Method and apparatus for reconcentrating liquid absorbent

An improved method of and apparatus for reconcentrating a liquid absorbent in an absorption system which method includes the steps of heating the rich liquid absorbent in a heating zone to vaporize a portion of the solute therefrom, flashing the partially reconcentrated absorbent into a flash separator and separating the lean liquid absorbent from the flashed vapors in the flash separator with the lean liquid absorbent being recycled to the absorption system and with the separated vapors being cooled to condense liquids and returning the stream of condensed liquids and vapors to the heating zone. The improved apparatus includes a reboiler with a heating chamber, a surge chamber, means for flowing partially reconcentrated liquid absorbent from the heating chamber to the surge chamber, a flash separator, a valve controlling the flashing of liquid from the surge chamber to the flash separator, means for discharging separated liquids from said flash separator and a vacuum pump for returning the condensed liquids and vapors to the reboiler.

BACKGROUND 
In the dehydration of gas streams, such as natural gas carbon dioxide and 
others, by use of a liquid absorbent, the degree of concentration 
accomplished in the regeneration of the liquid absorbent is of prime 
importance in the effectiveness of the gas stream dehydration. It is well 
known that very substantial improvement in dew point depression of the 
natural gas stream is achieved by using a liquid absorbent which has a 
99.9 percent concentration as compared to the same absorbent having a 99.5 
percent concentration. 
One early effort to improve this liquid absorbent concentration is shown in 
U.S. Pat. No. 3,105,748 wherein gas stripping was suggested to increase 
the concentration of the liquid absorbent flowing from the reboiler to the 
reservoir. This gas stripping is successful but the gas used is lost with 
the water vapors discharged from the reboiler. 
U.S. Pat. No. 3,616,598 discloses the use of subatmospheric pressure on the 
reconcentration system to remove additional water from the liquid 
absorbent. The reduced pressure is obtained by an ejector powered by the 
fuel gas flow to the reboiler heater and connected to reduce the pressure 
on a small separator connected to the reboiler column outlet downstream of 
a water condenser. This system can cause the loss of liquid absorbent 
through the liquid outlet of the separator and operates on the total rich 
glycol stream, not the partially regenerated stream, hence it must handle 
much more water vapor. 
U.S. Pat. No. 3,867,112 discloses the use of stripping gas to increase the 
concentration of the liquid absorbent in a closed system using a liquid 
seal compressor for gas recirculation and for contact between the gas and 
the lean liquid absorbent. This system does not discharge its stripping 
gas to atmosphere but when used to dehydrate gases, such as carbon 
dioxide, it requires much of the system to be of corrosive resistant 
material including a sophisticated stripping column. 
Another attempt to improve reconcentration of liquid absorbent by using 
subatmospheric pressure is disclosed in U.S. Pat. No. 3,824,171. This 
system conducts the heated partially reconcentrated liquid absorbent to a 
subatmospheric system so that additional water vapor is removed from the 
liquid absorbent. With this system, several of the vessels used are 
subject to subatmospheric pressure and, thus have to be designed for such 
conditions. A separate stream of lean absorbent is circulated to the 
subatmospheric contactor to contact the vapors from the flashing of the 
partially reconcentrated absorbent to ensure that no vaporized liquid 
absorbent escapes from the system. 
SUMMARY 
The present invention provides an improved liquid absorbent reconcentration 
method and apparatus. The improved method includes the steps of heating 
the rich absorbent to vaporize a substantial portion of the absorbed 
vapors, flashing the partially reconcentrated liquid to a subatmospheric 
pressure to vaporize additional absorbed, vapors from the liquid 
absorbent, separating liquids and vapors from said flashing step, cooling 
the separated vapors to condense a portion of said vapors and delivering 
the condensed liquids and the vapors to the heating step. The improved 
apparatus includes a regeneration vessel having a heater in a heating 
chamber in the vessel, a surge chamber, means for delivering partially 
reconcentrated liquid absorbent to the surge chamber, a flash separator, 
means for flashing liquid absorbent from the surge chamber to the flash 
separator responsive to the level of liquid absorbent from the separator, 
means for discharging vapors from the separator, a cooler receiving the 
discharged vapors to condense liquids, and a vacuum pump connected to the 
cooler to pump liquids and vapors to the regeneration vessel. 
An object of the present invention is to provide an improved method of and 
apparatus for reconcentrating a liquid absorbent to a high degree of 
concentration which does not discharge gas from the system. 
A further object is to provide an improved method and apparatus for 
reconcentrating a liquid absorbent to a high degree of concentration which 
does not utilize a side stream of lean absorbent to prevent loss of 
absorbent from the system. 
Another object is to provide an improved method of and apparatus for 
reconcentrating a liquid absorbent to a high degree of concentration more 
economically than the prior art. 
A still further object is to provide an improved method of and apparatus 
for reconcentrating a liquid absorbent to a high degree while using less 
energy than the methods and apparatus of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The improved system illustrated in the drawings is suitable for 
reconcentrating liquid absorbent which can be reconcentrated by heating 
and by reduction of vapor pressure. A typical example of such liquid 
absorbent is triethylene glycol which is commonly used in the dehydration 
of natural gas streams. 
As shown in FIG. 1 reboiler or regeneration vessel 10 has stripping column 
12 mounted thereon and in communication at its lower end with the interior 
of the heating chamber 14 of vessel 10. Stripping column 12 includes 
suitable contacting means 16 such as a packed section into which rich 
absorbent line 18 connects. Transverse bulkhead 20 extends across vessel 
10 with heating chamber 14 on one side thereof and surge chamber 22 on the 
other side. Heating element 24 is positioned to heat the liquid absorbent 
in heating chamber 14 to drive off the vapors of the material absorbed by 
the absorbent liquid. Transverse bulkhead 20 extends to the level desired 
for the liquid absorbent in heating chamber 14. 
Liquid absorbent from stripping column 12 falls into the end of heating 
chamber 14 away from bulkhead 20. Thus, the liquid absorbent has been 
regenerated, preferably to approximately 99 percent, when it flows into 
surge chamber 22. Further reconcentration of the liquid absorbent is 
provided by the additional equipment as hereinafter described. 
The rich liquid absorbent from the absorption tower or contacting zone (not 
shown in FIG. 1) is delivered through line 26 to reflux coil 28 in the top 
of stripping column 12, through line 30 to heat exchanger 32 and through 
line 18 into stripping column 12. Lean or reconcentrated liquid absorbent 
is delivered to pump 34 and is pumped therefrom through line 36. 
Partially reconcentrated liquid absorbent is discharged from surge chamber 
22 through line 38 and is flashed through valve 40 and heater 42 into 
flash separator 44. Valve 40 is controlled by liquid level controller 46 
on separator 44 to maintain a reservoir of liquid in the bottom of flash 
separator 44 so that vapors are not discharged through liquid outlet 48. 
It should be understood that heater 42 may be any source of heat, such as 
a heat exchange within or with the liquid in heating chamber 14. The 
purpose of heating the flashed stream is to replace the heat loss due to 
the vaporization of some of the liquids being flashed. This allows flash 
separator to operate at a higher temperature so that more vaporization 
occurs which results in a higher concentration of the regenerated liquid 
absorbent. Heater 42, however, is not required to achieve increased 
concentration of the absorbent. 
The regenerated liquid absorbent is discharged from separator 44 through 
liquid outlet 48 and through heat exchanger 32 where it is cooled and then 
delivered to pump 34 for delivery to the absorber (not shown). The vapors 
are discharged from flash separator 44 through vapor outlet 50 through 
overhead condenser or cooler 52 which cools the vapors to partially 
condense them and then to liquid seal vacuum pump 54 which creates the 
reduced pressure on flash separator 44 and returns the condensate vapor 
from condenser 52 through line 56 to stripping column 12. 
It is estimated that the approximate concentration of the condensed liquids 
flowing through line 56 will be about 95 percent and is shown to be 
introduced into stripping column 12 through line 58. Alternate inlets 60 
and 62 are provided to allow a range of introduction points for the 
condensed liquids and vapors returned to reboiler 10 through line 56. It 
is preferred that the liquids and vapors be introduced into reboiler 10 or 
its stripping column 12 at a point at which the liquid therein has 
substantially the same concentration as the returned liquid. 
If reboiler 10 is operated at a temperature of 400.degree. F., then it is 
estimated that the temperature of the flash stream flowing from valve 40 
would be 387.degree. F. Reheating the flash stream to 400.degree. F. can 
improve the concentration of the liquid absorbent to a concentration 
approaching 100 percent (99.95% for triethylene glycol) which provides a 
substantial improvement in the amount of absorption which the lean liquid 
absorbent can achieve in the absorber. 
The schematic diagram in FIG. 2 is substantially the same as the diagram in 
FIG. 1 with some modifications and with the inclusion of absorber 100. 
Those elements of the system which are unchanged are given the same 
numbers in both diagrams. For example, line 36 is shown connected to lean 
liquid absorbent inlet 102 in absorber 100 and rich liquid absorbent 
outlet 104 connects to line 26. The position of heat exchanger 32 may also 
be relocated to cool the regenerated glycol in line 48 downstream of pump 
34 in line 36 as shown in FIG. 2.