Internship Report Dawood Hercules

Fertilizers Limited

Muddassar Sultan School of Chemical & Materials Engineering

NUST, Islamabad

Ammonia Unit

Catacarb Section

School of Chemical & Materials Engineering, NUST 2

ACKNOWLEDGEMENT

I am thankful to Allah Almighty who has blessed me with the courage,

strength and wisdom so that I have been able to complete this internship

report.

I want to acknowledge the efforts but specially the positive consideration of

Senior General Manager Mr. Arshad Mehmood, Production Manager Mr.

Shafique Ahmad and Assistant Production Manager Mr. Amin Saleem.

The assistance and guidance of Training and ISO Manager Mr. Nasir Iqbal

Toor throughout the internship duration and especially for this internship

report has positioned me as a knowledgeable, hardworking and flexible

personality so that we can perform better in the practical field life. I am also

thankful to Incharge Training Mr. Imtiaz Ahmad for injecting practical

knowledge in our minds and for the guidance, he has given to me. I want to

acknowledge the efforts of my mentor Mr. Tauseef Basit who was every

time ready to give us guidelines in spite of his busy schedule.

Here I must not forget to thank the USEs’ who were also supportive

especially Mr. Imran, Mr. Mauzam & Mr. Ahmed Shah who always

welcomed our questions. I also want to give some credit to operators

especially Mr. Maqbool & Mr. Rafaqat.

School of Chemical & Materials Engineering, NUST 3

Table Of Content

Content Page #

Plant Overview 4

Catacarb Section 5

-High Temperature Shift Converter 6

-Heat Exchangers 7

-Low Temperature Shift Converter 8

-Desuperheating & Condensate Removal 10

-CO2 Absorber 11

-Rich Solution Flasher 13

-Power Recovery Turbine 13

-CO2 Stripper 14

-The Flow of Semi Lean

-Knock-out Drums

16

16

School of Chemical & Materials Engineering, NUST 4

PLANT OVERVIEW

There are 4 sections of DHFL plant:

Ammonia unit

Urea unit

Utility unit

Bagging & shipping unit

The topic studied during the internship duration was Ammonia Unit with

special attention to the details of Catacarb Section.

The further division of Ammonia Unit is as follow:

Reforming Section.

Catacarb Section.

Synthesis & Refrigeration Section.

Reforming Catacarb Synthesis Refrigeration

School of Chemical & Materials Engineering, NUST 5

CATACARB SECTION

The functionality of the Catacarb section is in fact the purification of

the synthesis gas from CO & CO2 up to a purity level of 1000 ppm CO2. At

first, the CO is converted to CO2 because of the fact that CO cannot be

removed from the gas simply as CO2. Afterward, all the CO2 is removed

from the synthesis gas with the help of Catacarb solution, which is mainly

K2CO3.

Following are the principal equipment in the Catacarb Section:

High Temperature Shift Converter

Low Temperature Shift Converter

Desuperheater

Absorber

Stripper

Rich Solution Flasher Drum

Knock Out Drums

Throughout the process, the flow is equipped with diverse kind of

instrumentation to have a better degree of control over the process. This

instrumentation includes high & low level alarms, temperature alarms &

indicators, flow recorder controllers, level indicator controllers, high & low

pressure alarms and differential pressure alarms. Certain precautions,

considerations and preferences are also to be followed during the process

and during the start-up & shut down procedures.

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o High Temperature Shift Converter (104-D):

The HTS is the first reaction vessel in the Catacarb section. Here the

temperature of the gases is about 675oF to 725oF. Design level is 700oF.

Only some of the reaction is allowed to proceed. Complete reaction cannot

be achieved because the reaction is exothermic and if the reaction is

allowed to proceed, the temperature rises above than the limit of 950oF

which can destroy the iron catalyst. The reaction being carried out is:

CO + H2O CO2 + H2

The reaction is also not allowed to complete because the rise in

temperature results in so much increase in temperature that the reaction

goes backward and the conversion achieved is nullified. Following are some

operating conditions:

Inlet Temperature: 675oF - 700oF

Outlet Temperature: 750oF

Catalyst: Iron

Pressure Drop: 5psi

The catalyst is placed over a bed through which the gas flow is

maintained and the reaction proceeds. Following is the comparison of the

Synthesis gas composition at the inlet and outlet of the HTS:

Composition Inlet % Outlet %

H2 56.25 60.25

CO 12.38 2.18

CO2 7.96 16.24

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o Heat Exchange:

The synthesis gas is cooled before its processing in the LTS. The

temperature is decreased from 750oF to about 400oF. Boiler feed water

and the methanator feed is preheated using the recovered heat of the

synthesis gas. Following is the arrangement and explanation of heat

exchangers:

These three heat exchangers are shell and tube heat exchangers and are

connected in parallel. The temperature is sometimes required to be

controlled if it falls below a certain level. The exchanger 103-C is by-passed

in that case. This alteration is governed by Temperature Recorder

Controller, which has a sensor of temperature at the inlet of LTS. It do not

allows the temperature to rise above 450oF. The temperature control

mechanism is equipped with a control valve and a Hand Control Valve is

also to be manipulated accordingly when the by-pass is operated.

104-C Heat exchanger is used to heat methanator feed. It actually

preheats that feed. This exchanger is to be operated in all conditions and

has no bypass or alteration.

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Similarly the heat exchanger 1112-C is used to heat up the Boiler Feed

Water and is operational all the time.

o Low Temperature Shift Converter (108-D):

The downstream of HTS is the LTS. Low temperature shift converter

carries out the reaction of the HTS and the remaining amount of CO is

further converted to CO2. The temperature in the LTS is about 410 F to 435

F. Copper catalyst is used in the LTS and there is a bed of few feet Zinc

Oxide that is placed to overcome any sulfur content not being desulfurized

in the desulfurizer. Following reaction of the HTS is carried out in the LTS:

CO + H2O CO2 + H2

Following are the conditions of LTS:

Inlet Temperature: 410 F

Outlet Temperature: 435 F

Catalyst: Copper

Pressure Drop: 5psi

The both vessels are equipped with a Pressure Differential Indicator. It

depicts the pressure drop if there is any deformation in voids or catalyst

spacing. In addition to that, the LTS is also by-passed in the start-up

conditions to heat up the piping system and also because the conversion is

quite achieved at the start. Following is the comparison of LTS inlet vs

outlet:

Composition Inlet % Outlet %

H2 60.25 61.2

CO 2.18 0.21

CO2 16.24 17.5

School of Chemical & Materials Engineering, NUST 9

In some conditions, LTS is also by-passed with the help of a by-pass

line that has a set of two motor operated valves. They help diverting the

flow when their controlling element, a Push Button is manipulated. It

alternates the flow by operating the MOVs in alternate flow paths.

This by-pass is only active during the start-up and is described just to

provide a better apprehension of flow lines and instrumentation.

o LTS Addition: The reasons of installing LTS in addition to HTS are as follow:

To avoid the reaction over a certain limit to control the

temperature.

The reaction in HTS may be reversed if the same conditions are

maintained with temperature increasing, so LTS is installed to avoid

complete conversion of CO to CO2.

School of Chemical & Materials Engineering, NUST 10

o Desuperheating & Condensate Removal:

After conversion of maximum CO to CO2, the syn gas is now ready to be

purified from CO2. The gas is made to pass through a Desuperheater that

absorbs the super heat of residual steam and this steam is then condensed

in a knock out drum, called Raw Gas Separator Vessel 102-F. The syn gas

during the process of losing its superheat is also made to heat up the

stripper’s solution. It is actually passed through 2

heat exchangers (1105-CA & B) that act as a reboiler for the stripper. These

two will be explained in detail further along with the stripper afterwards.

The Desuperheater is actually a part of the piping and is not any

equipment separately out of the flow line.

The Desuperheater can be replaced with a heat

exchanger that exchanges the gas’s heat with the

boiler feed water or cooling water. The condensate

used for Desuperheating can be saved. Plus the heat

will not be wasted!

The syn gas is then sent to the raw gas separator vessel that removes

the process condensate from the syn gas. The syn gas is now ready to be

charged to the Absorber where the CO2 removal is carried out.

Recommendation

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o CO2 Absorber (101-E):

The absorber is one of the core equipment of the Catacarb section. Here

the CO2 is removed from the syn gas using counter flow of the Catacarb

solution. The Catacarb solution mainly contains following main

constituents:

K2CO3

Diethylene Glycol (Defoaming Agent)

V2O5 (Corrosion inhibitor)

Absorber is a packed column that has 4 beds of two types of

packings. The upper two beds contain Polypropylene Flex rings. The two

bottom beds contain Stainless Steel Rings. In such a packed tower, there

can occur foaming or flooding. There is chemical foaming that occurs due

to certain particles of amine or fatty acids. Ethylene Glycol is added to avoid

that foaminess. Corrosion inhibitor is added to protect the corrosion that is

caused by the pyrophoric gases that oxidize the reaction vessel.

The pressure in the absorber is maintained at about 390 psig.

Temperature of the syn gas from the top is about 175OF.

Temperature of the Rich Solution at bottom is about 250OF.

The Catacarb lean solution that is in the recycle stream has about

16% of CO2. Attached with this column are following instrumentation

1) LICe

2) HLA

3) LLA

4) AR

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Level indicator controller is a device that maintains the level of

Catacarb solution at the bottom of the column. Along with it, there is HLA

& LLA two alarms that sound in event of higher level and lower level of

liquid respectively.

AR is the Analyzer Recorder that analyses the amount of CO2 in the

top flow of syn gas from the absorber. The amount of CO2 from here

should not increase 1000 ppm.

Following reaction occurs in the absorber and the CO2 is absorbed in

such way:

2KHCO3 K2CO3 + CO2 +H2O

School of Chemical & Materials Engineering, NUST 13

From absorber, there are two streams:

Rich Solution

CO2 free Synthesis gas

The CO2 free Synthesis Gas is supplied to the synthesis section for

furthur processing.

The Rich solution is sent to the Stripper via a flasher drum.

o Rich Solution Flasher (1103-E):

Rich solution flasher drum is a vessel that is used to flash out hydrogen

from the rich solution that is physically absorbed in the liquid. The

seperation takes place on pessure differential basis, the syn gas is flashed

from high pressure to low pressure. The hydrogen is seperated due to its

low molecular weight. The seperated gas is sent to the synthesis section.

Along with hydrogen, some amount of CO2 is also flashed out. The pressure

is reduced to about 60-65 psig.

The flashed out gas is sent to synthesis section. The Rich solution is sent

to the stripper for CO2 removal.

o Power Recovery Turbine (107-JHT):

The rich solution is being transferred from a vessel at 390 psig to a

vessel where is 5 psig. This pressure drop is used t run a turbine that drives

a pump of semi-lean solution from stripper to absorber. Along with the rich

solution, a steam turbine also assists the pumping of semi-lean from the

stripper to absorber. The let-down is hence used for the pumping purpose

and the system is made power efficient.

School of Chemical & Materials Engineering, NUST 14

o CO2 Stripper (102-E):

The stripper is a vessel that does the reverse reaction of the absorber. It

chemically strips off the CO2 from the rich solution. Following reaction

occurs in the stripper:

2KHCO3 K2CO3 + CO2 +H2O

The conditions in the stripper are lower pressure a higher pressure

than the absorber. There are stainless steel rings at the upper three beds,

polypropylene flexi rings on the two bottom beds, 5 beds in total. Glycol is

used as a defoamer. V2O5 is used as corrosion inhibitor.

Continuous air injection is carried out which is 65% air that is because

of the regeneration of the V2O5. Which is reduced by the time because of

the pyrophoric gases in the stripper.

The reaction proceeds on heat basis. The heat is provided from the

bottom with the help of three heat exchangers. Two of them are the

1105-CA & CB, which are heated by the syn gas. The other is 1111-C which

is steam heated.

School of Chemical & Materials Engineering, NUST 15

Attached with the stripper is the following instrumentation:

1) LRe

2) LLA

3) HLA

4) PDIe

5) HDPA

LRe is the level recorder that monitors the level in the stripper. Along

with it, there are HLA & LLA that sound accordingly in event of high & low

levels respectively.

PDIe is a device that measures the Differential pressure across the

stripper. The differential pressure indicates any signs of foaming, flooding

or any kind of deformation in the packed beds. HDPA is high differential

pressure indicator that warns about very high pressure drop along the

column. It indicates that there is foaming or flooding about to take place

and relevant measures are required to be taken.

From the Stripper, there exits two streams, one of them is relatively

richer in CO2 than the other.

Lean Solution 16% CO2

Semi Lean 35%CO2

Rich Solution 75%CO2

The flow of solution through the column is such that the solution

flows down to the bottom, heats up in the heat exchangers, strips off its

CO2. Hence there are two flows out from the stripper; one is the lean from

bottom to the top of absorber. The other from the middle of the stripper to

the middle of the absorber. The CO2 is retrieved from the top of the stripper

and sent to the Urea section for preparation of Urea.

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o The Flow of Semi Lean Solution:

There is a semi lean solution that contains about 35% of CO2 and is

taken from the middle of the stripper and is directed to the middle of

absorber. The point of this semi lean stream is power saving. Mean to save

the power that is used to generate the lean solution at the bottom of the

stripper. The flow of semi lean from stripper to the absorber is relatively

quite high as compared to the lean solution.

o Knock Out Drums 116-F & 113-F : There are two knock out drums at the top of each absorber and the

stripper that are used to remove the entrained liquid from the top flow

of these vessels. These knock-out drum work on the principle of

separation on momentum difference basis.