| INTRODUCTION TO PSA |
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Dear Visitor.
If you are in any way involved in the purification of hydrogen on an industrial
scale, we hope that we may raise your interest in learning how we accomplished
improvements of the process commonly referred to as
"PRESSURE SWING ADSORPTION".
During some 15 years of development with feedback from a benchscale unit,
two demonstration plants on industrial sites and further seven commercial units,
a sophisticated simulation model could be developed which gave us the profound
understanding, necessary for providing the breeding ground for new ideas
for improving
the performance of the process.
New additional parameters could be defined, enabling the realization of increased
recovery efficiencies and improved flexibility of plant operation. All improvements
have been subject of recent patent applications.
We hope that we will be able to raise your interest to learn more on what we have to
offer on this subject.
To proceed, please continue reading or jump to any item of your choice on the attached
listing of links.
PRESSURE SWING ADSORPTION
(PSA)is an adiabatic process
and is applied for purification of gases by removing the accompanying impurities by
adsorption through suitable adsorbents in fixed beds contained in pressure vessels under
high pressure.
Regeneration of adsorbents is accomplished by countercurrent
depressurization and by purging at low pressure with previously recovered near product
quality gas. To obtain a continuous flow of product, a minimum of two adsorbers is
needed, such that at least one adsorber is receiving feed gas and actually produces a
product of desired purity.Simultaneously, the subsequent steps of depressurization,
purging and repressurization back to the adsorption pressure are executed by the other
adsorber(s). After such adsorbent regeneration and repressurization the adsorber is
switched onto adsorption duty, whereupon an other adsorber is regenerated. Dependent on
the type of impurity to be adsorbed and removed, adsorbents to be used comprise zeolitic
molecular sieves, activated carbon, silica gel and activated alumina. Mostly,
combinations of adsorbent beds are used on top of one another, so dividing the adsorber
contents in a number of distinct zones. Monitoring and proper control of process
parameters ensures a stable operation. Stable operation means a pendulating swing in
each particular location, in adsorber bed or piping, of values for all
parameters, i.e. pressure, temperature, flow and composition of gaseous and adsorbed
phase.
TYPES OF GAS TO BE TREATED
Purification or separation of gases normally takes place at near ambient feed gas
temperatures, whereby the components to be removed are selectively adsorbed. Adsorption
should not be too strong and should be sufficiently reversible to enable regeneration
of adsorbents at similar ambient temperature. Use of different adsorbent zones enables the
reversible adsorption at ambiant conditions of certain components in a certain zone, thereby avoiding contact of these components with an unduly strong adsorbent for such components in an other downstream zone, such contact would spoil the capacity of this adsorbent. For instance a zone of activated carbon preceeding (i.e. in a cocurrent direction) a zone of molecular sieve 5A
(MS5A), removes carbon dioxide and thus avoids MS5A to become spoiled by carbondioxide.
The capacity of MS5A to remove nitrogen or carbon monoxide would otherwise be greatly
impaired.
PSA may be used for treatment of most common gases. The following descriptions will only concern the purification of HYDROGEN.
SCHEMATIC ARRANGEMENT PSA PROCESS
The general symplified flow diagram shows the schematic arrangement of the commonly known PSA process, which may comprise up to 12 adsorbers. Each adsorber is going through the following sequential stages,
While feedgas flows through one, or simultaneously and in parallel through more than
one adsorber, other adsorbers are in the various other stages of the cycling process.
Each step of de- and repressurization by transfer of gas between adsorbers takes place
until pressure equalization has been achieved. The stage of repressurization is
completed by a slip stream of high pressure primary product gas.
In the next pressure/time diagram of a basic 4-bed PSA unit, all the functions to which
each adsorber is subjected are shown in a fixed sequence in a diagram, representing one
cycle element. Each line from left to right represents the pressure of a different
adsorber with the function as indicated. After the end of a cycle element, a new
identical cycle element is started whereby the function of each adsorber is changed in
accordance with the indications in the diagram when switching from the right hand side
to the left hand side. Covering one complete cycle takes a number of cycle elements
equal to the number of adsorbers.
PSA may be used for treatment of most common gases. The following descriptions will only concern the purification of HYDROGEN.
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The general symplified flow diagram shows the schematic arrangement of the commonly known PSA process, which may comprise up to 12 adsorbers. Each adsorber is going through the following sequential stages,
In cocurrent direction
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In countercurrent direction
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LOSSES OF HYDROGEN
Losses of hydrogen are incurred during adsorbent regeneration, i.e. during
countercurrent depressurization called dumping, producing
DUMP GAS and during purging with purge gas, producing
PURGE OFFGAS.
The total of dump gas and purge offgas constitutes the
PSA OFFGAS;
the hydrogen content therein should be as low as possible for a maximum hydrogen recovery
efficiency.
Hydrogen recovery efficiency is defined as the rate of contained
hydrogen in the product as percentage of the rate of contained hydrogen in the feedgas.
The percentage hydrogen recovery efficiency (EFF) may be calculated from the average
hydrogen contents of feedgas (%H2F), productgas (%H2P) and offgas (%H2OG) as follows:
In the animated picture below, two typical
profiles of hydrogen concentration in PSA offgas are shown, starting at the left hand
side with dumping. Since the initial quantity of dump gas originates from the void
spaces of the adsorber bottom and associated piping, the composition of this part is
essentially the same as that of the feed gas. As dumping and offgas production
continues, a lower hydrogen content in the offgas appears until a minimum, after which
it increases again. When using low pressure purging, the hydrogen content in the PSA
offgas clearly drops more rapidly and to a lower level than in the case of high pressure
purging.
Therefore, low pressure purging generally gives a higher recovery efficiency unless
the purge gas quantity and consequently the purge offgas quantities are increased and
the hydrogen rich right hand side of the diagram is extended.
See also options /NPEn and /SM/SMEQ).
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During cocurrent (i.e. in the direction used during the adsorption step)
depressurization at closed adsorber inlet, near product quality gas is recovered at
gradually decreasing pressure, hereafter referred to as
SECONDARY PRODUCT GAS. It originates from the combined
effects of desorption and readsorption and from the void spaces of adsorbents, adsorber
vessel and associated piping. This gas is utilized for repressurization and for purging.
Quantity of secondary product gas
Under given process conditions and down to a fixed start-of-dump
pressure, the quantity of secondary product gas recovered in each consecutive cycle
is fairly constant and is only slightly affected by the degree of adsorbent loading
at end of adsorption.
The Utilization and Distribution of secondary product gas
A. Fraction for use as purge gas
Where the objective is to maximize the hydrogen recovery efficiency, adsorbent regeneration and therefore purging should take place at the lowest practical pressure. Practical in this sense means that the PSA offgas can be let down into an existing fuel gas system, without having to employ intermediate compression facilities. The more purge gas is used, the higher the degree of adsorbent regeneration, however also the higher the hydrogen content in the purge offgas and therefore the lower the hydrogen recovery efficiency. To avoid this, a lower fraction of the available secondary product gas should be taken for purging and the system should therefore be designed to enable the higher complementary part of said fraction to be used for the only other useful purpose, which is repressurization.
B. Fraction for repressurization
Where the objective is to maximize the hydrogen recovery efficiency, adsorbent regeneration and therefore purging should take place at the lowest practical pressure. Practical in this sense means that the PSA offgas can be let down into an existing fuel gas system, without having to employ intermediate compression facilities. The more purge gas is used, the higher the degree of adsorbent regeneration, however also the higher the hydrogen content in the purge offgas and therefore the lower the hydrogen recovery efficiency. To avoid this, a lower fraction of the available secondary product gas should be taken for purging and the system should therefore be designed to enable the higher complementary part of said fraction to be used for the only other useful purpose, which is repressurization.
B. Fraction for repressurization
By increasing the number of pressure equalizations, whereby a repressurizing adsorber receives gas from an increased number of depressurizing adsorbers in consecutive steps, a larger proportion of the secondary product gas is
used for repressurization.
See animation herebelow visualizing the effect of parameter "n".
by one the number of pressure equalization over said nominal
4)
Remark:The number of pressure equalizations however soon meets its limit for its positive effect on product recovery efficiency; beyond this limit the operation tends to become unstable at reduced recovery, which is predicted by our simulator and has been demonstrated by actual plant operation. However, where on the one hand any improvement of recovery efficiency may be negligable when increasing the number of adsorbers to 12 or more, on the other hand the plant capacity may be increased dramatically when the number parallel adsorbing duties is increased for instance from 3 in a 10-bed system to 5 in a 13-bed system, hence increasing the plant capacity by a factor 5/3. With due attention to limitation of gas velocities, options are available for inclusion in the plant control logic to avoid the afore-
mentioned instability.
In case PSA offgas should be sent to a higher pressure fuelgas system without intermediate compression, purge pressures should be consistently higher and a larger proportion of (a lower quantity of) secondary product gas is needed for purging, leaving less secondary product gas available for repressurization and therefore less need for multiple pressure equalizations. While dumping takes place at a higher pressure level, desorption at this stage is less, leaving more hydrogen in the dump gas. Altogether, relatively more product is lost through dumping and purging. See also under option DUMP RETURN /DUR.
See animation herebelow visualizing the effect of parameter "n".
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The number of participating adsorbers may be increased to as many as 12, enabling
up to 4 pressure equalizations, i.e. Option /NPE4. (See also under Sorption
Modulation), increasing |
Remark:The number of pressure equalizations however soon meets its limit for its positive effect on product recovery efficiency; beyond this limit the operation tends to become unstable at reduced recovery, which is predicted by our simulator and has been demonstrated by actual plant operation. However, where on the one hand any improvement of recovery efficiency may be negligable when increasing the number of adsorbers to 12 or more, on the other hand the plant capacity may be increased dramatically when the number parallel adsorbing duties is increased for instance from 3 in a 10-bed system to 5 in a 13-bed system, hence increasing the plant capacity by a factor 5/3. With due attention to limitation of gas velocities, options are available for inclusion in the plant control logic to avoid the afore-
mentioned instability.
In case PSA offgas should be sent to a higher pressure fuelgas system without intermediate compression, purge pressures should be consistently higher and a larger proportion of (a lower quantity of) secondary product gas is needed for purging, leaving less secondary product gas available for repressurization and therefore less need for multiple pressure equalizations. While dumping takes place at a higher pressure level, desorption at this stage is less, leaving more hydrogen in the dump gas. Altogether, relatively more product is lost through dumping and purging. See also under option DUMP RETURN /DUR.
In general, the number of pressure equilizations is increased by increasing the number
of participating adsorbers. Alternatively in some cases, without having increased this
number of participating adsorbers, the purge offgas stream may be cut off while
continuing the introduction of the tail end of secondary product as if
purging is continued. This tail end effectively starts the initial repressurization at
the expense of available purge gas until an (additional) pressure equilibrium is
achieved. This option is called Tail End Repressurization, or in short option /TAILR.
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- Remarks on /TAILR:
- The offgas stream will be interrupted during the time needed for this pressure equalization
- For shaving the peaks and filling the gaps in the offgas flow, an intermediate holding drum will be needed. This holding drum should be extra large because of the very low offgas pressure
OPTION /DD
In stead of the above mentioned option /TAILR, dumping follows immediately after
"providing purge gas" and is therefore indicated as option Direct Dump or /DD.
See next diagram.
In this case however, less secondary product gas will be used for repressurization, leaving a higher fraction of it for purging. This in turn will extend the hydrogen rich tail end part of the offgas, causing the product recovery efficiency to become lower.
By implementing options /SM/SMEQ, the quantity of purge gas may be further reduced advantageously beyond what may be achieved by increasing the number of participating adsorbers. When considering the final repressurization step, the initial part thereof is normally accomplished not only by the initial release of secondary product gas from an adsorber, but also and simultaneously by a slipstream of product gas (see above PRESSURE/TIME DIAGRAMS). Explanation : In order to keep a constant product flow, the withdrawal of this slipstream and its subsequent use for repressurization should not be interrupted, also when not strictly needed for that purpose. Consequently, pressure equilibration is reached at a level which is higher than if the slipstream had not participated in the repressurization. This makes the recovery and subsequent use of secondary product gas for repressurization less efficient. See also explanation under Apparant Pressure Recovery "APR"
To eliminate this element of ineffiency, option /SM may be used for a PSA unit employing at least two parallel adsorption duties, whereby the start of a new adsorption step is delayed, at least for as long the initial part of the final repressurization step of an adsorber, now with secondary product only, has not been completed. During the delay of the start of a new adsorption step, the number of parallel adsorption duties is reduced by one. The above may be more easily understood by comparing the pressure/time diagrams of the cycle elements of two 6-bed psa units, without and with option /SM.
As the animation below on the left diagram shows, an additional pressure equilibration is realized on extending option /SM, as such defined as option /SMEQ. The values of the parameters SMEQ and SM are defined as the moments in time expressed as fractions within one cycle element, i.e. SMEQ at the additional pressure equilibration and SM at the start of a new adsoption step, restoring the number of parallel adsorption duties to the nomimal value.(in this case n=2 in /PARn, and n=2 in NPEn, the nominal value for the number of full pressure equilibrations)
The above animation on the left shows the effect on Apparant Pressure Recovery "APR" if switching between the options /DD, /SM and /SMEQ. On the right it is shown for option /SMEQ, how by changing the values for "SMEQ" and "SM" the APR-value is changed and the product recovery efficiency for some typical case is changed with it. An additional tool has therefore become available, enabling the Pressure Recovery to be controlled and through this the product recovery efficiency to be modified on demand. See also remark under OPTION /NPEn.
Or putting it in other words, when for instance choosing a 6-bed system plus Sorption Modulation in stead of a regular 10-bed system, the benefit is obtained of needing not more than only six adsorbers although of larger size, also 40% less control valves and associated piping are required, plus the inherent sliding scale control flexibility of Sorption Modulation. In addition as in most cases a slight improvement of the recovery efficiency of one percentage point may be expected.
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See next diagram.
In this case however, less secondary product gas will be used for repressurization, leaving a higher fraction of it for purging. This in turn will extend the hydrogen rich tail end part of the offgas, causing the product recovery efficiency to become lower.
Because secondary product is reused in the process for
repressurization and finally for purging, its maximum allowable impuriry level by
its nature occurring in its tail end (see animation in frame at the left) is
restricted, the more so because this tail end is used as the last part of the purge
gas and the quality of this last part should of course not be spoiled. In contrast,
the LO-FIN process reverses the quality profile of purge gas such that the tail end
of the secondary product with the highest impurity level is used for the front end of
the purge gas. Consequently, the tail end of the purge gas is of the highest quality
for the "finishing touch". The overall effect is a higher recovery efficiency
and a more effective use of adsorbent.
SCHEMATIC ARRANGEMENT LO-FIN® PROCESS
The figure on the left shows the schematic arrangement of the PSA LO-FIN process, comprising adsorbers and a LO-FIN RETAINER.
LO-FIN RETAINER
Reversal of the quality profile of purge gas is achieved by temporarily preserving it in the LO-FIN RETAINER, a vessel packed with Raschig rings. Each time when a new charge of secondary product is produced for use as purge gas, it is sent through the LO-FIN retainer countercurrently to the previous charge. The acronym LO-FIN stands for Last Out-First IN, referring to the tail end part of recovered secondary product gas being returned as the front end part for purging, or for repressurization if combined with option /TAILR.
Operating the PSA process under option /LO-FIN means that a massive breakthrough of impurities during the recovery of secondary product gas may be permitted without a major adverse effect on product quality on subsequent use of this gas for purging or repressurization. For the same number and size of adsorbers and unchanged product quality and feed rate, it is seen that the feed quantity per step (FQPS) and consistently the cycle time should be increased.
On completion of cocurrent depressurization (i.e. ending production of secondary product gas) the minimum hydrogen content in the adsorber as well as in the PSA offgas is lower. This explains the increased product recovery efficiency. Also contributing in this respect is the increase of cycle time and the inherently lower gas velocities during adsorbent regeneration, despite the somewhat larger quantity of secondary product gas.
If the quantity of purge gas is relatively low, for instance in case of a higher purge pressure or in case a larger proportion of secondary product gas is used for repressurization, then the effect of LO-FIN is likewise small.
In a dual role, the significance of the LO-FIN Retainer is then shifted towards its
duty as a holding vessel, making the timing for purging less dependent on the timing
for recovery of secondary product gas. An other positive contribution by the Retainer
is the moderating effect on the gas velocity at the OVHD of purge gas providing adsorber,
especially near the end where a relatively higher purge gas rate is needed for the
same purge offgas rate. In spite of this higher demand, the rather flat depressurization
curve in this part, keeps the OVHD gas velocity of the depressuri-
zing adsorber at moderate levels. The LO-FIN PSA-Process, characterized by the use of the LO-FIN Retainer has been invented and patented by A.J. Esselink and is the property of ESSELINK B.V.
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The figure on the left shows the schematic arrangement of the PSA LO-FIN process, comprising adsorbers and a LO-FIN RETAINER.
LO-FIN RETAINER
Reversal of the quality profile of purge gas is achieved by temporarily preserving it in the LO-FIN RETAINER, a vessel packed with Raschig rings. Each time when a new charge of secondary product is produced for use as purge gas, it is sent through the LO-FIN retainer countercurrently to the previous charge. The acronym LO-FIN stands for Last Out-First IN, referring to the tail end part of recovered secondary product gas being returned as the front end part for purging, or for repressurization if combined with option /TAILR.
Operating the PSA process under option /LO-FIN means that a massive breakthrough of impurities during the recovery of secondary product gas may be permitted without a major adverse effect on product quality on subsequent use of this gas for purging or repressurization. For the same number and size of adsorbers and unchanged product quality and feed rate, it is seen that the feed quantity per step (FQPS) and consistently the cycle time should be increased.
On completion of cocurrent depressurization (i.e. ending production of secondary product gas) the minimum hydrogen content in the adsorber as well as in the PSA offgas is lower. This explains the increased product recovery efficiency. Also contributing in this respect is the increase of cycle time and the inherently lower gas velocities during adsorbent regeneration, despite the somewhat larger quantity of secondary product gas.
If the quantity of purge gas is relatively low, for instance in case of a higher purge pressure or in case a larger proportion of secondary product gas is used for repressurization, then the effect of LO-FIN is likewise small.
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zing adsorber at moderate levels. The LO-FIN PSA-Process, characterized by the use of the LO-FIN Retainer has been invented and patented by A.J. Esselink and is the property of ESSELINK B.V.
| SORPTION MODULATION, OPTIONS /SM/SMEQ |
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By implementing options /SM/SMEQ, the quantity of purge gas may be further reduced advantageously beyond what may be achieved by increasing the number of participating adsorbers. When considering the final repressurization step, the initial part thereof is normally accomplished not only by the initial release of secondary product gas from an adsorber, but also and simultaneously by a slipstream of product gas (see above PRESSURE/TIME DIAGRAMS). Explanation : In order to keep a constant product flow, the withdrawal of this slipstream and its subsequent use for repressurization should not be interrupted, also when not strictly needed for that purpose. Consequently, pressure equilibration is reached at a level which is higher than if the slipstream had not participated in the repressurization. This makes the recovery and subsequent use of secondary product gas for repressurization less efficient. See also explanation under Apparant Pressure Recovery "APR"
To eliminate this element of ineffiency, option /SM may be used for a PSA unit employing at least two parallel adsorption duties, whereby the start of a new adsorption step is delayed, at least for as long the initial part of the final repressurization step of an adsorber, now with secondary product only, has not been completed. During the delay of the start of a new adsorption step, the number of parallel adsorption duties is reduced by one. The above may be more easily understood by comparing the pressure/time diagrams of the cycle elements of two 6-bed psa units, without and with option /SM.
As the animation below on the left diagram shows, an additional pressure equilibration is realized on extending option /SM, as such defined as option /SMEQ. The values of the parameters SMEQ and SM are defined as the moments in time expressed as fractions within one cycle element, i.e. SMEQ at the additional pressure equilibration and SM at the start of a new adsoption step, restoring the number of parallel adsorption duties to the nomimal value.(in this case n=2 in /PARn, and n=2 in NPEn, the nominal value for the number of full pressure equilibrations)
The above animation on the left shows the effect on Apparant Pressure Recovery "APR" if switching between the options /DD, /SM and /SMEQ. On the right it is shown for option /SMEQ, how by changing the values for "SMEQ" and "SM" the APR-value is changed and the product recovery efficiency for some typical case is changed with it. An additional tool has therefore become available, enabling the Pressure Recovery to be controlled and through this the product recovery efficiency to be modified on demand. See also remark under OPTION /NPEn.
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Or putting it in other words, when for instance choosing a 6-bed system plus Sorption Modulation in stead of a regular 10-bed system, the benefit is obtained of needing not more than only six adsorbers although of larger size, also 40% less control valves and associated piping are required, plus the inherent sliding scale control flexibility of Sorption Modulation. In addition as in most cases a slight improvement of the recovery efficiency of one percentage point may be expected.
In case of higher PSA offgas pressures (not uncommon in oil refineries),
an interesting option concerns the recycle of a part of the dump gas, in particular the
hydrogen-rich front end part of it to the inlet part of a regenerated adsorber.
Said recycle is realized without the need for any compressor facility.
The difference between the start-of-dump pressure and the pressure in the regenerated
adsorber is the driving force for this recycle until equilibrium is attained, whereupon
dumping and repressurization is continued in the usual way.
Since the hydrogen rich part of the dump gas is not released with the offgas, the
average hydrogen percentage in the offgas is reduced, consistent with an improvement
of the recovery efficiency. However, where dump gas has contributed in the
repressurization of an adsorber, part of the secondary product gas otherwise being used
for repressurization has now become redundant for that porpose and will be added to the
amount of purge gas, extending the tail end part of the purge offgas and therefore partly offsetting the effect of this dump gas return.
Now by combining dump return option /DUR with sorption modulation /SMEQ, a further improvement of the recovery efficiency is achieved by reducing the amount of purge gas to an extend which even exceeds the extra thereof produced due to dump return.
Now by combining dump return option /DUR with sorption modulation /SMEQ, a further improvement of the recovery efficiency is achieved by reducing the amount of purge gas to an extend which even exceeds the extra thereof produced due to dump return.
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In the next animation these effects are shown for three conditions in a 6-bed system, 1. basic operation, 2. dump return, 3. dump return plus sorption modulation. |
| Examining condition (3) of sorption modulation reveals: | |
| 1. | combining sorption modulation and dump return is essential for achieving a lower average hydrogen percentage in the offgas and therefore a positive effect on the recovery efficiency, |
| 2. | effect of sorption modulation without dump return (if operating at high purge pressure!!) is negligable or could even be negative (increased average hydrogen percentage in net total offgas). |
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While the PSA process looks technically simple and straightforward, consisting of an
assembly of vessels, piping and valves, describing its functioning in detail on a
scientifically justifyable basis appears to be rather difficult. Ever since the first
commercial installations were built, some 50 years ago, the art has matured from an
empirical into a relatively sophisticated state, with due attention to developing an
appropriate model to describe the physical
adsorption process. Unlike for continuous processes where the parameters are
essentially constant and the description can be based on a single set of data, for PSA
such description can only be based on numerous snapshots taken at defined points within
the steady swing of parameter values. In the appending pictures a fairly complete
record of parameter values is given in graphical presentations, covering a complete
cycle for two cases. One for a 10-bed system based on options /PAR3/NPE3/SM/SMEQ/DD,
Pages AØ to A16
and a 6-bed system based on /PAR2/NPE2/SM/SMEQ/DUR,
Pages BØ to B18.
Whereas in the case of option /LO-FIN the differential loading of adsorbents is increased, the opposite effect results in the case of options /SM and /SMEQ; in both cases, the recovery efficiency is potentially increased in a first instance. The LO-FIN process produces more purge gas, containing a low proportion of high purity, whereas by introducing Sorption Modulation without LO-FIN, the total amount of purge gas (of higher quality)is reduced. In both cases, the hydrogen content in the purge offgas is reduced. Ultimately, the net effect of these different options depends on the secondary effect of changing gas velocities, due to respectively, increased, or reduced Feed Quantity per Step (FQPS) and cycle time. In any case, any potential of improved plant performance may be found in one or more of the following complementary factors: (1) product recovery efficiency, (2) product purity, (3) plant capacity.
As for case 2, product recovery efficiency may be increased by a few percentage points.
In still other cases, the effect of installing a retainer may be quite small, however in those cases improvements may also be found in an improved continuity of offgas flow.
- Adsorbent Packing Density. Next to using a sophisticated simulation program, it is of course essential that the adsorbents as loaded in the adsorbers should meet as closely as possible all specifications as defined. Neglecting this aspect could cause pockets with packed densities, differing by as much as 10%. It is therefore strongly recommended to use special adsorbent loading facilities to make sure that a homogeneous and dense packing and therefore a maximum attainable adsorber capacity and a maximum selectivity is realized. Probabilities of channeling in the adsorbent packing are so minimized.
- Offgas conditioning for export. When necessary, special attention may be focussed on the offgas conditioning for export, requiring one or more holding drums. Whereas product recoveries may be raised at the expense of offgas flow continuity, like in option /TAILR or /DUR, peakshaving - possibly including a controlled supply of fuelgas from a foreign source - could still produce a steady offgas flow within predetermined limits for heating value, specific gravity (WOBBE number) and pressure.
- Snap Switch Selector Valve. Because of the very large number of operating options envisaged for the 8-bed demonstration plant, a proprietary hydraulic rotary valve system, embracing 24 units of this Snap Switch Selector Valve was installed. This enabled extremely fast diversions of gas streams.
CONTACT/INFORMATION
For more and specific information please contact us either by letter, e-mail, phone or by completing and sending the form at the bottom of this page.
Our complete name and address are:
Whereas in the case of option /LO-FIN the differential loading of adsorbents is increased, the opposite effect results in the case of options /SM and /SMEQ; in both cases, the recovery efficiency is potentially increased in a first instance. The LO-FIN process produces more purge gas, containing a low proportion of high purity, whereas by introducing Sorption Modulation without LO-FIN, the total amount of purge gas (of higher quality)is reduced. In both cases, the hydrogen content in the purge offgas is reduced. Ultimately, the net effect of these different options depends on the secondary effect of changing gas velocities, due to respectively, increased, or reduced Feed Quantity per Step (FQPS) and cycle time. In any case, any potential of improved plant performance may be found in one or more of the following complementary factors: (1) product recovery efficiency, (2) product purity, (3) plant capacity.
OPTION /LO-FIN, CHANGE TO LO-FIN MODE
The first and major effect of adding
a Retainer to an existing set of adsorbers under fixed conditions, is an improvement
of product quality.-
A return to the original (lower) quality may result into:
- Increasing plant capacity (i.e. feed rate) and reducing cycle time, at an unchanged FQPS and product recovery efficiency.
- Increase of FQPS, cycle time and product recovery efficiency at an unchanged feed rate.
- Any compromise between above two cases.
As for case 2, product recovery efficiency may be increased by a few percentage points.
In still other cases, the effect of installing a retainer may be quite small, however in those cases improvements may also be found in an improved continuity of offgas flow.
OPTION /SM and /SM/SMEQ, CHANGE TO SORPTION MODULATION
By changing to Sorption Modulation, an extra control parameter is introduced which
permits the implementation of an additional pressure equilibration, resulting into a
reduction of the quantity of purge gas and therefore a shortening the tail end part of
the PSA offgas. Up to a certain point, the effect of this for the lower purge pressures
will be a lower average hydrogen content in the PSA offgas, i.e. a higher product
recovery efficiency. Part of existing analog control valves may become redundant, while
some on/off valves should be modified to enable analog control. Potentially, the
product recovery efficiency may be increased by 2 to 3 percentage points.
OPTION /DUR, CHANGE TO DUMP RETURN
The option Dump Return may be considered in cases where high purge pressures and
therefore high start-of-dump pressures are used. It requires controlled pressure
equilizations between adsorbers through each of the bottom outlet valves. Potentially,
the product recovery efficiency may be increased by 1 to 1.5 percentage points, up to
a total of about 3 percentage points if in addition option /SM/SMEQ is included.
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SCOPE OF SUPPLY
1. Feasibility studies
2. Process design
3. Commissioning Services
4. Licensing
REFERENCES:
- One 4-bed bench scale unit, evaluating the LO-FIN
concept.
- One 8-bed demonstration plant, processing various
synthesis gases.
- One 6-bed pilot plant, treating different types of
refinery gas.
- Two plants including extension for cryogenic offgas.
- Two plants for ethylene plant offgas.
- Two plants for synthesis gas
- One plant for refinery gas
- Adsorbent Packing Density. Next to using a sophisticated simulation program, it is of course essential that the adsorbents as loaded in the adsorbers should meet as closely as possible all specifications as defined. Neglecting this aspect could cause pockets with packed densities, differing by as much as 10%. It is therefore strongly recommended to use special adsorbent loading facilities to make sure that a homogeneous and dense packing and therefore a maximum attainable adsorber capacity and a maximum selectivity is realized. Probabilities of channeling in the adsorbent packing are so minimized.
- Offgas conditioning for export. When necessary, special attention may be focussed on the offgas conditioning for export, requiring one or more holding drums. Whereas product recoveries may be raised at the expense of offgas flow continuity, like in option /TAILR or /DUR, peakshaving - possibly including a controlled supply of fuelgas from a foreign source - could still produce a steady offgas flow within predetermined limits for heating value, specific gravity (WOBBE number) and pressure.
- Snap Switch Selector Valve. Because of the very large number of operating options envisaged for the 8-bed demonstration plant, a proprietary hydraulic rotary valve system, embracing 24 units of this Snap Switch Selector Valve was installed. This enabled extremely fast diversions of gas streams.
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ESSELINK BV Krommesteeg 1 3984 NE Odijk The Netherlands www.esselinkbv.com e-mail: aj@esselinkbv.com Phone: +31 343 531 046, Fax: +31 343 513 799 |