Membrane Gas Separation

Case Study: Hydrogen Separation in Refi neries 
The hydrogen upgrading in refi neries is currently carried out by means of pressure swing 
adsorption (PSA) and cryogenic separation processes. However, the application of mem-
brane systems for this type of separation is rapidly evolving to the commercial level, 
owing to the advantages related to the low capital costs, low energy requirements and 
modularity. 
In the following, after a brief description of the conventional separation processes 
(PSA and cryogenic) a comparison among these processes and membrane systems for 
hydrogen separation is reported, also introducing some project considerations such as 
process fl exibility, reliability, ease of response to the variations, expansion capability and 
versatility [76] . 
Pressure swing adsorption is a batch operation that uses multiple vessels to produce a 
constant product and off - gas fl ows. The PSA process is based on the capacity of some 
adsorbents (zeolites, molecular sieves, etc.) to adsorb more impurities at high gas - phase 
partial pressure. Proper selection of the adsorbents is critical to both the performance of 
the unit and adsorbent life. In the hydrogen separation, the impurities are adsorbed at 
higher partial pressure and then desorbed at lower partial pressure. The impurity partial 
pressure is lowered by ‘ swinging ’ the adsorber pressure from the feed pressure to the tail 
gas pressure, and by using a high - purity hydrogen purge. The hydrogen loss occurs 
through its use as a ‘ sweep gas ’ , to carry away the contaminants, amounting to about 
8 – 20% of the total hydrogen throughput [77] . Multiple adsorbers are used in order to 
provide constant feed, product and tail gas fl ows and each adsorber undergoes the same 
process steps in the same sequence. The separation is promoted by the impurity partial 
pressure difference between the feed and the tail gas. A minimum pressure ratio of 
approximately 4:1 between the feed and tail gas pressure is usually required for hydrogen 
separations. However, the absolute pressures of the feed and tail gas are also important, 
particularly to hydrogen recovery. The optimum feed pressure range for PSA units in 
refi nery applications is 15 – 30 bar. The optimum tail gas pressure is as low as possible. 
Since vacuum is normally avoided, tail gas pressure between 1.1 and 1.4 bar are typically 
used when high recovery is needed. The product hydrogen from a PSA unit is available 
at essentially feed pressure. 
Cryogenic separation is a low temperature separation process which uses the difference 
in boiling temperatures (relative volatilities) of the feed components to effect the separa-
tion. This process condenses the required amount of feed impurities by cooling the 
feed stream in multipass heat exchangers. The refrigeration required for the process is 
obtained by Joule – Thomson refrigeration derived from throttling the condensed liquid 
hydrocarbons. Additional refrigeration, if required, can be obtained by external refrigera-
tion packages or by turboexpansion of the hydrogen product. One of the main advantage 
of the cryogenic process is the ability to produce separated hydrocarbon streams rich in 
C 
4+
, ethane/propane, etc. In order to choose the best technology, some parameters, such 
as those reported below, have to be taken into account. 
14.4.1.1
Operating Flexibility 
Operating fl exibility is the ability to operate under variable feed quality conditions, either 
on a short - term or long - term basis. 

---

302
Membrane Gas Separation
The changes in feed composition occur very often in refi nery applications, particularly 
when the source of the feed is a catalytic process or when the feedstock to the upstream 
unit changes. 
In membrane processes, the increase in feed impurity concentrations tend to cause a 
decrease in product purity, which, however, can be maintained for small feed composition 
changes by adjusting the feed - to - permeate pressure ratio. In most refi nery membrane 
applications, however, the major product impurity is methane, and this can be allowed to 
increase slightly in the product without major downstream impact. The response time of 
membrane systems is essentially instantaneous, and corrective action has immediate 
results. The start - up time required by the process is extremely short. 
The PSA process shows a great ability to maintain hydrogen purity and recovery under 
changing conditions. The process is self - compensating and even relatively large changes 
in feed impurity concentrations have little impact on performance. As the concentration 
of a feed impurity increases, its partial pressure increases, increasing also the amount of 
the impurity which will be adsorbed. The purity of the hydrogen can be maintained con-
stant by a simple cycle time adjustment. The response time to variations is rapid but not 
abrupt, generally requiring 5 – 15 minutes for responding to a step change in feed quality. 
The new steady - state upon restart following shutdown is reached in about 1 hour. 
The cryogenic process has very low fl exibility, because changes in the concentration 
of the lower boiling components of the feed affect the product purity directly. Recovery 
is not strongly affected. Response time is not as rapid as for PSA or membrane systems. 
Start - up is 8 – 24 hours, depending on the procedure used.