We have kept discussion in this section brief to focus more on our Project ‘Heat exchange and integration in CDU ’. The main report is divided into 4 sections. To start with, the first Section is a basic introduction to Crude distillation unit, its process and products. Heat exchange is among the most important activity for crude distillation units. The 2nd part of the report focuses on this heat exchange process occurring in the crude distillation unit of Guwahati refinery. Necessary data regarding all the heat exchangers has been provided in a tabular manner to make understanding easier.
The concept of critical heat exchanger is introduced subsequently in the 3rd section along with the case study of some important heat exchangers of the process. Heat duty &other calculations presented in this section for heat exchangers are clearly explained in step by step manner along with formulas used. Through these calculations we have tried to analyze whether these exchangers are working efficiently. In the Last part of the report we have given an introduction to ‘heat integration’ one of the most important tool for increasing energy efficiency of a process.
Points discussed about heat integration in this section are: Meaning, Importance, its need in CDU, Tools. We have concluded this part with an example of heat integration done in 2010 in pre-heat train of CDU, showing how it helped saving energy & increasing efficiency. A final conclusion and glossary are provided to sum up the report. We hope you have a good time reading this report and find it informative and useful. Mudit, Amanpreet & Rishu Birla Institute of Technology & Science Pilani 333031 2 ACKOWLEDGEMENT
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A comprehensive report always requires the goodwill, encouragement, guidance and support of many people so we would like to start by thanking our college BITS pilani for initiating program like ps-1 thereby giving us the opportunities to visit real time industries and learn by working on hand in organizations as big as IOCL. Our sincere thanks to Mr. B K Das, CPNM and Mr. P S Sonowal for assigning us this important project on heat integration in refinery and getting us in contact with the required person in each unit. Without their help this whole program would not have been possible.
We are indebted by the constant support and mentoring provided by Mr. Vijay Kumar, TS for the preparation of this report. Also, we take this opportunity to thank all the chief engineers, and workers we have met in different units who have received us with open hearts and helped us in learning the vast array of knowledge that a refinery holds. Mr. E Edmund of CDU, Mr. A. Bairagi of OM&S are just the few of these names with whom we shared numerous informative talks which finally went into the preparation of this report. We also express our deep sense of gratitude to IOCL administration for providing us with necessary data nd making our stay a pleasant one. Thanks to our families & friends for their constant support and encouragement throughout the period of preparation of this report. It goes without saying that we are sincerely grateful to our instructor, Mr. Prasantha G for coordinating this ps-1 program and giving us opportunity to present before him this report. Thank you all Birla Institute of Technology & Science Pilani 333031 3 TABLE OF CONTENTS S No. Topic Page No. Preface Acknowledgement List of Illustrations Abstract 2 3 5 1. 1 1. 2 1. 3 Introduction IOCL Guwahati refinery Production Units of Guwahati refinery 6 7 8 9 2. 1
Crude Distillation Unit CDU : Process Description 11 3. 1 3. 2 3. 3 CDU Pre-heat Train (Heat exchange Process) Introduction Shell & tube Heat exchanger Pre-heat train : Description 15 16 17 Heat Exchanger Performance Calculation Critical heat exchangers Performance analysis: Examples ? S-25A/B ? S-11C ? S-23A/B 22 23 23 25 27 Heat Integration Heat Integration : Introduction Heat Integration in CDU ? Need for heat integration in CDU ? Heat integration in Guwahati Refinery ? Example from pre-heat train 30 32 32 32 33 Conclusions References Bibliography Glossary 35 36 37 38 4. 1 4. 2 5. 1 5. 2 Birla Institute of Technology & Science
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Pilani 333031 4 LIST OF ILLUSTRATIONS ILLUSTRATION NO. DESCRIPTION 1. 2. 1 1. 3. 1 IOCL Indian market share Table showing Production units 2. 1. 1 2. 1. 2 CDU flow diagram Table Showing CDU product properties 3. 2. 1 3. 3. 1 3. 3. 2 Types of Shell Tube heat exchanger Pre-heat train flow diagram Table Showing cold section heat exchangers Table Showing mid section heat exchangers Table Showing hot section heat exchangers 3. 3. 3 3. 3. 4 4. 2. 1 – 4. 2. 3 4. 2. 4 – 4. 2. 6 4. 2. 7 – 4. 2. 7 5. 1. 1 5. 2. 1 5. 2. 2 Properties, Design data, Practical data of S-25A/B Properties, Design data, Practical data of S-11C
Properties, Design data, Practical data of S-23A/B Steps of Heat Integration Heat integration in Pre-heat train S-11/c before & after heat integration Birla Institute of Technology & Science Pilani 333031 5 ABSTRACT Title of the Project: CDU: HEAT EXCHANGE & INTEGRATION Key Words: Crude distillation, Heat exchange, Heat integration, CDU-Pre heat train Project Areas: Heat exchange, Process design & optimization, Energy Abstract: In this work is on Crude distillation unit in a refinery & discusses the heat exchange process in it and introduction of the concept of heat integration which is of wide importance in CDU.
CDU is used for crude fractionation and requires a temperature of 354 oC. To increase the temperature of crude 2 step process is used; heat exchange with product streams & furnace. In the shell & tube type heat exchangers of Guwahati refinery Low temperature crude exchanges heat with high temperature streams of SRGO, RCO etc. in 3 stages(cold , mid, hot).
The performance of some of these heat exchangers have a major impact on production and are classified as ‘Critical’ on the basis of different criteria like frequent fouling, Large surface area.
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Calculation of heat duty, LMTD of some of these heat exchangers(s/15, S/23a-b,s-24) is done using design & practical data to check whether they are working efficiently as compared to design. Heat integration is an important concept to increase energy efficiency of process through heat recovery. Tools like pinch analysis are used in high energy demanding units like CDU to optimize the Heat Exchanger Network for maximum efficiency. Analysis of past heat integration done in Pre heat train of CDU is taken as an example for analysis. Signature(s) of Students(s) Date Signature of PS Faculty Date
Birla Institute of Technology & Science Pilani 333031 6 1. 1 INTRODUCTION This Report is an analysis of the one of the most key elements of any crude distillation unit i. e. ‘Heat Exchange’ & ‘Heat integration’. The report is based on Work in CDU of Guwahati refinery. Crude distillation units are used for the fractionation of crude into more valuable products using their difference in boiling points. Increase in temp for distillation us achieved in 2 steps; first step is the heat exchange of crude with product streams at high temperatures. In the next step a furnace is used to further elevate the temperature.
Our main point of discussion in this report is the first step of heat exchange called the Pre-heat train. The pre-heat train consists of 3 sections: Cold, Mid & hot. The purpose of each section is to recover heat from hot product stream of RCO, SRGO etc and supply it to crude through a shell-tube type heat exchanger. The details about various heat exchanger used in process are presented in the report. Performance of some heat exchanger is more critical on overall heat exchange than others; these heat exchangers are classified as ‘Critical heat exchangers’. Constant monitoring of efficiency of these heat exchangers needs to be done.
As Example calculations like heat duty, LMTD is done in this report for some of these critical heat exchangers (S-11C, S-23A/B) to check their efficiency. Crude distillation is an energy intensive process. With increasing oil costs, the focus is to maximize energy recovery. One way to do this is ‘heat integration’ which is a technique to design a process to minimise energy consumption. Tools like pinch analysis are used for obtain an optimized heat exchanger network for heat integration in CDU. The retrofitting of the HEN in CDU is taken as case study in report to realize importance of heat integration.
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Meaning of all the key terms used in the report can be found in the Glossary. Necessary data used in the report has been obtained through manuals & officials of Guwahati refinery. Birla Institute of Technology & Science Pilani 333031 7 1. 2 IOCL GUWAHATI REFINERY ? IOCL Indian Oil Corporation limited is largest state owned company in terms of revenue having ‘Maharatna’ status. It is ranked 98th in the Fortune global 500 listing. It’s biggest player in Indian downstream oil & Gas sector and operates10 of India’s 21 refineries with refining capacity of 65. 7 MMTPA.
Major ones are Panipat, Mathura, Guwahati, Gujrat, Haldia etc. Its products are Petrol, Diesel, LPG (Indane), ATF, lubricants, Naphtha, Kerosene etc. (1. 2. 1) Sector (India) IOCL Share Petroleum products 49% Refining capacity 37% Downstream pipelines 67% ? Guwahati Refinery Guwahati refinery is the first public sector refinery of India commissioned in 1962. It had an initial capacity of 0. 75 MMTPA which was extended to 1 MMTPA in 1986. Guwahati Refinery receives the raw crude from Oil India Limited & has a product line to Siliguri. Final products of Guwahati Refinery are Motor spirit (MS), ATF, Kerosene, LPG, Diesels, Petroleum Coke &Elemental Sulphur. It’s an environmentally Conscious refinery with modern effluent treatment facilities, Stack gas monitoring, Products with strict environmental specifications like BS-3 & BS-4. Birla Institute of Technology & Science Pilani 333031 8 1. 3 PRODUCTION UNITS ? Shown below in the table are the various production units of Guwahati refinery along with their use, feed & products(1. 3. 1): Unit Purpose Feed Output CDU Separation of Crude into useful products by distillation. Raw Crude
LPG , Naphtha, RCO, Kero-1 &2 DCU Thermal Cracking to obtain useful products from higher ends RCO CK, CGO, CFO, RFO and RPC HGU Production of hydrogen Light Naphtha (LN) Hydrogen INDMAX FCC unit for maximizing LPG production from residual stocks. CFO, CK, RCO LPG & Gasoline Quality improvement of Diesel, ATF by removing sulphur. SRGO, KERO 1 &2 BS 3 Diesel, ATF, SKO HDT Improving Octane No. of MS by Hydrotreating & Isomerization LN Isomerate (MS) HDT off gases Elemental Sulphur _ _ Waste water Effluent free water MSQU Recovering Sulphur SRU OM&S ETP Crude & product storage, Pumping & blending
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Treatment of waste water to remove oil and suspended solids Birla Institute of Technology & Science Pilani 333031 9 CRUDE DISTILLATION UNIT Birla Institute of Technology & Science Pilani 333031 10 2 CDU: Process Description INTRODUCTION Crude distillation unit or CDU in short is first & most important unit for any refinery. It’s called the ‘Mother unit’ as its products forms the feed of other units. CDU receives its crude from OM&S (oil movement and supply section) where it is supplied by Oil India Ltd from upper Assam oil fields. Incoming crude has following properties (2. 1. 1): Property
Density at 15 oC Water content Salt content(ppm) Sulphur content(ppm) Value 0. 8735 2. 05% 8. 7 0. 29 CDU then stripes the crude into various products like kero1, kero2, reduced crude oil, LPG, heavy gasoline and light gasoline using the distillation principle of difference in boiling points. KEY FACTS ? ? ? Installed in 1962. Revamped and modernized in 1986 & 2000. Capacity of 1MMTPA but currently processing 1. 3 MMTPA It’s an Atmospheric distillation unit (ATU) PROCESS DESCRIPTION A brief description of the different processes taking place in CDU is given on next page (2. 1. 2): 1.
Pre heating train before Desalter: Crude is pumped by pumps P1/1A at a pressure of 15 kg/cm2 through a series of heat exchangers where its temperature is elevated to 130o C. Crude is gaining heat in these exchangers from pre fractionated overhead vapours(in s-26), RCO( in s-29), SR Kero1 etc. Birla Institute of Technology & Science Pilani 333031 11 2. Desalter: Demulsifier mixed crude is fed at 130 o C to Desalter (v-101) where it is mixed with water through a mixing valve. The salt in crude dissolves in water and separated from the oil. Salt free crude on the other hand is pumped to Pre-topping column (CL-1) though 2 safety valves. 3.
Pre-topping Column (CL-1): The average temperature of crude before entering pretopping column is 238 oC (achieved through heat exchange).
The purpose of this column is to remove the straight run (SR) light gasoline and LPG from overhead and reduce the load on the main fractionating column. Shown below is the Flow diagram of CDU(2. 1. 1): 4. Atmospheric furnace (C 1 A): the crude coming from CL-1 bottom at 246 oC is fed to furnace. Furnace is fired by fuel oil (FO), Fuel gas (FG) or a mix of both supplied by DCU. The crude coming out of the furnace has a temperature of around 354 oC and goes to the flash zone of main fractionators’ column. 5.
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Main Fractionators column (CL-2): It’s the most important part of CDU. Cl-2 is where the actual distillation is taking place crude is separated into different products of different cut. Stripping stream enters at the bottom. The various subsystems of CL-1 are: Birla Institute of Technology & Science Pilani 333031 12 a) Overhead system: The overhead of the column of the water and the Straight rum Heavy Gasoline (SRHG) are separated. b) Stripper column: A stripping column (CL-3) is attached to the main fractionating column . Its function is to strip apart (separate) kero1, kero 2 and straight run gas oil coming from main fractionating column. ) CL-2 Bottom Section: The remaining crude oil called as the reduced crude oil (RCO) comes out of the bottom of CL-2. RCO forms the feed of DCU 6. Light Gasoline /Heavy Gasoline Processing System: Un-stabilized light gasoline from CDU pre-toping column is stabilized in the stabilizer where LPG is recovered from it while heavy gasoline is split in the two naphtha splitter to separate LN (Light Naphtha), RN (Reformer Naphtha) and HN (Heavy Naphtha).
The table below shows properties of the Products formed in the CDU(2. 1. 2): LN TBP cut HN Kero 1 Kero 2 SRGO RCO 65-135 135-165 165-190 190-300 300-370 370+ 11. 27 4. 90 8. 16 12. 11 22. 16 9. 7 0. 731 0. 786 0. 806 0. 866 0. 866 0. 9542 range Yield (% of crude) Density Birla Institute of Technology & Science Pilani 333031 13 CDU PRE-HEAT TRAIN (HEAT EXCHANGE IN CDU) Birla Institute of Technology & Science Pilani 333031 14 3. 1 INTRODUCTION As we have seen the temperature of the Crude oil needs to increase to 354 oC in CDU. This section is a discussion on how this is achieved. Why Crude needs to be heated in CDU? The various components of crude oil have different sizes, weights and boiling temperatures. Crude Distillation Unit is used to strip this crude into various products using the difference in their boiling points.
Therefore for distillation to take place the temperature of the Crude has to be elevated to around 354 oC so that all the fractions of crude oil can vaporize and get separated. How Temperature of Crude is increased in CDU? The Temperature increase of Crude oil consists of 2 major steps: 1. A 3 Stage Heat exchange (called CDU pre-heat train) of crude with product streams at high temperatures using shell & tube type heat exchangers. This step results in crude being supplied at 246 oC (elevated from ambient) to the next step ‘The atmospheric Furnace’. 2. The atmospheric furnace (C-1A) is then used to further elevate the temperature.
Modes of heat transfer in furnace are both convective and radiative the crude coming out of the furnace has a temperature of around 354 oC and goes to the flash zone of main distillation column (CL-2).
Furnace alone is not used because it would to too expensive and Energy consuming. By exchanging heat with products, the pre-heat train is serving twin purpose; The Crude is being heated at the same the temperatures of product streams of SRGO, RCO, KERO is brought down which was needed for their storage. This heat would have otherwise been wasted. Our point of discussion in this report is the First step i. e. The Heat exchanger Network’ which we will now study in detail. Birla Institute of Technology & Science Pilani 333031 15 3. 2 SHELL & TUBE HEAT EXCHANGER The Pre heat train uses shell & tube type heat exchager for heat transfer. ? It consist of two main things as its name implies Shell & Tubes ? The shell is a large vessel with a number of tubes inside it. ? The principle of operation: Two fluids of different temperatures are brought into close contact but they are not mixing with each other. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids.
The temperature of the two fluids will tend to equalize. The heat is simply exchanged from one fluid to the other and vice versa. No energy is added or removed. Common types of shell and tube exchangers: Birla Institute of Technology & Science Pilani 333031 16 3. 3 PRE-HEAT TRAIN: Description Our point of discussion in this report is first step of the 2 stages used for raising the temperature of crude i. e. the Heat exchanger network in which heat transfer occurs between crude and product streams of RCO, SRGO, and Kero 1. This network is also called the CDU pre-heat train.
In this process the temperature of the crude is elevated form ambient conditions at which it is supplied from OM&S to 246oC. The heat needed for this is coming from the product streams which are at high temperatures. Dual purpose is served in this way: heating of crude as well as the cooling of product streams (needed for their storage).
The pre-heat train is divided into three sections on the basis of temperature of crude flowing through them: 1. COLD section 2. MID section 3. HOT Section Shown below is the flow diagram of the CDU pre heat train (3. 3. 1): Birla Institute of Technology & Science Pilani 333031 17 1.
Cold Section (before Desalter): The crude from battery limit is pumped by pumps P-1/1A at about 15. 8 Kg/cm2 through a preheat train where it is heated to a temperature of 130 ? C before entering the Desalter (V-101).
The following table shows the shell side & tube side streams of all exchanger in this section along with their inlet-outlet temperatures (3. 3. 2):Exchanger Service Shell Tube Total Surface area Heat Duty Temperature shell tube I O I O S-26 Gasoline Crude 300 x 1 2. 11 107 90 40 65 S-29 RCO Crude 171 x 1 0. 47 116 100 54 60. 3 S-12A Crude Kero 2 148 x 1 0. 92 60. 3 72. 6 130 82 S-27A/B Crude SRGO 183 x 2 1. 39 72. 5 0. 3 173 85 S-7 Kero 2 Crude 93 x1 1. 48 199 130 90. 3 109 S-11A/B RCO Crude 134 x 3 1. 49 165 115 109 122 2. Mid Section (Between Desalter and pre-topping column): The crude from the Desalter is pumped by pump P-21 A/B and divided equally into 2 streams (Train A & B) for further heating before entering the Pre-topping column. P-21A/B? Train “A”: S-24, S-9A, S-23A/B, S-9B/C, S-22 CL-1 Train “B”: S-4A/B, S-12B, S-25A/B The crude in train A is preheated by hot-streams of SR Gas Oil (in S-24), RCO (in S-9A, S-23A/B), SR Gas Oil (in S-9B/C), RCO (in S-22) to about 236 ? C. While the crude in Birla Institute of Technology & Science
Pilani 333031 18 train B is preheated by SR kero circulating Reflux (CR) (in S-4A/B, S-12B) and RCO (in S-25A/B) to about 240 ? C. Desalted crude from both Train A and Train B streams are combined before entering the pre-topping column. Mixed preheat temperature achievable is around 238 ? C at which it is fed to pre-topping Column. Below is the table showing heat exchangers of mid section (3. 3. 3) Exchanger Shell Service Tube Total area Surface Heat Duty Temperature oC Shell Tube I S-9A Desalted Crude RCO S-23A/B RCO S-9B/C SRGO S-22 RCO S-4A/B Desalted Crude Desalted Crude RCO S-24 S-12B S-25A/B O I O SRGO – 0. 0 122 138 214 173 Desalted Crude Desalted Crude Desalted Crude Desalted Crude Kero CR 149 x 1 0. 61 184 195 138 152 148 x 2 1. 93 241 184 153 195 82 x 2 0. 54 272 214 195 218 149 x 1 0. 91 320 296 218 236 148 x 2 1. 67 122 161 190 164 Kero CR 148 x 2 1. 8 161 198 216 190 Desalted Crude 113. 15 x 2 2. 0 297 283 228 223 3. Hot Section (At CL-1 Bottom): The topped crude from CL-1 bottoms is pumped by pumps P-2/2A to exchangers S16/S-9D in series, where it is heated by SR Gas Oil Circulatory reflux. It then passes to exchanger number S-11C and further to S-21 where it is heated by Reduced Crude Oil (RCO) to about 246?
C. At this Temperature pre topped crude enters Atmospheric furnace (C-1A).
The table on next page gives details about every heat exchanger in this section (3. 3. 4).
Birla Institute of Technology & Science Pilani 333031 19 Exchanger Service Shell Total area Surface Heat Duty Tube Temperature oC Shell Tube I S-16/9D SRGO CR S-21 RCO S-11C RCO Skimmed crude Skimmed crude Skimmed crude O I O 186 x 2 2. 16 291 250 197 257 175 x 1 0. 61 336 320 239 246 134 x 3 1. 49 336 320 239 246 Birla Institute of Technology & Science Pilani 333031 20 HEAT EXCHANGER PERFORMANCE CALCULATION Birla Institute of Technology & Science
Pilani 333031 21 4. 1 CRITICAL HEAT EXCHANGER The critical heat exchangers are identified as the ones whose performance has a major impact on heat exchange rate, Production rate, product quality or environmental and health issues. Problem or inefficiency in any of the critical exchanger has a severe effect on overall heat exchange process occurring in the crude distillation unit. The engineers need to see that these exchangers are working efficiently for smooth running of the process (Though efficiency monitoring is important for every exchanger but the most attention has to be paid for Critical heat exchangers).
Criteria for Classification: 1. Fouling: is accumulation of unwanted material of heat exchanger surface is called fouling. Fouling is inevitable in heat exchanger but certain heat exchangers are more prone to fouling than others. These exchangers foul in short intervals and need to be cleaned frequently. Their regular maintenance is very necessary for the heat exchange process. E. g. is S-11/C in hot section is prone to frequent fouling. 2. Large surface area: Some heat exchanger have a very large heat transfer area which means they have a large contribution in total heat exchange taking place.
Their inefficiency or by-passing them would have a vital effect of final crude temperature. For example S-23 A/B in the mid section of heat exchanger trains. 3. Single heat exchanger in line: If a heat exchanger is single heat exchanger in the line like the one used in circulating reflux then it cannot be stopped or by-passed. For any maintenance work on them the whole unit has to shut down. Example is S16/9D. In next section the basic performance measurement calculations like heat, Duty, LMTD, Heat transfer coefficient is done for some of these critical heat exchangers. Birla Institute of Technology & Science
Pilani 333031 22 4. 2 PERFORMANCE ANALYSIS: Examples This section analyzes the performances of some main heat exchangers of pre-heat train through calculation of simple performance measurement tools like Heat duty, LMTD & Heat transfer coefficient. The definitions & formulas for these tools can be found in glossary at the end of the report. 1) S-25 A/B ? Type: Shell & tube ? Section: Mid ? Properties: Exchanger no. S-25A/B Service Shell No. of passes Shell Tube Tube RCO Desalted Crude 1 2 Total Surface area Heat Sq. M x no. of Duty element 113. 15 x 2 2. 0 (4. 2. 1) ? Design Data: Shell 54554 55454 296 241 (I) (O) 49. (uncorrected) Total flow (Kg/h) Operating temperature (? C) LMTD (? C) Tube 77586 77586 197. 7 240. 5 (I) (O) 47. 1 (corrected) (4. 2. 2) ? Practical data: Mass flow rate (RCO) = 59187. 5 Kg/hr S-25A/B Service Temperature, °C Shell Tube Post M&I Shell side I 297. 5 O 283. 7 Tube side I O 231. 9 245 Nov 11 Exchanger No. 263 253 198 RCO crude 203 (4. 2. 3) ? Calculations: 1. Heat Duty (design): M*Cp*(Ti –To) = 2002813. 7 Kcal/hr Birla Institute of Technology & Science Pilani 333031 23 2. Heat Duty (practical): 559336 Kcal/hr 3. Correction factor for LMTD (practical): 1.
01 4. LMTD (practical): 52. 12 (uncorrected), 51. 7(corrected) ? Observation The practical heat transfer of 559336 Kcal/hr is much lower than the design heat duty of 2002813. 7 Kcal/hr. ? Conclusions 1. The exchanger is not working efficiently 2. Due to fouling the temperature difference across the crude side is low which is reducing the total heat exchange in the exchanger. Birla Institute of Technology & Science Pilani 333031 24 2) S-11/C ? Type: Shell & tube ? Section: Hot ? Properties: It’s a critical heat exchanger because of the frequent fouling Service Exchanger no. Shell Tube S-11C RCO PreDesalter No. of passes Shell Tube 1 2 Total Surface area Sq. m x no. of lement 134 x 3 Heat Duty 1. 49 (4. 2. 4) ? Design data: Shell Tube Total flow (Kg/h) 54554 55454 155171 Operating temperature (? C) 336 (I) 320 (O) 239 (I) LMTD (? C) _ 1551 71 246 (O) _ (4. 2. 5) ? Practical data: Mass flow rate: 59871. 5 Exchanger No. S-11C Service Shell Tube RCO Skimmed Crude Post M&I Nov ‘11 Temperature, °C Shell side Tube side I O I O 327 315 257. 8 263 330. 2 275 231 255 (4. 2. 6) Birla Institute of Technology & Science Pilani 333031 25 ? Calculations: 1. Heat Duty (design): M*Cp*(Ti –To) = 6295320 Kcal/hr 2. Heat Duty (practical): 597382. 7 Kcal/hr 3. Correction factor for LMTD(practical) = 0. 75 4. LMTD (practical): 82. 84 (uncorrected), 80. 79(corrected) ? Observations: Heat transfer at present is 597382. 7 Kcal/hr which is satisfactory as compared to design heat transfer of 6295320. ? Conclusions: The present performance of the heat exchanger is satisfactory as compared to design. This little variation in the design heat duty and practical heat duty is due to variation in value of Cp of RCO with temperature. Birla Institute of Technology & Science Pilani 333031 26 3) S-23A/B ? Type : Shell & Tube ? Section: Mid (Train A) ? Properties: It is an critical heat exchager becase of its large heat transfer area
Service Total Surface area No. of passes Shell Tube Shell Tube (Sq. m x no. of element) RCO Desalted Crude 1 2 148 x 2 Heat Duty 1. 93 (4. 2. 7) ? Design Data: Property Shell Tube Total Flow (Kg/h) Temperature (? C) I 54554 241. 0 O 54554 184. 0 I 77585 152. 70 O 77585 195. 0 Specific Heat (Kcal/kg-? C) 0. 646 0. 597 0. 566 0. 617 LMTD (? C) 38. 21 35. 6 (4. 2. 8) ? Practical data: Mass flow rate (RCO) = 59187. 5 Kg/hr Temperature °C Service Shell RCO Tube crude Shell side Post M&I Nov ‘11 Tube side I 260. 2 O 230 I 145. 7 O 178. 8 253 230 142 160 (4. 2. 9) Birla Institute of Technology & Science Pilani 333031 27 Calculations: 1. Heat Duty (design): M*Cp*(Ti –To) = 1932602. 7 Kcal/hr 2. Heat Duty (practical): 1161000. 7 Kcal/hr 3. Correction factor: 4. LMTD (practical): (uncorrected), (corrected) ? Observations The practical heat transfer of 1161000. 7 Kcal/hr is much lower than the design heat duty of 1932602. 7. The value of LMTD on the other hand is actually higher in case of practical situation ? Conclusions The heat exchanger is not working efficiently. Due to fouling the temperature difference across the crude side is low which is reducing the total heat exchange in the exchanger even after having a high LMTD value.
Birla Institute of Technology & Science Pilani 333031 28 HEAT INTEGRATION Birla Institute of Technology & Science Pilani 333031 29 4. 1 HEAT INTEGRATION: Introduction In Today’s process industries like Guwahati refinery increasing energy efficiency is of prime importance. With the rising costs of input like crude, power the process has to be designed to have maximum energy recovery so as to reduce the costs. Heat integration is one of the ways to achieve this. 1. MEANING Heat integration is technique for designing a process to minimise energy consumption and maximise heat recovery.
Its part of the broader term ‘Process integration’ which is a holistic approach to process design which emphasizes the unity of the process and considers the interactions between different unit operations from the outset, rather than optimising them separately. 2. NEED FOR HEAT INTEGRATION Heat integration can lead to substantial reduction in the energy requirements of a process thus saving costs. It’s the answer to following questions: ? Are the existing processes as energy efficient as they should be? ? What changes can be made to increase energy efficiency without incurring any costs? What is the most important utility mix for the process? What investments can be made to increase energy efficiency? ? How to put energy efficiency & other targets like emission reduction, increasing plant capacity into one coherent strategic plan? 3. TOOLS FOR HEAT INTEGRATION ? Pinch Analysis The term pinch technology was introduced by Linnhoff to represent a set of thermodynamics based tools that that guarantee minimum energy levels in design of heat exchanger networks. Pinch Technology provides a systematic methodology for energy saving in processes & total sites. Its prime objective is to provide energy saving by better process heat integration. Here are some of its key features:
Birla Institute of Technology & Science Pilani 333031 30 1. Based on the first and second law of thermodynamics. 2. Pinch analysis is applicable for both new design as well as the retrofit systems. 3. It was developed for crude distillation systems but is now applicable to large number of process industries. 4. In addition to energy conservation Pinch technology provides general improvements 5. Some famous Pinch softwares are Pinch ExpressTM, Aspen PinchTM & SuperTargetTM ? Retrofit analysis Retrofit analysis is done to in old process processes to see what modifications suggested by pinch analysis are most suitable for the project.
It looks into the optimization of the process through energy capital trade off. In oil refining, retrofit designs are far more common than grass root applications. The retrofit targets are preferably achieved by re-using existing equipment more efficiently rather than installing new equipments and incurring new costs. 4. STEPS IN HEAT INTEGRATION Shown below are the different steps of heat integration (5. 1. 1) Birla Institute of Technology & Science Pilani 333031 31 5. 2 HEAT INTEGRATION IN CDU 1. IMPORTANCE OF HEAT INTEGRATION IN CDU Distillation is the largest single energy consumer in the Refinery.
Large section of oil is spent in fuelling the CDU itself. It is energy intensive process as the temperature of the crude has to be elevated to a high temperature of 354 oC. This increase in temperature is achieved by exchanging heat in various heat exchangers between crude and streams of RCO, SR Gasoline, Kero 2 etc which are at high temperature. Heat integration focuses on achieving maximum energy recovery from these streams through an optimized HEN so that the crude can be supplied at highest possible temperature to the furnace, thus saving energy.
A recent development in distillation technology has shown potential savings of up to 15-40 % through the heat? integrated exchanger network (pre-heat train) & distillation column. 2. HEAT INTEGRATION AT CDU GUWAHTI REFINEY What has been done? 1. The basis of heat integration in heat exchange process is putting process hot streams in thermal contact with process cold streams. We have already seen how product hot streams of RCO, SRGO etc at high temperature are used to exchange heat with the crude oil at low temperature. 2.
Designing of an Optimized Heat Exchanger Network in pre-heat train using heat integration tools like pinch analysis & retrofit analysis in July 2010. This design allows maximum energy heat recovery. What can be done? 1. Using heat integrated distillation columns. HIDC can save energy by recovering excess heat from the rectifying section for usage in the stripping section. Birla Institute of Technology & Science Pilani 333031 32 2. Seeing the interactions of HEN and distillation column and applying combined heat integration for whole unit.
