Commercial Applications of Paraxylene Adsorbents RAX-2000A and RAX-3000

Li Ben; Wang Huiguo; Yu Zhuo; Wang Dehua; Wu Wei

(Research Institute of Petroleum Processing, Sinopec, Beijing 100083)

Commercial Applications of Paraxylene Adsorbents RAX-2000A and RAX-3000

Li Ben; Wang Huiguo; Yu Zhuo; Wang Dehua; Wu Wei

(Research Institute of Petroleum Processing, Sinopec, Beijing 100083)

The RAX series paraxylene (PX) adsorbents RAX-2000A and RAX-3000, are developed by the Sinopec Research Institute of Petroleum Processing (RIPP) and manufactured by the Sinopec Catalyst Company. Performance test of RAX-2000A showed that the average purity of the PX product reached 99.81%, with an average PX yield of 98.6% per pass. The new generation of PX adsorbents, RAX-3000 not only retains most advantages of the adsorbent RAX-2000A, but also has higher selective adsorption capacity by at least 8%. The actual unit production capability of the adsorbent RAX-3000 was increased by about 18%. The RAX series PX adsorbents exhibited good adaptability to unfavorable feedstock containing high ethyl benzene (EB) fraction besides their better mechanical strength. Preliminary test results indicated that compared to the adsorbent RAX-2000A, the A/Faand D/F relating to the adsorbent RAX-3000 were notably decreased due to the higher selective adsorption capacity of the adsorbent RAX-3000.

paraxylene adsorbent; simulated moving bed; RAX-2000A; RAX-3000; aromatics adsorptive separation

1 Introduction

Paraxylene (PX) is an important basic raw material for polyester fiber industry[1]. Therefore, there is an increasing interest in separating and purifying the xylene isomers. Suspension crystallization has historically been the only technology for the separation and purification of PX from mixed xylene streams, and it could recover PX with any desired purity. Conventional crystallization, however, uses a great deal of energy, and the eutectic of meta-xylene (MX) and PX makes it impossible to obtain high PX yields when a dilute feedstock is treated. Adsorptive separation is far more efficient for processing dilute PX streams than crystallization process. Hence, most of the PX product is produced by adsorptive separation processes nowadays. The commonly applied purification technology for PX is the simulated moving bed (SMB) adsorptive process[2-3]. PX adsorbent is used for the selective adsorption and separation of PX from the mixed C8aromatic isomers and production of high-purity PX product. The PX adsorptive separation process is realized by the continuous countercurrent contact between the fluid and the adsorbent. The kernel of the process is the development and application of high-efficiency PX adsorbent. At the end of 2011, a total of 18 sets of PX adsorption and separation units were in operation in China. The overall PX production capacity exceeded 8.5 Mt/a. The total amount of adsorbent was over 15 kt. With the rapid development of the polyester industry, the demand for PX has been increasing year by year[4].

In 2004, the Research Institute of Petroleum Processing (RIPP) of Sinopec successfully developed a PX adsorbent RAX-2000A[5-6]and manufactured it at the Sinopec Catalyst Company. In October 2004, the adsorbent RAX-2000A was first commercially applied in the Parex unit at a Sinopec’s petrochemical company and achieved success at the first trial of start-up operation. In 2008, RIPP successfully developed another paraxylene adsorbent RAX-3000. This adsorbent RAX-3000, besides retaining most advantages of RAX-2000A, has higher selective adsorption capacity. The adsorbent RAX-3000 has been successfully applied at commercial plants engaging in PX production. Presented in this paper was a preliminary report of adsorbents RAX-2000A and RAX-3000 referring to their tests and applications in commercial scale.

2 Properties of Adsorbents

The RAX series PX adsorbents apply special cation exchanged faujusite as the active component prepared according to a patented process. These PX adsorbents are characteristic of high adsorption capacity, good adaptability to feed and better mechanical strength, which can be applied to any SMB separation processes. The properties of RAX series adsorbents are given in Table 1. The guaranteed product PX purity is ≥99.7% at ≥97% product recovery per pass. The adsorbent products and their preparation methods have been granted a number of invention patents with independent intellectual property rights.

Table 1 Properties of adsorbents RAX-2000A and RAX-3000

3 Commercial Applications of Adsorbents RAX-2000A and RAX-3000

3.1 Adsorbent RAX-2000A

In October 2004, the adsorbent RAX-2000A was applied in the Parex unit at a Sinopec’s petrochemical company using the steam-cracked C8aromatics blended with a small amount of reformer xylene as the feedstock. The average composition of feed is shown in Table 2. The adsorbent RAX-2000A achieved success in the first trial of start-up operation, producing qualified PX products. It can be seen from Table 3 that the average purity of PX reached 99.81% with the product yield reaching 98.6% on an average. The average daily production of PX was 217.5 tons. It should be mentioned that due to the constraints of the design capacity, the feed load could not be further improved, but actually the adsorbent RAX-2000A still had considerable room to cope with an increase in the processing capacity.

Table 2 Average composition of feed used during performance test m%

Table 3 Operating parameters of commercial PX plants

As shown in Table 2, the main feature of feed in this Parex unit was that the mass fraction of ethyl benzene (EB) was higher than PX. It is well-known that among the four isomers of C8aromatics, EB and PX are the most difficult components to be separated. Earlier results showed that, when a feed containing high EB fraction was processed, both the purity and yield of product PX would be decreased. Most of the mixed xylenes at domestic enterprises are originated from the heavy reformate. Therefore, the EB mass fraction is generally in the range of 9%—14%, and the mass ratio of PX to EB is 1.3—2.0. If a EB dealkylation type of C8aromatics isomerization catalyst is adopted, the mass fraction of EB could be further reduced to around 2%, and the mass ratio of PX to EB can reach up to around 13. These feedstocks are relatively easy to be separated. However, the feed in this unit was mainly delivered from the steam-cracked C8aromatics with EB mass fraction amounting up to 18%—24%, and the PX/ EB mass ratio was only 0.7—0.9, which could be regarded as the most difficult feed for recovering pure PX. A fairly good separation performance, however, has been realized under the relatively tough condition of feed C8by employing the adsorbent RAX-2000A which had fairly good adsorption selectivity and wide adaptability to C8feeds.

Pressure drop inevitably develops when fluid streams pass across the adsorbent bed. In order to ensure the stable operation of the system, it is generally required that the chamber pressure drop should be smaller than 0.35 MPa, in which a single bed pressure drop should not exceed 35 kPa. The bed pressure drop is generally related with the adsorbent itself and the equipment operating load. Tocompare the pressure drop between different devices or the same device under different conditions, the pressure drop coefficientKwas introduced, ΔP=K×um, where ΔPis the bed pressure drop,umis the fluid linear velocity in the chamber. The larger theKvalue, the higher the pressure drop of the adsorbent bed would be. The result is plotted in Figure 1. It can be seen from Figure 1 that theKvalue was first gradually increased and then kept almost unchanged after a definite operating time. Performance test showed that the whole chamber pressure drop was 0.18—0.19 MPa and a single bed pressure drop was 11.98 kPa, which was much lower than the upper limit of 35 kPa. The calculatedKvalue was 1.16. In contrast, Figure 1 also gives theKvalue of an adsorbent provided by another manufacturer. TheKvalue was increased from 1.17 to 1.60. Clearly, under the similar operation condition,Kvalue of the adsorbent RAX-2000A was significantly lower than that of the counterpart.

Figure 1 The pressure drop coefficient K vs. equipment running time

3.2 Adsorbent RAX-3000

In September 2011, an industrial demonstration unit for PX production equipped with the SMB technique developed by Sinopec was built in a local petrochemical company using RAX-3000 as the PX adsorbent. The adsorbent RAX-3000 achieved success in the first trial of start-up operation, producing qualified PX products, as shown in Figure 2. The average composition of feed is presented in Table 2. As shown in Table 4, the ratio of unit production capacity versus the adsorbent RAX-3000 was increased from 0.56 to 0.66 (kt/a)/t. Preliminary results also indicated that compared with the adsorbent RAX-2000A, the A/Faand D/F were notably decreased due to the higher selective adsorption capacity of the adsorbent RAX-3000. Undoubtedly, the adsorbent RAX-3000 will remarkably save the operating cost and increase economic benefits. The whole chamber pressure drop was 0.17 MPa to 0.22 MPa.

Table 4 Data on separation capacity and regeneration of RAX adsorbents

For a comprehensive understanding of the operation of the unit employing the adsorbent RAX-3000, the pumparound pump (PAP) survey is used to analyze the adsorbent chamber performance under an 120% feed load, with the results shown in Figure 3. The PAP survey is a series of samples taken at either the circulation pump outlet or the pumparound flow control valve with the feed passing through each adsorbent bed. Analysis of these samples reveals the composition of each zone as it passes through the PAP. Thus it can objectively reflect the operating conditions of PX stream from the adsorption phase to the desorption phase within the whole separation cycle. Generally, the two towers composed of totally 24 beds, are divided into four major areas by the feed (F), raffinate (R), desorption (D) and extract streams (E), namely: the adsorption zone (I), the purification zone (II), the desorption zone (III), and the buffer zone (IV). As it can be seen from Figure 3 that PX in the feed is preferentially adsorbed in zone I, where PX concentration is decreased rapidly and the PX curve is concave, indicating that the adsorbent has sufficient adsorption capacity characteristic of rapid adsorption rate. In zone II, the mass fraction of the liquid phase PX increases significantly, while the non-aromatics and other C8aromatic isomers decrease rapidly. The adsorbed PX is gradually purified indicating that the adsorbent has good adsorption selectivity. Zone III reflects the desorption rate of the adsorbed PX. PX is desorbed completely from the adsorbent after passingthrough three sections of adsorbent bed, indicating that the adsorbent has a faster PX desorption rate. The L3/A can be further improved to a considerable degree.

Figure 2 PX product purity and yield obtained during performance test

Figure 3 The pumparound curve of the adsorbent chamber

4 Application Prospect

In recent years, international market of PX adsorbents has been expanding rapidly. By the end of 2010, the global PX capacity was more than 30 Mt/a, and the total amount of PX adsorbents was estimated to be over 40 000 tons. By taking into account the average service life of adsorbents as 10 years, the average annual PX adsorbent replacement amount is about four thousand tons a year. In addition, the PX demand growth is projected to increase by more than 5% annually in the future.

5 Conclusions

The adsorbent RAX-2000A achieved success in the first trial of start-up operation at a Sinopec’s petrochemical company, while cranking out qualified PX products. The average purity of PX reached 99.81% with an average product yield of 98.6%. Actually the processing capacity of the adsorbent still has considerable room. Moreover, the chamber pressure drop was much lower than the specified upper limit. The adsorbent RAX-3000 not only retains most advantages of the adsorbent RAX-2000A, but also has a higher selective adsorption capacity. The RAX series PX adsorbents are on a par with the advanced level of similar overseas famous PX adsorbents. Moreover, they have exhibited good adaptability to unfavorable feedstock containing high EB fraction besides their better mechanical strength. An industrial demonstration unit for PX production by SMB developed by Sinopec has been built at a local petrochemical company, employing the adsorbent RAX-3000 for adsorptive separation of PX. In recent years, the rapid expansion of international PX market provides a good prospect for application and extension of the RAX series PX adsorbents.

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Recieved date: 2012-11-12; Accepted date: 2012-12-22.

Dr. Li Ben, Telephone: +86-10-82368215; E-mail: liben.ripp@sinopec.com.