Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene (MX) are often present together in C8 aromatic product streams from chemical plants and oil refineries. Of these C8 compounds, although EB is an important raw material for the production of styrene, for a variety of reasons most EB feedstocks used in styrene production are produced by alkylation of benzene with ethylene, rather than by recovery from a C8 aromatics stream. Of the three xylene isomers, PX has the largest commercial market and is used primarily for manufacturing terephthalic acid and terephthalate esters for use in the production of various polymers such as poly(ethylene terephthalate), poly(propylene terephthalate), and poly(butene terephthalate). While OX and MX are useful as solvents and raw materials for making products such as phthalic anhydride and isophthalic acid, market demand for OX and MX and their downstream derivatives is much smaller than that for PX.
Given the higher demand for PX as compared with its other isomers, there is significant commercial interest in maximizing PX production from any given source of C8 aromatic materials. However, there are two major technical challenges in achieving this goal of maximizing PX yield. Firstly, the four C8 aromatic compounds, particularly the three xylene isomers, are usually present in concentrations dictated by the thermodynamics of production of the C8 aromatic stream in a particular plant or refinery. As a result, the PX production is limited, at most, to the amount originally present in the C8 aromatic stream unless additional processing steps are used to increase the amount of PX and/or to improve the PX recovery efficiency. Secondly, the C8 aromatics are difficult to separate due to their similar chemical structures and physical properties and identical molecular weights.
A variety of methods are known to increase the concentration of PX in a C8 aromatics stream. These methods normally involve recycling the stream between a separation step, in which at least part of the PX is recovered to produce a PX-depleted stream, and a xylene isomerization step, in which the PX content of the PX-depleted stream is returned back towards equilibrium concentration, typically by contact with a molecular sieve catalyst. However, the commercial utility of these methods depends on the efficiency, cost effectiveness and rapidity of the separation step which, as discussed above, is complicated by the chemical and physical similarity of the different C8 isomers.
Fractional distillation is a commonly used method for separating different components in chemical mixture. However, it is difficult to use conventional fractional distillation technologies to separate EB and the different xylene isomers because the boiling points of the four C8 aromatics fall within a very narrow 8° C. range, namely from about 136° C. to about 144° C. (see Table 1 below). In particular, the boiling points of PX and EB are about 2° C. apart, whereas the boiling points of PX and MX are only about 1° C. apart. As a result, large equipment, significant energy consumption, and/or substantial recycles would be required for fractional distillation to provide effective C8 aromatic separation.
TABLE 1C8 CompoundBoiling Point (° C.)Freezing Point (° C.)Ethylbenzene (EB)136−95Para-xylene13813Meta-xylene139−48Ortho-xylene144−25
Fractional crystallization is an alternative method of separating components of a mixture and takes advantage of the differences between the freezing points and solubilities of the components at different temperatures. Due to its relatively higher freezing point, PX can be separated as a solid from a C8 aromatic stream by fractional crystallization while the other components are recovered in a PX-depleted filtrate. High PX purity, a key property needed for satisfactory conversion of PX to terephthalic acid and terephthalate esters, can be obtained by this type of fractional crystallization. U.S. Pat. No. 4,120,911 provides a description of this method. However, fractional crystallization is slow and the need to avoid the formation of binary eutectics between PX and MX and between PX and OX limits the amount of PX that can removed per pass through the crystallizer.
An alternative xylene separation method uses molecular sieves, such as zeolites, to selectively adsorb para-xylene from the C8 aromatic feedstream to form a PX-depleted effluent. The adsorbed PX can then be desorbed by various ways such as heating, lowering the PX partial pressure or stripping. For example, U.S. Pat. No. 3,997,620 discloses a process for separating para-xylene from a feed stream comprising para-xylene and at least one other C8 aromatic isomer which process comprises the steps of: (a) contacting said feed stream at adsorption conditions with an adsorbent comprising type X or type Y zeolite containing barium and strontium in a weight ratio of barium to strontium of from about 1:1 to about 15:1 at the exchangeable cationic sites to effect the selective adsorption of para-xylene; (b) removing a raffinate component comprising a less selectively adsorbed C8 aromatic from said adsorbent; (c) contacting said adsorbent with a desorbent material comprising para-diethylbenzene at desorption conditions to effect the desorption of para-xylene from said adsorbent; and, (d) removing from said adsorbent an extract component comprising para-xylene.
Similarly, U.S. Pat. No. 4,886,929 discloses a process for separating the para-isomers of a dialkyl-substituted aromatic hydrocarbon from a mixture of said para-isomer and at least one other isomer of said aromatic hydrocarbon, which process comprises contacting said mixture with a crystalline aluminosilicate adsorbent, particularly zeolite X or Y, containing barium and potassium at exchangeable cationic sites within the adsorbent crystalline structure in a BaO/K2O molar ratio of from about 0.6 to 1.2 at adsorption conditions selected to effect the adsorption of said para-isomer by said adsorbent and subsequently contacting said adsorbent with a desorbent material selected from meta-difluorobenzene and ortho-difluorobenzene and mixtures thereof at desorption conditions to effect the removal of said para-isomer from said adsorbent and recovering from said adsorbent a stream concentrated in said para-isomer.
Two commercially available xylene separation processes used in many chemical plants or refineries are the PAREX™ and ELUXYL™ processes. Both of these processes use molecular sieves to adsorb PX. In such molecular-sieve based adsorption processes, a higher amount of PX, typically over 90%, compared with that from a fractional crystallization process, typically below 65%, may be recovered from the PX present in a particular feed. Although this improved recovery is a significant advantage, it is accompanied by significant disadvantages, including a complex and expensive process scheme, large equipment sizes (up to 1 million pounds of adsorbent required) and high energy consumption. As a result the PX adsorption unit is generally the rate-limiting step in most PX production plants.
In addition to large pore molecular sieves, such as zeolite X and Y, attempts have been made to use adsorption with medium pore zeolites, such as ZSM-5 and ZSM-11, to separate PX from mixtures of C8 aromatics. However, a major disadvantage of these processes is that the time required to effect desorption of the adsorbed components is too long to provide a commercially useful process. In addition, with acidic zeolites, such as HZSM-5, the high temperatures used to obtain rapid desorption cause catalytic reactions to occur converting PX to MX and OX and converting EB to benzene. Furthermore, with HZSM-5, traces of olefins, which are usually present in commercial feeds, irreversibly chemisorb lowering the adsorption capacity of the zeolite. As a result, frequent reconditioning of the adsorbent (e.g., removal of coke deposits) is required.
In an attempt to avoid these problems, various proposals have been made to employ pressure swing adsorption at elevated temperature and pressure with a non-acidic, molecular sieve-containing adsorbent, such as silicalite, to selectively adsorb PX and EB from mixtures of C8 aromatics. Desorption is said to be significantly faster and reactions of the adsorbed molecules (PX and EB) do not occur. In addition, olefins do not adsorb irreversibly on the silicalite, so the adsorption capacity of the adsorbent remains high and frequent reconditioning is not required.
For example, U.S. Pat. No. 6,689,929 discloses a pressure swing adsorption process for separating para-xylene and ethylbenzene from a C8 aromatics stream produced by toluene conversion. The process uses a para-selective adsorbent, preferably orthorhombic crystals of silicalite having an average minimum dimension of at least about 0.2 μm, and is operated isothermally in the vapor phase at elevated temperatures and pressures. Para-xylene and ethylbenzene are preferentially adsorbed in a fixed bed of the adsorbent and, when the bed is saturated with para-xylene and ethylbenzene, the feed to the process is stopped and the partial pressure is lowered to desorb the para-xylene and ethylbenzene. A stream of non-adsorbed meta-xylene and ortho-xylene may be obtained before desorbing the para-xylene and ethylbenzene. Cycle times, that is the time between successive sorption/desorption cycles, of between 2 minutes and 200 minutes are disclosed.
In addition, U.S. Pat. No. 6,878,855 discloses a process for producing a para-xylene enriched product from a feedstream comprising xylenes and ethylbenzene, wherein the process comprises: (a) passing the feedstream through at least one isomerization reactor containing an isomerization catalyst to isomerize the xylenes and at least partially isomerize and/or at least partially destroy the ethylbenzene present in the feedstream to form an isomerization effluent; and (b) feeding the isomerization effluent in vapor phase through at least one swing adsorption unit containing a sorbent to produce alternately, at a cycle time, an exiting raffinate comprising a para-xylene depleted stream during an adsorption mode and a desorption effluent comprising the para-xylene enriched product during a desorption mode. Suitable adsorbents for use in the swing adsorption unit include zeolitic and non-zeolitic molecular sieves, especially medium pore zeolites having a pore diameter of smaller than about 7 Å, such as MFI type zeolites, for example ZSM-5 and silicalite, and pillared clays, carbons, and mixtures thereof. Suitable cycle times are said to be from about 0.1 second to about 120 minutes.
It is appreciated that swing adsorption of para-xylene from a mixed C8 aromatic feedstream is a cost-effective way of increasing the PX concentration in the feedstream. By increasing the PX concentration in the feedstream, swing adsorption can provide an inexpensive way of increasing the efficiency of a downstream separation unit, which could be a conventional PAREX™, ELUXYL™ or crystallization unit, and thereby assist in debottlenecking the xylenes loop in an aromatics complex. The present invention seeks to provide an improved swing adsorption process for recovering PX and an improved xylene production process incorporating swing adsorption of PX.