WO2019166726A1

WO2019166726A1 - Method for adjusting the swing air absorption process depending on a reduced oxygen production with respect to the nominal production of the system

Info

Publication number: WO2019166726A1
Application number: PCT/FR2019/050425
Authority: WO
Inventors: Guillaume Rodrigues; Maxime PEREZ; Pierre Petit; Stéphane PUSIOL
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2018-03-01
Filing date: 2019-02-25
Publication date: 2019-09-06
Also published as: FR3078492A1; FR3078492B1

Abstract

A method for adjusting the production of oxygen produced by a (V)PSA adsorption process by means of a unit comprising X adsorbers where X is greater than 2 each following, in an offset manner, a nominal pressure cycle of duration T_nominal, comprising at least the steps of producing, decompressing, purging and re-compressing and supplying a nominal production of oxygen P, comprising the following consecutive steps: a) determining the desired production P' with respect to the nominal production P, where P' expresses, in percent, the nominal production P; b) simultaneously determining the required dead time T_mort during which at least one adsorber must be isolated in order to obtain the desired reduction in production and the integer N indicating the application of this dead time, a phase time over N, such that the following formula is respected: T_mort=T_phase*N*[1/P'-1] where: - T_phase=T_nominal/X-N is an integer greater than or equal to 6 and is chosen so as to minimise the duration T_mort while respecting T_mort≥T_{mort_minimum}-T_{mort_minimum} > 1 s.

Description

METHOD FOR ADJUSTING THE SWING AIR ASORPTION METHOD BASED ON A

REDUCED OXYGEN PRODUCTION IN RELATION TO THE NOMINAL PRODUCTION OF THE SYSTEM

The present invention relates to a process for adjusting the production of oxygen produced by a PSA (V) type adsorption process.

The units VSA (Vacuum Swing Adsorption) 02 are units of separation of the gases of the air by adsorption process with modulation of pressure in which the adsorption is carried out substantially with the atmospheric pressure, called high pressure, it is between 1 bara and 1.5 bar, and the desorption takes place at a pressure below atmospheric pressure, typically between 0.3 and 0.5 bar. The production of gaseous oxygen reaches a purity of the order of 90% to 93% and the production range of this type of apparatus varies from 30t / d to 200t / d. These methods have applications in fields such as water purification, glass manufacturing, pulp processing, and the like.

A compressor and a vacuum pump are often used to reach cycle pressures.

Note that even if the present invention will apply primarily to VSA the present invention can also be applied to all PSA (pressure swing adsorption = pressure swing adsorption gas separation methods):

the VPSA processes in which the adsorption is carried out at a high pressure substantially greater than atmospheric pressure, that is to say generally between 1.6 and 8 bara, preferably between 2 and 6 bara, and the low pressure is below at atmospheric pressure, typically between 30 and 800 mbar, preferably between 100 and 600 mbar.

the PSA processes in which the adsorption is carried out at a high pressure clearly above atmospheric pressure, typically between 1.6 and 50 bara, preferably between 2 and 35 bara, and the low pressure is greater than or substantially equal to the atmospheric pressure, therefore between 1 and 9 bara, preferably between 1.2 and 2.5 bara.

Subsequently, we will use the term (V) PSA which will include VSA, PSA, and VPSA.

The (V) PSA cycles comprise at least the following steps: production, decompression, purge, recompression.

The units generally operate with a total cycle time greater than 30 seconds and employ one to three adsorbers.

When the need in 02 of the customer drops, it is necessary to adjust the production. The first level of adjustment is to reduce the opening of the production valve which has the effect of increasing the purity of oxygen. The output of the unit is then degraded. Moreover, beyond a certain level of purity, it can abruptly deteriorate due to the phenomenon of argon enrichment. For these 2 reasons, this solution is limited to moderate reductions in the production rate.

Alternatively, if it is possible to reduce the amount of feed gas and purge, the production can be adjusted by varying the cycle time. On a VSA 02 this type of operation is for example made possible by the installation of variable speed drives on the supply and pumping machines.

However, in most cases, for cost reasons, VSA 02s are not equipped with machine capacity adjustment systems. As a result, the flow rate per cycle is set and it is necessary to reduce the number of cycles per unit of time in order to reduce production. This is accomplished by isolating the adsorbers at a well defined cycle time (usually as the vacuum pump passes from one adsorber to the other), and rotating the compressor and vacuum pump with a minimal pressure differential ( by example injecting the gas discharged by the machines to their suction) in order to minimize their energy consumption. This pause step in the cycle is called dead time and is associated with the activation of the "Reduced Run RR" mode.

FIG. 1 represents a cycle with 2 adsorbers, 1 production buffer capacity, a blower, a vacuum pump and having 14 sub-steps.

Each adsorber has two sides, the first being the supply or pumping side and the second being the oxygen production side.

Simultaneously with the steps 1 to 7 undergone by the adsorber 1, the adsorber 2 undergoes the steps 8 to 14 Step 1 (Production 1): The blower feeds the first adsorber with air, the nitrogen is selectively adsorbed and a flow rich in Oxygen is sent to the production capacity Step 2 (Production 2): The blower feeds the first adsorber into air, the nitrogen is selectively adsorbed and a high oxygen flow is sent to the production capacity. Part of the flow of oxygen produced is taken directly at the outlet of the first adsorber and is used in a subsequent step (step 9)

Step 3 (Balancing 1): the blower no longer supplies the first adsorber. It is therefore closed on the power supply side and the output is connected to the second adsorber so that a part of the gas contained in the adsorber under non-adsorbed phase, rich in oxygen, is reused to reinflate and elute the second adsorber (step 10)

Step 4 (Reduced Run): the first and second adsorbers are closed and the machines operate with a minimum pressure differential. This step has a zero duration when the unit operates at its nominal flow rate (Figure 1). The duration corresponding to this step is called Tmort timeout ·

Step 5 (Equilibration 2 + purge): the first adsorber is connected to the vacuum pump on the supply side, which makes it possible to desorb a part of the nitrogen contained in the adsorbent, and its outlet is connected to the second adsorber of such so that part of the gas contained in the non-adsorbed adsorber phase is reused to reinflate the bottle 2 (step 12) Step 6 (Purge 1): the first adsorber is closed side of the oxygen production side and the side supply is connected to the vacuum pump which then extracts the nitrogen contained in the adsorbent. Step 7 (Purge 2): the first adsorber is closed on the oxygen side and the supply side is connected to the vacuum pump which then extracts the nitrogen contained in the adsorbent. Step 8 (Purge 3): the second adsorber is closed next to the oxygen production side and the supply side is connected to the vacuum pump which then extracts the nitrogen contained in the adsorbent. Step 9 (elution): the second adsorber is supplied to the oxygen side by the gas taken at the outlet of the first adsorber during step 2, which makes it possible to push back the adsorbed nitrogen front. On the supply side, the vacuum pump extracts the nitrogen contained in the adsorbent.

Step 10 (High 2 Balancing + Purge): the second adsorber is supplied to the oxygen side by the gas withdrawn at the outlet of the first adsorber during step 3, which makes it possible both to recover oxygen which would otherwise be lost and to push back the nitrogen front, while the supply side is connected to the empty vacuum pump extracts the nitrogen contained in the adsorbent. Step 11 (Reduced Run): the 2 adsorbers are closed and the machines operate with a minimal pressure differential. This step has zero duration when the unit is operating at its nominal flow rate. The duration corresponding to this step is called dead time Tmort.

Step 12 (Balancing 1 high): the second adsorber is closed on the supply side. It is inflated by the oxygen side thanks to the gas contained in the first adsorber, and from step 5, which allows both to recover oxygen that would otherwise be lost and push back the nitrogen front.

Step 13 (02 + air recompression): the blower is used to inflate the second adsorber by the supply side and at the same time oxygen is taken from the reservoir production buffer to inflate the adsorber from the top, which helps to push back the nitrogen front.

Step 14 (Final recompression): The blower is used to inflate the second adsorber by the supply side. The adsorber is closed on the oxygen side.

The "reduced running" mode does not start immediately when the demand in 0 ₂ is reduced but beyond a certain percentage of decrease allowing at the same time to be free from any variability of measure of purity 0 ₂ but especially to have a duration of the dead time (Tmon) compatible with the times of opening / closing of the valves.

Starting from these constraints, a problem that arises is to provide a method of adjusting the oxygen production produced by (V) PSA.

A solution of the present invention is a process for adjusting the oxygen production produced by a PSA type adsorption process by means of a unit comprising X adsorbers with X greater than 2, each of which is shifted one by one. nominal cycle time T _nominai pressure, comprising at least the steps of production, decompression, recompression and purge and providing a nominal output P oxygen, comprising the following successive steps:

a) determination of the desired production P 'with respect to the nominal production P, with P' expressed as a percentage of the nominal production P

b) simultaneous determination of the required dead time _Tm0 rt during which at least one adsorber must be isolated to achieve the desired production decrease and the integer N indicating the application of this dead time a phase of time on N, so that the following formula is respected:

T _m0 rt = Tphase * N * [l / P'-l]

With:

^" Tphase ⁼ Tnominai / X

- N is an integer greater than or equal to 6 and is selected so as to minimize the duration T _m ort while respecting Tmon> T _m0 rt_ .Minimum

Tmort_minimum ls.

Dead time means a time when the two adsorbers are closed. In order to minimize the consumption of the other machines of the installation during this period, they are generally in recycling (the suction and the delivery of the machines are placed in communication) or rejects the air sucked into the atmosphere. Preferably P 'is between 10% and 99%.

By way of example, N = 1 corresponds to a dead time T _m0rt applied to each phase time on each of the adsorbers, N = 2 corresponds to a dead time applied to one phase time out of 2, N = 3 corresponds to a dead time applied a phase time of 3 etc ... Note that a dead time can be applied two consecutive phase times; in other words, for N = 2 one could apply a dead time 2 consecutive phase times over 4 phase times.

Depending on the case, the method according to the invention may have one or more of the following characteristics:

- minimum Tmort> 2 s; Indeed, below 2 seconds it is not possible to perform the opening and the complete closing of the valves.

- N <4.

- Tnominai is between 30s and 90s.

the decompression step comprises a first decompression sub-step and the dead time succeeds this first decompression sub-step.

the first decompression sub-step corresponds to a partial pressure equalization with the other adsorber in recompression.

the two adsorbers follow the pressure cycle with a shift of half a cycle time.

- During the cycles not having a sub-step of time-out the speed of decompression is between 300-100 mbar / s.

during cycles having a substep of dead time, the decompression speed is between 150 and 5 mbar / s.

the adsorption process uses an adsorption unit comprising a buffer capacity.

When it has been chosen to apply the dead time to each bottle and each cycle, the constraint related to the opening times of the valves makes it necessary to trigger the reduced operating mode only at significant levels of flow reduction. By way of example, the column N = 1 of table 1 shows that for a cycle time of 36s and a valve opening time of 2 seconds, it will be chosen to trigger the "reduced running" mode only in below 90% of the nominal flow. Between 90% and 100% of the nominal flow, the unit will produce a stream enriched with oxygen to the detriment of the efficiency of the unit and an increase in the energy consumed per unit volume of oxygen produced. By the way, if the increase in purity is too much important, an argon enrichment phenomenon can occur, leading to a stall of purity (purity collapses). To avoid this phenomenon, beyond a certain level of purity (typically 95% oxygen) part of the production can be vented. At 90.1% of the nominal flow, the specific energy of the unit will then be increased by nearly 10%.

It is possible to apply a dead time only 2 consecutive cycles out of 4 (during 2 consecutive phase times the dead time is applied and the 2 phase times after the dead time is zero), or 1 phase time out of 2 ( the cycles of the two bottles are then not the same), which in both cases has the effect of doubling the duration of the dead time compared to the previous case and allows to trigger the "reduced running" mode to lower levels 0 ₂ production more reduced. The major drawback of such a solution is to induce a substantial magnification of the product buffer tank 02 to allow of 0 ₂ low product flow rates. For example, if the need for 0 ₂ produced represents 40% of the nominal flow rate of the unit, for a nominal phase time of 18 seconds, a duration of the production steps of 6 seconds, apply the dead time 2 out of 4 consecutive phase time amounts to increasing the volume of the oxygen buffer capacity produced by the order of 40% compared to a design made considering a dead time applied to each phase time, and this in order to preserve the same pressure variation in the buffer.

It thus appears difficult to obtain at the same time an efficient unit when the demand drop in 02 produced is low compared to nominal and at the same time preserve a reduced cost and a small footprint of the 0 ₂ buffer capacity produced.

However, the present invention provides a solution.

Indeed, the shaded area of Table 1 illustrates how the unit operates according to the desired production P 'for:

• N variable between 1 and 4

• A minimum Tmort ⁼ 2 S

• A nominal phase time of 18 seconds

• Production time of 6 seconds per phase time

For exemple :

-if 100%> P '> 97.3%, the reduced operation mode will not be activated because there is no frequency N such that T _m ort is greater than or equal to 2 seconds. The oxygen produced will then have a purity higher than its nominal purity and the specific energy will increase inversely proportional to P '

if P '= 96.5%, the frequency N = 4 will be selected and the duration of the Tmort will be 2.6 seconds

if P '= 95.1%, the frequency N = 3 will be selected and the duration of the Tmort will be 2.8 seconds

if P '= 91.8%, the frequency N = 2 will be selected and the duration of the Tmort will be 3.2 seconds

if P <90%, the frequency N = 1 will be selected

Thus, the unit will be able to operate in reduced operating mode from the low levels of reduced flow produced, thus preserving good specific energy while minimizing valve cycles and having an optimized buffer capacity of 0 ₂ .

Table 1 As previously explained, the minimum dead time is two seconds since under two seconds the opening / closing of the valves is impossible. But it turns out that when the "reduced running" mode is activated at each cycle, the constraint of a minimum duration of step of 2 seconds imposes a trigger when the fall of production approaches 10%. In the range 0% -10%, the flow rate of 0 ₂ is reduced simply by closing the production oxygen valve, inducing an increase in the purity 0 ₂ and a significant degradation of the specific energy of the VSA. Also, the adjustment method according to the invention makes it possible to adjust, without degradation of the VSA energy, oxygen production when the desired production drop is between 0% and 10%.

Moreover, if we chose to systematically introduce a dead time in one out of four cycles as soon as the production drop is greater than 2.5%, the buffer storage capacity to store GO2 downstream of the (V) PSA in order to to ensure continuous production will have to be considerably increased. Indeed, for a production drop of 60%, the VSA will only send oxygen in the capacity 10 seconds approximately every 140 seconds, compared to 10 seconds every 50 seconds when the dead time is introduced in all cycles. Also, the adjustment method according to the invention makes it possible to adjust the oxygen production without appreciably increasing the size of the production buffer capacity 0 ₂ .

Claims

claims

1. Process for adjusting the oxygen production produced by a PSA type adsorption process by means of a unit comprising X adsorbers with X greater than 2 each, each shifting a nominal cycle of pressure of duration T _nominai , comprising at least the steps of production, decompression, purge and recompression and providing a nominal production of oxygen P, comprising the following successive steps:

T _m0 rt = Tphase * N * [l / P'-l]

With:

^" Tphase ⁼ Tnominai / X

- N is an integer greater than or equal to 6 and is selected so as to minimize the duration T _m ort while respecting Tdead> T _m0 rt_ .Minimum

Tmort_minimum ls.

2. Adjustment method according to claim 1, characterized in that Tmort minimum> 2 s.

3. Adjustment method according to one of claims 1 or 2, characterized in that N <4.

4. Adjustment method according to one of claims 1 to 3, characterized in that T _n0 minai is between 30s and 90s.

5. Adjustment method according to one of claims 1 to 4, characterized in that the decompression step comprises a first decompression sub-step and the dead time succeeds this first decompression sub-step.

6. Adjustment method according to claim 5, characterized in that the first decompression sub-step corresponds to a partial pressure balance with the other adsorber recompression.

7. Adjustment method according to one of claims 1 to 6, characterized in that the two adsorbers follow the pressure cycle with a shift of half a cycle time.

8. Method according to one of claims 1 to 7, characterized in that during the cycles having no substep of dead time the decompression rate is between 300-100 mbar / s.

9. Method according to one of claims 1 to 8, characterized in that during the cycles having a substep of dead time decompression speed is between 150 and 5 mbar / s.

10. Method according to one of claims 1 to 9, characterized in that the adsorption process uses an adsorption unit comprising a buffer capacity.