Glycol De-hydrator design

K.SARKODIE

• Triethylene glycol dehydrators using tray or packed-column contactors may be sized from standard models by using the following procedures and associated graphs and tables (Sivalls).

Required gas stream data

• The following information must be available on the gas stream to be dehydrated:

• Gas flow rate, MMscfd.

• Specific gravity of gas.

• Operating pressure, psig.

• Maximum working pressure of contact, psig.

• Gas inlet temperature, oF.

• Outlet gas water content required, Ibm/MMscf.

Design criteria Having the above information, it is then necessary to

select two points of design criteria: • Glycol to water circulation rate based on water

removed. • A value of 2 to 6 gal TEG/IgH2O removed is adequate

for most glycol dehydration requirements. Use 2.5 to 4 gal TEG/Ib H2O for most field dehydrators.

• Lean TEG concentration from re-concentrator. 99.0 to 99.9% lean TEG is available from most glycol re-concentrators.

• A value of 99.5% lean TEG is adequate for most design considerations.

Inlet scrubber

• The required diameter of a vertical inlet scrubber may be selected using Fig. below, based on the operating pressure of the unit and gas capacity required. Two-phase inlet scrubbers are generally constructed with 7 1/2-ft shell heights.

• Additional data on typical standard vertical inlet scrubbers are contained in Tables A.14 and A.15 in the Appendix.

Glycol gas contactor

Selection based on • Operating pressure • Required gas capacity

• qo = gas capacity of contactor at operating conditions, MMscfd

• qs = gas capacity of contactor at standard conditions (0.7 specific gravity and 100oF) based on operating pressure, MMsfcd

• Ct = correction factor for operating pressure, MMscf • Cg = correction factor for gas specific gravity

Gas capacity for trayed glycol gas contactors based on 0.7 specific gravity, at 100oF. (After Sivalls.)

Gas capacity for packed glycol gas contactors based on 0.7 specific gravity, at 100oF. (After Sivalls.)

• The temperature and gas specific gravity correction factors for trayed glycol contactors are contained in Tables 4.1 and 4.2, respectively.

• The temperature and specific gravity factors for packed glycol contactors are contained in Tables 4.3 and 4.4, respectively.

Dew point depression and water removed from the unit

• The outlet dew point temperature can be found on the water vapour content graph using the outlet gas water content required and the operating pressure.

• The water content of the inlet gas can be determined from the same water vapour content graph using the inlet gas temperature and the operating pressure.

Dew point depression

• oF = inlet gas temp. oF – outlet dew point temp. oF

• Where

Wr = water removed, Ibm/hr

Wi = water content of inlet gas, Ib H2O/MMcf

Wo = water content of outled gas, Ib H2O/MMcf

q = gas flow rate, MMscfd

Effects of acid gases

• Since both carbon dioxide and hydrogen sulphide absorb considerably more water vapour than natural gas, they appreciably increase the total water content and dehydration requirements of the gas stream.

Trayed and packed Contactors To determine the number of trays

Use the required dew point depression and the selected glycol to water circulation rate

Glycol Re-concentrator

Calculate the required glycol circulation rate

Where

L = glycol circulation rate, gas/hr

Lw = glycol to water circulation rate, gal TEG/Ib H2O

Wt = water content of inlet gas, Ib H2O/MMscf

q = gas flow rate, MMscfd

Re-boiler • The required heat load for the re-boiler can be

estimated from the following equation:

Where

Ht = Total heat load on re-boiler, Btu/h

L = Glycol circulation rate, gph

• The above formula for determining the required re-boiler heat load is an approximation, which is accurate enough for most high-pressure glycol dehydrator sizing.

• A more detailed determination of the required re-boiler heat load may be made from the following procedure

• Where Ht = sensible heat required for glycol, Btuh Hw= heat of vaporization required for water, Btuh Hr= heat to vaporize reflux water in still, Btuh Hh = heat loss from reboiler and stripping still, Btuh Ht = total reboiler heat load, Btuh L = glycol circulation rate, gph ῥi = glycol density at average temperature in reboiler, Ibm/gal = (spgr)(8.34) C = glycol specific heat at average temperature in reboiler, Btu/IbmoF T2= glycol outlet temperature,oF T1 = glycol inlet temperature, oF 970.3 = heat of vaporization of water at 212oF 14.7 psia, Btu/Ibm Wi = water content of inlet gas Ib H2O/MMscf Wo = water content of outlet gas Ib H2O/MMscf q = gas flow rate, MMscfd

Glycol Circulating Pump

• The required size of the pump can readily be determined using the glycol circulation rate and the maximum operating of the contactor.

• This pump uses the rich glycol from the bottom of the contactor to power the pump and pump the lean glycol to the top of the contactor.

• Sizing data is contained in table A.21 BELOW

Glycol Flash Separator

Stripping Still

Stripping still size for glycol dehydrators

Number of trays needed