Improved separation and utilization of light naphtha stock by adsorption process

Abstract

An integrated process for separation and utilization of light naphtha stock in refineries is discussed in this paper. Normal paraffins present in light naphtha streams are first separated from nonnormal paraffins by adsorption technology. The adsorbed n-paraffins are recovered and can be used as an ideal feedstock for steam cracking, meanwhile iso-pentane or iso-pentane and iso-hexane blends are recovered by rectification of the nonadsorbed effluent and used as necessary components for modern gasoline or aviation gasoline products. From the results for a model feedstock, an isothermal adsorption and purging desorption approach is selected. Optimum parameters consist of adsorption and desorption duration of 30 min at 180℃ for each, nitrogen stream LHSV of 240 h⁻¹ in the desorption step, the dynamic capacity of the adsorbent reaches 0.042 g/g.

Introduction

Global production of hydrocracking and reforming units in petroleum refineries has reached 290 and 505 Mt/a, respectively, in 2014. Light naphtha is often separated as one of the products, reaching 10–20 V% in a hydrocracking process. It is known that the main products in a reforming unit are aromatics, wherein the carbon number of the stream often keeps unchanged. As shown in Table 1 (Han, 2001), light naphtha contains C₅ hydrocarbons, which cannot convert into aromatic compounds and the blending octane number of the aromatics formed is even lower than that of original C₆ cyclo-alkanes (Xu, 2006). Therefore, the initial distillation temperature of a reformer feedstock is limited to 80℃ for gasoline and 65℃ for BTX production. The topped light naphtha, which is lighter than these two temperatures, is removed by distillation before the feedstock is sent into the reformer. Obviously, it is actually from crude oil and some other units and accounts for 20% of the reforming feedstock. Up to now, all of these light naphtha stocks are mainly blended into gasoline pool to improve the evaporation characteristics. Clearly, it exceeds what is needed in this way. Moreover, the other challenge is the new specification requirement, imposed upon modern gasoline under lead phased off. Typical composition of light naphtha from hydrocracking and that topped from reforming are listed in Table 1. A detailed gasoline composition-based octane model has been published in a recent paper (Ghosh et al., 2006). From their work, the research octane number (RON) of each light naphtha is calculated to be 77.5 and 73.5, respectively, which are often higher than that naphtha from crude oil. It is clear these unleaded gasoline blends would not have a sufficient “front end octane number” to provide good cold start-up for the motor in winter time. Therefore, light naphtha stock especially that from crude oil often needs to be hydro-isomerized to convert their containing n-paraffins into iso-paraffins. This process is performed under hydrogen pressure in expensive equipment (Speight and Ozum, 2002). An integrated process as an alternative is highlighted in this paper. This approach aimed to prepare a nonadsorbed effluent with a satisfactory RON to meet the challenge discussed above by adsorption separation. In addition, it can also be used to prepare iso-pentane or blended iso-pentane and iso-hexane fractions by rectification for aviation gasoline components. Meanwhile, the adsorbed n-paraffin stream is recovered and used as an ideal feedstock blend for another processes, including steam cracking.

Table 1.

Compositions and properties of light naphtha from hydrocracking and reforming processes

Composition (V%)	Reforming topped light naphtha	Hydrocracking light naphtha	Boiling point (℃)	Octane number
				RON	MON	BRON
n-butane	9.36	2.8	−0.50	93.6	93.1	–
2-methyl butane	15.4	26.5	28	92.3	90.3	–
n-pentane	15.7	12.0	36	61.7	61.9	–
2,2-dimethyl butane	9.0	12.0	50	91.8	93.4	–
2-methyl pentane	12.8	5.4	60	73.4	73.5	–
3-methyl pentane	7.2	22.5	63	74.5	74.3	–
n-hexane	13.2	11.5	69	24.8	26.0	–
3-methyl hexane	3.36	–	90.1	42.4	46.4	–
n-heptane	2.88	–	98.4	0	0	–
2,2-dimethyl pentane	–	10.8	79.6	93	96	89
Methyl-cyclopentane	6.2	6.5	71.8	91	80	107
Cyclohexane	8.6	0.9	80.8	83	77	110
1,2-Dimethyl-cyclopentane	2.8	–	99.4	95	–	95
Methyl-cyclohexane	1.4	–	100.8	104	71	104
Toluene	2.9	–	110.5	115	104	124

MON: motor octane number; RON: research octane number; BRON: blend research octane number.

Experimental

Zeolite adsorbent and reagents

5A zeolite pellet with a diameter of 3 mm is purchased from UOP Inc. (Shanghai, China). It was pulverized and sieved to particles, ranging 0.2–0.8 mm. Reagents, including pentane, hexane, and cyclohexane are all analytical reagents and are produced by Shanghai Lin Feng Chemical Inc. To simulate pentane and hexane concentrations in the typical light naphtha stocks from hydrocracking and reforming units as shown in Table 1, a solution, which is composed of pentane, hexane, and cyclohexane at a ratio of 1:1:8, respectively, is initially used as a model compound in this work. Our main model sample is then composed of hexane and cyclohexane at a ratio of 2:8 for the reason given later.

Apparatus and operation for adsorption separation

A self-made adsorption bed system is operated at liquid state (Cao and Shen, 2013). Adsorption apparatus has been described in detail elsewhere (Zhou et al., 2016).

Pulverized and sieved 5A zeolite samples are activated at 723 K for 6 h and taken out at hot state and then it is transmitted to a vacuum dryer to be cooled to room temperature for use. Adsorption tests begin, when a first drop of reagent appears at the exit of the tube. Since then, liquid sample is taken at the exit with a plastic tube for analysis by using GC after every fixed time, until the concentration of the adsorbate in sampling tube has reached that of the model feedstock (saturated state). Then switch off the pump and turn this system to a desorption step. From this time, a nitrogen stream at a fixed flow rate is sent to desorb n-parrafins for a fixed time. It is separated from the stream in a liquid nitrogen trap and weighed with an electric balance. Composition of each liquid solution sample is analyzed by GC set equipped with a Φ0.25 mm×100 m SE-30 capillary column (dimethyl polysiloxane) and a FID detector. The degree of removal thus can be measured.

Adsorption breakthrough curves are plotted from the exit concentrations at varying time on stream. The adsorbed amount of an adsorbent at some time on stream is calculated by

$$\begin{array}{c}\multicolumn{1}{c}{q={\int }_0^{t}Q(C_0-C_t)\text{d}t=Q{C}_{0}t-Q{\int }_0^t{C}_t\text{d}t}\end{array}$$

(1)

where C₀: is the adsorbate composition at the entrance (%);

C_t: Adsorbate concentration at exit at the time of t (%);

Q: Feed inlet flow rate ml/min;

Results and discussion

From our preliminary work, isothermal liquid phase adsorption and purging nitrogen desorption cycles are suggested to decrease energy consumption. Related kinetics and equilibrium for C₅ and C₆ adsorption at liquid state have been studied in the previous literature (Silva and Rodrigues, 1997a, 1997b; Sun and Shen, 2009). In studies on adsorption stage, tests are first performed at 120℃ with a space velocity in volume (LHSV) of the model feedstock of 1.25 h⁻¹. The pressure was enough to keep the model compounds in liquid phase. It has been recognized that these parameters should be later adjusted from both adsorption and desorption behaviors to search the optimal operating parameters.

Competitive adsorption between pentane and hexane

To investigate the competitive adsorption behaviors of pentane and hexane in model compounds in this work, breakthrough curves for the model feedstock are plotted in Figure 1. This adsorption test is performed at 120℃ with a space velocity in volume of 1.25 h⁻¹. It is seen that the breakthrough curves are like S-type, demonstrating the good adsorption ability of 5A zeolite adsorbent for n-paraffin. Moreover, two curves, for pentane and hexane, respectively, in the model feedstock are very near each other, although the former breakthroughs a little bit earlier. The similarity between the adsorption separation characteristics of both pentane and hexane in the model compound demonstrates that it is enough to examine their total changes in concentration of the solution to examine the adsorption characteristic in the adsorption step. Therefore, a solution with n-hexane and cyclohexane ratio of 2:8 is used as a main model compound later on.

Figure 1.

Breakthrough curves of n-paraffins in model feedstock.

Effect of the adsorbate concentration in model samples on the adsorption behaviors

The breakthrough curves for varying adsorbate concentration in the model solutions at 120℃ are plotted in Figure 2. It is seen that the duration is shorter for the solution with higher paraffin concentration, because of higher paraffin concentration leads to faster saturation of adsorption bed.

Figure 2.

Adsorption breakthrough curves at different n-hexane concentrations in the model feedstock.

Effect of adsorption temperature on adsorption behaviors

Adsorption tests are here performed at the temperatures, ranging 120–200°, when the other parameters keep unchanged. Adsorption breakthrough curves on 5A zeolite adsorbent are shown in Figure 3. It is seen that all these curves are rather steep, resulting from the good adsorption behaviors of 5A zeolite adsorbent. In addition, the breakthrough appears slightly earlier at the increasing adsorption temperature. It is clear that they are a little decreased at higher temperature. The adsorption capacity at breakthrough is 0.072 g/g at 120℃, while it lowered to 0.055 g/g at 200℃.

Figure 3.

The adsorption breakthrough curves of model oil on 5A molecular sieves at different temperatures.

Effect of LHSV on adsorption behaviors

Changes in hexane concentration at exit with oil/sieve at varying feed space velocity are plotted in Figure 4. It is seen that they almost overlap for each LHSV, ranging from 0.5 to 2.5 h⁻¹, demonstrating the time necessary to reach breakthrough must be proportionally shorter at higher LHSV of the model solution.

Figure 4.

The effect of model oil feed flow rate on hexane exit concentration.

Studies on desorption

It is known that adsorption and desorption are performed in turn in a cycle. The optimal option and related operating parameters for desorption step plays an important role in the entire adsorption process. To investigate the effect of temperature on desorption behaviors, the bed is adsorbed to reach equilibrium at first, then the bed is desorbed with a nitrogen stream purging at atmospheric pressure for 30 min under a space velocity of 180 h⁻¹. The results are shown in Figure 5. It is indicated that the extent or degree of desorption raised with increasing desorption temperature, while this increase slows down from 180℃, resulting in incomplete desorption that would give negative impact to the succeeding adsorption.

Figure 5.

The extent of desorption of model oil at different desorption temperatures.

After adsorption has reached to an equilibrium, 5A zeolite bed is purged with a nitrogen stream for 30 min with varying space velocity for desorption at 180℃. Changes in desorbed n-hexane amount at varying nitrogen space velocity are plotted in Figure 6. It is seen that the former increases from 0.512 to 0.792 g, when the nitrogen space velocity changes from 120 to 300 h⁻¹, while it has reached to 0.769 g at nitrogen space velocity of 240 h⁻¹. These results reveal that it is enough to select the optimal nitrogen space velocity to be 240 h⁻¹, which would be advantageous to energy saving.

Figure 6.

The desorbed degree of model feedstock at different space velocities of nitrogen.

Effect of the duration of desorption on the degree of n-paraffin removal in percentage is investigated at 180℃, after the adsorption step has reached to an equilibrium. As seen in Figure 7, the extent of removal rapidly raises initially, but this increase significantly slows down at longer desorption time. From a duration of 30–60 min, the extent of removal changes from 69.84 to 76.81%, only an increase of 6.97%. In addition, it is advantageous to select a similar duration for both adsorption and desorption steps. Therefore, duration of desorption is selected to be 30 min.

Figure 7.

The effect of desorption time on the desorption extent of n-hexane.

Adsorption/desorption cycle experiment

Adsorption/desorption cycle experiment is performed under the conditions of 1.25 h⁻¹ LHSV for the model oil feed and that of 240 h⁻¹ for the purging nitrogen stream through a duration of 30 min for each adsorption or desorption step, respectively. A representative example is seen in Figure 8, wherein four cycles are performed at 150℃. It is seen the next three adsorption breakthrough curves after the first adsorption/desorption cycle almost overlap each other. Therefore, the dynamic capacity of the adsorbent can be deduced from the next overlapped curves. The results are shown in Table 2. As shown in that table, the dynamic capacity rises up with increasing temperature initially, reaches a maximum of 0.042 g/g at 180℃ and drops down afterward at 200℃. Therefore, 180℃ is an optimum temperature in our work.

Figure 8.

Results of adsorption/desorption cycle experiment.

Table 2.

The dynamic capacity of the adsorbent at different operating temperatures

Operating temperature (℃)	Dynamic capacity (g/g)
120	0.023
150	0.032
180	0.042
200	0.04

From the work described above, the optimum conditions duration of either adsorption or desorption step is selected to be 30 min under an isothermal temperature of 180℃. The LHSV of nitrogen stream for purging is 240 h⁻¹. Under these conditions, the dynamic capacity of the adsorbent is able to reach 0.042 g/g. Of course, there must be two subsidiary steps, including adsorption fill and void space purging from the issue, reported in the patent literature (Holcombe, 1980).

Product recovery and utilization

Gasoline component with enhanced RON rating

Hydrocracking is performed under high hydrogen pressure and a reforming unit often contains a prehydrogenation step, therefore, the unadsorbed effluent from each of the light naphtha of these two processes is mainly composed of saturated isomeric hydrocarbons without sulfur-containing compounds. Therefore, it can be typically used as a clean gasoline stock with satisfactory RON rating. By calculation (Ghosh et al., 2006) from the data listed in Table 1 for the compositions of the typical light naphtha stock without n-paraffin, RON value has boosted to 85.9, which has matched to that of C₅/C₆ hydro-isomerization products. These results demonstrate an easy option to improve the front-end RON of gasoline products as an alternative to C₅/C₆ hydroisomerization process. The latter is not economical owing to hydrogen pressure and complicated equipment.

Recovery of n-paraffins to provide an ideal steam cracking feedstock blend

Steam cracking is widely used to produce basic petrochemicals, including ethylene, propylene, and BTX. Naphtha stock is one of the most important feedstock, especially in Europe and Japan. It is reported that ethylene yields increase about 16% using n-paraffin as a feed (Liu and Shen, 2006). Therefore, the adsorbed n-paraffin can be recovered and blended to steam cracking feedstock.

Recovery and utilization of pure iso-pentane from this integrated process as one of necessary components for aviation gasoline blend

Aviation gasoline is the energy source for aircraft spark-ignition piston engines. It is still in use and new fields for utilization have been recognized in the latest decade. Specifications, imposed upon aviation gasoline products, are extremely rigorous, including motor octane number (MON) level, ASTM distillation profiles, RVP control, etc. Iso-pentane is widely used to play an important role in ASTM distillation control (Ghosh et al., 2006). As reported in the patent literature, aviation gasolines products are made up of a blend with about 15% by volume of iso-pentane (Braly, 2014). The latter is traditionally provided by C₅/C₆ hydroisomerization unit. This is also a high energy consuming rectification process, resulting from the very close volatilities between n-pentane and iso-pentane because of only 8℃ boiling point gap (Foley, 1991). As an alternative from economical consideration, an integrated process by adsorptive separation and rectification is suggested in this work. The nondesorbed effluent from a 5A zeolite adsorber as described in “Results and discussion” section is distilled for this purpose. As seen in Table 1, the nearest two components are iso-pentane (boiling point of 28℃) and 2,2-dimethyl butane (boiling point of 50℃) with a difference of 22℃. It is clear that the rectification expense must even be lower than the result in the published paper for the separation of C5 fraction, wherein the two key composition are n-pentane (boiling point of 36℃) and 2,2-dimethyl butane with a boiling point difference of 14℃ (Shen et al., 2011). Additionally, the products are only pure n-pentane and n-hexane chemicals.

In fact, it is not necessary to prepare pure iso-pentane, because the nearest heavier component is 2,2-dimethyl butane, which has rather high MON ratings (93.4). Therefore, there is a patent literature that selected iso-pentane and iso-hexane mixture for distillation range control of aviation gasoline (Schoppe and Laughlin, 2002). It must be a more economical and effective rectification option to leave some amount of 2,2-dimethyl butane in the iso-pentane fraction for this purpose. Comparison made to these rectification approaches will be given in detail elsewhere soon later.

Because of the nature of hydrocracking catalysts, iso-pentane yield is more than two times higher compared to that of n-pentane in the light naphtha stock as seen in Table 1. It is clear that the nonadsorbed effluent from hydrocracking light naphtha is preferably selected in this integrated process to produce iso-pentane or iso-pentane and dimethyl butane mixture.

Conclusion

An integrated process for separation and utilization of light naphtha stock is proposed in this paper. Light naphtha is separated to n-alkane and iso-alkane by adsorption technology. The adsorbed n-paraffin stream is used as an ideal feedstock for steam cracking, while iso-pentane or iso-pentane and iso-hexane fractions, recovered by succeeding rectification of the nonadsorbed effluent are used as motor gasoline or aviation gasoline good components.

The effect of operating parameters on adsorption and desorption behaviors is investigated. The optimum isothermal operation temperature is 180℃. Duration of either adsorption or desorption step is selected to be 30 min. The purging nitrogen space velocity is 240 h⁻¹. Under these conditions, the dynamic capacity of the adsorbent reaches 0.042 g/g. All the results described above would be useful to pave the way for the application of this integrated process.