Cold-model investigation of effects of operating parameters and overflow outlet diameter on separation with a liquid–liquid cyclone reactor for isobutane alkylation catalyzed by ionic liquid

https://doi.org/10.1016/j.cherd.2018.07.037Get rights and content

Highlights

  • A liquid–liquid cyclone reactor (LLCR) for ionic liquid catalyzed isobutene alkylation (ILA).

  • The integration of reaction and separation process in the novel LLCR.

  • The construct models predicting LLCR performance were established.

  • A empirical adjustment range of optimal overflow ratio was proposed (1.02–1.14).

Abstract

A liquid–liquid cyclone reactor (LLCR) was designed to enhance mixing and accelerate separation between reaction products and catalysts during isobutane alkylation catalyzed by ionic liquid. The LLCR was designed on the basis of the axial-flow hydrocyclone. The effect of total inlet flow (Qt), feed ratio (rf), overflow ratio (ro) and overflow outlet diameter (Do) on the performance of LLCR (pressure drop, ΔP, and recovery, R) was investigated. The LLCR’s performance indices underflow pressure drop (ΔPu), overflow pressure drop, (ΔPo), glycerin water solution recovery (Rg), kerosene recovery (Rk), reduced efficiency were determined. The results were used to establish an empirical models predicting LLCR efficiencies from total inlet flow, feed ratio, overflow ratio and overflow outlet diameter. Besides, an optimal overflow ratio, ror, was proposed to realize the optimization of two-phase recovery, which is linear with the expression of feed ratio. The results show that the range of the empirical coefficients (k) in the expression of ror is 1.02–1.14.

Introduction

Alkylation of isobutane with light olefins is widely used in the petroleum industry to produce high-quality gasoline. Alkylate is characterized by a high octane number, low vapor pressure, and low contents of sulfur, olefins, and aromatics, therefore, it is an ideal component of high-octane gasoline. Concentrated sulfuric acid (H2SO4) and hydrofluoric acid (HF) are commercial liquid catalysts for isobutane alkylation, but have disadvantages such as severe corrosion, high operation costs, and safety, environmental, and disposal issues. Yoo et al. (2004) and Zhang et al. (2007) showed that acidic ionic liquids (ILs) are promising substitutes for H2SO4 and HF as alkylation catalysts because of their safer operation, low consumption, and strong catalytic performance. A composite ionic liquid developed by the China University of Petroleum (Beijing) shows high selectivity for high octane alkylate and is considered an ideal substitute for traditional alkylation catalysts from safety and environmental points of view (Qi et al., 2015).

Liquid acid catalyzed isobutane alkylation is a heterogeneous system wherein reactions occur in or near the interface between the catalyst and hydrocarbon phases (Albright and Eckert, 2001). The intrinsic reaction rate is extremely fast owing to the highly reactive carbonium intermediate (Schmerling, 2002). Too long a residence time of the ionic liquid–hydrocarbon mixture will lead to side reactions, such as isomerization, disproportionation, hydrogen transfer and cracking, which have detrimental effects on alkylate quality. Thus, it is necessary to study and develop a reactor for ionic liquid catalyzed isobutane alkylation (ILA). Liu et al. (2006) used a Stratco reactor and Zhou et al. (2008) proposed a novel liquid–solid loop reactor for ILA. However, the ideal time to separate alkylate from the catalyst is unclear, and the reactors suffer from occurrence of side reactions and low product yield.

To solve these issues, we developed a liquid–liquid cyclone reactor (LLCR) for ILA. The design of the LLCR was based on an axial-flow hydrocyclone developed by Wang et al., 2011a, Wang et al., 2011b. Ma et al. (2011) and Zhang et al. (2013) showed that a short-contact cyclone reactor can potentially accelerate gas–solid reactions during the fluid catalytic cracking process (FCC). The basic structure of the LLCR is shown in Fig. 1. The reaction and product separation occur in the same reactor to accelerate separation between the product and catalyst, and thereby reduce the occurrence of side reactions. The main components of the LLCR are the reaction chamber and separation chamber, and there are two different inlets for the two phases. The C4 hydrocarbon mixture is injected through symmetric tangential slots, while the ionic liquid flows axially from inlet 1 into the LLCR. A guided vane of four runners (50 mm length, 11° outlet angle) lies below the ionic liquid inlet. After entering the LLCR, the ionic liquid gradually flows along the guided vane, a strong swirling motion is developed within the LLCR, which forms a layer of ionic liquid near the wall surface. After entering the LLCR through the tangential slots, the centrifugal force causes the C4 hydrocarbon mixture to move to the axis that promotes mixing between the two phases (Narasimaha et al., 2005). Finally, the alkylate flows away via the overflow outlet, meanwhile, the ionic liquid flows away via the underflow outlet. Remarkably, the rotation direction of the guided vane is consistent with the orientation of the tangential slots. The LLCR has several advantages such as integrated mixing, reaction and separation processes, compact geometry, easy maintenance, intensive mixing, and high separation efficiency.

However, as the LLCR was only recently developed, the reaction experiment has not yet been carried out. Therefore, we want to use the concentration distribution, residence time distribution (RTD) and droplet size distribution (DSD) of the dispersed phase, as well as the recoveries of two phases in the cold-model experiment system to evaluate the mixing and separation performance of LLCR. The studies on the concentration distribution and RTD of dispersed phase have been completed and related papers are published (Zhang et al., 2017a, Zhang et al., 2017b, Zhu et al., 2017). Eventually, we hope to obtain a suitable range of operational and structural parameters for the reacting system through the cold-model experiments.

The specific objectives of this research were (1) to study the effect of operational (total inlet flux, feed ratio and overflow ratio) and structural parameters on the pressure drop and separation efficiency of LLCR and (2) to develop the empirical equation for predicting the LLCR efficiency and performance.

Section snippets

Rationality of cold model experiment

The main reaction equation of ILA is shown in Fig. 2. In the LLCR, the mixture of isobutane and butene is injected into the reactor as a single phase, while the ionic liquid as the other phase. Eventually, the alkylate flows away from the overflow outlet and ionic liquid flows away from the underflow outlet for re-utilizing. Consequently, the phase holdup distribution and droplet size distribution of the disperse phase can be used to evaluate the mixing performance in the LLCR through the cold

Effect of Qt, rf, ro and Do on ΔPu and ΔPo

Pressure drop is one of the important performance parameters of LLCR (Rietema, 1961, Huang et al., 2013, Li et al., 2014). The pressure drop consists of two parts: overflow pressure drop, ΔPu, and underflow pressure drop, ΔPo.

Equations for pressure drop, ΔPu and ΔPo could be generally defined as follows,ΔPo=PidPoΔPu=PicPuwhere Pid and Pic is the inlet pressure of disperse phase and continuous phase, Po and Pu is the outlet pressure of overflow and underflow, respectively.

Fig. 4 presents the

Conclusions

The effect of total inlet flux (Qt), feed ratio (rf), overflow ratio (ro) and overflow outlet diameter (Do) on the performance of LLCR (underflow pressure drop, ΔPu, overflow pressure drop, ΔPo, recovery of continuous phase, Rc, and recovery of disperse phase, Rd) was investigated. The results show that a higher total inlet flux resulted in a higher ΔPu and ΔPo due to the increase of inlet velocity. Besides, the increase of the overflow ratio shows opposite effect on the ΔPu and ΔPo,

Acknowledgments

The authors gratefully acknowledge support from the National Natural Science Foundation of China: No. 2020619, The National Science Fund for Distinguished Young Scholars: No. 21425626, China Postdoctoral Science Foundation: No. 2016M592266, The Fundamental Research Funds for the Central Universities: No. 17CX02016A, The Natural Foundation of Shandong Province: No. ZR2018BB068 and the Innovation Project for Postgraduate of UPC: No YCX2017029.

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