Device and a membrane process for separating gas components from a gas stream having varying composition or flow rate
A device for separating a gas stream which has a compressor and three membrane separation units in series, connected to pass the retentate stream of each of the first two units to the subsequent membrane separation unit, comprises conduits for recycling the permeate streams of the second and the third membrane separation unit to upstream of the compressor and a control device controlling the fraction of the second permeate stream which is recycled. Adjusting which fraction of the second permeate is recycled can be used to maintain a target composition of the retentate obtained in the third membrane separation unit when the flow rate or the composition of the gas stream changes.
1. A device for separating a gas stream, comprising a first gas component and
a second gas component, comprising:
(a) a first membrane separation unit receiving said gas stream through a feed conduit, said first membrane separation unit comprising a gas separation membrane having higher permeance for said first gas component than for said second gas component, providing a first permeate stream enriched in said first gas component and a first retentate stream;
(b) a first permeate conduit connected to said first membrane separation unit to receive said first permeate stream;
(c) a first retentate conduit connected to said first membrane separation unit to receive said first retentate stream;
(d) a second membrane separation unit, connected to said first retentate conduit to receive the first retentate stream as feed, said second membrane separation unit comprising a gas separation membrane having higher permeance for said first gas component than for said second gas component, providing a second retentate stream and a second permeate stream;
(e) a second retentate conduit connected to said second membrane separation unit to receive said second retentate stream;
(f) a second permeate conduit connected to said second membrane separation unit to receive said second permeate stream;
(g) a third membrane separation unit, connected to said second retentate conduit to receive the second retentate stream as feed, said third membrane separation unit comprising a gas separation membrane having higher permeance for said first gas component than for said second gas component, providing a third retentate stream and a third permeate stream;
(h) a product conduit connected to said third membrane separation unit to receive said third retentate stream;
(i) a first recycle conduit connected to said third membrane separation unit to receive said third permeate stream, and connected to a first recycle feedpoint on said feed conduit;
(j) a gas compressor arranged in said feed conduit between said first recycle feed point and said first membrane separation unit or arranged in said first recycle conduit;
(k) a second recycle conduit connected to a second recycle feed point on said feed conduit or on said first recycle conduit, said second recycle feed point being located upstream of said gas compressor and said second recycle conduit being connected to said second permeate conduit to receive all or a fraction of said second permeate stream; and
(l) a control device controlling the fraction of said second permeate stream being passed to said second recycle conduit and passing the remainder of said second permeate stream to a discharge conduit;
wherein said first permeate conduit and said discharge conduit are connected to a joint discharge conduit; and wherein:
i) said first membrane separation unit comprises a gas separation membrane with:
a pure gas selectivity at 20° C. for the first gas component over the second gas component that is from 1.05 to 2 times higher than the pure gas selectivity of a gas separation membrane in the second membrane separation unit for said first gas component over said second gas component at the same temperature and for the same gas components;
a permeance at 20° C. for said first gas component that is lower than the permeance of a membrane in the second membrane separation unit;
ii) said second membrane separation unit comprises a gas separation membrane with:
a pure gas selectivity at 20° C. for the first gas component over the second gas component that is higher than the selectivity of a membrane in the third membrane separation unit for said first gas component over said second gas component; and
a permeance at 20° C. for said first gas component that is lower than the permeance of a membrane in the third membrane separation unit.
2. The device of claim 1 , wherein a check valve is arranged in said product discharge conduit preventing passage of the first permeate stream from said first permeate conduit to said second recycle conduit.
3. The device of claim 1 , additionally comprising a first vacuum pump arranged in said first recycle conduit, providing subatmospheric pressure on the permeate side of said third membrane separation unit.
4. The device of claim 1 , additionally comprising a second vacuum pump arranged in said second permeate conduit upstream of said discharge conduit, providing subatmospheric pressure on the permeate side of said second membrane separation unit.
5. The device of claim 2 , additionally comprising:
a) a first vacuum pump arranged in said first recycle conduit, providing subatmospheric pressure on the permeate side of said third membrane separation unit;
b) a second vacuum pump arranged in said second permeate conduit upstream of said discharge conduit, providing subatmospheric pressure on the permeate side of said second membrane separation unit.
6. The device of claim 1 , additionally comprising a gas analyzer connected to said product conduit for determining a gas property of said third retentate stream, said gas property being selected from the content of said first gas component, the content of said second gas component and the calorific value, said gas analyzer being connected to said control device to transmit information on said gas property and said control device being configured to maintain said gas property at a target value by controlling the fraction of said second permeate stream being passed to said second recycle conduit.
7. The device of claim 1 , wherein said third membrane separation unit comprises at least two membrane modules arranged in parallel and at least one shut-off valve for taking a membrane module off stream.