Article pubs.acs.org/IECR
Multicycle Investigation of Normal Paraffin Separation from Naphtha To Improve Olefin and Aromatic Feed Jichang Liu,* Xiang Chen, Shimin Zhao, Xin Cao, and Benxian Shen State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *
ABSTRACT: A programmed control double-column adsorber was built to investigate the steady operation of the normal paraffin separation from naphtha. The naphtha from Shanghai Gaoqiao Petrochemical Company (SGPC) was separated into a raffinate oil rich in non-normal hydrocarbons and a desorption oil rich in normal paraffins, using zeolite 5A as an adsorbent. In this paper, we investigate the effects of the adsorption/desorption temperature, feed space velocity, desorption gas space velocity, and switch time on the steady separation of normal paraffins. The adsorbability observed from the breakthrough curves of normal paraffins is consistent with the adsorption energies. The optimal conditions for the adsorption/desorption process were an operation temperature of 270 °C, a naphtha feed space velocity of 90.1 h−1, a switch time of 30 min, a desorption gas N2 space velocity of 127.5 h−1, and an intermediate oil cutting time of 2 min. With the Cutting Intermediate Oil (CIO) process, the normal paraffin content in raffinate oil was 3.42% and the potential aromatic content was 12.14% higher than that of naphtha. The normal paraffin content of desorption oil was 96.61%, and the ethylene yield was 15.08% higher than that of naphtha.
1. INTRODUCTION Naphtha, which is the lightest fraction of crude oil, serves as one of the feedstocks of the steam cracking process, to produce ethylene, and the catalytic reforming process, to produce aromatics or upgrade to a hydrocarbon composition with high octane number. The full range naphtha is used as cracker feed in many countries without a sufficient naphtha supply. However, the components in naphtha (i.e., normal paraffins, iso-paraffins, cyclanes, and aromatics) have distinct performances during the reactions. Normal paraffins are the best cracking feeds with high ethylene yield, while cyclanes are highquality reactants for aromatics. Separation of different group compositions in naphtha through the adsorption process using zeolites 5A has attracted considerable interest from researchers.1,2 Zeolite 5A compounds have LTA (Linde Type A) lattice structures with Ca cations.3 The uniform three-dimensional (3D) channels with a diameter of 0.51 nm, which is between the diameters of linear alkanes and branched alkanes,4,5 provide high shape selectivity in the separation of hydrocarbons. The adsorption equilibrium6−8 and kinetics9,10 of normal paraffins using zeolite 5A compounds have attracted much research interest. C5−C6 range normal paraffin separation, coupled with the isomerization process, was used in the production of high-octane gasoline.11,12 In 2002, UOP developed a simulated moving bed technology,13−15 using zeolite 5A compounds as the adsorbent and n-pentane as the desorbent to separate the normal paraffin from naphtha. East China University of Science and Technology developed a fixedbed adsorption process, known as Molecular Sieve Fixed-bed Adsorption (MSFA) technology using zeolite 5A compounds as the adsorbent and nitrogen as the desorbent.16,17 However, all of the studies on the normal paraffin separation only focus on the adsorption capacity of fresh zeolite 5A compounds, which varies after the adsorption/desorption cycles. © XXXX American Chemical Society
An adsorption process must be followed by a desorption process to recover the adsorption capacity of the zeolites and obtain the desorption oil simultaneously. The performance of adsorption separation is dramatically influenced by adsorption and desorption conditions, which interact with each other and provide feedback after multiple cycles. The adsorption/ desorption cycles cannot reach their steady state until several adsorption/desorption cycles have been carried out, because the fresh zeolites are more energetic than the zeolites that are used. The switch between adsorption and desorption processes by manual operation causes notable random errors, because of the disaccord between cycles. To investigate the steady operation of the normal paraffin separation from naphtha, a double-column fixed-bed absorber, which was conducted by 10 programmed control electromagnetic valves, was built and operated. Different desorbents have very different desorption performances, especially after adsorption/desorption cycles. The advantage of nitrogen desorbent is that the desorbent can be separated with the normal paraffins in a simple gas−liquid separator after being cooled and condensed in the desorption process. While for the hydrocarbon desorbent, an additional distillation tower is necessary for the separation of the mixture of the desorbent and the normal paraffins. The total separation cost for the technology in this work is relatively low at the similar recovery efficiency. In addition, the purity of normal paraffins can be improved to >98% in our study, by using the cutting intermediate oil process to void the contamination of feed holdup in the adsorbent bed. This work revealed the Received: August 11, 2015 Revised: November 30, 2015 Accepted: November 30, 2015
A
DOI: 10.1021/acs.iecr.5b02955 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research interactions of C4−C10 normal paraffins during the adsorption separation, using nitrogen gas as a desorbent after multicycles.
naphtha passes by the zeolite bed after being vaporized, normal paraffins in naphtha are adsorbed in the pores of the zeolite 5A compounds. A raffinate oil that is rich in non-normal hydrocarbons is discharged. Using N2 as the desorption gas, the desorption process is countercurrently conducted in each bed. The desorption product is separated into the gas phase and a desorption oil that is rich in normal paraffins in a gas-oil separator. The desorption gas is subsequently recycled. The intermediate oil, which is the holdup of the feed naphtha, is cut at the beginning of the desorption process to obtain desorption oil with high purity. The switch time of the adsorption/ desorption process and the cutting time of intermediate oil are set in the control program. The procedure for running the cyclic process with an operation period of 60 min and an intermediate oil cutting time of 3 min is shown in Table 2. 2.3. Steam Cracking Process. The desorption oil, which was rich in normal hydrocarbons, was used as the feedstock for the steam cracker, to produce ethylene. The flowchart of the steam cracking equipment is shown in Figure 2. The cracking tube is 600 mm in length, with an inner diameter of 6 mm. The cracking feed is vaporized and mixed with the diluting steam. After being heated in the preheater, the hydrocarbons in the oil/steam mixture are subsequently cracked into small molecules by heating in the furnace tube. The cracking products are quenched and separated into corresponding gaseous and liquid phases.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. The naphtha used was from the atmospheric tower of Shanghai Gaoqiao Petrochemical Company (SGPC), Sinopec with a boiling range of 33−174 °C. The n-paraffins, iso-paraffins, olefins, naphthalenes, and aromatics (PIONA) composition and the normal paraffins in SGPC naphtha are presented in Table 1 and Table S1 in the Supporting Information, respectively. Table 1. n-Paraffins, Iso-paraffins, Olefins, Naphthalenes, and Aromatics (PIONA) Composition of Shanghai Gaoqiao Petrochemical Company (SGPC) Naphtha hydrocarbon
content (%)
n-paraffins iso-paraffins olefins cyclanes aromatics
30.94 29.54 0.00 30.91 8.61
total
100
The adsorbent zeolite 5A compounds with diameters of 2−3 mm were obtained from Shanghai UOP Company (China). The properties of zeolite 5A are shown in Table S2 in the Supporting Information. The desorbent nitrogen gas (99%) was supplied by Shanghai No. 5 Steel Company. 2.2. Double-Column Fixed-Bed Adsorber. The flowchart of the programmed-control double-column fixed-bed adsorption process is shown in Figure 1. Two parallel columns are packed with 4.0 kg of zeolite 5A compounds as the absorbent. Ten programmed-control electromagnetic valves switch the adsorption/desorption process, including the intermediate oil cutting process, automatically. When the feed
3. RESULTS AND DISCUSSION 3.1. Breakthrough Curves of C4−C10 Normal Paraffins in the Fixed-Bed Adsorber. The hydrocarbons in naphtha consist of normal paraffins, iso-paraffins, cyclanes, and aromatics. Different groups of hydrocarbons can be separated by the zeolites via shape selective adsorption. Normal paraffins with straight long chains have smaller footprints than those of iso-paraffins, cyclanes, and aromatics. The primary channels of zeolites A are 8-membered rings. When the cations are exchanged by Ca2+, the channel diameter is ∼5.1 Å. According
Figure 1. Flowchart of the double-column fixed-bed adsorption/desorption process for naphtha. Legend: 1, naphtha tank; 2, naphtha pump; 3, naphtha vaporizer; 4, relief valve; 5, electromagnetic valve; 6, intermediate oil condenser; 7, desorption oil condenser; 8, #1 raffinate oil condenser; 9, #2 raffinate oil condenser; 10, intermediate gas-oil separator; 11, desorption gas-oil separator; 12, fixed bed; 13, desorption gas preheater; and 14, N2 cylinder. B
DOI: 10.1021/acs.iecr.5b02955 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 2. Procedure for Running the Cyclic Process time
valves ON
valves OFF
bed #1
bed #2
0:00−0:03 0:03−0:30 0:30−0:33 0:33−0:60
b, d, g, i d, f, g, i a, e, h, j c, e, h, j
a, c, e, f, h, j a, b, c, e, h, j b, c, d, f, g, i b, c, d, f, g, i
adsorption adsorption intermediate oil cutting desorption
intermediate oil cutting desorption adsorption adsorption
Figure 2. Flowchart of the steam cracking equipment. Legend: 1, feed tank; 2, water tank; 3, feed pump; 4, water pump; 5, oil vaporizer; 6, water vaporizer; 7, preheater; 8, cracking furnace; 9, quench cooler; 10, gas/liquid separator; 11, gas mass flowmeter; and 12, tail oil tank.
to the quantum chemistry calculation, the diameters of normal paraffins are in the range of 4.8−5.0 Å, while the diameters are in the range of 6.1−6.6 Å for iso-paraffins and 6.7−7.4 Å for cyclanes and aromatics.18 The SGPC naphtha with 30.94% of normal paraffins was fed to the zeolite bed after being vaporized. The normal paraffins were adsorbed in the micropores of the zeolite 5A compounds. From the determination of the raffinate gasoline, the breakthrough curves were depicted. Figure 3 shows the S-shaped
Figure 4. Adsorption curves of the individual n-paraffins in naphtha. Adsorption temperature: 300 °C.
paraffins. When the normal paraffins enter the micropores, each CH3− or −CH2− group engenders a certain force with the wall of the zeolite 5A micropore. The adsorptivity is related to the resultant force, as well as the number of the CH3− or −CH2− groups. The normal paraffins with low carbon numbers occupying the micropores of zeolite 5A compound can be partly replaced by normal paraffins with larger carbon numbers. The roll-up phenomenon19 can be observed in the adsorption breakthrough curves of the low-carbon-number normal paraffins. The normal paraffin content in a certain segment of raffinate oil was higher than that in the feed naphtha, because of the displacement effect. To obtain raffinate oil with low normal paraffin content, the adsorption process should be switched to the desorption process when n-butane breaks through the zeolite bed. For the normal paraffin separation from naphtha, the normal paraffin content in the last drop of raffinate oil in the adsorption process is the decisive parameter for the switch time. The threshold value of the normal paraffin content in the last drop raffinate oil is 5%, to ensure that the normal paraffin content in the total raffinate oil is
