FIELD OF INVENTION
The present disclosure relates to a process for recovering high purity hydrogen fi-om an off gas
stream and a system to carry out said process.
BACKGROUND OF INVENTION
There is an ever increasing demand of hydrogen in today's refinery. The major factors
responsible for this trend have been increased processing of heavier crude oils, increased demand
of clean fuels, tighter legislative controls and reduced demand of black products. These in turn
impact the refinery margins adversely. As the demand of hydrogen is increasing, the loss of
hydrogen as off gases from the hydrogen consuming processes and purification processes is also
increasing. Efforts are being made continuously to reduce these hydrogen losses. Despite these
efforts large amount of hydrogen is lost as low purity hydrogen streams fi-om the refineries.
Low purity (-20-70 mol%) hydrogen containing hydrocarbon streams can be sourced fi-om the
secondary processing units in oil refineries like Naphtha Hydrotreater Uunit (NHT), Naphtha
Isomerization Unit, Hydrocracker Unit (HCU), Diesel Hydrotreater Unit, Product
desulphurization Units and Refinery Off Gas (ROG) Pressure Swing Adsorption Unit as low to
moderate pressure refinery purgeloff gas. Petrochemical plant, fertilizer plant and other chemical
plants have Hydrogen stream from where hydrogen can be recovered (Ethylene plant in
petrochemical complex, Ammonia plant in fertilizer complex etc). These low purity hydrogen
containing streams contains C1, C2, C3 & heavier hydrocarbon as major hydrocarbon entity along
with impurities like HzS, HzO, NH3, COY COZ. Other inerts like nitrogen, argon in varying
proportion depending upon source of the purgeloff gas and operating practice. Conventionally
these off gas streams are burnt as fuel gas in process heaters for heating purpose. A significant
value addition and reduction in carbon emission is possible by recovering hydrogen from these
offlpurge gas streams.
In prior art various processes for the purification of the hydrogen from a crude hydrogen stream
has been proposed. In one of such processes, feed gas is cooled and partially condensed in
exchanger by indirect contact with cold hydrogen and hydrocarbon streams. Condense
hydrocarbon stream is throttled to reduce temperature. When higher purity of hydrogen product
is required, a portion of the hydrogen product is mixed with throttled hydrocarbon stream for
further reduction of feed temperature. Though, this process is simple, it is observed that subcooled
liquid hydrocarbon (temperature -148OC) upon throttling gives Reverse Joule-Thompson
effect and consequently increase in stream temperature. As a consequence throttled hydrocarbon
stream is not capable of giving required temperature approach in cold side of the feed exchanger.
The mentioned scheme works only upon mixing a significant part of the product hydrogen
stream with the throttled hydrocarbon stream for temperature reduction. This reduces recovery of
hydrogen drastically.
In another proposed method, feed is compressed, cooled and partially condensed to separate
hydrogen and light hydrocarbon from mixture of hydrogen & hydrocarbon stream. Cooling
required for this process is provided by indirect heat exchange with refrigerant in closed-loop gas
expander refrigeration cycle. This process produces purer hydrogen along with light hydrocarbon
liquid product with help of external refrigeration cycle.
In another proposed method, pressure difference between the high pressure feed gas and
expanded low pressure is used for cooling of the feed stream to separate hydrogen and
hydrocarbon from a feed gas containing hydrogen and hydrocarbon, preferentially C2-Cs. Vapor
from .partially condensed feed is turbo expanded to cool and is again mixed with liquid part of
the partially condensed feed. In such scheme separation occurs at pressure range of 20-27
kg/cm2g and temperature range of -1.5OC. This level of pressure and temperature is not suitable
to get high purity hydrogen -from a stream containing C1, C2, C3 hydrocarbon.
Another process involves stepwise cooling by a series of multi-pass heat exchangers and inter
stage separation of condensates for separating condensable constituents like methane from
hydrogen containing streams. In this process external re-frigeration (C3 refrigeration) is used for
additional cooling requirement.
Most of the schemes in literature are targeted to use external refrigeration utility for cooling and
partially condensation of feed gas. This increases process dependency towards cooling utility,
overall power consumption and hence higher operating cost. Also the operating conditions of the
few schemes are unsuitable to remove C1 hydrocarbon from hydrogen.
Thus, in the view of the aforementioned drawbacks of the processes disclosed in the prior art,
there exists a need of an improved process for recovering high purity hydrogen from an off gas
stream and a system to carry out said process. Also, an efficient process is required which does
not involve the use of the external refrigeration for the cooling and condensation of the feed gas.
In order to overcome shortcoming with the recovery of hydrogen from an off gas stream, the
applicant has developed an improved process for the recovery of high purity hydrogen from an
off gas stream without use of any external refrigeration, thereby, making the process
economically benign and environment friendly.
Therefore, it is an aim of the present disclosure to provide an improved process for the recovery
of high purity hydrogen from an off gas stream with minimal loss of hydrogen as purgeloff gas.
Yet another aim of the present disclosure is to provide a process to maximize hydrogen product
purity without use of external refrigeration utility by integrated auto-refrigeration.
Yet another aim of the present disclosure is to provide a system for recovery of high purity
hydrogen from an off gas stream.
SUMMARY OF THE INVENTION
The present disclosure describes a process for the recovery of high purity hydrogen from an off
gas stream and a system to carry out such process. It provides a process for the recovery of high
purity hydrogen from an off gas stream with minimal loss of hydrogen as purgeloff gas. The
present process maximizes the hydrogen product purity without use of external refrigeration
utility by integrated auto-refrigeration. The purity of the hydrogen recovered from the off gas
stream by the present process is 90-99 %, with the recovery of hydrogen up to 99.9 %.
Accordingly, the present disclosure provides a process for recovering hydrogen from an off gas
streams by allowing to pass off gas stream (Sl) through a separator (Bl) to separate
condensables like water & Heavy hydrocarbon and subsequently passing the dry off gas stream
in a compressor (B2) and further in a cooler (B3) and repeating the separation, compression and
cooling process at least two times for obtaining a compressed gas stream (S 14).
The compressed gas stream (S14) is then passed through at least one molecular sieve adsorber
(B11) and dry and purified gas stream (S17) is obtained which is further cooled in a multi-stream
Heat exchanger (B 13) to obtain cooled gas stream (S 18) having temperature in the range of -100
to -180 OC. This cooled gas stream is flashed in an insulted flash column (B14) and separating
the cooled high pressure flashed vapor (S 19) and light hydrocarbon rich stream (S20) containing
C1 to C3 and heavier hydrocarbons. The cooled high pressure flashed vapor (S19) is then passed
through the turbo-expander (B 15) and thereby obtaining the cooled hydrogen rich flashed vapor
(S21) having temperature in the range of -1 10 to -190 "C. Cooled hydrogen rich flashed vapor
(S2 1) is then passed through the multi stream exchanger (B 13) so as to carry out heat exchange
with the incoming dry and purified gas stream (S 17) of high temperature and thereby obtaining
the hydrogen rich gas (S22) at an ambient temperature.
In another embodiment of the present disclosure, the off gas stream is a hydrogen containing
refinery off gas or petrochemical plant off gas or fertilizer plant off gas stream.
In another embodiment of the present disclosure; the off gas stream in step (i) is compressed
sequentially to a pressure ranging from 30 -90 kg/cm2g.
In another embodiment of the present disclosure, the molecular sieve adsorbers (B11) are
banged in a series or parallel combinations to remove the impurities preferably HzO, H2S, COY
CO2, NH3, N2.
In another embodiment of the present disclosure, the compressed, dry & purified gas stream of
step (ii) is optionally passed through the filter (B12) to prevent carryover of adsorber bed
material to multi stream heat exchanger (B 13).
In another embodiment of the present disclosure, the light hydrocarbon rich stream (S20)
obtained from the insulated flash column (B14) of step (iv) is throttled and splitted into, high
pressure gas stream (S27) having pressure in the range of 5-10 kg/cm2g and low pressure gas
stream (S28) having pressure in the range of 1-5 kg/cm2g.
In another embodiment of the present disclosure the throttling of light hydrocarbon rich stream
(S20) of step (iv)and low pressure gas stream (S28) in throttle valves (B25, B26) thereby
increasing the temperature of the streams in the range of -143 to -140°C and obtaining throttled
hydrocarbon streams (S27, S30).
In another embodiment of the present disclosure, the passing of the throttled hydrocarbon
streams (S27, S30) along with the cooled hydrogen rich stream high pressure flashed vapor (S21)
of step (v) through the Multi stream heat exchanger (B13) in the reverse direction to the dry and
purified gas stream (S17), thereby causing heat exchange by the counter current flow and
obtaining hydrogen rich gas (S22) at ambient temperature.
Also, the present disclosure provides a system for recovering hydrogen from an off gases stream
comprising:
a series of separators (Bl, B4, B7, BlO), compressors (B2, B5, B8) and coolers (B3, B6,
B9) are configured to separate, compress and cool the off gas stream (Sl) and obtaining
a compressed gas stream (S 14);
molecular sieve adsorbers (B11) arranged in a series or parallel combination and
optionally a filter (B 12) coupled with 'the separator (B 10) to remove the impurities
preferably H20, H2S, COz, NH3, N2 and obtaining dry and.purified gas steam (S17);
an multi-stream exchanger (B 13) connected with a molecular sieve adsorbers (I31 1) to
cool down the further dry & purified gas steam (S 17) by the counter current flow of the
steams (S21, S27 and S30) and obtaining cooled gas stream (S18);
an insulated flash column (B14) coupled with a multi-stream exchanger (B13) for
flashing the cooled gas stream (S18) and obtaining cooled high pressure flashed vapor
(S 19) and light hydrocarbon rich stream (S20);
a turbo-expander (B15) coupled with an insulated flash column (B 14) to cool down the
cooled high pressure flashed vapor (S19) and obtaining the significantly cooled hydrogen
rich flashed vapor (S21) having temperature in the range of - 1 10 to - 1 90°C and
throttle valves (B25, B26) coupled with an insulated flash column p14) for throttling of
light hydrocarbon rich stream (S20) and low pressure gas stream (S28) and allowing the
same to pass through an multi stream exchanger (B13) for heat exchange.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Further aspects and advantages of the present disclosure will be readily understood horn the
following detailed description with reference to the accompanying figure of the drawing. The
figure together with a detailed description below, are incorporated in and form part of the
specification, and serve to further illustrate the embodiments and explain various principles and
advantages but not limiting the scope of the invention.
In the accompanying drawing, figure 1 shows a,schematic representation of the process1 system
for the recovery of hydrogen hom an off gas stream.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
While the invention is susceptible to various modifications and alternative forms, specific
embodiment thereof has been shown by way of example in the figure and will be described in
detail below. It should be understood, however that it is not intended to limit the invention to the
particular forms disclosed, but on the contrary, the invention is to cover all modifications,
equivalents, and alternative falling within the spirit and the scope of the invention as defined by
the appended claims.
Before describing in detail the various embodiments of the present invention it may be observed
that the novelty and inventive step that are in accordance with the present invention resides in the
process for the recovery of high purity hydrogen from an off gas and a system to carry such
process. It is to be noted that a person skilled in the art can be motivated from the present
invention and modify the various steps of the current process and/ or constructions of current
system. However, such modification should be construed within the scope and spirit of the
invention.
Accordingly, the drawing is showing only those specific details that are pertinent to
7
understanding the embodiments of the present invention so as not to obscure the disclosure with
details that will be readily apparent to those of ordinary skill in the art having benefit of the
description herein.
The terms "comprises", c'comprising", "including" or any other variations thereof, are intended
to cover a non-exclusive inclusion, such that a process or a system that comprises a list of steps
or components does not include only those steps or components but may include other steps or
components not expressly listed or inherent to such process or system, mechanism or setup. In
other words, one or more elements1 parts in the current process or system proceeded by
c L ~ ~ m p rdio~ese n~oyt, 'w ithout more constraints, preclude the existence of other steps or elements
or additional elements in the system or mechanism. The following paragraphs explain present
invention and the same may be deduced accordingly.
Referring to figure 1, the off gas stream (Sl) containing Hydrogen (-10 - 70 mol%) along with
light hydrocarbon and impurities like H20, H2S, COY C02, NH3, N2 is being passed through the
separator (Bl) to separate condensables and subsequently passed through the compressor (B2)
and cooler (B3). This process is repeated at least two times to obtain a compressed gas stream
(S14) having pressure in the range of the 30-90 kg/cm2g.
Compressed gas stream (S14) is passed through at least one molecular sieve based adsorber
(B11) to remove the impurities like H20, H2S, CO, Cozy m, N2 etc. to the extent to prevent
freezing of impurities. This results in dry and compressed gas stream (S16) having temperature
in the range of 20 to 50°C.
Dry and compressed gas stream (S16) is then optionally passed through the filter (B12) to
prevent carryover of adsorber bed material to multi stream heat exchanger and obtaining dry and
purified gas stream (S 17).
Dry and purified gas stream (S 17) is being passed through the multi-stream Exchanger (B 13) by
counter current contact with the cooled hydrogen rich flashed vapor (S21) and purge
Hydrocarbon streams (S27, S30) which results in cooled gas steam (S 18) having temperature in
the range of -100 to -1 80°C.
Cooled gas steam (S 18) is being passed through an insulated flash column (B14) which results in
the separation of the cooled high pressure flashed vapor (S19) and light hydrocarbon rich strearn
(S20) containing C1 to C4 and heavier hydrocarbons.
Cooled high pressure flashed vapor (S19), rich in hydrogen is then passed through a turboexpander
(B15) to drop the gas pressure to 20 to 25 kg/cm2g and consequently get the cooled
hydrogen rich flashed vapor (S21) having temperature in the range of -110 to -190°C.
Light hydrocarbon rich stream (S20) fsom flash column bottom is throttled and split into two
streams (S23, S28), one with pressure ranging 5-10 kg/cm2g (S27) and another with pressure
ranging 1-5 kg/cm2g (S28).Throttling of this cold & pressurized hydrocarbon liquid streams in
throttle valves (B25, B26) gives Reverse Joule-Thompson Effect and eventually increase in
temperature of the stream.
Splitting of hydrocarbon stream fsom flash column bottom in different pressures is required to
limit maximum temperature approach in multi-stream Exchanger (B13) below 20- 25°C. The
throttled hydrocarbon streams (S27 and S30) along with the cooled hydrogen rich flashed vapor
(S21) are passed through the multi-stream Exchanger (B13) in the reverse direction to the dry
and purified gas stream (S17), ther.eby resulting in heat exchange by the counter flow. As the
throttling of liquid hydrocarbon stream increases temperature, only the turbo-expanded cooled
hydrogen rich flashed vapor (S21) is capable of giving the required temperature difference in
multi-stream Exchanger (B13) cold end. This step results in the hydrogen rich gas (S22), low
pressure hydrocarbon stream (S31) and high pressure hydrocarbon stream (S32) having
temperature in the range of 10- 40°C.
A part of the high pressure hydrocarbon stream (S34) is heated in a heater (B17) up to
temperature ranging from 250°C to 300°C and used for regeneration of the adsorber bed (Bl I).
Hot regeneration gas stream (S36) from adsorber bed (B11) is passed through cooler (B18) and
results in cooled gas stream (S37). This cooled gas stream (S37) is mixed with other hydrocarbon
stream in block (B 19).
When additional decrease in temperature is required in insulated flash column (B14), a portion
of the hydrogen product fsom flash column top (S24) is mixed with bottom hydrocarbon stream
(S23) through valve (B16). This can further enhance hydrogen product purity by reducing flash
temperature and recovery of hydrogen product.
In one of the examples of the present disclosure the compositions of off gases fiom which
hydrogen can be recovered are given in Table 1.
Table 1: Composition of off gas as feed for hydrogen recovery
The purity of the hydrogen recovered from the off gas stream is 90-99%, with the recovery of
hydrogen up to 99.9%.
Component
H2
c 1
c2
c3
c4
c5+
Mol%
5 5
17.4
10.5
7.6
6.2
3.3

We claim:
1. A process for recovering hydrogen from an off gas stream comprising the steps
of:
(i) allowing to pass off gas stream (Sl) in a separator (Bl) to separate
condensables and subsequently passing the dry off gas stream in a
compressor (B2) and further in a cooler (B3) and repeating the separation,
compression and cooling process at least two times for obtaining compressed
gas stream (S14);
(ii) passing the compressed gas stream of step (i) in at least one molecular sieve
adsorber (B11) and obtaining dry & purified gas stream (S17);
(iii) cooling the dry & purified gas. stream (517) of step (ii) in a multi-stream
Heat exchanger (B13) and obtaining cooled gas stream (S18) having
temperature in the range of -100 to -1800C
(iv) passing the cooled gas stream (S18) of the step (iii) in an insulted flash
column (B14) and separating the cooled high pressure flashed vapor (S19)
and light hydrocarbon rich stream (520);
(v) passing the cooled high pressure flashed vapor (S19) of step (iv) through the
turbo-expander (B15) and obtaining the cooled hydrogen rich flashed vapor
(S21) having temperature in the range of -110 to -190 "C and
(vi) passing the cooled hydrogen rich flashed vapor (S21) of step (v) through the
multi-stream exchanger (B13) so as to carry out heat exchange with the
incoming dry and purified gas stream (517) of high temperature and
obtaining the hydrogen rich gas (S22) at an ambient temperature.
2. The process as claimed in claim 1, wherein off gas stream is a refinery off gas or
petrochemical plant off gas or fertilizer plants H2 rich purge stream.
3. The process as claimed in claim 1, wherein the off gas stream in step (i) is
compressed sequentially to a pressure ranging from 30 -90 kg/cm2g.
4. The process as claimed in claim 1, wherein molecular sieve adsorbers (B11) are
arranged in a series or parallel combinations to remove the impurities preferably
H20, HzS, CO, C02, NH3, N2.
5. The process as claimed in claim 1, wherein gas stream of step (ii) is optionally
passed through the filter (B12) to prevent carryover of adsorber material to
multi-stream heat exchanger (B13).
6. The process as claimed in 1, wherein the light hydrocarbon rich stream (520)
obtained from the insulated flash column (B14) in step (iv) is throttled and
splitted into high pressure gas stream (S27) having pressure in the range of 5-10
kg/cm2g and low pressure gas stream (S28) having pressure in the range of 1-5
kg/cm2g.
7. The process as claimed 1 or 6, wherein throttling of light hydrocarbon rich
stream (S20) of step (iv)and low pressure gas stream (S28)in throttle valves (B25,
B26) thereby increasing the temperature of the streams in the range of -1430C to -
1400C and obtaining throeled hydrocarbon streams (527,530).
8. The process as claimed in claim 7, wherein the passing of the throttled
hydrocarbon streams (S27, S30) along with the cooled hydrogen rich flashed
vapor (S21) of step (v) through the multi-stream heat exchanger (B13) in the
reverse direction to the dry and purified gas stream (S17), thereby causing heat
exchange by the counter current flow.
9. A system for recovering hydrogen from an off gaqes stream comprising:
a series of separators(B1, B4, B7, BlO), compressors (B2, B5, B8) and coolers (B3,
B6, B9) are configured to separate, compress and cool the off gas stream (Sl) and
. obtaining a compressed gas stream (S14);
molecular sieve adsorbers (B11) arranged in a series or parallel combination and
optionally a filter (B12) coupled with the separator (B10) to remove the
impurities p~eferably H20, H2S, CO, C02, NH3, N2 and obtaining dry and
purified gas steam (S17);
multi-stream exchanger (B13) connected with a molecular sieve adsorbers (B11)
to cool down the further dry & purified gas steam (S17) by the counter current
flow of the steams (S21, S27 and S30) and obtaining cooled gas stream (S18);
an insulated flash column (B14) coupled with multi-stream exchanger (B13) for
flashing the cooled gas stream (S18) and obtaining cooled high pressure flashed
vapor (S19) and light hydrocarbon rich stream (S20);
a turbo-expander (B15) coupled with an insulated flash column (B14) to further
cool down the cooled high pressure flashed vapor (S19) and obtaining the
sigruficantly cooled hydrogen rich flashed vapor (S21) having temperature in the
range of -110 to -1900C and
throttle valves (B25, B26) coupled with an insulated flash column (B14) for
throttling of light hydrocarbon rich stream (S20) and low pressure gas stream
(528) and allowing the same to pass through an multi-stream exchanger (B13)
for heat exchange.