As stated in the previous section, decreasing dense membrane thickness is the most straightforward conceptual approach to increasing oxygen flux through a perovskite membrane. However, surface treatments are a promising avenue for improvement if thickness decreases alone do not yield sufficient oxygen transport. If membrane stability were assigned its proper priority in the membrane reactor design, the need to enhance diffusion in other ways than decreasing dense layer thickness would become critical.
Air separation and partial oxidation of methane are integrated in a single unit, in order to eliminate the need for an extremely costly air separation unit. In the proposed mechanism of a tubular reactor, as seen in Figure 4.1, air is introduced into the shell side of the reactor through which oxygen is transported to the other side (membrane tubes) where it reacts with the natural gas (flare gas) feedstock, which is introduced into the tube side, to produce synthesis gas. This takes place at elevated temperatures, at
62 about 750°C or higher. Nickel-based catalysts facilitate the partial oxidation of methane, which indicate their appropriate use in a ceramic tubular membrane.
Figure 4.1: Dense membrane reactor mechanisms
The studies on oxygen permeable dense membranes in the literature (Tsai et al., 1997, Bouwmeester, 2003, Diethelm, 2003and Li, 2007) indicate that the oxygen permeation flux is proportional to the membrane temperature and the logarithm of the ratio of oxygen partial pressures across the membranes and is inversely proportional to the membrane thickness.
Tubular membranes’ reactors with thick walls were developed to lessen the engineering design difficulties. Practically, these membrane reactors are not favourable as they
63 reduce the oxygen flux due to their small surface area to volume ratio and thick walls (Wang et al., 2002). Membrane tubes with a thin wall can overcome these obstacles.
The assumptions made for this study are based on the use of thinner membranes to improve oxygen flux and to lower the costs found in conventional operations. These assumptions and the calculation procedures for a feasibility study of this process are given in the following sections, where a multitubular assembly of membrane tubes is enclosed in a shell through which air passes.
It was assumed that flare gas that was previously flared, i.e. methane, is fed to the reactor tube side packed with Ni catalyst and air is fed to the shell side of the reactor in co-current flow pattern. At the air inlet, the oxygen concentration was assumed at 21%. Some of oxygen permeates the membrane and reacts with methane, as air flows through the reactor. Therefore, oxygen concentration on the air side decreases and methane on the catalyst side is converted.
The 10 ml/m2.min maximum oxygen flux is assumed and it is used only at this stage to show that at this flux the process is feasible. A flare gas flow rate of 25,000 m3/day is assumed which is an average rate for a typical plant.
Assume O2 permeation rate i.e. 10 ml/ (cm2.min).
Assume tube size diameter, do = 0.015 m and length, L = 1.5m.
4.2.4.1 Number of Tubes Required Calculation
The procedure is adapted from Chapter 8 of (Kakac and Liu, 2002).
Assume flare gas flow rate of a typical oil and gas production plant is 25,000 m3/day. Subtract 20% of the flare gas flow rate for emergency flaring.
64 25000 – 5000 = 20000 m3/day
The next step is to determine the approximate number of tubes needed. All estimations are based on the following overall equation for one reactor:
CH4 + 1/2O2 CO+2H2, ΔH = -36 kJ/ mol (4.1)
Based on the above equation, as the stoichiometric oxygen flow rate is half that of methane, the flare gas flow rate (20,000 m3/day) requires 10,000 m3/day of oxygen and the oxygen level in the air is in the range of 21 %, so the air flow rate required is 47,920 m3/day.
First, oxygen permeation is assumed at 10 ml/ (cm2.min) and then the calculations for other values (12, 15 and 20 ml/ (cm2.min)) were performed using Excel spreadsheet.
10 ml/ (cm2.min) x 1440 min/day = 14400 cm3/ cm2.day O2 permeation = 1.44 x10-2 m3/ day.cm2
The approximate surface area, for the reactor:
As= O2 flow rate / O2 permeation rate (4.2)
As= (10000 m3/day)/ (1.44 x10-2m3/ day.cm2)
Area = 694,444 cm2 = 69.44 m2
Assume the tubes are made of LSCF (6428) powder due to its high oxygen permeation rate and oxygen stability (Li, 2007):
As = NTπdo L (4.3)
Where NT is the number of tubes, do is the outside diameter of a tube and L is its length.
For the above assumptions,
65
4.2.4.2 Amount of LSCF (6428) and Cost
Assume thickness of tube = 0.002 m Tube thickness bm = (do-di)/2
From the above equation; di = do-2bm = 0.011 m
Net volume of LSCF powder needed per tube:
Volume, V= AL, = (πd2/4) L = π* L /4 (do2 - di2) (4.4)
For easy calculation, the units are converted to centimetres = π*150 /4* (2.25 –1.21) = 122 cm3
Volume of LSCF (6428) powder needed for one membrane tube = 122 cm3 ρ (density) = Mass/Volume
Density of LSCF ( 28) powder ≈ 2 gm / cm3
Approximate weight of powder per tube ≈ Density x volume = 2 gm/ cm3 x 122 cm3 = 244 gm
Price of powder ≈ ₤ 00/kg ≈ ₤0. /gm
Price of powder per tube ₤0. /gm x 2 gm ₤1 . Total price of powder for the tubes = 982 x 1 . ₤1 3,7 5 Flare gas flow rate assumed at 20,000 m3/day (mainly CH4)
Half of the flow rate through the reactor (oxygen flow rate) = 10,000 m3/day =10000*106/ (3600*24*982) = 118 cm3/s
Velocity = flow rate /tube cross area =118/1.767 = 67cm/s
Thus, it can be seen that with the above assumptions it is feasible to construct a reactor system of LSCF tubes which is capable of producing syngas (H2 + CO) from the high
66 the costs quoted above are for laboratory quantities and would be lower in practice for the levels required in the present proposal.
A theoretical mechanism of the oxygen permeation through a pirovskite membrane suggested by Tsai et al., 1997 is followed in this work.
⁄ ( ) ( .5)
Where is the oxygen permeation rate, is the activation energy and the pre- exponential factor A for the LSCF (6482) material are 62,700 J/mol and 7.34x10-7 mol/ (ms K), respectively (Tsai et al., 1997), Tm is the membrane temperature, bm is the
membrane thickness and
is the ratio of oxygen partial pressure at the air shell side
(feed) and tube side (permeate).