Membrane Gas Separation

14.4.1.2
Turndown 
Turndown is the capability of the system to operate at reduced capacity. 
Membrane systems are highly capable of maintaining product purity even though the 
capacity is reduced down to 10% of the initial design. This is done by either reducing 
feed pressure, increasing permeate pressure, or by isolating modules from the system. 
The fi rst two methods can be used for short - term operation, and the latter when operating 
at signifi cantly reduced capacity for extended periods. No previsions are required in the 
design phase. 
PSA units can maintain both recovery and product purity at throughputs of ca. 30 –
100% of design by adjustment of the cycle time. Between 0 and 30% of design product 
rates, purity can be maintained but recovery is reduced unless special provisions are made 
in the design. 
The turndown capability of cryogenic systems strongly depends on the design. Partial 
condensation units can maintain product purity at slightly reduced recovery at fl ows down 
to 30 – 50% of design.
14.4.1.3
Reliability 
Reliability takes into account the on - stream factors which can cause unscheduled shut-
downs. It is an important project consideration, particularly if the process is a primary 
source of make - up hydrogen to a hydroprocessor or other mainstream refi nery processes. 
Membrane systems are extremely reliable with respect to the on - stream factor. The 
membrane separation process is continuous and has few control components which can 
cause a shutdown. Typically, the response to unscheduled shutdowns is rapid. 

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Membrane Engineering: Progress and Potentialities in Gas Separations
303
PSA systems are moderately reliable. The numerous valves associated with the process 
can cause unexpected shutdowns. The new PSA are designed with alternate modes of 
operation, in which 100% of design capacity can be achieved while bypassing any failed 
valve or instrument, with only a slight loss of recovery. Failures are automatically detected 
and bypassed by the microprocessor - based control system. However, stronger and peri-
odic control cycles are required. 
The cryogenic process is considered by refi ners to be less reliable than the PSA or 
membrane processes; this is mainly due not to the process itself but to the need for feed 
pre - treatments. Failure of the pre - treatment system usually results in contaminants freez-
ing in the cold box, leading to shutdown. In this case, a ‘ thaw ’ is necessary prior to restart. 
The pre - treatment system itself is often more complex than the cryogenic system.
14.4.1.4
Ease of Expansion 
In some cases, future expansion is contemplated even during the initial phase of a project. 
Expansion of membrane systems is very easy, since this only requires the addition of 
identical modules. 
PSA systems can also be expanded, but it requires additional design considerations and 
adds cost in the initial phase of the project. 
The cryogenic units cannot be expanded if it is not foreseen during the design phase. 
Generally they can be over - dimensioned and a capacity increase is often obtained without 
modifi cation to the cold box itself through addition of a tail gas compressor.
14.4.1.5
By - product Value 
By - product value is a measure of the capability of producing a high - value tail gas. In 
many refi nery hydrogen upgrading processes, the impurities to be rejected include hydro-
carbons which have value signifi cantly above fuel value. This is particularly true for 
olefi n 
- 
containing streams. The relative amounts of high 
- 
value hydrocarbons and the 
incremental cost of further separation determine whether by - product recovery is an impor-
tant parameter to be considered. 
The cryogenic process is best suited to applications involving by - product hydrocarbon 
recovery. It is possible to separate hydrocarbon streams containing C 
2
/C 
3
and C 
4+
com-
ponents, with recovery of these components generally quite high, with typical overall 
values of 90% for the C 
2
/C 
3
components and up to 100% of the C 
4+
. 
The membrane process is not capable of providing separate streams rich in specifi c 
hydrocarbon fractions; however, it is also used in these applications, since the hydrocar-
bon - rich stream is available at high pressure. 
By - product hydrocarbon recovery is normally not economical when the PSA process 
is used, because the single hydrocarbon - rich tail gas is delivered at too low a pressure. 
Combination of separation processes, such as membrane and cryogenic processes, may 
be applicable to these applications.