Carbon dioxide removal process

Details Carbon dioxide removal process Carbon dioxide removal process

2003-01-28

2003-11-18

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Compressing and indirect cooling of gaseous fluid mixture to...

C095S050000, C095S051000, C096S009000

Reexamination Certificate

active

06648944

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the treatment of natural gas and other gas streams containing carbon dioxide and methane. More particularly, the invention relates to the use of gas separation membranes to remove excess carbon dioxide from the gas.
BACKGROUND OF THE INVENTION
Natural gas is the most important fuel gas in the United States and provides more than one-fifth of all the primary energy used in the United States. Natural gas is also used extensively as a basic raw material in the petrochemical and other chemical process industries. The composition of natural gas varies widely from field to field. For example, a raw gas stream may contain as much as 95% methane, with only minor amounts of other hydrocarbons, nitrogen, carbon dioxide, hydrogen sulfide or water vapor. On the other hand, streams with large proportions of one or more of these contaminants are common. For example, gas that is extracted as a result of miscible flood enhanced oil recovery may be very rich in carbon dioxide, as well as being saturated with C
3+
hydrocarbons.
Overall, about 10% of gas exceeds the typical gas pipeline specification for carbon dioxide of no more than 2%.
Before such gas can be sent to the supply pipeline, the carbon dioxide content must be reduced. Various techniques for acid gas removal, including absorption into an amine solution, cryogenic separation and membrane separation, have been used in the industry. Each has its own advantages and disadvantages.
If membrane separation is used, cellulose acetate membranes, which can provide a carbon dioxide/methane selectivity of about 10-20 in gas mixtures at pressure, have been the membranes of choice, and about 100 plants using cellulose acetate membranes are believed to have been installed around the world.
It would be desirable in many more cases to use membrane separation, because membrane systems are relatively simple, have few moving parts, can operate under moderate temperature and pressure conditions and, unlike amine scrubbing, do not require a regeneration cycle. Also, the wellhead gas pressure may be high enough to provide the total driving force for transmembrane permeation. However, cellulose acetate and other polymeric membranes are not without problems.
Natural gas often contains substantial amounts of water, either as entrained liquid, or in vapor form, which may lead to condensation within the membrane modules. The gas separation properties of cellulose acetate membranes are destroyed by contact with liquid water. Therefore, care must be taken to remove all entrained liquid water upstream of the membrane separation steps and to lower the water vapor dew point comfortably below the lowest temperature that the gas under treatment is likely to encounter.
Likewise, many membranes are irreparably damaged by liquid hydrocarbons, and similar precautions must be taken to avoid the risk of condensation of C
3+
hydrocarbons on the membranes at any time. The presence of more than modest ppm levels of hydrogen sulfide, especially in conjunction with water and heavy hydrocarbons, is also potentially damaging.
Furthermore, carbon dioxide readily sorbs into and interacts strongly with many polymers, and in the case of gas mixtures such as carbon dioxide/methane with other components, the carbon dioxide tends to have a swelling or plasticizing effect, thereby adversely changing the membrane permeation characteristics. Although some membrane materials, such as polyimides, exhibit a high ideal selectivity for carbon dioxide over methane when measured with pure gases at modest pressures in the laboratory, the selectivity obtained under mixed gas, high-pressure conditions is much lower. This means it is often very difficult under field conditions to meet target specifications for carbon dioxide content without resorting to impractically large amounts of membrane area and/or unacceptably complicated processing schemes.
These issues are discussed in more detail in U.S. Pat. No. 5,407,466, columns 2-6, which patent is incorporated herein by reference.
Thus, although membranes have been, and are, used to remove carbon dioxide from natural gas, there are many situations where the composition of the gas, the size of the stream to be processed, or the site geography render a membrane-based process technically or economically unrealistic.
That membranes can separate C
3+
hydrocarbons from gas mixtures, such as natural gas, is known, for example from U.S. Pat. Nos. 4,857,078, 5,281,255 and 5,501,722. It has also been recognized that condensation and membrane separation maybe combined, as is shown, for example, in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843 and 5,374,300.
In spite of the above knowledge and practices, technology that can process gas containing excess quantities of carbon dioxide, C
3+
hydrocarbons and water in a cost-effective manner is still needed. The challenge of treating gas that contains relatively large amounts of carbon dioxide, such as more than about 8% or 10%, for example, is particularly difficult.
SUMMARY OF THE INVENTION
The invention is a process and apparatus for separating carbon dioxide from gas, especially natural gas, that also contains C
3+
hydrocarbons. The invention uses two or three membrane separation steps, optionally in conjunction with cooling/condensation/phase separation under pressure, to yield a lighter, sweeter product natural gas stream, and/or a carbon dioxide stream of reinjection quality and/or a natural gas liquids (NGL) stream.
In a basic embodiment, the process of the invention includes the following steps for treating a gas stream:
(a) providing a first membrane having a first feed side and a first permeate side and being selective for C
3+
hydrocarbons over methane;
(b) passing the gas stream, under conditions in which the gas stream has a carbon dioxide partial pressure of at least about 30 psia and a C
3+
hydrocarbons combined partial pressure of at least about 30 psia, to the first membrane on the first feed side;
(c) withdrawing from the first feed side a first residue stream depleted in C
3+
hydrocarbons compared with the gas stream;
(d) withdrawing from the first permeate side a first permeate stream enriched in C
3+
hydrocarbons compared with the gas stream;
(e) providing a second membrane having a second feed side and a second permeate side and being selective for carbon dioxide over methane;
(f) passing the first residue stream to the second membrane and across the second feed side;
(g) withdrawing from the second feed side a second residue stream depleted in carbon dioxide compared with the first residue stream;
(h) withdrawing from the second permeate side a second permeate stream enriched in carbon dioxide compared with the first residue stream;
(i) providing a third membrane having a third feed side and a third permeate side and being selective for carbon dioxide over methane;
(j) passing the second permeate stream to the third membrane and across the third feed side;
(k) withdrawing from the third feed side a third residue stream depleted in carbon dioxide compared with the second permeate stream;
(l) withdrawing from the third permeate side a third permeate stream enriched in carbon dioxide compared with the second permeate stream;
(m) optionally recirculating at least a portion of the third residue stream for further treatment within the process;
(n) optionally recirculating at least a portion of the first permeate stream to step (b).
Such an embodiment can be used if no removal of C
3+
hydrocarbons prior to treatment in the membrane separation units is required. The first permeate stream will be enriched in C
3+
hydrocarbons compared with the feed stream, and may be treated, for example by compression/cooling/phase separation, to recover condensed hydrocarbons as an NGL stream.
A typical embodiment in which compression, cooling and phase separation steps are used upstream of the membrane separation units includes the following steps:
(a) compressing the gas stream;
(b) cooling the gas stre

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