EP2121166A1

EP2121166A1 - Process for producing hydrogen from a hydrogen-rich gas

Info

Publication number: EP2121166A1
Application number: EP07858699A
Authority: EP
Inventors: Guillaume De Souza; François-Xavier HAULLE
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2006-12-08
Filing date: 2007-11-15
Publication date: 2009-11-25
Also published as: WO2008068436A1; CN101573170A; FR2909570A1

Abstract

The invention relates to a process for producing hydrogen from a hydrogen-rich gas, in which N adsorbers are used, which each follow, in a standard manner, a cycle where phases of adsorption, of regeneration, of elution and of repressurization follow one another and in which at least part of the stream or streams exiting the adsorber or adsorbers in the regeneration phase is recycled, by compressing said recycled part to the high cycle pressure and by supplying at least one of the adsorbers in the adsorption phase with said recycled part. The regeneration phase comprises a depressurization substep that is composed of a substep of co-current depressurization with partial equilibration. The equilibration ratio Neq is between 1.8lnPHP-3.5 and 0.6lnPHP-1.8, PHP representing the value of the high cycle pressure.

Description

PROCESS FOR PRODUCING HYDROGEN FROM HYDROGEN-RICH GAS

The present invention relates to a process for producing hydrogen from a hydrogen-rich feed gas.

It is known to use hydrogen purification units with the dual purpose of purifying a hydrogen-rich gas stream with high pressure and additional hydrogen recovery in one or more low-pressure streams. These units can implement membranes or pressure swing adsorption processes (English or PSA). One of these processes has notably been described in application WO 03/070358: it consists of an adsorption cycle in which, during the depressurization step, a partial pressure balance is achieved between at least one adsorber at the beginning of depressurization at co-current and at least one adsorber repressurization step, until the pressure of said adsorber at the beginning of co-current depressurization at a partial equilibration pressure strictly lower than the high pressure of the cycle. However, it has been observed that in the case of very high adsorption pressure, partial equilibration may be insufficient to achieve optimal performance (hydrogen yield, adsorption capacity, compression cost).

The object of the present invention is to propose a means for determining the conditions of the balancing step of the adsorption process as a function of the high and low cycle pressures so as to meet the above problem, in particular a means of evaluating the number of optimal balancing of the adsorption process as a function of said pressures.

For this purpose, the invention relates to a process for producing hydrogen from a hydrogen-rich main feed mixture, wherein N adsorbers are used, with N greater than or equal to one, each cycle where one succeeds:

a phase of adsorption substantially at a high pressure of the cycle producing a production flow,

a regeneration phase, which regeneration phase comprises a substep of depressurization up to a low pressure of the cycle comprising a substep of co-current depressurization,

an elution step at the low pressure of the cycle and

a repressurization step up to the high pressure of the cycle, in which, during the depressurization step:

at least one sub-step of partially balancing pressures between at least one adsorber in a co-current depressurization stage and at least one adsorber in a repressurization stage, until the pressure of said adsorber is depressurized with co-current at a partial equilibrium pressure strictly lower than the high pressure of the cycle, and

at least one sub-step in which the flow (s) leaving the adsorber (s) in co-current depressurization (s) is (are) sent to the adsorber or to the adsorbers in the elution stage, and in which at least a part of the or outflow of the adsorber or adsorbers regeneration phase, compressing said recycled portion to the high pressure of the cycle and supplying at least one of the adsorbers adsorption phase by said recycled portion, characterized in that the ratio of 'Neq balancing is between 1 8lnP _H p - 3.5 and p _H 0,6lnP -1, 8, P _HP representing the high pressure value of the cycle, in bar,

and Neq being defined by the formula: V - - in which N represents the number of

, Qi max substeps of partial equilibrations implemented in the process, Qi represents the quantity of gas actually exchanged between the adsorber in cocurrent depressurization and the adsorber in the repressurization stage during this substep partial equilibrium i and Qi max the maximum amount of exchangeable gas between the adsorber cocurrent depressurization and the adsorber repressurization step during this partial equilibration sub-step i, when these two adsorbers are contacted and isolated from any other gas inlet or outlet. In the text, the following terms are used: - "total balancing of pressures": any balancing i for which Qi = Qi max, and

- "partial balancing of pressures": any balancing i for which Qi <Qi max,

"hydrogen-rich main feed mixture" means a gaseous mixture comprising at least 60% by volume of hydrogen.

According to the invention, the cycle exhibits, during the depressurization step, at least a partial pressure equalization between at least one cocurrent depressurization adsorber and at least one adsorber in the repressurization stage. As a function of the value of the high (P _PH ) and low (P _PB ) pressures of the cycle, the invention makes it possible to determine the ratio of partial equilibria and therefore the quantity of gas to be exchanged between an adsorber under co-current depressurization. and adsorbers in repressurization step corresponding to each of these balances concerned. According to the invention, a partial equilibrium between at least one co-current depressurization adsorber and at least one adsorber in a repressurization stage covers three types of balancing:

the sub-step of co-current depressurization which transmits, during this substep, only a part Qi of the maximum quantity Qi max of transferable gas to the adsorber in the repressurization stage because of the stopping the balancing step before balancing the pressures between adsorbers,

the sub-step of co-current depressurization which transmits, during this substep, only a part Qi of the maximum quantity Qi max of transferable gas to the adsorber in the repressurization stage because of the introduction into the adsorber in the repressurization stage of a part of the production flow,

the sub-step of co-current depressurization which transmits, during this substep, only a part Qi of the maximum quantity Qi max of transferable gas to the adsorber in the repressurization stage because of the sending a portion of this transferable gas to at least one elution step. According to the invention, another type of partial balancing can be a combination of at least two of these three types of partial balancing.

Preferably, the method is implemented according to at least one of the following variants, whether or not combined:

during the depressurization step, at least two partial balances are carried out; during the depressurization step, the first substep is a partial balancing sub-step;

the first depressurization sub-step is a partial equilibration sub-step in which the totality of the flow coming from the adsorber in a co-current depressurization stage and a part of the adsorber is introduced into the adsorber in a repressurization stage; production flow;

it comprises a partial equilibration sub-step in which the flow coming from the adsorber in the depressurization stage is introduced partly into an adsorber in a repressurization stage and partly into an adsorber in the elution stage;

it comprises at least two elution sub-steps and the depressurization step comprises a sub-step in which the flow from the adsorber in the depressurization stage is divided to be introduced into at least two adsorbers in sub-stages of elution;

during the depressurization step, at least partial equilibration and total equilibration are carried out; it comprises at least one step of recycling at least a portion of the flows leaving the adsorber or adsorbers in the regeneration phase by compressing said recycled portion to the high cycle pressure and feeding at least one of the adsorbers adsorption phase by said recycled portion;

if the low pressure PPB is less than or equal to 2 bars, then, preferably, the Neq balancing ratio is closer to the value of 1. 8lnP _H p-3.5 than the value of 0.6lnP _HP -1, 8, P _HP ;

- if the low pressure PPB is greater than or equal to 6 bars, then preferably, the balancing ratio Neq is closer to the value of 0,6lnP -1 _H p, 8, P _HP as the value of 1, 8IΠP _HP - 3.5.

Other characteristics and advantages of the invention will appear on reading the description which follows. Embodiments and embodiments of the invention are given by way of non-limiting examples, illustrated by the accompanying drawings in which:

FIG. 1 is a schematic view of a PSA unit,

FIG. 2 is a diagram illustrating the method of the invention implemented using the unit of FIG. 1.

In what follows, the terms "input" and "output" refer to the inlet and outlet ends of an adsorber in the adsorption phase; the term "co-current" designates the flow direction of the gas in the adsorber during this adsorption phase, and the expression "countercurrent" designates the opposite direction of circulation.

FIG. 1 shows a unit 1 for producing hydrogen from a gas rich in hydrogen, for example implanted in a petroleum refinery. Unit 1 is adapted to produce, from the feed gas conveyed by a line 2, a high purity hydrogen stream (with a hydrogen content generally greater than 99% by volume) via a production line 3, while evacuating a stream of waste gas through a discharge line 4 intended to be connected to an evacuation network, currently located in oil refineries. Unit 1 comprises an adsorption purification apparatus 5 provided with a recycling line 6. This recycling line is equipped, from upstream to downstream, with a mixing capacity 7 and a compression apparatus 8, for example a compressor. The purification apparatus 5 comprises seven adsorbers R1 to R7, each comprising an adsorbent material suitable for adsorbing the impurities contained in the feed mixture. Different types of adsorbent materials are possible, such as activated carbon, silica gels and / or molecular sieves. The purification apparatus 5 is of the PSA type. It comprises for this purpose pipes, valves and control means, not shown, adapted to follow each adsorber R1 to R7 a cycle of period T, which consists of seven phase times of substantially the same duration, and a first example is shown in FIG. 2. Considering that the cycle represented applies from the instant t = 0 to t = T at the adsorber R7, the operation of the adsorber R6 is deduced therefrom by the time shift of JIl. , that of the adsorber R5 by time shift of 2T / 7 and so on until that of the adsorber R1 obtained by shifting in time of 6T / 7. By phase / adsorber duality, this amounts to considering that, in FIG. 2, the adsorber R7 follows the first phase time represented between the instants t = 0 and t = JI1, the adsorber R6 follows the second stage of phase represented between the instants t = JIl and t = 2T / 7, and so on until the adsorber R1 which follows the seventh phase time represented between the instants t = 6T / 7 and t = T. In the figure 2, where the times t are plotted on the abscissa and the absolute pressures

P on the ordinate, the lines marked by arrows indicate the movements and destinations of the gas streams, and, in addition, the direction of flow in the adsorbers R1 to R7: when an arrow is in the direction of increasing ordinates (upwards of the diagram), the current is said to cocurrent in the adsorber. If the upward arrow is below the line indicating the pressure in the adsorber, the current enters the adsorber through the inlet end of this adsorber; if the upwardly directed arrow is above the pressure line, the stream exits the adsorber through the outlet end of the adsorber, the inlet and outlet ends being respectively those of the adsorber gas to be treated and gas withdrawn in production; when an arrow is in the direction of decreasing ordinates (towards the bottom of the diagram), the current is said against the current in the adsorber. If the downward arrow is below the line indicating the pressure of the adsorber, the stream exits the adsorber through the inlet end of this adsorber; if the downward arrow is located above the line indicating the pressure, the current enters the adsorber through the outlet end of this adsorber, the inlet and outlet ends being always those of the gas to be treated and gas withdrawn in production. The inlet end of the adsorbers is their lower end.

Thus, for example for the adsorber R7, the cycle comprises an adsorption phase of t = 0 at t = 2T / 7 and a regeneration phase of t = 2T / 7 at t = T. More precisely, the phase of adsorption comprises:

- From t = 0 to t = JIl, a first step of treatment of the feed gas during which the impure hydrogen to be treated arrives at the inlet of the adsorber via line 2 at a high adsorption pressure, noted PH on the cycle. A flow of substantially pure hydrogen is then withdrawn at the top, under the same pressure, and partially feeds the production line 3, the remainder being sent to another adsorber during repressurization step described later; - From t = JIl t = 2T / 7, a second step of treatment of a gas from the recycling line 6, formed by the discharge of the compressor 8, which carries the gas at the adsorption pressure PH. In the same way as in the preceding step, a part of the stream of substantially pure hydrogen withdrawn at the top constitutes the production flow at 3, the remainder being sent to the adsorber during the repressing step mentioned above. -above.

The regeneration phase comprises, from t = 2T / 7 to t = AJI1, a depressurization step comprising:

from t = 2T / 7 to t = t1, t1 being greater than 2T / 7 and less than 3T / 7, a sub-step of co-current depressurization, during which the outlet of the adsorber R3 is connected that of the adsorber R7 at the end of the repressurization step described below; the flow leaving the adsorber R3, called the first depressurization flow, is completed by a part of the production flow 3 before the adsorber R7 is repressurized according to a partial equilibrium,

- From t1 to t2, t2 being greater than t1 and less than 3T / 7, a substep of co-current depressurization, during which the outflow co-current of the adsorber R3, said second flow of depressurization, is decompressed and sent in part at the outlet of the adsorber R5 in elution stage and partly at the outlet of the adsorber R6 also in the elution stage;

from t2 to t = 3T / 7, a sub-step of co-current depressurization, during which the flow leaving the adsorber R3, called the third depressurization flow, is sent to the adsorber R6 at the beginning of repressurization step and the adsorber R5 in elution step;

from 3T / 7 to t3, t3 being greater than 3T / 7 and lower than AJI1, a sub-step of co-current depressurization, during which the outflow co-current of the adsorber R4, said fourth depressurization flow, is decompressed and sent partly to the outlet of the adsorber R5 in the elution stage and partly to the outlet of the adsorber R6 also in the elution stage;

- From t3 to AJIl, a substep of countercurrent depressurization during which the outflow of the adsorber R4 is sent to the discharge line 4. This substep continues to the low pressure of the cycle , noted PB and may have a value between 1, 6 bar and 10 bar, preferably 6 bar.

The regeneration phase then comprises, from t = 4T / 7 to t = t7, t7 being greater than 4T / 7 and less than 6T / 7 an elution step during which the adsorbent material is swept by an elution gas so to desorb substantially all previously adsorbed impurities. This elution step includes:

from t = 4T / 6 to t = t4, a sub-step of elution towards evacuation, during which the adsorber is purged against the current by means of a part of the fourth flow of depressurizing the adsorber R4, evacuating a waste gas under the low pressure PB in line 4, and

- From t4 to t5, a sub-step of elution to recycling, during which the adsorber is also purged against the current by means of a part of the second depressurization flow of the adsorber R3, forming a gas of recycling under low pressure PB, sent to the input of line 6, and

- From t5 to 5T / 7, a sub-step of elution to recycling, during which the adsorber is also purged against the current by means of a part of the third depressurization flow of the adsorber R3, forming a recycle gas under the low pressure PB, sent to the inlet of line 6,

from 5T / 7 to t6, a substep of elution towards recycling, during which the adsorber is also purged against the current by means of a part of the fourth depressurization flow of the adsorber R4, forming a recycle gas under the low pressure PB, sent to the inlet of the line 6, - t6 to t7, a sub-step of elution to recycling, during which the adsorber is also purged against the current by means of a part of the second depressurizing flow of the adsorber R3, forming a recycle gas under the low pressure PB, sent to the inlet of the line 6.

Thus, starting from t4, the line 6 receives a gas richer in hydrogen than the gas sent to the waste line 4, which amounts to recycling only the flows, issued against the current of the adsorbers in the regeneration phase, the richest in hydrogen, the impurities being mainly desorbed at the end of depressurization against the current and early elution. Depending on the gas volume desired at the inlet of the recycling line 6, the durations of the intervals [t3; t4] and [t4; 6T / 7] can be modified. Once the gas of the line 6 homogenized by the mixing capacity 7 and compressed from the low pressure PB to the high pressure PH of the cycle by the compressor 8, it forms the feed gas of the adsorber in the second stage treatment (from T / 7 to 2T / 7 as described above).

By repeating the description of the cycle of FIG. 2, the regeneration phase finally comprises, from t7 to T, a counter-current repressurization stage, comprising:

- From t7 to 6T / 7, a counter-current repressurization sub-step, during which the adsorber is R6 is recompressed by means of a part of the third depressurization flow of the adsorber R3,

from 6T / 7 to t8, t8 being greater than 6T / 7 and lower than T, a counter-current repressurization sub-step, during which the adsorber is R7 is recompressed by means of the first depressurization flow of the adsorber R3 and a part of the workflow 3, - from t8 to t9, t9 being greater than t8 and lower than T, a counter-current repressurization sub-step, during which the adsorber is R7 is recompressed by means of a part of the workflow 3,

from t9 to T, a counter-current repressurization sub-step, during which the adsorber is R7, is recompressed by means of a part of the production flow 3.

The secondary feed gas conveyed by the line 6 is poorer in hydrogen than the main feed gas conveyed by the line 2, and these two feed gases form an asymmetry, in terms of hydrogen content, for the successive feeding of each adsorber in the adsorption phase. This dissymmetry makes it possible to achieve higher productivity than that of a PSA unit with a single feed stream. In addition, this gain is even higher than the recycled stream, resulting from adsorbers of the apparatus 5, is important because of an increase in this dissymmetry by lowering the hydrogen content of the second feed gas . Thus, instead of having in the case of the prior art a deterioration of the productivity when increasing the amount of recycle gas to increase the hydrogen yield of a PSA device, we observe a maintenance of this productivity.

By the implementation of the invention, it is possible to implement a method

PSA of the type described in the application WO03 / 070358 at a very high adsorption pressure, typically greater than 25 bar, with optimal performance in terms of hydrogen yield, adsorption capacity and compression cost.

EXAMPLE

The process of FIG. 2 is carried out with a high pressure of the P _HP cycle of 35 bar and a low cycle pressure P _B p of 3.5 bar absolute. According to the invention, the balancing ratio Neq must therefore be between 2.9 and 0.33. Since this process has two partial equilibrium stages ei and e ₂ , the following relation on the balancing ratio must therefore be respected:

0.33 <- ^ - + - ^ - <2.9 Q _el max Q _e2 max Since the low pressure P _HB = 3.5 bar absolute is of intermediate value, it is preferable to take an intermediate value of the order of 1, 6 for the balancing ratio

Neq equal to ^ ^el + - ® ^el

It is generally preferred to have a Q _e i as large as possible and to adjust Q _{Θ 2} to achieve the _desired sum. For the partial balancing sub-step e1, Q _e i is close of Q _e i max since during [2T / 7 - t1] the entire flow of the adsorber R3 goes into the adsorber R7 and part of the production flow 3 completes the volume of the adsorber

R7. We therefore set the value of 0.9.

Therefore, we come to the relation: 0.9 H - - = 1, 6, which comes back

Q _e2 max

then to = 0.7, hence the knowledge of Q _Θ2 , amount of gas to be transferred that one

adjusted by controlling, at the end of the second partial equilibration step e2, the pressures of the upstream adsorber in depressurization and the downstream adsorber in repressurization. In the case of a partial equilibration with elution supply, the quantity of gas sent to the elution is then deduced. This calculation makes it possible to obtain a economically interesting hydrogen PSA because of an optimal combination of hydrogen yield and investment in PSA and compressor.

Claims

A process for producing hydrogen from a hydrogen-rich main feed mixture, wherein N adsorbers are used, with N greater than or equal to one, each of which is shifted one cycle in succession:

a phase of adsorption substantially at a high pressure of the cycle producing a production flow 3,

an elution step at the low pressure P _BP of the cycle and

a repressurization step up to the high pressure P _H p of the cycle, in which, during the depressurization step:

at least one sub-step in which the flow (s) leaving the adsorber (s) in co-current depressurization (s) is (are) sent to the adsorber or to the adsorbers in the elution stage, and in which at least a part of the or outflow of the adsorber or adsorbers regeneration phase, compressing said recycled portion to the high pressure of the cycle and supplying at least one of the adsorbers adsorption phase by said recycled portion, characterized in that the ratio of 'Neq balancing is between 1 8lnP _H p - 3.5 and p _H 0,6lnP -1, 8, P _H p is the high pressure value of the cycle, in bar,

and Neq being defined by the formula: V - - - in which N represents the number of

, QimΑx substeps of partial equilibrations implemented in the process, Qi represents the quantity of gas effectively exchanged between the adsorber in cocurrent depressurization and the adsorber in the repressurization stage during this sub-step d partial equilibration i and Qi max the maximum quantity of exchangeable gas between the adsorber cocurrent depressurization and the adsorber repressurization step during this partial equilibration sub-step i when these two adsorbers are set contact and isolated from any other gas inlet or outlet.

2. Method according to claim 1, characterized in that it performs, during the depressurization step, at least two partial balances.

3. Method according to claim 1 or 2, characterized in that, during the depressurization step, the first substep is a partial balancing sub-step.

4. Method according to claim 3, characterized in that the first depressurization sub-step is a partial equilibration sub-step in which is introduced into the adsorber repressurization step all of the flow from the adsorber in co-current depressurization step and part of the production flow 3.

5. Method according to claim 2 or 3, characterized in that it comprises a sub-partial balancing step in which the flow from the adsorber depressurization stage is introduced in part into an adsorber repressurization step and partly in an adsorber in the elution stage.

6. Method according to one of the preceding claims, characterized in that it comprises at least two elution sub-stages and in that the depressurization step comprises a substep in which the flow from the adsorber in depressurization step is divided to be introduced into at least two adsorbers in elution sub-stages.

7. Process according to claim 1, characterized in that during the depressurization step at least partial equilibration and total equilibration are carried out.

8. Method according to one of the preceding claims, characterized in that it comprises at least one step of recycling at least a portion of the stream leaving the adsorber or adsorbers regeneration phase by compressing said recycled portion to the high pressure of the cycle and feeding at least one of the adsorbers adsorption phase by said recycled portion.

9. Method according to one of the preceding claims, characterized in that the low pressure P _BP is less than or equal to 2 bar and the balancing ratio Neq is closer to the value of 1, 8lnP _H p - 3,5 that the value of 0.6 lnP _H p -1, 8, P _H p.

10. Method according to one of claims 1 to 8, characterized in that the low pressure P _BP is greater than or equal to 6 bar and the balancing ratio Neq is closer to the value of 0.6lnP _H p -1 , 8, P _H p than the value of 1. 8lnP _H p - 3.5.