Gas dehydration using solid bed
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This document describes the process of solid bed gas dehydration. It begins with an introduction to gas dehydration and why it is needed to meet contractual water content specifications. It then covers determining the water content in a gas stream and corrections that must be made. The main part of the document discusses how solid bed dehydration systems work using adsorption onto a desiccant and provides details on the process, design considerations, example calculations, and heat requirements for regeneration. It concludes with a solved example to design a solid bed dehydration unit to treat a specific gas feed.
![The pressure drop assume ⅛ - in .bead
and μ=0.01 cp
∆P =
[(0.056*0.01*55)+(0.00009*1.7*55²)]*30
∆P = 14.8 psia > 8 psi
this is more than the recommended 8
psi
Choose dia. Of 5 ft 6 in
Vm = 39.2 ft/min
L = 21.5 ft/min
∆P = 5.5 psi < 8 psi accepted
](https://image.slidesharecdn.com/gasdehydration-131211144508-phpapp02/85/Gas-dehydration-using-solid-bed-32-320.jpg)
Gas dehydration using solid bed
- 1. Gas Dehydration Using Solid Bed Dehydration By Engineer Mohammed Bedair Yossof Graduated from Faculty of Petroleum & Mining Engineering Suez Canal University E-mail : bakar_zezo99@yahoo.com
- 2. CONTENTS What is gas dehydration ? 2. Water content calculations 3. Solid bed dehydration process 4. Solved example about it 1.
- 3. WHAT’S THIS ? This is the process of removing water vapor from a gas stream to lower it’s ‘’ Dew point’’ The gas sales contracts specify a maximum value for the amount of water vapor allowable in the gas .
- 4. EX : AT 1000 PSI Place Value lb / MMscf Southern U.S. 7 Dew point temperature °F 32 Northern U.s 4 20 Canada 2:4 Zero
- 5. WATER CONTENT DETERMINATION For sweet gas containing over 70 % methane and small amounts of ‘’heavy ends ‘’ The Mc Ketta-Wehe pressure –temperature correlation ,as shown can be used Ex : Assume Molecular weight of 26 that is in equilibrium with 3 % brine at a pressure of 3000 psia an temperature of 150 F
- 7. SO FROM FIGURE 1 At 3000 psia and 150 F water content = 104 lb water per MMscf of wet gas The correction for salinity is 0.93 and for molecular weight is 0.98 Then the total water content = 104*0.93*0.98 =94.8 lb / MMscf
- 8. CORRECTION FOR ACID GAS Correction for acid gas should be made when the gas stream contains more than 5 % CO2 and /or H2S EX : Assume the previous gas example contain 15 % H2S
- 11. SOLUTION From figure 3 , the water content of H2S is 400 lb / MMscf So the effective water content of the stream is equal to =(0.85*94.8)+(0.15*400) = 141 lb / MMscf
- 13. SOLID BED DEHYDRATION Solid bed dehydration system work on the principle of adsorption . Adsorption involves a form of adhesion between the surface of the solid desiccant and the water vapor in the gas . The water forms an extremely thin film that is held to the desiccant surface by the forces of attraction , but there is no chemical reaction .
- 14. The desiccant is a solid , granulated drying or dehydrating medium with an extremely large effective surface area per unit weight because of a multitude of microscopic pores and capillary openings . A typical desiccant might have as much as 4 million square feet of surface area per pound The initial cost for a solid bed dehydration unit generally exceeds that of a glycol unit .
- 15. However the dry bed has the advantage of producing very law dew points ,which are required for cryogenic gas plants Cryogenics : Is the study of the production of very low temperature materials (below −43°C) and the behavior of materials at those temperatures . Disadvantages are that it is a batch process , there is a relatively high pressure drop through the system , and the desiccant are sensitive to poisoning with liquids or other
- 16. PROCESS DESCRIPTION 1. 2. 3. 4. 5. 6. Inlet gas separator . Two or more adsorption towers (contactors )filled with a solid desiccant . A high –temp heater to provide hot regeneration gas to reactive the desiccant in the towers A regeneration gas cooler to condense water from the hot regeneration gas . A regeneration gas separator to remove the condensed water from the regeneration gas . Piping , manifolds , switching valves and controls to direct and control the flow of gases according to the process requirements .
- 18. DESIGN CONSIDERATION 1- Temperature Adsorption plant operation is very sensitive to the temperature of the incoming gas , Generally , the adsorption efficiency decrease as the temperature increases The maximum hot regeneration gas temp. depends on the type of contaminants.
- 19. If a wet gas is used to cool the desiccant , the cooling cycle should be terminated when the desiccant bed reaches a temperature of approximately 215 °F .Additional cooling may causes water to be adsorbed from the wet gas and preload the desiccant bed before the next adsorption cycle 2- PRESSURE The adsorption capacity of a dry bed unit decreases as the pressure is lowered
- 20. 3- CYCLE TIME Most adsorbers operate on a fixed cycle time and , frequently , cycle time is set for the worst conditions However , the adsorbent capacity is not fixed value ; it decline with usage 4- GAS VELOCITIES Generally , as the gas velocity during the drying cycle decreases ,the ability of the drying desiccant to dehydrate the gas increases
- 21. MAXIMUM SUPERFICIAL VELOCITY FT/MIN
- 22. Low velocity require towers with large cross sectional areas to handle a given gas flow d² = 3600*(Qg*T*Z) / (P*Vm) d Qg T Z P Vm = vessel internal diameter , in = gas flow rate ,MMscf = gas temp , R = compressibility factor = gas pressure , psia = gas superficial velocity , ft/min
- 23. 5- Bed height to Diameter Ratio A bed height to diameter ratio (L/D) of more than 2.5 is desiable 6- PRESSURE DROP ∆P / L =B *μ* Vm + ρ *C* Vm² ∆P = pressure drop , psi L = length of bed , ft μ = gas viscosity , cp Vm = gas superficial velocity , ft/min ρ = gas density , lb/ft³ Note , pressure drop greater than approximately 8 psi are not recommended
- 24. B and C are constants given by Practical Type B C ⅛ - in . Bead 0.0560 0.0000889 ⅛ - in . extrudate 0.0722 0.000124 ⅟16 - in . Bead 0.152 0.000136 16 - in . extrudate ⅟ 0.238 0.000210
- 25. 7- Moisture content of the inlet gas How much lb of water vapor per MMscf , and to how much you want to be . 8- DESICCANT SELECTION No desiccant is best for all applications The most common desiccant used is the Molecular sieves as It less affected by temperature increases (capacity decrease ) Less contaminated with liquid
- 26. Desiccant disadvantages Molecular sieves and alumina gels acts as a catalyst with H2S to form COS , when the bed is regenerated , sulfur remains and plugs up the spaces Silica gels will shatter in the presence of free water and are chemically attacked by many corrosion inhibitors
- 27. PROPERTIES OF COMMERCIAL DESICCANTS Molecular sieves Silica gel Sorbeads ‘’R’’ Design loading ( % weight of water / weight bed ) 8-10 6-7 6-7 Regeneration temperature (°F) 450-550 350 300-500 Bulk density Lb/ ft³ 40-46 45 49 Specific heat Btu / lb-°F .25 .22 .25
- 28. EXAMPLE OF DRY DESICCANT DESIGN Feed rate 50 MMscfd Molecular weight of gas 17.4 Gas density 1.7 lb/ft³ Operating temperature 110 °F Operating pressure 600 psia Inlet dew point 100 °F (equivalent to 90 lb H2O / MMscf ) Desired outlet dew point 1 ppm H2O
- 29. SOLUTION Assume an 8-hours on stream cycle with 6 hours regeneration Water absorbed =(8/24)*50 MMscfd * 90 lb/MMscf =1,500 lb H2O/ Cycle Loading Use sorbeads as a desiccant and use design loading = 6%
- 30. 1500 lb H2O .06 lb H2O / lb desiccant = 25,000 lb desiccant per bed 25,000 lb desiccant per bed 49 lb desiccant / ft³ = 510 ft³ per bed
- 31. Tower sizing Assume Z = 1 From chart max Vm = 55 ft / min d²= 3600*(50*570*1)/(55*600) d = 55.7 in = 4.65 ft The bed height is : L= 510 ft³ / ((π*4.65²)/4) ft² L= 30 ft
- 32. The pressure drop assume ⅛ - in .bead and μ=0.01 cp ∆P = [(0.056*0.01*55)+(0.00009*1.7*55²)]*30 ∆P = 14.8 psia > 8 psi this is more than the recommended 8 psi Choose dia. Of 5 ft 6 in Vm = 39.2 ft/min L = 21.5 ft/min ∆P = 5.5 psi < 8 psi accepted
- 33. Leaving 6 ft above and blow the bed , so the total length including the space to remove the desiccant and refill would be about 28 ft So L/D = 28/5.5 = 5 >2.5 accepted Regeneration heat requirement Assume the bed and the tower is heated to 350 °F , so the average temperature will be (350+110)/2 = 230 °F The approximate weight of the 5.5 ft *28 ft tower is 53,000 lb including the shell , head , nozzels and support for the desiccant
- 35. Q = m Cp ∆T Heating requirement /cycle Desiccant Tower Desorb water 25000 lb*(350-110)*0. 25 = 1,500,000 Btu 53000 lb*(350-110)*0. 12 = 1,520,000 Btu 1,500 lb *1,100 Btu/lb 1500 lb*(230-110)*1 = 1,650,000 Btu = 200,000 Btu 4,870,000 Btu +10% for heat losses ,etc 490,000 Btu total heat = 5,360,000 Btu / cycle 0.12 specific heat of steel The number 1100 Btu/lb is the heat of water desorption , value supplied by the desiccant manufacturer The majority of the water will adsorb at the average temperature . This heat requirement represent the sensible heat require to raise the temperature of the water to the adsorption temperature
- 37. Cooling requirement / cycle Desiccant 25000 lb*(350-110)*.25 = 1,500,000 Btu Tower 53000 lb*(350-110)*.12 = 1,520,000 Btu 3,020,000 Btu + 10 % for non-uniform cooling ,etc 300,000 Btu Total cooling heat =3,320,000 Btu /cycle Regeneration gas heater Assume the inlet temperature of the regeneration gas is 400 F , The initial outlet temp. of the bed will be temperature of 110 F , outlet temp will be the designed value of 350 F , So the average outlet temperature will be (350+110)/2 =230 F
- 39. Then the volume of the gas required for the heating will be V heating 5,360,000 Btu/cycle (400-230) °F*0.64 Btu/lb/°F V heating = 49,400 lb/cycle QH is then : QH =49,400*(400-110) °F*0.62 =8,900,000 Btu/cycle The regeneration gas heater load For design , add 25 % for heat losses and non-uniform flow . Assuming a 3-hours heating cycle , the regenerator gas heater must be sized for QH=8,900,00*1.25/3 =3,710,000 Btu/hr 0.62 →specific heat of gas at average temperature
- 41. Regeneration gas cooler The regeneration gas cooling load is calculated assuming that all the adsorbed water is condensed during a ⅟2 hr of the 3 hrs cooling cycle Regeneration gas 49,400(230-110)*0.61/3 = 1,205,000 Btu/hr Water Btu/hr 1,500*(1,157-78) / 0.5 = 3,237,000 4,442,000 Btu/hr +10 % losses Btu/hr 444,000 Total cooling load Qc =4,886,000 Btu/hr
- 43. Cooling cycle V cooling = 3,320,000 Btu/cycle (230-110)°F*0.59 Btu/lb/°F V cooling = 46,900 lb/cycle 0.59 → Specific heat at average temperature
- 44. References Chapter 8 Gas dehydration Book Gas Reservoirs
- 45. THE END