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-0902009

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.

Project Definition

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

Process Block Diagram

Process Flow Diagram

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

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

Reboiler duty Reboiler duty can be calculated from the following equation Q=Ql+Qw+Qr = Lρc(T2-T1)+ 970.3x(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=208500.38 Btu/h

Stripper diameter & height Using the following table from Gas production engineering-Sanjay Kumar (pg 230) Reboiler duty = 208500.38 Btu/hr Glycol circulation rate = 120 gph Stripper diameter=8.625 in = 0.71875 ft ≈ 1ft (0.3048m) Stripper height= 56.68 inch=4.42 ft ≈ 5 ft (1.52 m)

Packing size & height Size range for stainless steel pall rings Here column diameter =0.71875 ft =0.22 m So packing size has to be less than 25 mm Assuming packing size to be 18 mm

For packed type design, Maximum packing height is about 8ft for a 1000000 Btu/h unit [Gas production engineering-Sanjay Kumar (pg 230)] So for a 208500.38 Btu/hr reboiler duty, maximum packing height=1.67 ft ≈ 2ft (0.6096m)

Reboiler diameter & height Using the table above from Gas production engineering-Sanjay Kumar (pg 230) Here reboiler duty = 208500.38 Btu/hr Glycol circulation rate = 120 gph So reboiler diameter= 24 inch = 2 ft (0.61m) Reboiler length= 5.895 ft ≈ 6 ft (1.83m)

Fired tube diameter & length Surface area of the firetube, A= (Reboiler duty/8000) [Gas production engineering-Sanjay Kumar] = 26.063 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=26.063 ft2 So length of the firetube, L= 4.61 ft ft ≈ 5 ft (1.52m)

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

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

Mechanical Drawing Reboiler Stripper

Piping & Instrumentation Diagram

Plot Plan

Plant Layout

Economic Analysis

Cost of utility Cost of steam = steam generation + steam distribution = 4% of fixed capital investment = 4% of ($2364747) = $94,589 Cost of electricity = electricity generation + electric distribution = 2% of fixed capital investment = 2% of ($ 2364747) = $ 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 = 94589 + 47294 + 47294 = $ 1,89,178

Calculation of Rate of Return Assuming Tax=25% Net profit after paying al taxes=$442283.9598 Thus =15.3267%

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 = -2885694 + 442284 (P/A,i%,25) + 69069.3 (P/F,i%,25) Or, 0 = -2885694 + 442284 + 69069.3 (1+i)-25 By trial and error, i = 19.99 % Since i is greater than MARR, so the project is acceptable.  

ERR Method Let, MARR = 12% Total capital investment (F/P,i%,25) = Salvage value + Annual revenue (F/A,12%,25) Or, 2885694 (F/P,i%,25) = 69069.3 + 442284 (F/A,12%,25) Or, 2885694 (1+i)25 = 69069.3 + 442284 Or, i = 13% Since, i is greater than MARR, so project is acceptable.

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.

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