Order Experimental Design……64
4.1.6 Graphical Analysis of the Various Models……………66
4.1.7 Visual Observations of the Fluid Flow Pattern of the Novel Tray
Column………………………………………….…………….101
4.2 Discussion of the Results…………………………………..117
4.2.1 Discussion of the Results of the Pure Linear Model…….……..117
4.2.2 Discussion of the Results of the Linear Model with Interactions..118
4.2.3 Discussion of the Results of the Linear Model with Triple Factor
Interactions…………………………………..119
4.2.4 Discussion of the Results of the Pure Quadratic Model Fit….120
4.2.5 Discussion of the Results of the Quadratic Model with
Interactions…………………………………………..121
4.2.6 Discussion of the Results of the Graphical Analysis…123
4.2.7 Discussion of the Visual Observations of the Fluid Flow Pattern of
the Tray Column………………………….124
4.2.8 Summary of the Discussion of the Results…………..125
CHAPTER FIVE (CONCLUSION AND RECOMMENDATIONS) …..126
5.1 Conclusion……………….………………………………………126
5.2 Contribution to Knowledge…………………………………….127
5.3 Recommendations…….…………………………………………128
REFERENCES ……………………………………………………………..129
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APPENDIX 1: Table for Obtaining the Tabulated F-Values for Comparison
with the Calculated F-Values for Regression Diagnostics………..135
APPENDIX 2: Tables Showing Variations in Percentage of Fe (II) Oxidised
as Obtained from the Various Models …………………………..138
APPENDIX 3: The Null Hypothesis and Statistical Inference…………………156
APPENDIX 4: Catalogue of developed trays and facilities for tray testing, development and troubleshooting………………………………..158
APPENDIX 5: Archimedes principle and principle of floatation……………..165
APPENDIX 6: Construction of the Factorial Design Matrices………………..166
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND OF STUDY
Gas-liquid operations constitute one of the major modes of mass transfer encountered in chemical engineering applications (Sinnot, 1993). The following gas-liquid mass transfer systems are listed by Perry and Green (1997): distillation, flashing, rectification, absorption, stripping, evaporation, humidification, and spray drying. All these operations are designed to contact liquid and gas (vapour) phases for the purpose of mass, heat and momentum transfer between them (Hanley, 2012). According to Treybal (1981) “the rate of mass transfer is directly dependent upon the interfacial surface exposed between the phases, hence the nature and degree of dispersion of one fluid in the other are therefore of prime importance”. Most mass transfer operations are motivated by the need to make maximum contact between phases in which mass transfer is expected to occur (Benitez, 2009, Nnolim, 1993).Properly designed gas-liquid mass transfer equipment should therefore provide efficient interphase diffusional interchange by dispersing the gas phase, liquid phase or both phases during operation in a cost effective manner (Fard et al, 2007, Liu et al, 2011, Naziri et al, 2012, Smith, 1963). The fields of application of mass transfer theories have become widespread, from traditional chemical industries to bioscience and environmental industries, where the design of new processes, the optimization of existing processes, and solving pollution problems are all heavily dependent on knowledge of mass transfer (Asano, 2006, Negrea et al, 2008, Zang et al, 2012).
1.2 PROBLEM STATEMENT
Mass transfer equipment are designed with suitable internals, either tray/plates or packings which increase the surface area available for contact of the gas and liquid phases (Baehr & Stephan, 2006, Hoon et al, 2011).The overall column efficiency, defined as the ratio of the number of theoretical plates to actual plates required for a given separation is of ultimate concern during the design and operation of contacting devices. For a properly designed column, the efficiency depends almost entirely on proper contact between vapour and liquid streams. Any condition which leads to poor liquid distribution or short-circuiting will therefore lower the efficiency of the column. One way of avoiding liquid channelling is by ensuring the column remains vertical and does not sway during operation. The need may however arise to operate contacting devices on moving platforms. The term “moving platform,” implies that the column may not always be vertical, but can sway depending on the motion of the platform. Such sways will result in tilting of the Vapor Liquid Equilibrium (VLE) equipment mounted on the platforms and lead to a loss or total collapse of the efficiency of such equipment (Lockett &
Billingham, 2003; Locket & Billingham, 2002; Tanner et al, 1996; Waldie, 1996). The design of VLE equipment which can operate under such conditions without loss of efficiency still remains a major Chemical Engineering challenge and forms the forms the basis of this research.
1.3 RESEARCH OBJECTIVES
The objectives of this research are as follows;
- To design and construct a Separation Tray which can operate without loss of efficiency on mobile platforms.
- To evaluate the mass transfer performance of this tray using the amount of
Fe (II) oxidized to Fe (III) as the system response.
- To test the tray in both vertical and tilted positions to determine the significance of the following key parameters; The gas flowrate x1
(in litres per minute)
- The liquid flowrate x2 (in litres per minute)
- The angle of column tilt x3 (in degrees)
1.4 RESEARCH HYPOTHESIS
Polynomial models will be developed from the experimental data to be obtained that will have the 3 key parameters as variables. The p-values in the student’s t-test will be used as a tool to check the significance of each of the coefficients which in turn may indicate the pattern of the interactions between the variables. The Null Hypothesis as defined in Appendix 3 will be used as the Research Hypothesis.
1.5 JUSTIFICATION OF STUDY
Offshore oil and gas exploration activities have in recent times been on the increase. This is because as the number of new discoveries of oil and gas reserves decreases, there is need to fully exploit existing resources (Lye et al, 2007). These offshore exploration activities have their concomitant technological challenges, which are quite different from those experienced during onshore exploration.
These challenges have led to the rampant use of Floating production, Storage and Offloading (FPSO) platforms, and major modifications of the process equipment used on these platforms such as coil-wound heat exchangers and contacting columns. A typical offshore exploration challenge is the need to economically recover “stranded gas”. “Stranded gas” refers to gas reserves in offshore locations that cannot be transported to shore via pipelines due to the prohibitive and uneconomic cost of such a venture. These gases also cannot be flared due to the deleterious environmental consequences of such large scale gas flaring. According to Goldstone et al (1998), remote conversion of natural gas to liquid fuels presents a unique set of challenges where offshore locations are involved. The lack of infrastructure, field marginality, inability or undesirability to flare gas, etc may require a floating production, storage and offloading installation to process the associated crude oil, convert gas to liquid fuel, store, and subsequently offload all products into shuttle tankers to the mainland.
The solution therefore lies in the conversion of such gases to high density liquids on a floating production plant either by refrigeration to Liquefied Natural Gases (LNG) or by Gas to Liquid (GTL) chemical conversion to higher molecular weight liquids which can be economically transported in shuttle tankers. In both cases, the process plant would have to be mounted on a large ship, barge, or other tethered support and hence would be subjected to tilt and motion from wave and wind forces (Waldie, 2004). A further desirable feature of such a floating platform is that it can be moved from one stranded gas source to another thus taking full advantage of a series of small reserves (Lye et al, 2007).
Although Baker and Waldie (1996) have developed a packed column which can operate in tilt and motion conditions with better efficiency than conventional packed columns, there is to date no plate column design for such operations.
The need to develop a plate column that can operate without loss of efficiency under vertical, tilt and motion conditions is the motivation for this research work.
The scope of applications of columns that can operate on mobile platforms without loss of efficiency is wide. Another classic example of this need is seen in “Stripping Ships”. Stripping ships are vessels used to convey crude oil from offshore rigs to onshore refineries. These ships are equipped with stripping columns. After each discharge, the tanks are washed to avoid vaporization of the left over crude when tanks get heated (by sunlight, for example) in transit. These volatile oils are inflammable and pose fire and explosion hazards. Because of the large capacity of these tanks, the amount of oil washed down is substantial. This oil is usually recovered by stripping the wash liquid. The ship must however be docked until the stripping operation is completed. With a stripping column operating efficiently on a moving platform, no time need be lost since the separation can be carried out while the ship is in motion.
With this technology also, it will be possible if desired to erect columns on floating platforms, which may not remain rigidly vertical continuously. Hence if there should be a marine tilt or motion due to a typhoon, tsunami, tidal wave, or any other violent and adverse condition that causes the column to sway/tilt, the column would still be operating efficiently despite the sways these may cause.
Apart from these needs described above, because the operation of this novel plate column will be liquid controlled as described in the next chapter, such a column will also compete favorably with others during normal operation. It may even be found to offer certain operational efficiency advantages over the conventional plate columns in normal operating conditions
1.6 SCOPE OF STUDY
The scope of this research is to design, construct and test a separation tray which can operate without loss of efficiency on mobile platforms. The novel tray being proposed will achieve this by ensuring that intimate contact of the vapour and liquid phases is maintained at all times even if the tray experiences a tilt from its normal horizontal position.
The operation dynamics and performance of the novel tray will be evaluated on air-water system to determine its vapour and liquid flow capacity, and its tray efficiency will be evaluated based on oxidation of Fe (II) to Fe (III) in water. This test will be carried out in both vertical and tilted positions to determine the impact of the following key parameters:
- The gas flowrate
- The liquid flowrate
- The angle of column tilt
To achieve this, a Factorial Design of Experiment will be used to afford simultaneous varying of all 3 factors during the experimental runs as against the one factor at a time method where excessive amount of experimentation will be required to evaluate the simultaneous impact of the 3 factors. The experimental runs will be done in replicates to reduce random experimental errors and the data obtained will be fitted to polynomial models. The adequacy of the models will be ascertained statistically and the effects of the model parameters will also be ascertained both statistically and otherwise. From these analyses we will be able to assess the tray column performance over a range of operating conditions including
tilting.
DEVELOPMENT AND PERFORMANCE EVALUATION OF A TRAY COLUMN UNDER VERTICAL AND TILT CONDITIONS
