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UAEU

United Arab Emirates University

College of Engineering

Industrial Training and Graduation Project Unit

Design and optimization of a fractionation unit

Mariam Ali Albraiki

Hanaa Saeed Al-Shamsi

Lamya Lari

Fatima Al-shehhi

Advisor: Dr. Rachid Chebbi

- Introduction.
- Summary of GP1
- Objective of GP2
- Project units

- Refrigeration unit
- Fractionation unit
- Sweetening unit

Chiller

Compressor

Sweetening

Heat exchanger

Absorber

Fractionation unit

Distillation column

- Sizing of the equipments
- Cost of the plant
- Conclusion

The project is about designing and optimizing fractionating plant in Al-Ruwais Fractionation Unit. Our target from the fractionation unit is to design and optimize a unit in order to separate the liquid components from natural gas into DEO, C3, butanes and C5+.

- Complete literature survey.
- Two simulator packages were used simulate chemical processes such as :
- HYSYS simulator used for the fractionation and refrigeration sections
- ASPEN-PLUS used for sweetening section

- Design Fractionation Unit.
- Design equipments of the Refrigeration and Sweetening Unit.
- Determine efficiency of compressor and pump.
- Estimate Fractionation, Refrigeration and Sweetening Units Cost.
- Study Environmental Aspect.

Separation Process

Separation process operates basically on the principle of pressure reduction to achieve the separation of gas from a liquid inlet stream.

- Fluid physical properties required for sizing, were obtained form HYSYS simulator:
- Density for liquid and vapor phases
- Operating pressure
- Volumetric flow rate of vapor and liquid phases

- The settling velocity of liquid droplets

- Relation between operating pressure and Lv/Dv

- The cross sectional area for vapor flow :

- The vapor velocity :

- Vapor residence time required for the droplets to settle to liquid surface :

- Actual residence time:

- The actual residence time is set equal to vapor residence time, and from this step , Dv, and Lv are determined.

- Logarithmic mean temperature

- The rate of heat transfer:

- Heat transfer area was determined assuming U

- Shell side heat transfer coefficient

Tube side heat transfer coefficient

- Calculate Uo and compare it with the assumed U :

Definition of Compressor

Compressors are described as mechanical device that takes in a gas and increases its pressure by squeezing volume of it into a smaller volume

Types of Compressor

- Reciprocating compressors
- Centrifugal compressors
- Axial flow compressors

29023

8.06

The vapor area of the holes varies between 5 to 15% of the tray area

Sieve tray

In the sieve plate, vapor bubbles up through simple holes in the tray through the flowing liquid

Hole sizes range from 3 to 12 mm in diameter, with 5 mm a common size

vapor or gas rises through the opening in the tray into bubble caps

This type has been used over 100 years

The gas flows through slots in the periphery of each cap and bubbles upward through the flowing liquid.

modification of the sieve tray, they are essentially sieve plate with large diameter holes.

stage efficiency is the performance of a practical contacting stage to the theoretical equilibrium stage. Murphree plate efficiency is the ratio of the actual separation achieved to that which would be achieved in an equilibrium stage

- Calculate the maximum and minimum vapor and liquid rates which can be obtained from simulation results.
- Collect the system physical properties from simulation results.
- Select a trial plate spacing.
- Estimate the column diameter based on flooding considerations.

Decide the liquid flow arrangement.

Check the weeping rate.

Check the pressure drop.

Check downcomer back up.

Recalculate the percentage flooding Check entrainment.

The liquid- vapor flow factor was determined for the top and bottom part using the following equation:

Flv ( Top) = 0.359

Flv ( Bot) = 0.394

Where,

Lw = Liquid mass flow rate in Kg/s

Vw = Vapour mass flow rate in Kg/s

Ρv = is the vapor mass density in Kg/m3

Ρl = is the liquid mass density in Kg/m3

From the graph ,it the KTop and KBotwere found as 8*10-2 and 7.6*10-2 respectively.

Next step was to correct the surface tension for top either bottom as shown below , from HYSYS simulator the surface tension are

S Top = 2.771 *10-3 N/m

S Bot = 2.155 *10-3 N/m

K'Top = 0.0538

By using the equation

K’Bot = 0.0486

Then the flooding velocity was estimated using the equation

Where,

u f is the flooding vapor velocity in m/s

K' is the correction value for the surface tension in top and bottom part

u fTop=0.1387 m/s

u’ fTop=0.1178m/s

With 85 % flooding

u fBot=0.1387 m/s

u’ fBot=0.097 m/s

In order to find the Maximum volumetric flow rate , the following equation was used

V top =0.074 m3/s

V bot=0.077 m3/s

Where,

Vw = Vapour mass flow rate in Kg/s

Ρv = is the vapor mass density in Kg/m3

In this step , the net area was calculated using the following equation

Top = 0.628 m2

Bot = 0.802 m2

In this part, it was assumed that the downcomer area is 12% from the total as a trial step , the column cross –sectional area are

Top =

Bot =

= 0.713 m2

= 0.911 m2

Finally , the column diameter column at top and bottom

D top =0.952 m

Where,

D is the column diameter in m

A is the net area

D bot =1.01 m

By taking all assumption as below

Weir height = 50 mm

Hole diameter = 5 mm

Plate thickness = 5 mm

Total pressure drop

ht = hd +( hw+ how)+ hr

100 mm liquid

Actual minimum vapor velocity =

= 0.709 m/s

Downcomer liquid back up

Back –up in downcomer was estimated by

hb = (hw + how) + ht + hdc

= 178 mm

=

= 0.0875 m/s

Number of holes

Area one hole = 1.964 *10-5 m2

Number of holes =

= 3869.6 = 3870

Device that facilitate the exchange of heat between two fluids that are at different temperature without allowing them to mix

Most heat exchangers are classified in one of several categories on the basis of configuration of the fluid flow path through heat exchanger.

The most common types of flow path configuration are:

- Double-Pipe Exchangers
- Compact Exchangers
- Shell and Tube Exchangers
- Plate and Frame Exchangers
- Regenerative Exchangers

The advantage of this type are:

The configuration gives a large surface area in small volume

Easily cleaned

Can be constructed from a wide range of materials

Fluid location: shell or tube

Corrosive fluid Tube

Fouling fluid Tube

Higher temperature Tube

Higher pressure Tube

More viscous Shell

Low Flow rate Shell

Shell and tube fluid velocities

For Tube (1-2) m/s

For Shell (0.3-1) m/s

Stream temperature

The closer the approach temperature used, the larger will be the heat transfer area required.

Minimum approach temperature = 20oC

Pressure drop

Selection of pressure drop depends on the economical analysis that gives the lowest operating cost.

Heat Exchanger Design

Shell-side

T1=121 oC

Baffle Spacing

Bundle Diameter

Shell Diameter

Tube Diameter

Tube-side

t1=31 oC

Tube-side

t1=55 oC

Shell-side

T1=89 oC

Tube Length

Baffle

Physical properties needed for calculation:

Heat Exchanger Design

Step 1 : calculating the ΔTlm

Step 2 : Selecting Number of shell and tube passes

Ft : Temperature correction factor

Step 3 : Calculating the ΔTm

Step 4: Selecting a trial value for the overall heat transfer coefficient U

Step 5 : Calculating the heat transfer area A

Where,

Q : The heat load, was taken from ASPEN PLUS simulation result

Step 6: Calculating tube side heat transfer coefficient hi

Step 7: Calculating shell side heat transfer coefficient ho

Where,

- k is thermal conductivity of fluid
- G is the mass velocity
- jh isfriction factor obtained from figure
- μ is viscosity

Step 8: Calculating calculate Uo and compare it with the assumed U

- Where,
- Kw is thermal conductivity of the tube wall material
- hd is fouling factor of the fluid in tube and shell

Trial and error is used if the computed Uo is different than the assumed U

Tube-sidePressure drop

Shell-side

Where,

L = tube length, m

LB= baffle spacing

Np=number of tube-side passes,

u = tube-side velocity

- The design will consist of calculating the following:
- Column diameter.
- Total cross-sectional area.
- Height.
- Downcomer area.
- Active area over the tray.
- Weir length.
- Distance from tray center to weir.
- Total hole area.
- Pressure drop.
- Weeping and Entrainment

Fractional entrainment:

Entrainment mass flow rate :

What happens to hydrogen sulfide when it enters the environment ?

- It is released primarily as a gas and will spread in the air.

- It will form sulfur dioxide and sulfuric acid in the atmosphere.

- It causes acid rains.

- It remains in the atmosphere

for about 18 hours.

How can hydrogen sulfide affect my health?

Breathing very high Concentrations:

Death within just a few breaths

Loss of consciousness after one or more breaths

Breathing Low Concentrations:

Eye irritation

Sore throat and cough

Shortness of breath

Long-term, low-level exposure:

Loss of appetite

Headaches

Irritability

Dizziness

Poor memory

Has the federal government made recommendations to protect human health?

Occupational Safety and Health Administration (OSHA):

Acceptable concentration of (20 ppm) in the workplace.

National Institute of Occupational Safety and Health (NIOSH(

recommends a maximum exposure level of 10 ppm.

Training should be provided for workers in h2s operation

The characteristics, sources, and hazards of Hydrogen

Sulfide.

Proper use of the Hydrogen Sulfide detection methods

used on the site.

Gas detector

Recognition of, and proper response to, Hydrogen Sulfide

warnings at the workplace.

Symptoms of Hydrogen Sulfide exposure

Proper rescue techniques and first-aid

procedures to be used in a Hydrogen Sulfide exposure.

Worker awareness of workplace practices

and maintenance procedures to protect

personnel from exposure to hydrogen sulfide.

General consideration in finding the cost for equipments as the following steps ( example : depropanzier column)

Using the process engineering index in Figure as :

Value of index at 1998 = 390

Value of index at 2003 = 395

Purchase cost, $ for vessel = (bare cost from Figure) x (material factor) x (pressure factor ).

Purchase cost, $ for vessel = (bare cost from Figure 5.1.3)x(material factor)x (pressure factor ).

- = 25000x 2x 1= 50,000
- Purchase cost, $ for single plates = (bare cost from Figure 5.1.2) x (material factor)
- = 400x1.7 = 680
- Purchase cost for 40 plates, $ = 40x680 = 27,200

Total purchase column will be = 77200 x

With 1.5% inflation from 2003 to 2004 it will become as:

Purchased and installation cost for depropanizer column = $87312

Total physical plant cost (PPC) = PCE (1+ f1 + f2 + f3+ f6)

= (377,028) (1+ 0.7+0.7+0.2+0.5)

= $1,168,786

Fractionation columns Fixed capital = PPC (1+ f10 + f11 + f12)

= 11,168,786.8 x (1+ 0.3+0.05+0.1)

= $1,694,740

Three units were studied in this project. Simulation was done for the two units using two different simulators. GASCO specifications were met. In sweetening unit, hydrogen sulfide concentration was reduced to 4ppm. Process equipment design, sizing and cost estimation were done in this project. Fixed capital cost for the whole plant is found to be $6,556,844.

- Study the effect of using different amine solution in sweetening unit.
- Study the effect of using different sweetening process like membrane.

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