Membrane permeation treatment with adjustment of the number of membranes used as a function of the pressure of the feed gas flow

Abstract

A facility and method for membrane permeation treatment of a feed gas flow containing at least methane and carbon dioxide that includes a compressor, a pressure measurement device, at least one valve, and first, second, third, and fourth membrane separation units for separation of CO.sub.2 from CH.sub.4 to permeates enriched in CO.sub.2 and retentates enriched in CH.sub.4, respectively. The at least one valve adjusts the number of membranes combined and connected to the flow of gas entering into at least one of the membrane separation units as a function of the pressure recorded by the pressure measurement device.

Claims

1. A method for membrane permeation treatment of a feed gas flow containing at least methane and carbon dioxide, comprising: compressing the feed gas flow with a compressor; receiving the compressed feed gas flow from the compressor at a first membrane separation unit and supplying therefrom a first permeate and a first retentate, the first membrane separation unit comprising a plurality of membranes more permeable to carbon dioxide than methane; receiving the first retentate at a second membrane separation unit and supplying therefrom a second permeate and a second retentate, the second membrane separation unit comprising a plurality of membranes more permeable to carbon dioxide than methane; receiving the first permeate at a third membrane separation unit and supplying therefrom a third permeate and a first retentate, the third membrane separation unit comprising a plurality of membranes more permeable to carbon dioxide than methane; receiving the third retentate at a fourth membrane separation unit and supplying therefrom a fourth permeate and a fourth retentate, the fourth membrane separation unit comprising a plurality of membranes more permeable to carbon dioxide than methane; measuring a pressure of either the compressed feed gas flow received from the compressor by the first membrane separation unit or a pressure of the second retentate; comparing the measured pressure with a setpoint pressure and determining a difference therebetween; and adjusting, through use of at least one valve, a number of the plurality of membranes of at least one of the membrane separation units that receive a flow of gas as a function of the determined difference between the measured pressure and the setpoint pressure so as to keep a methane concentration of the second retentate constant.

2. The method of claim 1, wherein said step of adjusting comprises, successively: adding one of the plurality of membranes of the first membrane separation unit to the number of the plurality of membranes of the first membrane separation unit that receive the compressed feed gas flow from the compressor, adding one of the plurality of membranes of the third membrane separation unit to the number of the plurality of membranes of the third membrane separation unit that receive the first permeate, and finally adding one of the plurality of membranes of the second membrane separation unit to the number of the plurality of membranes of the second membrane separation unit that receive the first retentate.

3. The method of claim 1, wherein said step of adjusting comprises, successively: withdrawing one of the plurality of membranes of the first membrane separation unit from the number of the plurality of membranes of the first membrane separation unit that receive the compressed feed gas flow from the compressor, withdrawing one of the plurality of membranes of the third membrane separation unit from the number of the plurality of membranes of the third membrane separation unit that receive the first permeate, and finally withdrawing one of the plurality of membranes of the second membrane separation unit from the number of the plurality of membranes of the second membrane separation unit that receive the first retentate.

4. The method of claim 1, wherein said steps of measuring, comparing, and adjusting step are carried out automatically by a programmable controller.

5. The method of claim 1, wherein said step of measuring comprises measuring a pressure of the compressed feed gas flow received from the compressor by the first membrane separation unit and said step of adjusting is performed according to the following rules: when the measured pressure goes above a previously fixed threshold pressure value S1: one of the plurality of the membranes of the first membrane separation unit is added to the number of membranes receiving the flow of gas from the compressor, one of the plurality of the membranes of the third membrane separation unit is added to the number of membranes receiving the first permeate, and one of the plurality of the membranes of the second membrane separation unit is added to the number of membranes receiving the first retentate; and when the measured pressure goes below a previously fixed threshold pressure value S2: one of the plurality of the membranes of the first membrane separation unit is withdrawn to the number of membranes receiving the compressed feed gas flow from the compressor, one of the plurality of the membranes of the third membrane separation unit is withdrawn to the number of membranes receiving the first permeate, and one of the plurality of the membranes of the second membrane separation unit is withdrawn to the number of membranes receiving the first retentate.

6. The method of claim 5, wherein said steps of measuring step and adjusting step are carried out automatically by a programmable controller.

7. The method of claim 1, wherein the feed gas flow is biogas.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic of an example of a facility incorporating the inventive technique.

(2) FIG. 2 is a schematic of a membrane separation unit incorporating the inventive technique.

(3) FIG. 3 is a graph indicating the number of membranes per separation unit as a function of events.

DETAILED DESCRIPTION OF THE INVENTION

(4) Preferably, the membranes combined in one and the same membrane separation unit are placed in parallel as illustrated in FIG. 2. FIG. 2 clearly shows that, by means of the valves, the flow entering said membrane separation unit does or does not pass through the membranes.

(5) An example of a facility according to the invention is depicted in FIG. 1. Depending on the case, the facility according to the invention may have one or more of the following features: the pressure measurement device measures the pressure of the gas flow at the inlet of the first separation unit or the pressure of the second retentate of the second separation unit; the means for adjusting the number of membranes comprises at least one valve. Preferably, said valve will be included in the membrane separation unit in which it is desired to adjust the number of membranes; the fourth retentate is recycled to the compressor for compressing the feed gas flow; the membranes used in the membrane separation units have the same selectivity; at least one membrane separation unit comprises at least two membranes with the same selectivity; at least one membrane separation unit comprises at least two membranes with different selectivities; at least one membrane separation unit uses a membrane with a selectivity different from the selectivity of the membranes of the other membrane separation units.

(6) A subject of the present invention is also a method for controlling a facility according to the invention, comprising the following steps: measuring a pressure of the feed gas flow at the inlet of the first membrane separation unit, comparing this measurement with a setpoint value, and of determining the difference with respect to this setpoint value, and adjusting the number of membranes combined and connected to the entering gas flow, in at least one of the membrane separation units as a function of the difference determined, so as to keep the methane concentration in the second retentate constant.

(7) Depending on the case, the method according to the invention can exhibit one or more of the features below: the step of adjusting the number of membranes comprises: successively adding a membrane in the first membrane separation unit, then in the third membrane separation unit and, finally, in the second membrane separation unit, or successively withdrawing a membrane in the first membrane separation unit, then in the third membrane separation unit and, finally, in the second membrane separation unit; the measuring step, the comparing step and the adjusting step are carried out automatically by data transmission and data processing device.

(8) According to a second alternative, the method for controlling a facility according to the invention comprises the following steps: a step (i) of measuring the pressure of the feed gas flow at the inlet of the first membrane separation unit, a step (ii) of adjusting the number of membranes combined and connected to the entering gas flow in at least one of the membrane separation units according to the following rule: When the pressure measured in step (i) goes above a previously fixed threshold S1, a membrane is added in the first membrane separation unit, then in the third membrane separation unit and, finally, in the second membrane separation unit; When the pressure measured in step (i) goes below a previously fixed threshold S2, a membrane is withdrawn in the first membrane separation unit, then in the third membrane separation unit and, finally, in the second membrane separation unit.

(9) Depending on the case, the process according to this second alternative can exhibit one or more of the following characteristics: the measuring step and the adjusting step are carried out automatically by data transmission and data processing means. A data transmission and data processing means may for example be an industrial processor of the programmable controller type; the feed gas flow is biogas.

(10) The present invention will be described in greater detail by means of FIG. 3. FIG. 3 is a graphic indicating the number of membranes per separation unit as a function of the events. In the graphic, the first membrane separation unit is referred to as the first stage, the second membrane separation unit is referred to as the second stage and the third membrane separation unit is referred to as the third stage.

(11) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(12) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(13) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

(14) Providing in a claim is defined to mean furnishing; supplying, making available; or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(15) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(16) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(17) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.