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09.12.2014, admin
Science, Technology and Medicine open access publisher.Publish, read and share novel research. Superficial velocity against time for (a) the feed pressurization step, (b) first minute of the feed adsorption step and (c) the counter-current depressurization and the purge stepsTable 6.
Natural Gas is a mixture of Methane(C1), Ethane (C2), Propane (C3), Butane isomers (C4) and Pentanes, to name only the more significant components. Higher hydrocarbons are there as the gas comes off the well, but are removed in the gas treatment plants. Winterization and dehydration are two major aspects of gas treatment before it is transported to the user through pipelines [1].When the use of Natural Gas is as a fuel replacing conventional sources such as coal, or furnace oil or petroleum fractions, the most important thing is its calorific value. Even this small percentage of higher hydrocarbons can change the calorific value significantly.
Users of Natural Gas as fuel thus would like more presence of the heavier hydrocarbons in the supply gas.Gas based fertilizer plants, metallurgical units which need to carry out ore reduction using gases such as CO and H2 would like to use Natural Gas as mainly a feedstock for reformers. Even a small presence of C2+ could be detrimental to the reformer catalyst and necessitate its regeneration frequently. Natural Gas is thus necessarily stripped of its C2+ content before use as reformer feedstock. One of the techniques used is cooling the gas below the boiling points of Propane so that C2+ hydrocarbons are condensed. Joule-Thompson expansion is often employed to achieve temperatures as low as 203 K for this purpose. C2+ alkanes are recovered almost entirely and can contribute to Liquefied Petroleum Gas (LPG) pool. An alternative separation technique is desired for large scale gas treatment for removal of C2+ from Natural Gas.This paper deals with an adsorption based technique developed and implemented for this purpose.
Due the large volume that needed to be treated, we could conceive a significant modification in PSA technology.
By suitable application of model based design and operation, we could convert the PSA to a surge-less PSA technology.
This eliminated or minimized the need for surge vessels, so integral to any PSA process because of pressure swings. This paper discusses the conceptual design process design and process plant engineering of a surge-less PSA plant designed to process a feed rate as high as 2000000 m3 per day of Natural Gas at standard temperature and pressure conditions.
Design problemThe separation plant’s objective was to reduce C2+ content of Natural Gas to below 0.6% (mole fraction) for all feed compositions with C2+ content not exceeding 2%.
It was even this relatively low content of C2+ hydrocarbons which was perceived to be causing catalyst deactivation due to coking in downstream reactors. The main components of the Natural Gas are given in Table 1 and a typical feed gas composition used in design was as given in Table 2. Limited data on adsorption thermodynamics is available in terms of adsorption isotherms [2]. The adsorption capacities increase significantly with increase in the number of Carbon atoms in the homologous series of alkanes of interest, namely C1 to C4. Typical pure-component isotherms at the operating temperature and pressures up to 35 bara and the parameters of the fitted curves for the five hydrocarbons are shown in Figure 1 and Table 1 respectively.
Carbon Dioxide has also been reported for its adsorption behavior and its isotherm and isotherm expressions are also included in Figure 1 and Table 1. The isotherms show typical favorable isotherm shape and fit very well with Langmuir equation. The isotherms and their mathematical expression forms show the adsorption equilibrium constant increasing with the number of Carbon atoms in the alkane molecule.
The mathematical expressions can be used to appreciate the separation potential offered by Silica Gel for the desired separation task.
If the Natural Gas composition given above is equilibrated with Silica Gel, the equilibrium adsorbed phase concentration of each of the six components is tabulated in Table 2 based on Extended Langmuir expression for multi-component adsorption.

If the feed composition is the same, but equilibration is done at 34 bara, the equilibrium adsorbed phase concentration of each component is also tabulated in Table 2. Physical adsorption being reasonably weak, multi-layer adsorption is less possible and monolayer saturation capacities are reached at the pressures considered. At both pressures, the solid phase has considerable amount of adsorbed CH4 (74% on molar basis or 52% on mass basis of adsorbed phase is Methane).
It is the major component of the adsorbed phase as its mole fraction in the feed itself exceeds 95%. If a bed of Silica Gel is subjected to a flow of Natural Gas, the simultaneous adsorption of the hydrocarbons and CO2 will occur. The concentrations of higher hydrocarbons (due to their lower value and stronger preference by the adsorbent) would deplete fast and one can expect a raffinate gas leaner in the C2+ content. The bed would get gradually saturated and C2+ gases would begin to break through into the raffinate. If the feed gas is then diverted to another identical bed and the saturated bed is regenerated, continuous raffinate production is possible. The desorption of C2+ can also be facilitated by passing through the bed a fraction of the lean gas (LG) produced earlier or purge-in gas (PG) which will reduce the partial pressure of C2+ components in the bulk phase surrounding the adsorbent particles. The gas collected during these blow down and purging steps will be richer in C2+ components than the feed gas and is termed as rich gas (RG). The adsorbed bed switched between alternate high and low pressure regimes thus offer a potential to separate the Natural Gas into two streams, namely a raffinate leaner in C2+ than the feed and an extract gas richer in C2+ components. We thus have a potential to employ a suitably designed PSA process for the desired separation task. The design was carried out using a comprehensive simulation model for PSA processes developed by the authors [3].
In Natural Gas-Silica Gel system, the desired product is the weakly adsorbed component (Methane and Ethane). Conventional 4-step cycle of the Skarstrom type is used for dealing with raffinate PSA systems [4]. The steps of the Skarstrom cycle in sequence are: pressurization of the bed with feed gas to the feed tank pressure which is the adsorption pressure, adsorption of feed gas at the adsorption pressure to give raffinate product, depressurization of the bed to the desorption pressure in a direction counter-current to that of the feed gas flow direction and counter-current purge with fraction of raffinate product at the desorption pressure. The bed starts getting pressurized with the feed Natural Gas mixture and at the end of this step, the bed is pressurized to a value nearly equal to the feed tank pressure. Since the raffinate tank pressure is less than that of the pressurized bed, material starts flowing out from the bed to the raffinate tank and fresh feed flows into the bed.
The raffinate tank collects a gas that is relatively free of Propane and higher alkanes and Carbon Dioxide as these components get preferentially adsorbed in the adsorbent. Desorption of the species will also occur simultaneously with depressurization and the model considers it. This step is included to improve the recovery of the weaker adsorbates and reduce the velocity surges that would have occurred in Step 4 if the bed is directly depressurized counter-currently to the extract tank after finishing the adsorption step.
This step is used to flush the desorbed heavy components in the void spaces towards the extract end. One cycle is completed at the end of this step (at t1+t2+t3+t4+t5 = tcycle) and a fresh cycle begins starting with Step 1.2.
Simulation modelPSA is a very versatile process and the process embodiments most suited for a given separation task depend entirely on the adsorbent-adsorbate system and desired separation performance.
To study various process options, an elaborate and costly experimental program needs to be launched and executed. Another equally effective, much less costlier and more exhaustive option that is possible is to systematically carry out simulation based design. The authors have developed a generic simulator for PSA process genre covering all its possible variations such as PSA, Vacuum Swing Adsorption (VSA), Pressure-Vacuum Swing Adsorption (PVSA), etc. The model allows putting together any PSA cycle and predict its effect on performance measured in terms of product purity, recovery and adsorbent throughput.

A relevant summary along with the definitions of performance indices for the present case is presented here.The model mainly assumes-(i) perfectly plug flow of the fluid, (ii) fluid behaves like an ideal gas, (iii) multi-component adsorption equilibrium behavior as per Extended Langmuir isotherm, (iv) isothermal operation, (v) mass transfer rate described by the linear driving force approximation model, (vi) flow at the ends of the bed described by a valve equation with the ability to vary the valve coefficient with respect to time in case of controlled opening or closing of the valve and (vii) pressure drop across the packed bed is described by the Ergun equation. The model can consider- (i) sorption occurs during the pressurization and depressurization steps (no frozen solid assumption), (ii) fluid velocity variations, (iii) fluid density variations and (iv) presence of inert or empty layer below or above the adsorbent layer. Recovery in this case is therefore defined as the percentage of these components from the feed gas which end up in the extract stream over a complete cycle.
Purity is therefore defined as the difference between 100 and mole percent of these undesirable components in the raffinate over a complete cycle. The definition is given as Equation (9).3) Additionally, the feed rate was the average hourly volumetric flow rate of Natural Gas to a single bed in a complete cycle at Standard Temperature and Pressure conditions. It was estimated from the total moles of Natural Gas fed in a cycle to a bed and the cycles completed in an hour. The mass balance equations in the form of differential algebraic equation (DAE) are discretized using finite difference technique.
This converts the model equations into a set of simultaneous algebraic equations, which relate all the variables at an advanced time step to their values at the immediately previous time interval (initial conditions). Pressure and flow velocity, bulk and adsorbed phase concentrations for all the components at all grid points along the bed become the algebraic variables. For each step of the cycle, the boundary conditions as above are then specifications of relevant variables at the bed ends. After discretization and applying suitable boundary conditions, the number of variables becomes equal to the number of equations. The number of such variables in each step of the cycle depend upon the number of divisions the bed is discretized into, and the number of components as given in Table 5.Then the variables were normalized and non-dimensionalized. An upwind implicit scheme is used for the spatial discretization and a backward difference scheme is used for the temporal discretization. Discretization results in a system of simultaneous non-linear algebraic equations linking the variable values at an advanced time increment to the values at immediately previous time increment. Convergent temporal and spatial grid sizes were found by successively reducing them and simulating the cyclic steady state till differences in the concentration profiles vanished signifying convergence.
Initial guess for each unknown variable, maximum number of iterations, step size for Jacobian evaluation and tolerance have to be provided by the user.
Powell’s method is a combination of Newton’s method and the steepest descent method and offers improved convergence as compared to Newton’s method. As we march from one time step to another, the known conditions at the current time are provided as the initial guess for the respective unknowns of the advanced time step. These values are used as the initial guess for the unknowns of the next time step and so on.
The system of non-linear algebraic equations is converted to linear ones by estimating the Jacobian based on a finite difference approximation. The inverse of the Jacobian is calculated and used for giving the improved solution.At start-up, the bed is assumed to be at the pressure of extract tank and contains pure CH4 (most weakly adsorbed component) in both the phases.
For simulation with component lumping, the bed is assumed to be filled with the pseudo-component having the lowest adsorption equilibrium constant. The adsorbed phase and the bulk phase are assumed to be in adsorption equilibrium at this pressure. After the first cycle, the initial conditions of the bed are the conditions of the bed at the end of the last step of the first cycle. Similarly, the initial conditions for any step are the conditions at the end of the previous step in the cycle. Each step is uniquely described by a boundary condition on velocity and the inlet fluid stream composition.

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