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INTRODUCTION
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Page 1: Design Project

INTRODUCTION

Page 2: Design Project

Urea is an organic compound with the chemical formula (NH2)2CO. Urea is also known by the International Non-proprietary Name (INN) carbamide, as established by the World Health Organization. Other names include carbamide resin, isourea, carbonyl diamide, and carbonyldiamine.

Synthetic urea It was the first organic compound to be artificially synthesized from inorganic starting materials, in 1828 by Friedrich Wöhler, who prepared it by the reaction of potassium cyanate with ammonium sulphate. Although Wöhler was attempting to prepare ammonium cyanate, by forming urea, he inadvertently discredited vitalism, the theory that the chemicals of living organisms are fundamentally different from inanimate matter, thus starting the discipline of organic chemistry.This artificial urea synthesis was mainly relevant to human health because of urea cycle in human beings. Urea was discovered; synthesis in human liver in order to expel excess nitrogen from the body. So in past urea was not considered as a chemical for agricultural and industrial use. Within the 20th century it was found to be a by far the best nitro genic fertilizer for the plants and became widely used as a fertilizer. Urea was the leading nitrogen fertilizer worldwide in the 1990s.Apart from that urea is being utilized in many other industries. Urea is produced on a scale of some 100,000,000 tons per year worldwide. For use in industry, urea is produced from synthetic ammonia and carbon dioxide. Urea can be produced as prills, granules, flakes, pellets, crystals, and solutions. More than 90% of world production is destined for use as a fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use (46.7%). Therefore, it has the lowest transportation costs per unit of nitrogen nutrient. Urea is highly soluble in water and is, therefore, also very suitable for use in fertilizer solutions (in combination with ammonium nitrate).

Commercial production of urea Urea is commercially produced from two raw materials, ammonia, and carbon dioxide. Large quantities of carbon dioxide are produced during the manufacture of ammonia from coal or from hydrocarbons such as natural gas and petroleum-derived raw materials. This allows direct synthesis of urea from these raw materials. The production of urea from ammonia and carbon dioxide takes place in an equilibrium reaction, with incomplete conversion of the reactants. The various urea processes are characterized by the conditions under which urea formation takes place and the way in which unconverted reactants are further processed. Unconverted reactants can be used for the manufacture of other products, for example ammonium nitrate or sulphate, or they can be recycled for complete conversion to urea in a total recycle process. Two principal reactions take place in the formation of urea from ammonia and carbon dioxide.The first reaction is exothermic:2 NH3 + CO2 ↔ H2N-COONH4 (ammonium carbamate) Whereas the second reaction is endothermic:H2N-COONH4 ↔ (NH2)2CO + H2O

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Both reactions combined are exothermic.

PROPERTIES OF UREA

Molecular Formula NH2CONH2

Molecular Mass 60.07 g/moleAppearance White odourless solidSpecific Gravity 1.335 (at 20 0 C ) Heat of Fusion 57.08 Cal/gm.Heat of Combustion 2531 Cal/gm.Crystal Form TetragonalNitrogen Content 46.6%Boiling Point Decompose on boiling at 1600CMelting Point 132.7 0CBulk Density 740 to 750 kg/m3

Angle of Repose 23 to 300CViscosity 2.58 (at 132.7 0 C ) Triple point 1020CDielectric Constant 3.52 to 0.2Specific Heat 0.42Kcal/cm2

RAW MATERIALS OF UREA MANUFACTURING

1. AMMONIA Ammonia (NH3) is a comparatively stable, colourless gas at ordinary temperatures, with a boiling point of -330C. Ammonia gas is lighter than air, with a density of approximately 0.6 times that of air at the same temperature. Ammonia is highly soluble in water, although solubility decreases rapidly with increased temperature. Ammonia reacts with water in a reversible reaction to produce ammonium (NH4) + and hydroxide (OH) - ions, as shown in equation. Ammonia is a weak base, and at room temperature only about 1 in 200 molecules are present in the ammonium form (NH4) +. The formation of hydroxide ions in this reaction increases the pH of the water, forming an alkaline solution. NH3 + H2O (NH4) + + OH –

Ammonia Production:Essentially all the processes employed for ammonia synthesis are variations of the Haber-Bosch process, developed in Germany from 1904-1913. This process involves the reaction of hydrogen and nitrogen under high temperatures and pressures with an iron based catalyst.N2 + 3 H2 2NH3

The source of nitrogen is always air. Hydrogen can be derived from a number of raw materials including water, hydrocarbons from crude oil refining, coal, and most commonly natural gas. Hydrogen rich reformer off-gases from oil refineries have also been used as a source of hydrogen. Steam reforming is generally employed for the production of hydrogen from these raw materials. This process also generates carbon dioxide, which can then be used as a raw material in the production of urea.

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Ammonia storage Anhydrous ammonia is usually stored as a liquid in refrigerated tanks at -33.30C and atmospheric pressure, often in doubled-walled tanks with the capacity for hundreds or thousands of tonnes. The low temperature is usually maintained by the venting of ammonia gas.

2. CARBON DIOXIDE CO2 is odourless and colourless gas, which contain 0.03% in the atmosphere. It is emitted as a pollutant from number of industries. CO2 can be obtained from ammonia production process as a by-product.

APPLICATIONS OF UREA

1. Agricultural use More than 90% of world production is destined for use as a fertilizer. Urea is used as a nitrogen-release fertilizer, as it hydrolyses back to ammonia and carbon dioxide, but its most common impurity, biuret, must be present at less than 2%, as it impairs plant growth. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use (46.4%N). It therefore has the lowest transportation costs per unit of nitrogen nutrient. In the past decade urea has surpassed and nearly replaced ammonium nitrate as a fertilizer. In the soil, urea is converted into the ammonium ion form of nitrogen. For most floras, the ammonium form of nitrogen is just as effective as the nitrate form. The ammonium form is better retained in the soil by the clay materials than the nitrate form and is therefore less subject to leaching. Urea is highly soluble in water and is therefore also very suitable for use in fertilizer solutions, e.g. in “foliar feed” fertilizers. 2. Industrial use Urea has the ability to form 'loose compounds', called catharses, with many organic compounds. The organic compounds are held in channels formed by interpenetrating helices comprising of hydrogen-bonded urea molecules. This behaviour can be used to separate mixtures, and has been used in the production of aviation fuel and lubricating oils. As the helices are interconnected, all helices in a crystal must have the same 'handedness'. This is determined when the crystal is nucleated and can thus be forced by seeding. This property has been used to separate racemic mixtures. 3. Further commercial uses A stabilizer in nitrocellulose explosives A component of fertilizer and animal feed, providing a relatively cheap source of nitrogen

to promote growth A raw material for the manufacture of plastics, to be specific, urea-formaldehyde resin A raw material for the manufacture of various glues (urea-formaldehyde or urea-melamine-

formaldehyde); the latter is waterproof and is used for marine plywood An additive ingredient in cigarettes, designed to enhance flavour An ingredient in some hair conditioners, facial cleansers, bath oils, and lotions A flame-proofing agent (commonly used in dry chemical fire extinguishers as Urea-

potassium bicarbonate)

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An ingredient in many tooth whitening products A cream to soften the skin, especially cracked skin on the bottom of one's feet An ingredient in dish soap

4. Medical useUrea is used in topical dermatological products to promote rehydration of the skin. If covered by an occlusive dressing, 40% urea preparations may also be used for nonsurgical debridement of nails. This drug is also used as an earwax removal aid. Like saline, urea injection is used to perform abortions. It is also the main component of an alternative medicinal treatment referred to as urine therapy.5. Textile use

Urea is a raw material for urea-formaldehyde resins production in the adhesives and textile industries. A significant portion of urea production is used in the preparation of urea- formaldehyde resins. These synthetic resins are used in the manufacture of adhesives, moulding powders, varnishes and foams. They are also used for impregnating paper, textiles and leather. In textile laboratories they are frequently used both in dyeing and printing as an important auxiliary, which provides solubility to the bath and retains some moisture required for the dyeing or printing process.

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PROCESS SELECTION

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Several processes are used to urea manufacturing. Some of them are used conventional technologies and others use modern technologies to achieve high efficiency. These processes have several comparable advantages and disadvantages based on capital cost, maintenance cost, energy cost, efficiency and product quality. Mainly two types of processes are used:1. Toyo Koatsu process (Total recycle process)2. Stripping process

Some other processes for direct synthesis of urea are: Chemico process Moniticatini process CPI allied Morsk hydro – pressure

Toyo Kotsu Process (Total Recycle Process):

Urea is synthesized in a urea reactor under condition of 2000C and 250 kg/cm2 from CO2, liquid ammonia and recycled carbamate. Reactor is lined with titanium. The product consists of urea, water, carbamate and excess ammonia.

Stripping Process:

In this process decomposition of ammonium carbamate is done at the synthesis pressure by using either CO2 (Steam-Carbon stripping process) or ammonia vapors (Snam Progetti Stripping process) in a falling film type decomposer normally called stripper. The reactor effluents are allowed to fall through the stripper tube in form of a liquid film form the top while the stripping agent is introduced from the bottom, which travels upwards. Medium pressure steam condensing on the shell side of the stripper supplies heat for decomposition of ammonium carbonate.

Currently Snam Progetti process doesn’t require injection of ammonia vapours at stripper bottom; only higher mole ratio of ammonia and CO2 is maintained in the reactor. At higher mole ratio, ammonia liberated in stripper is adequate to carry out stripping effectively.

Since the reactor stripper condenser steam operates at nearly same pressure, it is possible to feed recycled carbamate either by gravity flow which however necessitates the installation of carbamate condenser at a much higher elevation so as to give the required head or to use a recycle ejector using a part of the feed reactants as a motive fluid. In both Snam Progetti and steam carbon stripping process low pressure stem is generated in carbamate condenser, which has resulted in lower consumption of steam and less need of cooling water.

SNAM-PROGETTI PROCESSES In the first generation of NH3 and self-stripping processes, ammonia was used as stripping agent. Because of the extreme solubility of ammonia in the urea containing synthesis fluid, the stripper effluent contained rather large amounts of dissolved ammonia, causing ammonia overload in downstream section of the plant. Later versions of the process abandoned the idea of using ammonia as stripping agent; stripping was achieved only by supply of heat. Even without using ammonia as a stripping agent, the NH3:CO2 ratio in the stripper effluent is relatively high. So the recirculation section of the

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plant requires an ammonia-carbamate separation section.The process uses a vertical layout in the synthesis section. Recycle within the synthesis section, from the stripper via the high pressure carbamate condenser, through the carbamate separator back to the reactor, is maintained by using an ammonia-driven liquid-liquid ejector. In the reactor, which is operated at 150 bars, has a molar feed ratio of NH 3:CO2

equal to 3:5. The stripper is of the falling film type. Since stripping is achieved thermally, relatively high temperatures (200-2100C) are required to obtain a reasonable stripping efficiency. Because of this high temperature, stainless steel is not suitable as a construction material for the stripper from a corrosion point of view; titanium and bimetallic zircornium-stainless steel tubes have been used Off gas from the stripper is condensed in a kettle type boiler. At the tube side of this condenser the off gas is absorbed in recycled liquid carbamate from the medium pressure recovery section. The heat of absorption is removed through the tubes, which are cooled by the production of low pressure steam at the shell side. The steam produced is used effectively in the back end of the process. In the medium pressure decomposition and recirculation section, typically operated at 18 bar, the urea solution from the high pressure stripper is subjected to the decomposition of carbamate and evaporation of ammonia. The off gas from this medium pressure decomposer is rectified. Liquid ammonia reflux is applied to the top of this rectifier; in this way a top product consisting of pure gaseous ammonia and a bottom product of liquid ammonium carbamate are obtained. The pure ammonia off gas is condensed and recycled to the synthesis section. To prevent solidification of ammonium carbamate in the rectifier, some water is added to the bottom section of the column to dilute the ammonium carbamate below its crystallization point. The liquid ammonium carbamate-water mixture obtained in this way is also recycled to the synthesis section. The purge gas of the ammonia condenser is treated in a scrubber prior to being purged to the atmosphere.

The urea solution from the medium pressure decomposer is subjected to a second low pressure decomposition step. Here further decomposition of ammonium carbamate is achieved, so that a substantially carbamate -free aqueous urea solution is obtained. Off gas from this low pressure decomposer is condensed and recycled as an aqueous ammonium carbamate solution to the synthesis section via the medium pressure recovery section.

Concentrating the urea water mixture obtained from the low pressure decomposer is performed in a single or double evaporator depending on the requirement of the finishing section. Typically, if prilling is chosen as the final shaping procedure, a two stage evaporator is required, whereas in the case of a fluidized bed granulator a single evaporation step is sufficient to achieve the required final moisture content of the urea melt. In some versions of the process, heat exchange is applied between the off gas from the medium pressure decomposer and the aqueous urea solution to the evaporation section. In this way, the consumption of low pressure steam by the process is reduced. The process condensate obtained from the evaporation section is subjected to desorption hydrolysis operation to recover the urea and ammonia contained in the process condensate.

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PROCESS DESCRIPTION

AND FLOW SHEET

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The process which is used in formation of urea is Snam - Pragetti Process at IFFCO Plant. This is self-stopping process.

The basic raw material for the formation of urea is Ammonia & Carbon Dioxide. The formation of urea is taking place in following manner:

2NH3 + CO2 NH4COONH2 + Heat (1)

(Ammonium carbamate)

NH4COONH2 NH2CONH2 + H2O - Heat (2)

(UREA)First reaction is takes place at high pressure and temperature that is P = 150kg/cm 2 (g) & T = 1700C. In this reaction carbamate is formed. At high pressure reaction is taking place at in forward direction and at low pressure reaction is taking place in backward direction. It is exothermic reaction. In the 2nd reaction carbamate is dehydrated to form Urea. This is endothermic process. The heat which is generated in reaction first is utilized in reaction two. At a very high temperature reaction two precede backward direction.

The process root is summarized in the following steps:

1. COMPRESSION OF CARBON DIOXIDE In this step carbon dioxide is compressed through compressor. The carbon dioxide enters in the compressor at 1.4 atm. & around 400C for increasing pressure up to 155kg/cm2 (g). This is achieved by using two centrifugal pumps driven by an extraction cum condensing turbine. Ammonia is comes from the Ammonia Plant or from the Ammonia Storage Tank. The ammonia is passed through the preheated tank to high pressure synthesis loop. The high pressure synthesis loop is combination of booster centrifugal pump and reciprocating pressure pump. The pressure of ammonia comes out from the high pressure synthesis loop is 240kg/cm2. The high pressure liquid ammonia is also provided for motive force for ejector, which recycles carbamate solution to urea reactor. The ammonia is kept in excess for the complete conversion of carbon dioxide. The ration of ammonia to carbon dioxide is 3.33:1.

2. UREA SYNTHESIS AND HIGH PRESSURE RECOVERY This section consists of reactor, high pressure stripper, horizontal carbamate condenser (two units placed in series). The compressed carbon dioxide and excess ammonia are entered in the reactor to form the urea at the temperature of 1900C & pressure 150kg/cm2 (g). The concentration of urea formed in the reactor is nearly 32%. The effluent of reactor is consisting of ammonia, Carbon dioxide, carbamate, vapour and urea. This effluent is passed to stripper in which CO2 is absorbed according to the Henery Law. Heat required for stripping is supplied by 26kg/cm2 (g) steam obtained from extraction of carbon dioxide compression turbine. The concentration of urea obtained from the stripper is 45%. The off gases obtained from the stripper ammonia, CO2 and vapour are entered into horizontal carbamate condenser where the total mixture, except for some inert, is condensed as carbamate and recycled to the reactor by means of ejector.

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3. UREA PURIFICATION AND LOW PRESSURE RECOVERIESUrea purification takes place in two stages at decreasing pressures as follows:i. Medium Pressure at 18 atm. pressure

ii. Low Pressure at 4.5 atm. pressure Medium Pressure Purification and Recovery stage (at 18 atm.)The solution, with a low residual CO2 content, leaving the bottom of the stripper at synthesis pressure is let down to18 atm. and enters medium pressure decomposer The M.P. decomposer and divided in two parts:1) Top separated: where the released flash gases are removed before the solution enters the

tube bundle2) Decomposition section (falling film type): where residual carbamate is decomposed and

the heat require for the decomposition is applied by means of 26 atm. steam condensate flowing out of the shell side of stripper

The NH3 and CO2 rich gases leaving the top separator are sent to medium pressure condenser where they are partially absorbed in aqueous carbonate solution coming from low pressure recovery section .The absorption heat is removed by tempered cooling water circulation in the tube side of the medium pressure condenser. In the M.P. condenser CO2 is almost totally absorbed. The effluents flow to medium pressure absorber. The gaseous phase enters the rectification section of the M.P. absorber. The rectification section has bubble trays. The bubble cap trays are fed by pure reflux ammonia at the top trays which eliminates residual CO2 and H2O from gases leaving M.P. absorber. The reflux ammonia is pumped to rectification column.NH3 with inert gases leaving the M.P. absorber is condensed in ammonia condenser.

The inert gases, saturated with ammonia enter ammonia preheater where an additional amount of ammonia is condensed by heating cold ammonia coming from ammonia storage area and used as make up feed to Urea plant.

The inert gases with residual ammonia content are sent to medium pressure ammonia absorber, which is a falling film type and where they meet a condensate flow which absorbs ammonia from bottom of ammonia absorber the water ammonia solution is pumped to medium pressure absorber. The inerts leaving the top are free from ammonia.

Low pressure purification and recovery stage (at 4.5 atm.)Low pressure decomposer consists of:1) Top separator: where the released gases are removed before the solution enters the

lower tube bundle2) Decomposition section (falling film type): where residual carbamate is decomposed and

the heat require for the decomposition is applied by means of saturated steam at 4.5 atm.The urea solution from the M.P. decomposer bottom enters the L.P. decomposer after expansion through a level controller. Consequently most of the residual carbamate is decomposed and in the process urea solution gets concentrated. The remaining carbamate is decomposed in a falling film exchanger, which is a part of L.P. decomposer.

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The vapors from the L.P. decomposer enter the L.P. condenser where they get cooled and liquefied. Prior to the entry of L.P. off gases in L.P. condenser the vapor gets mixed with the aqueous solution from waste water section. The vapor thus formed get condensed in L.P condenser goes to carbonate solution tank from where it is send back to M.P. condenser. The inert gases in the tank contains considerable amount of ammonia and thus are absorbed in cool condensate before being sent to vent stack.

The urea solution at the bottom of the L.P. decomposer is sent to pre vacuum concentrator through a level control valve.

Urea concentration section:As it is necessary in order to prill urea to concentrate urea solution up to 99.8 % wt., a vacuum concentration section in two stages is provided.

The two concentrator use saturated steam at 4.5 atm. The liquid - vapor phase coming out of second vacuum concentrator enters gas - liquid separator where the vapors are extracted by second vacuum system.

First vacuum system:First evaporator is operated at 1300C and 0.3 Kg/cm2 pressure. Overhead vapor from the top of the first vacuum separator is directed to the shell side of pre condenser and heat of condensation is removed by cooling water in the tube side. Ammonia vapor and residual CO2

is absorbed in condensate forming dil. Ammonium carbonate sol. and flows down through barometric leg of waste water tank.

Uncondensed gases are sucked by the ejector (motive fluid being 44.5 atm. steams) and discharged in the shell side after condenser, which also receives uncondensed gases from second vacuum system. Heat of condensation is removed by cooling water in the tube side.

Second vacuum system:It operates at 14000C and 0.03 Kg/cm2 pressure. Overhead gases from second vacuum separator are sucked by a booster ejector and discharged at slightly higher pressure where heat of condensation is removed by cooling water in the tube side.

Uncondensed gases are drawn by ejector and discharged to shell side of second inter condenser where heat of condensation is again removed by cooling water.

Urea prilling:The molten urea leaving second vacuum separator is pumped to the prilling bucket by means of centrifugal pump.

The molten urea coming out of the prilling bucket in the form of drops fall along the prilling tower and encounters air flow which causes its solidification and subsequent cooling solid prills are sent to the conveyer belt by rotary scraper which carries urea to bagging plant or storage. The heated air containing few ppm of NH3 is released from the top into the atmosphere.

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REACTOR

SEPERATOR

CO

ND

EN

RE

R

STRIPPER

H.

Ex.

M.P

. D

EC

OM

PO

SE R

ABSORBER

L.P

. D

EC

OM

POS

ER

VA

CU

UM

CO

NC

EN

TR

AT

OR

PRILLTNG TOWER

CO

2

To

cond

ense

rU

RE

A

A ir

FLO

W-S

HE

ET

OF

SNA

M P

RO

GE

TT

I PR

OC

ESS

CO

2

NH

3

1 T

1 B

5 B

2 T

3T 3 B

2B

6B

7B

6T

Page 14: Design Project

MATERIAL BALANCE

Page 15: Design Project

BASIS: 100 kmol /hr. of feed.

In this process the ratio of NH3 to CO2 is 3:6.1. This can be approximately taken as 1:2.

NH3 in the feed= 33.3 kmol

CO2 in the feed= 66.6 kmol

BALANCE ACROSS THE REACTOR

2 NH3 + CO2 -- NH4COONH4

NH3 reacted = 33.3 kmol

CO2 reacted = 16.65 kmol

NH4COONH4 produced = 16.65 kmol

NH4COONH4 -- NH2CONH2 + H2O

This reaction takes place upto 64%

NH4COONH4 reacted = 0.64 × 16.65

= 10.656 kmol.

NH2CONH2 produced = 10.656 kmol

H2O produced = 10.656 kmol.

1B 1TKmol/hr Kg/hr Kmol/hr Kg/hr

Ammonia 33.3 566.1 --- ---Carbon dioxide 66.6 2930.4 49.95 2197.84Ammonium carbamate --- --- 5.994 467.532Urea --- --- 10.656 639.36Water --- --- 10.656 191.808

3496.5 3496.54

BALANCE ACROSS THE STRIPPER

NH4COONH2 -- 2 NH3 + CO2

Decomposition is about 64%

Decomposed NH4COONH2 = 0.64 × 5.994 = 3.83616 kmol

NH3 produced = 7.67232 kmol

CO2 produced = 3.83616 kmol.

Reactor

1T

1B

Stripper

1T

2B

Page 16: Design Project

12% water evaporates. (Assume)

2B 2T 1TKmol/

hrKg/hr Kmol/hr Kg/hr Kmol/hr Kg/hr

Ammonia --- --- 7.672 130.429 --- ---Carbon dioxide --- --- 53.786 2366.591 49.95 2197.8Ammonium carbamate 2.157 168.311 --- --- 5.994 467.532Urea 10.656 639.36 --- --- 10.656 639.36Water 9.377 168.786 1.278 23.017 10.656 191.808

976.457 2520.037 3496.5

1T = 2B + 2T = (976.457 + 2520.037) = 3496.49 kg

BALANCE ACROSS MEDIUM PRESSURE DECOMPOSER

NH4COONH2 -- 2 NH3 + CO2

This decomposition is about 85.5%

Decomposed NH4COONH2 = 0.855 × 2.15784

= 1.8449 kmol.

NH3 produced = 3.6899 kmol.

CO2 produced= 1.8449 kmol.

12% water evaporates. (Assume)

3B 3T 2BKmol/hr Kg/hr Kmol/hr Kg/hr Kmol/hr Kg/hr

Ammonia --- --- 3.6899 62.7283 --- ---Carbon dioxide --- --- 1.8449 81.1756 --- ---Ammonium carbamate 0.31294 24.40932 --- --- 2.15784 168.31152Urea 10.656 639.36 --- --- 10.656 639.36Water 8.252 148.536 1.1253 20.2554 9.37728 168.79104

812.305 164.159 976.4625

3T + 3B = 812.305 + 164.159 = 2B = 976.4625 kg

BALANCE ACROSS THE LOW PRESSURE DECOMPOSER

NH4COONH2 - 2 NH3 + CO2

Medium Pressure Decomposer

3T

2B3B

Page 17: Design Project

Decomposition takes up to 50%

Decomposed NH4COONH2 = 0.50 × 0.31294

= 0.15647 kmol.

NH3 produced = 0.31294 kmol

CO2 produced= 0.15647 kmol

12% water evaporates.

6B 6T 3BKmol/hr Kg/hr Kmol/hr Kg/hr Kmol/hr kg/hr

Ammonia --- --- 0.31294 5.31998 --- ---Carbon dioxide --- --- 0.15647 6.88468 --- ---Ammonium carbamate 0.15647 12.20466 --- --- 0.31294 24.40932Urea 10.656 639.36 --- --- 10.656 639.36Water 7.26176 130.71168 0.99024 17.82432 8.252 148.536

782.276 30.0289 812.305

6B + 6T = 782.276 + 30.0289 = 3B = 812.305kg

RECYCLE STREAM

5B is the recycle stream.

5B = 2T + 3T

5BKmol/hr Kg/hr

Ammonia 11.3622 193.1574Carbon dioxide 55.631 2447.764Ammonium carbamate --- ---Urea --- ---Water 2.40402 43.27236

2684.19

BALANCE ACROSS VACUUM CONCENTRATOR

% composition of 6B:

Urea = 78.8%

Water = 19.6%

Low Pressure Decomposer

6T3B

6B

Vacuum concentrator6B 7B

Page 18: Design Project

Ammonium carbamate = 1.6%

% of urea in 7B = 98%

Applying component balance:

0.788 × 782.276 = 0.98 × 7B

7B = 629.013 kg /hr.

We get 629.013 kg of 98% urea.

Scaling factor = moles product per hour specified

moles product produced100

kmol of raw material

Moles of product specified = 2500 tpd

= 1736.1kmol/hr.

Scaling factor = 165.60

Actual inlet

Ammonia 5514.48 kmol/hr.Carbon dioxide 11028.96 kmol/hr.

Page 19: Design Project
Page 20: Design Project

HEAT EXCHANGER

DESIGN

Page 21: Design Project

DESIGN EQUATIONS1. Heat load = m cp ΔT2. Assume U from typical values.

3. Find ΔTlm= ∆ T 1−∆ T 2

ln∆ T 1

∆ T 2

Where ΔT1= T1 – t2, ΔT2= T2 – t1.T1 and T2 are hot fluid inlet and outlet temperature, t1 and t2 are cold fluid inlet and outlet temperature.For more than one pass correct ΔTlm= Ft ΔTlm (assuming counter current flow)

For Ft, R =T1−T 2

t2−t1, S =

t 2−¿ t1

T1−t1¿.

Ft is calculated from graphs or from equation for 1 shell & 2 tube pass heat exchanger.

4. Ft = √( R2+1 ) ln [ (1−S )(1−RS ) ]

( R−1 ) ln ¿¿¿

5. Total area A0 = q

UΔT .6. Area of one tube: πdoL

7. Bundle diameter: Db= do (N 1

K 1¿¿

1n 1

8. Heat transfer coefficient for tube side : hi d i

k f = jhRePr0.33( μ

μw)

0.14

9. Heat transfer coefficients for water: hi = 4200(1.35+0.02 t)ut

0.8

d i0.2

hi = inside heat transfer coefficient for watert = water temperatureut = water velocitydi = tube inside diameter

KERN’S METHOD

10. Area for cross flow: As = ( p t−dο ) Ds lB

pi

pt = tube pitch,do = tube outside diameter,Ds = shell inside diameter,LB = baffle spacing.

11. Shell side mass velocity Gs = W s

A s

Ws = fluid flow rate on the shell side, kg/s.

Page 22: Design Project

12. Shell side linear velocity us = Gs

ρρ = shell side fluid density, kg/m3.

13. Shell – side equivalent diameter for triangular pitch:

de = 4 (

pt

2× 0.87 p t−

12

πd ο

2

4)

π dο

2de = equivalent diameter, m.

14. Shell – side Reynolds number:

Re = Gs de

μ15. Shell side heat transfer coefficient :

Nu = hs de

k f = jhRePr

13 ( μ

μw)

0.14

16. Overall heat transfer coefficient:

1U ο

= 1hο

+ 1hod

+dο ln

dο

d i

2kw+(

d ο

di× 1

hid)+(

d o

d i× 1

hi)

Uo = the overall coefficient based on the outside area of the tube, W/m2 ̊C, ho = outside fluid film coefficient, W/m2 ̊C, hi = inside fluid film coefficient, W/m2 ̊C, hod = outside dirt coefficient (fouling factor), W/m2 ̊C, hid = inside dirt coefficient, W/m2 ̊C, kw = thermal conductivity of the tube wall material, W/m ̊C, di = tube inside diameter, m, do = tube outside diameter, m.

17. Pressure drop in tube side: ∆ Pt=N p ¿ΔP = tube side pressure drop, paNp = number of tube side passes,ut = tube side velocity, m/s.L = length of one tube, m.

18. Pressure drop in shell side: Δ P s=8 jf ( Ds

de )( LlB ) ρ ut

2

2 ( μμw )

– 0.14

L= tube length,lB = baffle spacing.

Page 23: Design Project

Step 1: Specification

105878.016 kg/h of feed at 2050C (T1) cooled to 1450C (T2), by shell and tube heat exchanger with water at 250C (t1).

Step 2: Physical Properties:

Properties Feed Water

Density 920 kg/m3 1000 kg/m3

Viscosity 0.73 m N s/ m2 1.005 m N s/ m2

Specific Heat 2.78 kJ/kg ̊C 4.2 kJ/kg ̊CThermal Conductivity 0.8073 W/m ̊C 0.59 W/m ̊C

Heat load = [{(105878.016 / 3600} * 2.78 * (205-145)] [From eq. 1]

= 4905.681 kW

Water flow: let us assume cooling water flow rate = 40 kg/h

4905.681 = 40*4.2*(t2-25)

t2 = 54.200C

Step 3: Overall coefficient

For a heat exchanger of this type the overall coefficient will be in the range of 250 & 750 W/m20C, (Figure 1 and Table 1); so start with 600 W/m2 0C.

Step 4: Exchanger type and dimensions

Tlm = (205–54.20) – (145–25) [eq. 3]

ln[(205-54.20)/(145–25)]

= 134.8140C

R = T1 – T2 = 205–145 = 2.05

t2 - t1 54.20–25

S = t2 – t1 = 54.20–25 = 0.162

T1 – t1 205–25

For a 1 shell: 2 tube pass exchanger, the correction factor is: [From Fig. 20]

Ft = 0.98

Which is acceptable, so Tm = (134.814*0.98) = 132.1180C.

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Step 5: Heat transfer area

Aₒ = 4905681 / (400 * 132.118) = 61.885 m2 [From eq. 5]

Step 6: Layout and tube size

We have used a split-ring floating head heat exchanger, for efficiency and ease of cleaning.

The feed is corrosive, so assign to tube-side and the coolant (water) in the shell.

Taking 25.40 mm (1 inch) outside diameter, 18.59 mm inside diameter, 4.88 m Long tubes (a popular size) on a triangular 31.75 mm pitch (pitch/dia. = 1.25). Allowing for tube-sheet thickness, take L = 4.83 m.

Step 7: Number of tubes

Area of one tube (neglecting thickness of tube sheets) [From eq. 6]

= * 4.83 * 25.4 * 10-3 = 0.38 m2

Number of tubes = 61.885 / 0.38 = 162.855, say 164

So, for 4 passes, tubes per pass = 164 / 2 = 82

Tube cross-sectional area = ( * 18.592) / 4 = 271.424 mm2

So area per pass = 82 * 344. 055 * 10-6 = 0.0222 m2

Volumetric flow rate of feed = 0.0319 m3/s.

Tube-side velocity, ut = 0.319 / 0.241 = 1.43 m/s

The velocity is satisfactory, between 1 & 2 m/s.

Step 8: Bundle and shell diameter

From table 2, for 2 tube passes, K1 = 0.249, n1 = 2.207,

Db = 25.40 164 1/2.207 = 483.46 mm [From eq. 7]

0.249

For a split-ring floating head exchanger the typical shell clearance from Figure 3 is 53 mm, so the shell inside diameter,

Ds = 483.46 + 58 = 551.46 mm

Step 9: Tube-side heat transfer coefficient

Re = (920 * 1.43 * 18.59 * 10-3) / (0.73 * 10-3) = 33502.74

Pr = (2.78 * 103 * 0.73 * 10-3) / 0.8073 = 2.51

Page 25: Design Project

L / di = 4830 / 18.59 = 259.82

From figure 4, jh = 3.5 * 10-3

hi = (0.8073 * 3.5 * 10-3 * 33502.74 * 2.510.33) / (18.59 *10-3) [From eq. 8]

= 6899.16 W/ m2 ̊C

Step 10: Shell-side heat transfer coefficient

For standard baffle spacing (0.3 * Ds) < bl < (1.0 * Ds), shell side velocity and overall heat transfer coefficient were not satisfying the specifications, so we elected baffle spacing (b l) = (0.90 * Ds)

bl = 0.90 * 551.46 = 496.314 mm

As = {(31.75 – 25.4) / 31.75} * 551.46 * 496.314 *10-6 [From eq. 10]

= 0.0547 m2

De = (1.1/ 25.40) / {31.752 – (0.917 * 25.402)} [From eq. 13]

= 18.03 mm

Volumetric flow-rate on shell-side = (40/1000) = 0.04 m3/s

Shell-side velocity us = 0.04 / 0.0547 = 0.73 m/s

The velocity is satisfactory, between 0.3 & 1 m/s.

Re = (18.03 * 10-3 * 0.73 * 1000) / (1.005 * 10-3) [From eq. 14]

= 13096.42

Pr = (4.2 * 103 * 1.005 * 10-3) / 0.59

= 7.15

Use segmental baffles with a 25% cut. This should give a reasonable heat transfer coefficient without too large a pressure drop.

From figure 5, jh = 5.2 * 10-3

Neglecting the viscosity correction:

hs = (0.59 * 5.2 * 10-3 * 13096.42 * 7.150.33) / (18.03 * 10-3) [From eq. 15]

= 4265.14 W/ m2 ̊C

Step 11: Overall coefficient

Thermal conductivity of low carbon steel = 50 W/ m ̊C.

Page 26: Design Project

Take the fouling coefficients from Table 6; light organic = 5000 W/ m2 ̊C, brackish water (sea water), 2000 W/ m2 ̊C. From eq. 16:

1 = 1 + 1 + 25.40 * 10-3 ln (25.40 / 18.59) + 25.40 1 + 1

U0 4265.14 5000 2 * 50 18.59 6899.16 2000

= (2.34 + 2 + 0.8 + 1.98 + 6.83) 10-4

U0 = 716.845 W/ m2 ̊C

Ucal - Uass = 716.84 – 600 = 0.194

Uass 600

This is within the range of specification (0 – 0.3), so our calculations are right.

Step 12: Pressure drop

Tube-side:

From Figure 7, jf = 3.5 * 10-3

Pt = 2{8 * 3.5 * 10-3 * (4830 / 18.59) + 2.5} *{(920 * 1.432) /2} [From eq. 17]

= 18389.558 N/m2 = 18.38 k N/m2

This is within the range of specification i.e. < 35 k N/ m2.

Shell-side:

From figure 8, jf = 4.0 * 10-2

Ps = (8* 4.0 *10-2 * 551.46 * 4830 * 1000 * 0.732) / (18.03* 496.314* 2) [From eq. 18]

= 25379.00 N/m2 = 25.38 k N/m2

This is within the range of specification i.e. < 35 k N/ m2.


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