PHASE - II
LINEAR ALKYL BENZENE
(LAB)
Table of contents
List of figures ... i
List of Tables ... iii
Abbreviations ... iv
Nomenclature ... vi
CHAPTER 1: INTRODUCTION TO PRODUCT ... 1
1.1 History ... 2
1.2 Details of LAB Capacity ... 3
1.3 Market value of LAB ... 3
1.4 Competitors ... 3
1.5 Technology provider ... 3
1.6 Application ... 3
1.7 Physical, Chemical properties of LAB ... 4
CHAPTER 2: SELECTION OF PROCESS ... 5
Introduction ... 6
2.1 Raw Material Specifications ... 6
2.1.1 Availability & Transportation of Raw Material ... 6
2.1.2 Cost of raw materials ... 6
Properties of raw materials ... 6
2.2 Discussion on alternative technologies for the production of LAB ... 9
2.3 Selection of technology ... 10
2.4 Overview of the process ... 10
2.4.2 Block diagram of Process ... 11
2.4.3 Process description ... 11
CHAPTER 3: MATERIAL BALAN ... 49
3.1 INTRODUCTION ... 50
3.2 Overall Material Balance ... 50
3.3 BLOCK DIAGRAM OF OVERALL M.B ... 81
CHAPTER 4: ENERGY BALANCE ... 82
4.1 INTRODUCTION ... 83
CHAPTER 5: UTILITIES ... 109
5.1 Hydrogen Plant ... 110
5.3 Hot Oil Heater ... 114 5.4 Cooling tower ... 116 5.5 D.M.Water plant ... 117 5.6 Boiler ... 119 5.7 Flare system... 120 5.8 Instrument Air ... 121 5.9 Tank Farm ... 121 5.10 Pump house ... 123 5.11 Loading-Unloading ... 124
CHAPTER 6: DETAILED DESCRIPTION OF EQUIPMENTS... 125
CHAPTER 7: EQUIPMENT DESIGN ... 130
7.1 Stripper Column Design ... 131
7.2 Heat Exchanger ... 136
CHAPTER 8: PUMPS & CONTROL VALVE ... 148
8.1 Introduction ... 149
CHAPTER: 9 FIRE, SAFETY AND POLLUTION ... 154
9.1 Introduction to Safety ... 155
9.1.1 Safety equipments used in plant are: ... 155
9.1.2 The safety measures taken in the tanks are: ... 156
9.1.3 The safety measures taken in case of fire are: ... 156
9.1.4 Fire hazards: ... 156
9.1.5 Principle of protection & prevention: ... 158
9.2 Pollution Control ... 159
9.2.1 Effluent Treatment Plant ... 159
9.2.2 Process flow diagram... 160
9.2.3 Process flow description ... 160
CHAPTER 10: PLANT LOCATION & PLANT LAYOUT ... 163
10.1 Plant location ... 164
10.2 Plant layout ... 165
CHAPTER 11: COST ESTIMATION ... 169
11.1Introduction ... 170
CHAPTER 12: CONCLUSION... 179
i
List of figures
Pg no Figure 1.1 Chemical structure of LAB 03 Figure 2.4.1 Process block diagram 11
Figure 2.4.2 Stripper column 12
Figure 2.4.3 Rerun column 12
Figure 2.4.4 Nitrogen removal 15
Figure 2.4.5 Halide removal 16
Figure 2.4.6 Union fining reactor 17 Figure 2.4.7 Product stripper column 17 Figure 2.4.8 Light end stripper 18
Figure 2.4.9 Molex feed 23
Figure 2.4.10 Moving bed system 24 Figure 2.4.11 Adsorption chamber 26
Figure 2.4.12 Extract column 27
Figure 2.4.13 Desorbent stripper column 27 Figure 2.4.14 Raffinate column 28
Figure 2.4.15 Pacol reactor 33
Figure 2.4.16 Product stripper 34
Figure 2.4.17 Define reactor 39
Figure 2.4.18 Pep adsorption system 40 Figure 2.4.19 Desorbent column 41 Figure 2.4.20 Depentanizer column 41
Figure 2.4.21 Detal reactors 44
Figure 2.4.22 Benzene column 45
Figure 2.4.23 Paraffin column 45
Figure 2.4.24 Rerun column 46
Figure 2.4.25 Recycle column 46
Figure 3.1 Block Dia. Of Overall M.B. 81 Figure 5.1 Process flow diagram of H2 Plant 110
ii Figure 5.3 Process flow diagram of Hot oil heater 115
Figure 5.4 Diagram of cooling tower 116 Figure 5.5 Process flow diagram of D.M water plant 117 Figure 5.6 Boiler process block diagram 119
Figure 8.1 Cascade control 151
Figure 8.2 Ratio Control 152
Figure 9.1 Process flow dia. Of ETP 159 Figure 10.1 Outside battery limit of plant 165 Figure 10.2 Inside battery limit of plant 166
iii
List of Tables
Pg no Table 1.1 Physical & Chemical Properties of LAB 4 Table 2.2.1 Properties of Kerosene 6 Table 2.2.2 Properties of n-Pentane 7 Table 2.2.3 Properties of iso-octane 8 Table 2.2.4 Properties of Benzene 8 Table 2.2.5 Properties of Hydrogen 9 Table 2.4.1 Contents of hydrotreater catalyst 15 Table 2.4.2 Suction and discharge pressure for MUG compressor 21 Table 2.4.3 Contents of Molex adsorbent 22 Table 2.4.4 Contents of Pacol catalyst 33 Table 2.4.5 Data of Pacol CFE inlet-outlet temperature 36 Table 2.4.6 Contents of Define catalyst 38 Table 3.1 Overall material balance for frontend 63 Table 3.2 Overall material balance of backend 80 Table 3.3 Overall material balance of Plant 81 Table 4.1 Energy balance summary table 107 Table 5.1 Water specification for D.M water plant 118 Table 5.2 Data of Steam production 119 Table 5.3 Data of Steam consumption 120 Table 5.4 Data of Water specification for boiler 120 Table 5.5 Data of storage Tank and its capacity 122 Table 5.6 Data of Pump type and its capacity 123 Table 5.7 Data of unloading point for raw material 124
Table 9.1 Fire extinguishers 157
Table 9.2 Data of Final treated quality of ETP 160 Table 10.1 Color coding of plant 167
iv
Abbreviations
LAB Linear alkyl benzene HAB Heavy alkyl benzene
PF Pre-fractionation
UF Union fining
MOLEX Molecular extraction TNP Total normal paraffin TNN Total non normal paraffin PACOL Paraffin converted to olefin PEP Pacol enhancement process DETAL Detergent Alkylation
HO Hot oil
MUG Make up gas compressor
HOH Hot oil heater
ETP Effluent treatment plant LPFD Low pressure feed drum HPS High pressure separator DSD Desorbent surge drum CMI Coplanar manifold index KSC Kilogram per square centimeter LES Light end stripper
v
FFC Fin fan cooler
SWS Sour water stream
UOP Universal oil product EMD Extract mixing drum RMD Raffinate mixing drum
vi
Nomenclature
Symbol Full Form SI Unit Cp Specific heat kJ/kg k
Latent heat of vaporization kJ/kg
M Mass flow rate Kg/hr
Q Heat flow rate kJ/hr
Eo Overall tray efficiency
V Vapor flow rate m3/hr
Vc Column velocity m/s D Diameter M A Area m2 Ρ Density Kg/m3 Co Orifice co-efficient Vo Hole velocity m/s H Height M P Pressure KPa T Temperature C Μ Viscosity Kg/ms E Efficiency T Thickness Mm
vii tr` Roof plate thickness Mm
1
2
1.1 History
In 1939, the soap industry began to create detergents using surfactants that were supplied to the soap manufacturers by the petro-chemical industry. Because the synthetic detergents produced from these surfactants were a substantial improvement over soap products in use at the time, they soon gave rise to a global synthetic detergent industry.
In late 1940s, UOP developed a process to economically produce commercial quantities of Do-Decyl Benzene Sulphonates (DDBS), which became one of the surfactants most widely used in synthetic detergents at that time.
In the late 1950s, it was found that DDBS had a slow rate of biodegradation that resulted in generation of large amounts of foam in surface waters, such as rivers and streams. UOP responded to the industry need for the more bio-degradable detergents by developing process technology in the 1960s to produce Linear Alkyl Benzene (LAB), a new surfactant raw material used to make Linear Alkyl Benzene Sulphonate (LAS). LAS were deemed to be a much more bio-degradable surfactant and to this day, they are one of the main building blocks in the manufacture of detergents.
The popularity of LAB can be attributed to excellent LAS surfactant properties, it’s bio-degradability, and it’s low cost of manufacture compared to other surfactant raw materials. Over the past several decades, worldwide demand for LAB has continued to grow.
Linear alkyl benzene referred to as LAB is an intermediate in detergent production.
The chemical structure of LAB is shown in figures below:
Figure 1.1 Chemical structure of LAB
3
1.2 Details of LAB Capacity
INDIA is one of the largest producer of LAB in the world. In Indian LAB market there are five major producers of LAB. Namely NIRMA (Savli), IOCL, RELIANCE, Tamilnadu Petro-Chemicals.
India is reeling under the oversupply of LAB, as the domestic demand is lower than the domestic capacity and production. The total domestic demand is estimated at around 300000 TPA, while the production capacity is close to 500000 TPA. They are exporting their surplus to ensure higher capacity utilization.
Reliance is the largest industry in the Indian LAB sector with the dominant share of the capacity amounting to 185000 TPA. RIL is also the 5th largest producer of LAB in the world.
Tamilnadu petro-chemicals and IOCL have installed LAB capacity of 120000 TPA. And NIRMA located at Savli has the installed capacity of 75000 TPA.
Overall capacity of LAB in the world is around 3.5 million TPA.
1.3 Market value of LAB
Market value of LAB in the India is ranging from 78000 to 100000 Rs./tons
1.4 Competitors
Technical
Mainly all the LAB plants are prepared by the UOP (Universal Oil Product) – A Honeywell Company. But it has competition with BASF, DOU etc.
Commercial
Commercially IOCL, RIL, NIRMA & Tamilnadu Petro-Chemicals are major competitors.
1.5 Technology provider
Technology provider for all the major LAB plants is UOP (Universal Oil Product) – A Honeywll Company.
1.6 Application
LAB is the most common raw material for the manufacture of bio-degradable household detergents. It is sulphonated to produce linear alkyl benzene sulphonate (LAS).4
2% Agricultural Herbicides
Emulsion polymerisation
Electrical cable oil
Wetting agent
Ink solvent
Paint industry1.7 Physical, Chemical properties of LAB
Table 1.1 Properties of LAB
Property Specification
Appearance Clear colorless liquid
Odor Odor less
Boiling Point 282 – 302 oC
Flash Point 130 oC
Aniline Point 15.9
Average Molecular Weight 235 – 239 Kg/Kmol Specific Gravity at 20oC 0.855 – 0.870
Kinematic Viscosity at 40oC 4.3 centistokes Vapor Pressure at 20oC 0.01mmHg Bromine Index 10 max mg/100g
5
6
Introduction
In this chapter we discussed about raw material specification, its suppliers, and properties of materials to be used, various routes by which the LAB product produced and final selection of the process which is most suitable for the .
The commercial development of LAB focused on the extraction of high purity linear paraffin derived from kerosene feed. This linear paraffin was dehydrogenated to linear internal mono-olefins. Using a catalyst dehydrogenated effluent was used to alkylate benzene to produce LAB. The resulting LAB product became the detergent intermediate for the production of linear alkyl benzene sulfonate which is a major biodegradable synthetic surfactant which replaced do-decyl benzene having slow rates of biodegradation.
2.1 Raw Material Specifications
2.1.1 Availability & Transportation of Raw Material
Mainly raw materials are supplied from Kerosene – IOCL (Pipe-line)
Benzene – Reliance (By road through tankers)
N-pentane – PPL, Oriented Ltd. (By road through tankers)
Fuel oil, Naphtha – HPCL, BPCL, IOCL (Pipe-line)
LPG – IOCL, HPCL, BPCL (Pipe-line)
2.1.2 Cost of raw materials
Benzene - 55000 Rs./ton
N-paraffin – 75000 Rs./ton
Kerosene – 15000 Rs./ton
Properties of raw materials
2.1.1 Kerosene
Table 2.1 Properties of Kerosene
Formula C7 to C17.
State Liquid.
7 Boiling Point Range 175-265 0C.
Specific Gravity 0.8
Smoke point 18 mm
Flammable Yes.
Water Solubility Very less.
Bromine index 2 max
Flash point 42C
2.1.2 n-Pentane
Table 2.2 Properties of n-Pentane
Formula C5H12.
State Liquid
Color Colorless
Molecular weight 72.2Kg/Kmol Boiling Point Range 360C
Specific Gravity 0.63
Flammable Yes
Water Solubility Partially soluble Melting point 129.70C
Flash point -35C
8
Table 2.3 Properties of iso-octane
Formula (CH3)3.CH2.CH.(CH3)2
State Liquid
Color Colorless
Molecular weight 119.2Kg/Kmol Boiling Point Range 99.2 0C
Specific Gravity 0.692
Flammable Yes
Water Solubility Insoluble
Flash point -10C
2.1.4 Benzene
Table 2.4 Properties of Benzene
Formula C6H6
State Liquid
Molecular weight 78 Kg/Kmol Boiling Point Range 80 – 85 0C Specific Gravity 0.87
Flammable Yes
Water Solubility Very less
Flash point -11C
9
Table 2.5 Properties of Hydrogen
Appearance Colorless
Odor Odorless
Stability Stable
Specific Gravity 0.069 Auto Ignition Temperature (oC) 570
Flammability Flammable
2.2 Discussion on alternative technologies for the production of LAB
There are five production processes of LAB[1] UOP/HF n-paraffin process:
The HF process involving dehydrogenation of n-paraffin to olefins & subsequent reaction with benzene using HF as catalyst. These process accounts for the majority of the installed LAB production in the world, It includes a PACOL stage where n-paraffin are converted to mono-olefins a Define unit whose primary function is to convert residual diolefin to mono-olefin a Pep unit and alkylation step where alkylation of benzene is done by reaction between benzene & paraffin by using HF acid as catalyst.
[2] UOP/Detal process:
This is a newer technology & has several of stages same as in the HF process but it is principally different in the benzene alkylation step, during which a solid-state catalyst (AlSiF4) is employed.
[3] Friedel-craft alkylation:
Friedel-craft involves chlorination of n-paraffin to mono chloro paraffin followed by benzene Alkylation with AlCl3 catalyst. This is the oldest process.
[4] HF /olefin process:
Purchased olefins reacted with benzene in presence of HF or AlCl3 catalyst.
10 In this process chlorination of n-paraffin to mono-chlorinated paraffin followed by dechlorination to produce olefins & subsequent benzene alkylation.
2.3 Selection of technology
Several LAB production processes are reviewed. The emphasis is on the Detal & HF processes as these are the dominant technologies in the LAB industry today.
UOP HF process involve the problem of corrosion, catalyst neutralization, disposal of HF & environmental concerns while Detal technology is very safe, non-corrosive ,ecofriendly & zero discharge. Detal process uses solid catalyst which is re-generable over life of 2 years, so it is also economically viable.
From the overall observations Detal process is preferred for LAB production.
2.4 Overview of the process
The process plant is divided into two main sections. These two sections contain process units.
[1] Front end
Pre-fractionation (PF) Union fining (UF)
Molecular extraction (MOLEX)
[2] Back end
Paraffin converted to olefin (Pacol) Pacol Enhancement Process (Pep)
Di-olefins Conversion to Mono Olefins (Define) Detergent Alkylation (Detal)
Kerosene Pre-fractionation is used to tailor the kerosene feed to the desired carbon range. Kerosene is stripped off light ends and heavier ends so that the heart cut, containing the desired n-paraffin for the production of LAB of a certain range of molecular weight is produced. The Distillate Union Fining process hydro treats kerosene at sufficient severity to remove sulphur, nitrogen, olefins, and oxygenates compounds which might poison the Molex adsorbent.
The Molex process is a liquid state separation of n-paraffin from branched and cyclic components using Sorbex Technology. The simulated moving bed adsorptive separation results from using a proprietary multiport rotary valve. The extract stream is a high purity n-paraffin
11 stream. The raffinate stream, consist mainly of iso-kerosene or cyclic-kerosene range compounds.
I
n Pacol process, the n-paraffin is de-hydrogenated in a vapor phase reaction to produce corresponding mono-olefins over a highly selective and active catalyst. The Define process is a liquid phase selective hydrogenation of di-olefins in the Pacol reactor effluent to corresponding mono-olefins over a catalyst bed.The P.E.P process allows the selective removal of aromatics in the feed to the Detal. The Detal process is a solid catalyst fixed bed process in which benzene is alkylated with mono-olefins produced in Pacol Unit to produce LAB
2.4.2 Block diagram of Process
Figure 2.4.2.1 Process block diagram
2.4.3 Process description
A. Front end:
2.4.3.1 Pre-fractionation (PF)
2.4.3.1.1 Introduction
LAB manufacturing requires special type of feed. To get this specification Pre-fractionation is used. The feed to the Pre-Pre-fractionation unit is straight run Kerosene, which contains carbon range C7 to C17. This stream contains considerably more nonlinear
12 and one rerun column. The carbon range for LAB feed is from nC10 to nC13 for light LAB
product and nC11 to nC14 for heavy LAB product. Stripper column removes lighter components
up to C9 and rerun column removes C14 to C17 the heavier components. The product stream
from rerun column is called “Heart-cut” which contain C10-C13 carbon range along with
contaminants like organic sulphur, nitrogen & metal compounds.
2.4.3.1.2 Process flow diagram
Figure 2.4.1: Stripper column
Figure 2.4.2: Rerun column
13 Supply kerosene from the storage tank is pumped through fresh feed/rerun bottom exchanger where bottom exchanger pre-heats the feed to 910C & then to feed/rerun pump around exchanger where it is heated from 910C to1380C & fed to 26th tray of the stripper column. In the Stripper columns the lighter ends C7-C9 are stripped on temperature difference
& removed from top. The heat load to the stripper column is supplied by thermo siphon type re-boiler & the heating medium used is circulating hot oil [Therminol].
The stripper column overhead vapours are condensed in fin fan cooler, where it is cooled from 158 °C to 770C. The condensed liquid is collected in receiver. Before the stripper
overhead is send to fin fan cooler, water wash is given for dilution of the halide impurities which may corrode the fin pipes. From the receiver, the non-condensable goes to the flare header. The receiver floats on the flare header pressure, positive nitrogen pressure given as purge eliminates any possibility of back flow to the receiver which may lead to contamination. The condensed liquid in the receiver separates into water & kerosene. The sour water collects in the receiver boot and is send to Effluent Treatment Plant (ETP).One stream of the receiver liquid is sent as reflux to the column & the other stream is sent to return kerosene storage tanks. The bottom product from stripper column is pumped by stripper column bottom pump & fed to the 27th tray of the rerun column. This stream contains C10 & other heavier
hydrocarbons and will be at about 236°C.This stream is sent to the rerun column. This column is provided with two re-boilers [one as stand by].Thermo siphon re-boiler supplies heat to the rerun column. The heating medium used is circulating hot oil [Therminol] & its flow is controlled by Flow Control Valve. This column is operated under vacuum & its vacuum is maintained by the vacuum pump.
The overhead of rerun column is C10-C13 heart cut. The overhead vapours (O/H
vapours) are condensed in built in packed bed contact condenser by the flow controlled cold reflux & collected in the O/H accumulator located below contact condenser. The temperature of the accumulator tray is around 159 °C. The rerun column O/H pumps take suction from the accumulator and delivers into three separate streams. The first stream is sent as hot reflux on the first tray controlled by Flow Control Valve (FCV), which is cascaded with TRC [41st tray
temperature].The second stream is taken as a side stream, downstream of feed/rerun pump around exchanger routed through rerun pump around cooler cooled to 55°C and sent to the top of contact condenser as cold reflux controlled by FCV. The third stream is the feed to UF unit. It has a carbon range of C10-C13 hydrocarbons. The bottom products from the rerun column are
pumped by rerun bottom pump to the kerosene tanks via feed/rerun bottom exchanger and return kerosene cooler. This stream will have carbon range of C14-C17 hydrocarbons.
14
2.4.3.1.4 Process equipments
[1] Stripper column
Stripper column consists of 50 trays. The feed enters on the 26th tray. It has a narrow
cross sectional area at the top while it is broad from bottom. The input of heat is from bottom through horizontal Thermo-siphon type re-boiler. The heat input is the only independent variable which will affect the reflux rate and as a result the distillation efficiency of the column. The stripper bottom is pumped from column on level controller and sent directly to rerun column.
[2] Rerun column
The Rerun column consists of 50 trays. There is no separate storage tank on top but there is an inbuilt accumulator which stores the heart cut. Heat input to this column is provided by Hot Oil circulation to the Re-boiler. The Rerun column is operated under vacuum to minimize the required heat input. The vacuum conditions are maintained by a line from top of column to LRVP.
2.4.3.2 Union fining (UF)
2.4.3.2.1 Introduction
Contaminants like Sulphur, Nitrogen and Metal compounds are present in the
petroleum fraction. Purpose of Union fining process is to remove these contaminants as they lead to problems like increase in air pollution, corrosion & difficulties in further processing of material [damage the molecular sieves used in MOLEX].Union fining is a catalytic, fixed bed process developed by UOP for hydro treating a wide range of feed stocks. This process uses a catalytic hydrogenation method to upgrade the quality of petroleum fractions by decomposing contaminants with negligible effect on the boiling range of the feed. This process removes sulphur & nitrogen & saturates olefin & aromatic compounds while reducing other contaminants like oxygenates & organ metallic compounds.
The hydrogenation of feed is obtained by processing the feedstock over a fixed bed of catalyst in the presence of large amount of hydrogen. UNIONFINING is a fixed bed catalytic process in which “NIMOX” catalyst with alumina base is used for removal of these contaminants by hydro treating. After hydro treating reaction 0.2wt% sulphur & 0.02wt% nitrogen are permissible.
15 UF reactor catalyst: The Hydro treater catalyst consists of oxides of nickel and molybdenum impregnated on an alumina base. The catalyst is prepared either as a sphere or an extrudate with special shapes. The catalyst is yellowish green in colour and odourless.
Table 2.4.1 Contents of Hydro treater Catalyst
Content Weight Percent
Aluminum Oxide 65 – 80
Molybdenum Trioxide 10 – 19
Phosphorus Oxide 02 – 08
Nickel Oxide 01 – 05
2.4.3.2.2 Hydro treating chemistry
The following chemical steps and reactions occur during hydro treating process
[1] Sulphur Removal
Typical feed stocks of crude oil contain simple Mercaptans, Sulphides and Di-sulphides, which can be easily converted to Hydrogen Sulphide (H2S). Feed stocks containing heteroatom
molecules difficult to process. De-sulphurization of heteroatom compounds proceeds as: Initial ring opening.
Sulphur removal
Saturation of resulting olefin
[2] Nitrogen removal
De-Nitrogenation is more difficult than De-sulphurization. Side reactions may yield nitrogen compounds more difficult to hydrogenate than the original reactant. Saturation of heterocyclic rings is also hindered by large attached groups. The de-Nitrogenation of the heteroatom rings proceeds as:
Aromatic ring Saturation Ring hydrogenolysis De-Nitrogenation For example Quinoline
Fig 2.4.4: Nitrogen removal 4
16
[3] Oxygen Removal
The organically combined oxygen is removed by hydrogenation of the carbon – hydroxyl bond forming water and the corresponding hydrocarbon.
[4] Olefin Saturation
The Olefin saturation reaction proceeds very rapidly. It has very high heat of reaction. a. Linear Olefin
R-C=C-C-C-R’ + H2 R-C-C-C-C-R’ (and isomers)
[5] Aromatic Saturation
Aromatic Saturation reactions are most difficult and are highly exothermic in nature.
[6] Halides Removal
Organic Halides such as chlorides and bromides are decomposed in the reactor.
Decomposition of organic halides is considered difficult with a maximum removal of ~90%.
Fig 2.4.5: Halide removal [7] Metal Removal
Crude Oil contains metals like nickel, vanadium, lead etc. Iron is also present, which is corrosion product. Sodium, Calcium and Magnesium are also present due to contact of feed with salt water or additives. Improper use of additives to protect stripper overhead systems from corrosion or to control foaming account for the presence of phosphorus and silicon. The mechanism of the decomposition of organ metallic compounds is not well understood. However, it is known that metals are retained on the catalyst by a combination of adsorption and chemical reaction. The catalyst has a certain maximum tolerance for retaining metals. Removal of metals normally occurs in plug flow fashion with respect to the catalyst bed. Metal removal is essentially complete above temperature of 315oC to a metals loading of 2 – 3 wt% of the total catalyst.
17
Figure 2.4.6: Union fining reactor
18
Figure 2.4.8: Light end stripper
2.4.3.2.4 Process flow description
Product from pre fraction unit is stored in the feed surge drum of union fining unit. By using sun dyne pump it is pumped to combined feed heat exchanger, here the temperature of feed is increases up to 2920C and again it is sent to charge heater for further increasing of temperature. Outlet of charge heater is at 3110C. The heat exchanger is known as combined feed heat exchanger because here H2 and feed both are heated with the outlet of catalytic bed
reactor by using sun dyne pump the pressure is increases up to 75 – 80 kg/cm2.
In the catalytic bed there are two beds provided. In the reactor hydrogenation reaction is carried out, & hydrogenation reactions are exothermic in nature. The reactor contains two beds to maintain temperature by providing quenching hydrogen in between the beds. In this catalytic bed reactor NIMOX catalyst is used. It is Nickel with Molybdenum oxide catalyst on alumina base. In the reactor, temperature and pressure requirement are high because for sulfur removal high temperature is required and for nitrogen removal high pressure is required. At the output of reactor water injection and hydrogen addition is carried out. Water injection is carried out in order to dissolve the (NH3)2S formed during the reaction. In the reactor the H2/ HC ratio
19 The makeup gas comes from MUG compressor’s 4th stage. By using number of compressors in series the pressure is increases from 2 kg/cm2 to 80 kg/cm2.This outlet of reactor goes to fin fan cooler and is send to high pressure separator. Here the pressure is reduced by upto7 kg/cm2. At the top of separator hydrogen is separated and that hydrogen is sent to recycle gas compressor. The liquid of high pressure separator is then sent to low pressure separator. The reduction of pressure is carried out by using angle valve.
From low pressure separator the liquid is sent to product stripper column, and off gases are removed from the top. These off gases are used in Hot Oil Heater (H.O.H.) as a fuel. Over head of product stripper column is cooled in fin fan cooler and sent to receiver. From the boot of receiver the sour water is collected, which goes to STP plant. One fraction of liquid from receiver is recycled back to product stripper and the other fraction is sent to light end stripper column. The bottom of product stripper column is the final product of the Union fining unit which is feed for the MOLEX unit. The light end stripper column bottom is sent to return kerosene tank. This column is on the total reflux condition
2.4.3.2.5 Process Equipments
[1] Reactor:A Kerosene Union Fining reactor is typically constructed of 1.25 Cr-0.25 Mo, 2.25 Cr–1
Mo base metals with S.S lining. The alloy is selected on excellent corrosion resistant properties. Reactor has two beds of catalysts with one inter bed quenching zone.
The Reactor consists: 1. Inlet Diffuser
It is inserted into the inlet nozzle to eliminate a symmetric flow pattern, reduce fluid velocity and distribute the liquid evenly across the tray.
2. Vapor/Liquid Distribution tray
Optimum catalyst performance is achieved when efficient contact of reactant is provided. The tray is fabricated sections by beams and a ring on vessel wall. Cylindrical risers with slotted caps are evenly spaced across top of tray.
3. Quenched Section
The reaction system is divided into multiple catalyst bed with each bed separated by quench section. The quench assembly is designed to thoroughly mix quench gas with effluent from previous bed and re-distribute the reactants uniformly over the top of next catalyst bed.
20 This is a vertical vessel constructed of carbon Steel. It is made up of number of sieve trays which will vary depending on units designed. Feed is introduced towards the middle of columns. The stripper is typically re-boiled with circulating Hot Oil. The stripper bottom is pumped out from bottom of column while vapor flows to overhead condenser. Liquid reflux is returned to top of column above tray number 1.
[3] Light ends stripper column:
The column is a vertical carbon steel vessel which is fitted with internals to support two packed beds for vapor liquid contact.
[4] Charge heater:
The Charge Heater of UF section is made up of S.S 34%. It produces the desired reaction temperature of 311°C.
It consists of two sections
(i) Radiation Section: Consists of 28 vertical tubes. Feed passes through the tubes.
(ii) Convection Section: Consists of 18 horizontal tubes. Feed passed through the tubes where
it is heated by convection currents of flue gases rising. The charge heater is single pass. The source of heat is 3 burners. Fuel Oil is used as fuel &
reaction temperature of 311°C is obtained. [5] Sun dyne pump:
Sun dyne pump is also called vertical pump, it is used for high flow and high pressure. Here we need high pressure to keep the kerosene in liquid phase. It consists of one main shaft which is coupled with motor which rotates at 30000 rpm. There are two gears, one having small grooves fixed with other having large grooves. Again this large gear is grooved with small. The arrangement is such that one revolution of large gear produces 3-4 revolutions of smaller grooves. The pump produces a discharge pressure of 117 kg/cm2 .It is high speed pump with 20600 rpm.
[6] Recycle gas compressor:
It is constructed of killed carbon steel with 316 SS mesh blanket for entrained liquid removal located towards top of the reactor. Gas enters side of vessel and leaves out from the top and condensed liquid is drained periodically from bottom. It is single stage double acting compressor. There are two pistons and two cylinders for continuous discharge. H2 gas from
HPS goes to cooler. Thus liquid particles get separated and gas then goes to separator. It has a mesh blanket. The suction pressure of R.G compressor is 68kg/cm2 and discharge pressure is 78 kg/cm2.
21 [7] Make up gas compressor:
It is constructed of killed carbon steel. It is four stage single acting compressor. The suction and discharge pressure of the four stages are:
Table 2.4.2 Suction & Discharge pressure for MUG compressor
Suction pressure Discharge pressure
1st Stage 1.8 7
2nd Stage 7 18
3rd Stage 18 39
4th Stage 39 80
The H2 gas feed to this compressor is from PACOL unit. If PACOL unit is closed H2 gas is
added to the third stage from hydrogen plant. The first two stages run on spill back. Thus it increases pressure from 1.8 kg/cm2 to 80 kg/cm2.
2.4.3.3 Molecular extraction (Molex)
2.4.3.3.1 Introduction
Molex stands for Molecular Extraction. The product of Union Fining has a mixture of Normal (Linear) and Non-normal (Branched) Paraffin. They have almost the same boiling point. The UOP MOLEX Process is an effective method of continuously separating Normal Paraffin from a stream of Normal and Non-normal by means of physical selective Adsorption. The feedstock, essentially having same properties of kerosene is separated into high purity Normal Paraffin section at high recoveries and a Non-normal fraction.
The Process includes Counter-Current contact between a fixed bed Adsorbent and the feed stream. It uses a solid adsorbent, liquid desorbent and a flow directing device called a Rotary Valve. The Molex Process does the separation by adsorption Process. Adsorption can be defined as the adheration of liquid or gas on solid surface. The solid surface is called the Adsorbent. It is convenient to visualize the Adsorbent as a porous solid having certain characteristics. When the solids are immersed in a liquid mixture, the pores become filled with liquid. The Adsorbent employed in the Molex Process is a specially designed Molecular Sieve which is made of Zeolite Crystals.
The pore diameter of the Sieve is selected so that Normal Paraffin can pass through the pores and other species are retained or excluded because of their sizes. The non adsorbed
22 Branched and Cyclic Paraffin referred to as Non-normal may become entrained in the large sieve voids but easily removed by washing Adsorbent with a Non-desorptive Hydro-carbon, such as iso-Octane (iC8). This effectively flushes the Non-normal, while easily leaving the
Adsorbed Paraffin intact. To displace Normal Paraffin from the selective pore, short linear chained Paraffin such as Normal Pentane (nC5) must be used. By virtue of its short length and
small diameter the nC5 is extremely mobile and can pass into selective pores of the Sieve and
displace the larger C10-C13 Normal Paraffin.
Table 2.4.3 Contents of Molex Adsorbent
Content Weight % Silicon Oxide < 50 Aluminum Oxide < 40 Calcium Oxide < 20 Sodium Oxide < 15
Adsorbent theory:
The adsorbent is a porous solid having certain characteristics. Each adsorbent piece is
composed of crystals of Zeolite. When the solid is immersed in a liquid mixture, the pores become filled with liquid. At equilibrium (Equilibrium is the term used to describe a situation where no net change is occurring.), the composition of the liquid in the pores will be different from that of the liquid surrounding the particles. The adsorbent is said to be physically selective for the component that is more concentrated in the pores than in the surrounding liquid.The structures of the Molex Feed constituents are shown in the figures below.
N-Paraffin
23 Alkyl Naphthene
Alkyl Aromatic
Fig 2.4.9 Molex feed
From the structures, it may be seen that the n-paraffin has a much smaller maximum
diameter (in plane normal to the carbon – carbon bonds) than the other species present. The pore diameter of the sieve is selected so that the n – paraffin can pass through the pores and into the cavities within the crystal structure, while the other species are excluded because of their size. It is shown in Figure 4.6 below.The non – adsorbed branched and cyclic paraffins referred to as non – normals, may become entrained in the large sieve voids but are easily removed by washing the adsorbent with a non – desorptive hydrocarbon such as iso-octane (iC8). The iC8 effectively flushes away
the non – normals while leaving the adsorbed n-paraffins intact.
To displace the n-paraffins from the selective pores, a short linear chained paraffins such as n-pentane (nC5) must be used. By virtue of its short length and small diameter, the nC5
is extremely mobile and can easily pass into the selective pores of the sieve and displace the larger C10 – C13 n-paraffins.
Adsorptive separation with moving bed
24
The adsorbent circulates continuously as a dense bed, in a closed cycle, and moves
up the adsorbent chamber from bottom to top. Liquid streams flow down through the bed, countercurrent to the solid. For simplicity, the feed is assumed to be a binary mixture A and B, with component a being more selectively adsorbed relative to B. Feed is introduced to the bed as shown.Desorbent, D is introduced to the moving bed model at a point above the extract
location. The desorbent is a liquid of a higher boiling point than the feed components and having a high adsorbent selectively. This means that the desorbent can desorbs the feed components from the adsorbent and in downstream fractionation can be separated from the feed components.Raffinate product, consisting of the less strongly adsorbed component B mixed with desorbent is withdrawn as shown from a position below the feed entry. Extract product, consisting of the more strongly adsorbed component A mixed with desorbent is withdrawn from the chamber above the feed point. Only a portion of the flowing liquid in the bed is withdrawn, and the remainder continues to flow in a closed loop.
The positions of introduction and withdrawal of net streams divide the bed into four main zones, each of which performs a different function. The zones are described below:
Zone I Adsorption Zone:
Zone I is defined as the section between the Feed and Raffinate points. The primary function of Zone I is to adsorb A from the liquid. The solid entering the bottom of this zone carries only B and D in its pores. As the liquid stream flows downward, countercurrent to this solid, component A is transferred from the liquid stream into the pores of the solid. At the same time, some of the components B and D are desorbed from the pores due to concentration driving forces and selectivity differences. This means it is transferred from the pores to the liquid stream making room for A in the pores. Zone 1 is the zone in which normal paraffin is adsorbed from the liquid phase. Thus, it is referred to as the adsorption zone.
Zone II Purification Zone:
Zone II is defined as the section between the extract and feed points. The primary function of Zone 2 is to remove B from the pores of the solid. When the solid arrives at the feed point, the pores will contain the quantity of A that was adsorbed in Zone I. However, the pores will also contain a large quantity of B, because the solid does not make a perfect separation. The liquid entering the top of Zone 2 contains no B – only A and D. As the solid moves upward, B is gradually displaced from the pores and is replaced by A and D. Thus, when
25 the solid arrives at the top of Zone 2, the pores will contain only A and D. By proper regulation of the liquid rate in Zone 2, B can be desorbed almost completely from the pores. This can be done without simultaneously desorbing all of A, because A is more strongly adsorbed than B. Zone 2 is the zone in which normal paraffin is purified. Thus, is referred to as the purification zone.
Zone III Desorption Zone
Zone III is defined as the section between the desorbent and extract points. The function of this zone is to desorb A from the pores. The solid entering the bottom of the zone carries A and D in the pores; the liquid entering the top of the zone consists of pure D. As the solid rises, A in the pores is displaced by D. Zone 3 is the zone in which normal paraffin is desorbed from the solid. Thus, it is referred to as the desorption zone.
Zone IV Buffer Zone:
Zone IV is defined as the section between the Raffinate and desorbent points. The purpose of Zone 4 is to keep components B, which is at the bottom of Zone 1, from entering Zone 4 and flowing through Zone 4 to Zone 3 where it can contaminate the extract material. If the flow rate is set such that desorbent flows up in Zone 4, raffinate material would be prevented from gaining access to Zone 3 where it would contaminate the purified extract stream. This means that the main function of Zone 4 is to separate Zone 3 from Zone 1 and as a result it is referred to as the buffer zone.
For the liquid-solid system each stage has to mix the solid with the liquid and subsequently separate the two phases after equilibrium is reached. The liquid and solid can then be passed on to the next stages. To make another comparison with distillation, the liquid could be seen as passing up through a column with trays that the solid is passing down through. At each tray the solid would be pushed across. The solid would fall to the tray below and the liquid would pass to the tray above.
26
2.4.3.3.2 Process flow diagram
Figure 2.4.11: Adsorption Chamber
27
Figure 2.4.13: Desorbent stripper column
28
2.4.3.3.3 Process flow description
Feed is pumped from the union fining process unit & is charged in to feed surge drum of Molex unit. In this tank the level is maintained for continuous supply of feed & nitrogen blanketing is provided for preventing of vapor loss & fire formation. From feed surge drum the feed is transferred to screen feed filters for removal of suspended impurities in the size range of 10 microns & greater so as to prevent clogging in further treatment & poisoning of beds. As the feed filters, flush filters & desorbent filters provide zone flush, feed, and line flush and desorbent. All these materials are inlet of rotary valve which does the function of sending proper material to the proper bed in the chamber at proper time.
It is the heart of the Molex unit. The feed enters the adsorbent chambers where adsorption of n-paraffin from its mixture of non-normal paraffin for this adsorption the molecular sieve beds are provided in which selective & non selective pores are provided. There are two chambers in which 12 molecular sieve beds are provided in each adsorption tower. The pump around system is provided to circulate material from 12th bed to 13th bed & from 24th bed to 1st bed. In the selective pores the n-paraffin are adsorbed while the non-normal paraffin are adsorbed in the non-selective pores. The n-paraffin are displaced by n-C5 & non normal paraffin
are displaced by i-C8.
The n-paraffin with i-C8 & n-C5 are obtained as extract while non normal paraffin nC5
& iC8 are obtained raffinate. The raffinate, extract & line flush out are obtained as product from
the chambers which goes to the rotary valve the line flush does the work of flushing the bed before entry of feed while zone flush does the flushing of the proper zone. The rotary valve again sends the extract; raffinate & line flush out lines to the proper destination. The extract goes to extract mixing drum which is provided for continuous flow to the extract column & for mixing of the extract effectively as extract comes in short intervals. In the extract column feed enters at the 26th tray. In here simple distillation occurs on temp. difference where the n-C
5 is
removed as the top product, i-C8 gets removed as side cut & the n-paraffin are removed as the
bottom product & sent for further processing. The n-C5 obtained as top product is recycled as
some part while other part gets divided in to two parts one of each goes to desorbent surge drum & other goes to desorbent stripper column. The i-C8 is obtained as side cut which is sent to
desorbent stripper column for obtaining it in pure form.
The raffinate goes to the raffinate mixing drum which has the same function as that of 11extract mixing drum. The raffinate from there goes to raffinate column where top and side products are same as that of the extract column while the bottom product is non normal paraffin
29 which is sent to the return kerosene. In the desorbent surge drum 60:40 ratio by volume of n-C5 & i-C8 is maintained.
In desorbent stripper column the separation of n-C5 & i-C8 is done where i-C8 is obtained
as 99% pure & sent to desorbent surge drum, filters, zone & line flush for the same process while the n- C5 is sent to surge drum. Some portion of n-C5 & i-C8 is also sent to storage tank.
2.4.3.3.4 Process equipment
[1] Rotary valve:It is a device through which the bed mechanism is controlled in the adsorption chamber. In rotary valve there is a rotor and a stator plate. Each plate containing 24 holes in its periphery. Bottom plate which is static in nature is having all holes in open condition and the top plate which is rotary has 7 open holes. These whole openings is followed by the mechanism of 6-1-5-1-7-1-3.
After a fix time, a stroke is applied on the system so that the feed position is changes from one bed to second one. This means that each stream goes to one number higher position than the previous one. This is done by hydraulic system in which the oil is used at a pressure of 80 Kg/cm2.
[2] Adsorption chamber:
The vessels that contain the Molecular Adsorbent and the Distributor Grids are called Chambers. Between two adjacent beds of adsorbent is a special distributor grid which also acts as a support plate for the bed above it. Distributors between each bed are connected to peripheral parts of the Rotary Valve. In addition to these, grids are provided at top and bottom of each chamber. Liquid is pumped to and from the chambers. The two process variables for the chambers that need to be controlled are Temperature and Pressure. The chamber Temperature is controlled by incoming feed and desorbent system at approximately 177 0C.
The Pressure is set at 24.6 kg/cm2 which are high enough to prevent Hydro-carbon from vaporizing.
If pressure falls below the bubble point, liquid will boil and vaporize and this is to be prevented as vaporization may damage adsorbent structure. Pressure also is an important factor. There is an emergency system for preventing loss in pressure. The switch on chamber to control the pressure closes the Extract valve if pressure falls below the determined point.
30 [3] Extract and raffinate column:
The primary purposes of both columns are to separate the recyclable desorbent and yield a purified bottom product, Normal paraffin from the extract and Non-normal paraffin from raffinate, as well as to provide feed source to desorbent stripper column. Bottom product level controller and pure products are sent to storage after cooling. Bottom product is recycled for high recovery. The overhead vapor is condensed and dropped to receiver. Bypass line connects the receiver with vapor line to control pressure by Butterfly valve.
It is desirable to run outlet condenser at temperature slightly less than condensing temperature because if the temperature is high all vapors will condense and if it is too low heat will be wasted. The side cut product from raffinate column is pumped to desorbent stripper on flow control. Suction for this is provided. The side cut’s major portion is given back to column tray below weir. This rate is controlled by TRC located few trays below weir. The net overhead from the raffinate column is pumped out on flow control by level in receiver to desorbent surge drum.
The reflux to extract column is pumped by reflux pump. The amount is reset by overhead receiver level controller. The other net draw is sent to desorbent surge drum. The iC8 rich side
cut is sent to the stripper desorbent. It is important to maintain tight and accurate control as loss of normal Paraffin decreases purity and recovery causing the loss of desorbent.
[5] Desorbent stripper column:
This is typically a 20-30 tray vessel. The objective is to produce high purity of iC8 as nC5
contamination will reduce purity. The Extract and Raffinate Column side cut streams merge and enter the Stripper Column. Purified iC8 exists at bottom and is pumped through Desorbent
Stripper Column bottom heater to filter and the same process repeats again. Temperature of Zone Flush leaving Exchanger is regulated by flow control of Hot Oil. Desorbent Stripper overhead is nC5 and is returned to the Raffinate Column to a point just above the side cut tray.
[6] Filters:
Filters are located in three streams leading to Adsorbent Chambers feed, Desorbent and Flush. Filters remove particles that could damage Turbine Meters, Vortex Meter or the Rotary Valve Teflon Sheet. The Filters have replaceable Cartridges. These should be initially placed to remove particles from the system. It can remove particles of diameter 10 microns and larger. Strainers are provided to protect the turbine meters if Filters are out of Streams.
31
B. Back end
2.4.3.4 Paraffin converted to olefin (Pacol)
2.4.3.4.1 Introduction
The process is a fixed bed catalytic process designed to selectively dehydrogenate a high purity, normal paraffin feed to the corresponding mono olefins. The feed to the Pacol unit must be free from impurities which could harm the catalyst and contains maximum of four carbon range of normal paraffin [C10 – C13].The catalyst is a 1/16” spherical dehydrogenation
catalyst of stabilized platinum on alumina base. It is non- regenerable and pre-reduced as received by the refinery. The catalyst used in the Pacol section is of high selectivity for the desired reaction and thereby minimizing the side reactions. If selectivity of normal mono-olefin is maintained reasonably high, the conversion of paraffin to mono-olefin is limited at low levels. Thus recycling of untreated normal paraffin is moderately high.
PACOL reactor reaction is carried out in a low pressure hydrogen environment at moderately high temperature (low hydrogen partial pressure) in presence of Nickel catalyst (DEH-7). The life of this catalyst is around 30-45 days depending upon operating conditions. The product also produces small amount of Di-olefins which forms undesirable byproduct thereby decreasing the yield of LAB and degrading LAB quality.
These Di-olefins are converted into mono-olefins in DEFINE section.
Reaction [Dehydrogenation Reaction]
[1] Olefin formation R-C-C-R’ R-C=C-R’ + H2 N-paraffin mono-olefin [2] Diolefin formation R-C-C=C-R’ R=C-C=C-R’ + H2 Mono-olefin Di-olefin [3] Aromatics Formation R” R=C-C=C-R’ R”’
The primary reaction of Pacol unit is dehydrogenation of normal paraffin into
32 In this dehydrogenation reaction of normal paraffin because of high temperature [450-500 °C] and low pressure [1.4 Kg/cm2] subsequently Di-olefins and aromatics are also formed by the side reactions to minor extent.
The dehydrogenation reaction of n-paraffin is an endothermic reaction. The percentage conversion of n-paraffin is 10-13% into mono-olefins, Di-olefins, light ends, aromatics & hydrogen.
Pacol catalyst: The catalyst is a 1/16” spherical dehydrogenation catalyst of stabilized platinum on alumina base & it is non- regenerable. It is dark gray in color and is odorless. It is in the form of spheres.
Table 2.4.4 Contents of Pacol catalyst
Content Weight%
Platinum <1
Silicon Oxide 40 – 60
Aluminum Oxide (non-fibrous) 40 – 60
Magnesium oxide 10-20
2.4.3.4.2 Process flow diagram
33
Figure 2.4.16: Product separator
2.4.3.4.3 Process flow description
The main sources of feed for Pacol unit are: Recycled n-paraffin from Detal unit, Fresh feed from tank farm, Fresh feed from Front End Molex unit. The feed is stored in the feed surge drum. In the surge drum 50-60 % level of material is maintained. It is preheated in a vertical combined feed exchanger from 209°C to 390°C. The fresh cold feed passes from the tube side and gets heated. At such high temperature hydrocarbon gets vaporized. The feed from the feed surge drum mixes with recycled hydrogen at temperature of 45 -55 °C and at pressure of 2.2kg/cm2 from recycle gas compressor [KOBE COMPRESSOR].
The combined feed exchanger is especially vertically designed heat exchanger to maximize the amount of heat recovered from the reactor effluent at shell side while minimum pressure drop. The vapors are heated more in charge heater to raise temperature to 445-465°C. This charge heater is a radiant convection type.
DM Water is injected into the combined feed outlet stream before it enters the charge heater to reduce the light ends formation. This is then charged in the PACOL reactor containing Platinum catalyst. The reactor effluent is recovered by heating fresh feed in combine feed exchanger. Reactor inlet temperature varies from 445-465°C at SOR [Start of Run of PACOL
34 Catalyst)] to 487-505°C at EOR [End of Run of PACOL Catalyst]. In the reactor dehydrogenation reaction [endothermic gas phase reaction] is taking place in presence of Pt catalyst.
In the reactor there are two hydrogen streams given: (i) At top-for moisture removal/catalyst reduction (ii) At bottom-for coke removal from catalyst surface.
The temperature of effluent stream reduces from 473°C to 210°C.This effluent stream contains untreated n-paraffin, mono-olefins, di-olefins, aromatics, light ends [Less than C10].Here in reactor the high selectivity of mono-olefins is desired for maximum LAB
production. The lower conversion of 13 % gives higher selectivity of n-olefins [90%]. Hence to stabilize the upstream of the combined feed exchanger water injection in the upstream of the combined feed exchanger is done.
The dehydrogenation reaction produces hydrogen gas which must be separated from the reactor effluent. This is separated in a product separator. It is a packed bed of metal Pall Rings. The vapor is cooled by passing up through the bed by cold re-circulated liquid flowing down. As the vapor cools, the condensable hydrocarbon [n-Paraffin] condenses and collects at the bottom of the separator with the re-circulated liquid [pump around].The cold gas passes out through the top of the separator where it is returned to the KOBE COMPRESSOR.
Pure hydrogen gas cooled by hydrocarbon quenching is compressed Kobe Compressor from 0.86 kg/cm2 pressure to 2.04 kg/cm2 and distributed in five different streams.
Stream distribution of the KOBE Compressor is as shown below: (1) U.F. unit for makeup gas.
(2) To CFE tubes. (3) To PACOL reactor.
(4) Spill back to separator to maintain the pressure of the separator unit. (5) DEFINE unit for hydrogenation reaction purpose.
35
2.4.3.4.4 Process Equipments
[1] Vertical combined feed exchanger:
The main purpose of combined feed exchanger is to recover heat of reactor effluent and to pre-heat fresh feed. The Vertical combined feed exchanger is a simple shell and tube heat exchanger. This is specially designed vertically to maximize the amount of heat recovered from the reactor effluent while minimizing pressure drop.
The material of construction of vertical combined feed exchanger is carbon steel. The fresh feed which needs to be preheated passes through the tube side entering form bottom and the reactor effluent stream passes through the shell side entering from the top and out form bottom and exchanges its heat with tube side fluid.
Table 2.4.5 Data of Pacol CFE inlet-outlet temperature
Initial Temp Final Temp Feed [Tube] 209°C 390°C Effluent [Shell] 473°C 213°C [2] Charge heater:
The main purpose of charge heater is to preheat the feed to high temperature. This charge heater is U – Tube Type. There are seven burners which uses fuel oil, air and steam for combustion. There are 42 single pass U – Tubes. From the inlet of header feed is entered which flow through the tube joining the outlet header. The temperature of heater is 1000°C.
The feed enters the inlet header from bottom passes through the curved part and then enters the outlet header from where it is removed from bottom again.
[3] Pacol Reactor:
The main purpose for the Pacol reactor is dehydrogenation reaction in presence of catalyst.
The main shell of the reactor contains the cylindrical bed of catalyst with perforated holes over them. The Pacol reactor is divided into three zones:
36 This zone contains one fresh batch of catalyst. Hot hydrogen purging is done to remove any moisture present and oxygen from catalyst.
(ii) Reaction Zone:
The feed initially enters the main shell of reactor. The vaporized feed enters the top of the reactor passes down through an opening along the vessel wall, the flows across the catalyst bed and collects in the pipe located in the center of the reactor. This type of reactor is called a radial flow reactor because the feed passed in radial direction. The advantage of radial flow reactor is that the pressure drop through reactor is very low. PACOL catalyst is confined to an annulus between the outer basket and inner center pipe in this zone (JOHNSON BASKET).
(iii) Collection zone:
This part helps in changing of catalyst. This contains Valve, Funnel and Drum for storage of used catalyst.
Catalyst is fed through the Lock Hopper-1 in the reactor at the top and the spent catalyst is unloaded through Lock Hopper-2 at the reactor bottom. The reactor shell and heads are fabricated from 1Cr, ½ Mo alloy material suitable for hydrogen services.
[4] Separator:
The main purpose of separator is to separate hydrogen and hydrocarbon. The top of the separator contains a random packed bed of pall rings, which provides the residence time. Above the pack bed mist eliminator is also provided.
The reaction product contains H2 gas of hydrocarbon and liquid hydrocarbon. This product
stream is charged centrally. The liquid part moves downwards and gets cooled with the help of fin fan cooler and condenser while the vapor rises through the packed bed. The cooled liquid bottom is quenched at the top of the packed bed. It passes through the packing and condenses the hydrocarbon vapors. This quenching cools hydrogen gas before it is charged to the Kobe Compressor & if after this any liquid hydrocarbon particle is carried with H2, the mist
eliminator is used to discard it. The reactor shell and heads are fabricated from Killed C.S and each Metal Pall Rings are 50mm C.S. rings.
[5] Kobe Compressor:
This is the heart of entire plant. In case if compressor gets tripped, there is no alternated of shutdown of plant and should be immediately closed. There is no stand by compressor because it is very costly [about 12 crores] and is designed by Kobe Compressor manufacturing Co. Japan.
It is a screw type compressor with two screws, Male rotor and Female rotor. Both are having screwed lobes and these rotors are inter-meshed in parallel sealed casing. Clearance between
37 two rotors is only 3 mm. Both rotors are supported by bearing at two ends so that they can rotate without contacting each other.
Uniqueness of the compressor is that no lubrication is required except in bearing. To prevent ingress of bearing lubrication oil into chamber or leakage of compressed gas to outside, an oil shield and shaft scaling deceive is provided. A clearance is kept between rotor and casing to prevent metal to metal contact, so that is not necessary to supply lubrication oil to compressor chamber.
2.4.3.5 Define
2.4.3.5.1 Introduction
The effluent from the Pacol unit is alkylated with Benzene in the Detal unit. But, it is required that side products are obtained in minimum quantity and to get high yield of LAB. Di-olefins are also formed in the Pacol reactor with Mono-Olefins which forms undesired by-product as HAB hence they are required to be removed. For this purpose Define unit is required. The Define process significantly improves the overall efficiency and profitability of LAB complex.
Di-Olefins present in Pacol reactor effluent are selectively hydrogenated to the
corresponding mono-olefins in presence of Sulphur. At 90% Di-olefins conversion, the selectivity to mono-olefins is about 50%Table 2.4.6 Contents of Define Catalyst
Content Weight%
Nickel < 6
Nickel Oxide < 10
Aluminum Oxide (non fibrous)
> 94
Reaction [Hydrogenation Reaction]
R –C=C-C-C=C-R’ + H2 R—C=C-C-C-C-R’
Di-olefin Mono-olefin
R—C=C-C-C-C-R’ + H2 R—C-C-C-C-C-R’
38
2.4.3.5.2 Process flow diagram
Figure 2.4.17: DEFINE reactor
2.4.3.5.3 Process flow description
The feed to the Define unit originates from the Pacol product separator. The Nickel catalyst present is highly active which may convert di-olefin to mono-olefin and mono to paraffin. So to reduce the conversion Sulfur is injected which will passivate the catalyst activity. Hydrogen gas from recycle gas compressor is boosted to 35-38 kg/cm2 from 2.2 kg/cm2 by net gas compressor and is mixed with the define feed as a reactant.
Hydrogen flow depends upon selectivity & conversion of di-olefin in Define reactor. Define feed is heated by Define charge heater to 190-200°C.Two Define feed filters at the inlet of Define reactor are there to filter out impurities from Define feed.
Define reaction is a hydrogenation reaction which is exothermic in nature where di-olefin is converted to mono-di-olefin [desired reaction] and mono-di-olefin is converted to paraffin [undesired reaction].Bottom of the DEFINE reactor is sent to the Stripper Column. Here the light ends formed by cracking are removed from the top and off gases are sent to HOH. The column bottoms paraffin and olefins are pumped to the PEP unit from the stripper bottom.
2.4.3.5.4 Process equipment
39 Reactor Heads and Shells are fabricated of Killed C.S. head is 2:1 Ellipsoidal head. It is designed for 22.2KSC and 260°C.200Kg of Di-methyl Disulfide [DMDS] is used as a catalyst passivation agent.
[2] Feed surge drum & Stripper Column:
Reactor Heads and Shells are fabricated of C.S. Head is 2:1 ellipsoidal head.
2.4.3.6 Pacol enhancement process (PEP)
2.4.3.6.1 Introduction
The UOP Pacol Enhancement process (PEP) is a fixed bed adsorption unit for the selective removal of aromatics components forms the Pacol product stream using adsorption technology. Removal of aromatics to a level less than 1% may be expected.
Primary source of aromatics are:
(1) Aromatics produced in the Pacol reactor.
(2) Aromatics in fresh n-paraffin feed to the Pacol unit. (3) Light alkylated in the recycle paraffin.
Build up aromatics in the recycle paraffin will drop the LAB yield and product quality decreases. Removal of aromatics compounds from the alkylation unit feed will reduce the production of heavy alkylated by 60-70% and increases the yield of LAB by 3-5%.
In Detal process, the reduction of aromatics will increase the activity of the catalyst, allowing the reactor temperature to be reduced by 10-15°C. This will increase the linearity of the LAB by about 1-5%.
2.4.3.6.2 Process flow diagram
40
Figure 2.4.19: Desorbent column
41
2.4.3.6.3 Process flow description
The pep unit consists of adsorption section and fractionation section.
The adsorption section consists of six absorbers while the fractionation section consists of desorbent column and depentanizer column.
[i] Adsorption section:
The section consists of six adsorbers. Temperature and pressure maintained in each adsorber are 130°C and 15 kg/cm2. The feed to the Pep unit is Pacolet from product stripper bottom in Define unit. The temperature is 224°C, which is reduced to 130°C.
The removal of aromatics can be divided into six steps as follows: (1) Desorbent displacement step
The PEP unit operates in cyclic manner. At the beginning of new cycle the adsorbers are full of desorbent (benzene). In this step benzene from the void spaces in the adsorbers is removed by feed charge which is sent to the desorbent column.
(2) Adsorption step
When all the benzene from void space is removed adsorption of aromatics from feed starts causing desorption of benzene.
(3) Treated feed displacement step
Treated feed [olefins + benzene] from the voids is removed by introduction of N-pentane. The treated feed is fresh feed for Detal unit.
(4) Purge step
The treated feed left after the displacement step is recovered by n-Pentane purge which removes nearly all of the treated feed and sends to the depentanizer column.
(5) Pentane displacement step
N-pentane from void space is removed from adsorbers by introduction of benzene. The absorber’s effluent is charged to the Depentanizer column.
(6) Desorption step
The adsorbed aromatics are displaced by benzene Adsorption. Nearly all of the aromatics are removed from the sieve during this step and the adsorber effluent goes to the Desorbent column. At the end of the Desorption step, the Adsorber is full of benzene and returns to the Desorbent displacement step. Thus adsorption section containing six adsorbers work in continuous manner. From here the effluent goes to the fractionation section.
