1. Natural Gas Processing Plant
Design of a 20 MMSCFD
Natural Gas Processing Plant
Supervised By
Professor Dr. M. A. A. Shoukat Choudhury
Department of Chemical Engineering ,BUET
Nayer Sultana ID

2. Project Definition
A Natural Gas processing plant is to be set up at Brahmanbaria having a capacity of 20 MMSCFD of sales gas for an effective operation of 365 days, including offsite, utilities and other facilities using 500 kg of Triethylene Glycol and 1000 kgmol of raw gas per hour.

3. Project Definition

4. Process Selection Steps of NG processing Dehydration by absorption
Selection criteria for glycols
Alternatives for dehydration of NG

5. Process Block Diagram

6. Process Flow Diagram

7. Design of Stripper & Reboiler
Inlet
Outlet
Temperature (oC)
Rich TEG
Lean TEG
Water vapor
Pressure (KPa)
176
202.9
170.1
Flowrate (kmole/h)
379.2
103.4
101.4
4.279
3.686
0.5934

8. Steps in calculation Determination of reboiler duty
Determination of stripper diameter & height
Determination of packing height & size
Determination of reboiler length & diameter
Determination of fire tube length & diameter

9. Reboiler duty
Reboiler duty can be calculated from the following equation
Q=Ql+Qw+Qr
= Lρc(T2-T1) x(q/24)x(Wi-Wo)+0.25 x Qw
Ql= Sensible heat required for glycol
L=Glycol circulation rate
ρ= Glycol density at average reboiler temperature
c= Specific heat of glycol at average reboiler temperature (3100F)
T2= Temperature of glycol leaving the reboiler
T1= Temperature of glycol entering the reboiler
Qw= Heat of vaporization required for water
q= Gas flow rate
Wi= Water content of inlet gas
W0= Water content of outlet gas
Qr= Heat required to vaporize reflux water in the still
Assuming 25% reflux and 10% heat loss, Q= Btu/h

10. Stripper diameter & height
Using the following table from Gas production engineering-Sanjay Kumar (pg 230)
Reboiler duty = Btu/hr
Glycol circulation rate = 120 gph
Stripper diameter=8.625 in = ft ≈ 1ft (0.3048m)
Stripper height= inch=4.42 ft ≈ 5 ft (1.52 m)

11. Packing size & height Size range for stainless steel pall rings
Here column diameter = ft
=0.22 m
So packing size has to be less than 25 mm
Assuming packing size to be 18 mm

12. For packed type design,
Maximum packing height is about 8ft for a Btu/h unit
[Gas production engineering-Sanjay Kumar (pg 230)]
So for a Btu/hr reboiler duty, maximum packing height=1.67 ft ≈ 2ft (0.6096m)

13. Reboiler diameter & height
Using the table above from Gas production engineering-Sanjay Kumar (pg 230)
Here reboiler duty = Btu/hr
Glycol circulation rate = 120 gph
So reboiler diameter= 24 inch
= 2 ft (0.61m)
Reboiler length= ft
≈ 6 ft (1.83m)

14. Fired tube diameter & length
Surface area of the firetube, A= (Reboiler duty/8000) [Gas production engineering-Sanjay Kumar]
= ft2
Where 8000 Btu/hr-ft2= Estimated design heat flux for direct fired reboiler [Gas production engineering-Sanjay Kumar]
Firetubes typically range from 0.5 to 2.5 ft ID [GPSA engineering databook (12th edition)].
Let D= 1.8 ft
Surface area of the firetube πDL= ft2
So length of the firetube, L= 4.61 ft ft
≈ 5 ft (1.52m)

15. Mechanical design of Stripper & Reboiler
Diameter of Feed Nozzle=0.2 ft
Diameter of Exit Nozzle=0.07 ft
Thickness of Shell=0.251 ft
Thickness of elliptical head=0.251 ft
Reboiler
Shell thicknes: 0.25 ft
Head thickness: 0.25 ft
Exit nozzle diameter: 0.21 ft

16. Material of Construction
Stripper & reboiler = Carbon steel
Packing material= Random packing of stainless steel pall rings

17. Mechanical Drawing
Reboiler
Stripper

18. Piping & Instrumentation Diagram

19. Plot Plan

20. Plant Layout

21. Economic Analysis

23. Cost of utility
Cost of steam = steam generation + steam distribution
= 4% of fixed capital investment
= 4% of ($ )
= $94,589
Cost of electricity = electricity generation + electric distribution
= 2% of fixed capital investment
= 2% of ($ )
= $ 47,294
Cost of cooling water = water treatment + water supply, cooling and pumping
= $47,294
Cost of utility = cost of steam + cost of electricity + cost of cooling water =
= $ 1,89,178

24. Calculation of Rate of Return
Assuming Tax=25%
Net profit after paying al taxes=$
Thus = %

25. Project feasibility IRR Method Let, MARR = 15%
PW = -Total capital investment + Annual income (P/A,i%,25) + Salvage value(P/F,i%,25)-
Or, 0 = (P/A,i%,25) (P/F,i%,25)
Or, 0 = (1+i)-25
By trial and error, i = %
Since i is greater than MARR, so the project is acceptable.

26. ERR Method
Let, MARR = 12%
Total capital investment (F/P,i%,25) = Salvage value + Annual revenue (F/A,12%,25)
Or, (F/P,i%,25) = (F/A,12%,25)
Or, (1+i)25 =
Or, i = 13%
Since, i is greater than MARR, so project is acceptable.

28. So, the payback period with interest is equal to 12 years since cumulative present worth appears positive at EOY=12 years.
The payback period without interest is equal to 7 years since cumulative PW appears positive at EOY=7 years.

29. THANK YOU