CN215310297U - Crystallizer ejector vapor supply - Google Patents

Abstract

An apparatus in a Purified Terephthalic Acid (PTA) plant having: an evacuated crystallizer vessel for crystallizing Crude Terephthalic Acid (CTA) having a crystallizer ejector in fluid communication with a top discharge line of the evacuated crystallizer; a condenser located in the top discharge line between the evacuated crystallizer vessel and the crystallizer discharger; and a top fluid line directing pressurized top fluid from another separation vessel in the PTA plant through the crystallizer ejector.

Description

Crystallizer ejector vapor supply

Technical Field

The present invention relates to an improved apparatus in a purified terephthalic acid plant.

Background

In a process for producing purified terephthalic acid by the oxidation of para-xylene, a crude terephthalic acid slurry is initially formed in a solvent medium comprising acetic acid. After the oxidation step, the crude terephthalic acid produced can be conventionally separated from the reaction medium by centrifugation or filtration and subsequently dried, and then reslurried in an aqueous medium for purification. This transfer step of the crude terephthalic acid (from the reaction medium to the aqueous medium) is commonly referred to as solvent exchange. However, it is more convenient to exchange (preferably continuously) the reaction medium for an aqueous medium in a filter press, for example a rotary filter press. The crude terephthalic acid/aqueous medium is then heated to effect dissolution of the crude terephthalic acid and passed to a purifier where the crude terephthalic acid is hydrogenated to convert the undesirable components to components that are readily removed from the aqueous medium, thereby forming a purified terephthalic acid solution. The purified terephthalic acid solution is crystallized in one or more crystallization vessels and collected as a slurry, then washed and filtered.

Typically, a Purified Terephthalic Acid (PTA) plant employs a series of Crude Terephthalic Acid (CTA) crystallizers, such as three CTA crystallizers, downstream of the para-xylene oxidation system. The third CTA crystallizer is typically operated below atmospheric pressure (0.5-0.99 bar a (bara)) and also below the normal operating pressure of the discharge header (1.0-2.0 bar a). Therefore, to maintain the required operating pressure, a vent pump driven by steam is required to evacuate the vapor from the third CTA crystallizer to maintain the required operating pressure. This steam usage is low, but it does represent the feed water to the process as well as the use of steam that could otherwise be sent to a steam turbine for power generation.

It would be advantageous to find a source of pressurized fluid within the PTA plant (plant) suitable for supplying the third CTA crystallizer ejector (ejector), and to find an inexpensive means for delivering the pressurized fluid to the ejector. Such a system is disclosed below.

SUMMERY OF THE UTILITY MODEL

There is provided an apparatus in a Purified Terephthalic Acid (PTA) plant comprising: an evacuated crystallizer vessel for crystallizing Crude Terephthalic Acid (CTA) having a crystallizer ejector in fluid communication with a top discharge line (overhead vent line) of the evacuated crystallizer; a condenser located in the top discharge line between the evacuated crystallizer vessel and the crystallizer discharger; and a top fluid line directing pressurized top fluid from another separation vessel in the PTA plant through the crystallizer ejector.

In one form, the apparatus further comprises a pressurized crystallizer vessel having a top discharge line upstream of the evacuated crystallizer vessel and having a top fluid line in communication with the top discharge line of the pressurized crystallizer vessel.

In another form, the apparatus further comprises a methyl acetate recovery column to separate methyl acetate from acetic acid having an overhead discharge line for methyl acetate vapor in communication with the overhead fluid line and the crystallizer ejector. In this configuration, the top fluid line communicates with the top discharge line of the evacuated crystallizer vessel between the condenser and the evacuated crystallizer vessel.

Drawings

The disclosure is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown in the drawings and are herein described in detail. However, it should be understood that the description herein of specific exemplary embodiments is not intended to limit the disclosure to the particular forms disclosed herein.

The disclosure is to cover all modifications and equivalents as defined by the appended claims. It should also be understood that the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the exemplary embodiments of the present invention. Furthermore, some dimensions may be exaggerated to help visually convey these principles. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. Furthermore, two or more blocks or elements depicted in the drawings as being different or separate may be combined into a single functional block or element. Similarly, individual blocks or elements illustrated in the drawings may be implemented in multiple steps or by cooperation of multiple elements.

The aspects of the present disclosure are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which corresponding reference numerals refer to similar elements and in which:

fig. 1 is a schematic diagram of a system that uses the top fluid from a first CTA crystallizer to drive an ejector for pulling a vacuum on a third CTA crystallizer.

FIG. 2 is a schematic diagram of a system using the overhead stream from the methyl acetate recovery column to drive an ejector for pulling a vacuum on the third CTA crystallizer.

Detailed Description

Various aspects will now be described with reference to specific forms selected for purposes of illustration. It should be understood that the spirit and scope of the devices, systems, and methods of the present disclosure is not limited to the selected forms. Further, it should be noted that the drawings provided herein are not drawn to any particular scale or size and that many variations to the forms shown are possible.

As used herein, each of the following terms in the singular grammatical forms "a", "an" and "the" can also refer to and encompass a plurality of the recited entities or objects, unless specifically defined or stated otherwise, or unless the context clearly dictates otherwise. For example, as used herein, the phrases "a device," "an assembly," "a mechanism," "a component," and "an element," respectively, may also refer to and encompass a plurality of devices, a plurality of assemblies, a plurality of mechanisms, a plurality of components, and a plurality of elements.

As used herein, each of the following terms "including," comprising, "" having, "" including, "and" containing, "as well as linguistic or grammatical variations, derivatives, and/or conjugates thereof, means" including, but not limited to.

In all of the illustrative descriptions, embodiments and appended claims, numerical values for parameters, features, objects or dimensions may be stated or described in terms of a numerical range format. It is to be fully understood that this numerical range format is provided to illustrate the practice of the aspects of the present disclosure, and is not to be understood or interpreted as inflexible limiting the scope of the aspects of the disclosure.

Further, for the purpose of stating or describing a numerical range, the phrase "in a range between about a first numerical value and about a second numerical value" is considered to be equivalent to and have the same meaning as the phrase "in a range from about the first numerical value to about the second numerical value," and thus, these two equivalent meaning phrases may be used interchangeably.

Concentrations, dimensions, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a range of about 1 to about 200 should be interpreted to include not only the explicitly recited limits of 1 and about 200, but also to include individual dimensions (e.g., 2, 3, 4, etc.) and sub-ranges (e.g., 10 to 50, 20 to 100, etc.). Similarly, it should be understood that when numerical ranges are provided, such ranges should be construed as providing written support to claim limitations that only recite the lower value of the range as well as claims limitations that only recite the upper value of the range. For example, a disclosed numerical range of 10 to 100 provides written support for a claim that lists "greater than 10" (no upper limit) and a claim that lists "less than 100" (no lower limit). Corresponding numbers indicate corresponding or analogous structures and/or features in the drawings; and various illustrated structures and/or features may be discussed in detail herein without reference to the illustrations. Likewise, each structure and/or feature may not be explicitly labeled in the figures; and any structure and/or feature discussed herein with reference to the figures may be used with any other structure and/or feature without departing from the scope of the present disclosure.

It is also to be understood that, unless otherwise specifically defined or stated, all technical and scientific words, terms, and/or phrases used throughout this disclosure have the same or similar meaning as commonly understood by one of ordinary skill in the art. The phraseology, terminology, and symbols employed throughout this disclosure are for the purpose of description and should not be regarded as limiting.

CTA purification typically comprises: an aqueous medium is added to the CTA to form a slurry thereof, which is then heated to dissolve the CTA in the medium to provide an aqueous solution of terephthalic acid. The solution is then passed through a reduction step in which the solution is contacted with hydrogen under reducing conditions and in the presence of a heterogeneous catalyst to chemically reduce the organic impurities (e.g. 4-carboxybenzaldehyde (4-CBA)). The hydrogenated solution is passed through a pressure reduction (crystallization) vessel in which PTA crystals are formed to provide a PTA slurry in an aqueous medium. PTA is then recovered from the aqueous medium.

In the first embodiment, instead of using steam to drive the ejector, the apparatus of the present disclosure provides for using the overhead vapor from the first CTA crystallizer (which is a mixture of inert gas and solvent) as a supply of high pressure vapor to drive the third crystallizer ejector, thus reducing operating costs through steam savings and elimination of water feed.

Figure 1 shows a plant in a PTA plant, particularly in a CTA oxidation-crystallization system 1000, having crystallization vessels C1 through C3, where C3 is the evacuated crystallizer vessel C3. The CTA slurry from the para-xylene oxidation reaction system (not shown) enters a series of three crystallizers C1, C2, and C3, each having an overhead fluid discharge line 1020, 1021, and 1022, respectively, via line 1010. This third CTA crystallizer is typically operated at sub-atmospheric pressure and functions to flash cool the CTA solids and mother liquor to about 90 to 115 ℃ before recovering the solids. Heat is removed by flashing the vapor (acetic acid mother liquor vapor) and then condensed in a water-cooled heat exchange, followed by return to the slurry at typically 50 to 65 ℃. The top fluid in each discharge line decreases relative to the upstream discharge line until there is a partial vacuum in line 1022 resulting at least in part from the crystallizer ejector 1040 being in fluid communication with the top discharge line 1022 of the evacuated crystallizer C3. Condenser 1030 is located within top discharge line 1022, between evacuated crystallizer vessel C3 and crystallizer discharger 1040. An overhead fluid line 1025 from another separation vessel in the PTA plant directs a small portion of the pressurized overhead fluid through the crystallizer discharger 1040. In this embodiment, top fluid line 1025 originates from a pressurized crystallizer vessel C1, which has a top discharge line 1020, upstream of the evacuated crystallizer vessel C3. The top fluid line 1025 is in fluid communication with the top discharge line 1020 of the pressurized crystallizer vessel C1. In the other branch of the vent system, line 1026 passes the remaining vapor from C1 to a first CTA crystallizer condenser (not shown). It should be appreciated that a similar system may be derived from the top discharge 1021 from crystallizer C2. The fluid exiting from the ejector 1040 is sent through another condenser 1050 and into an oxidation plant discharge header 1060 for subsequent processing. However, a portion of the fluid exiting the ejector 1040 may be recycled back through an ejector gas return system 1045 connected to the top vent 1022 to help maintain a steady pressure in C3. The ejector gas return system may include valves and pressure controllers, and associated piping required to connect the system between the ejector and the crystallizer top drain 1022. Optionally, if desired, inert gas may be filled into the ejector gas return system 1045 through line 1046 to supplement the pressure in C3. This embodiment provides a small operating cost reduction by eliminating the water feed to the process that involves the use of steam and by freeing the steam previously used for the ejector to be available for power generation in the plant steam turbine.

In a second embodiment, as shown in fig. 2, the apparatus of the present disclosure provides an alternative to using a steam driven ejector by using the overhead gas from the methyl acetate recovery column. In the PTA plant, methyl acetate stripping (recovery) column 2000 is used to remove methyl acetate from the acetic acid solvent. Stripped acetic acid passes from the bottom of the column and a methyl acetate rich stream is recovered as an overhead product, while a small amount of non-condensables enriched in methyl acetate continue through oxidation discharge header 1060 for discharge VOC treatment. Because methyl acetate is a relatively low boiling organic (57 ℃ at normal atmospheric pressure) and acetic acid has a much higher boiling point (about 119 ℃ at atmospheric pressure), methyl acetate can be effectively stripped to the vapor phase by the inert gas. The exhaust gas from column 2000 is filled into overhead fluid line 2025 through line 2010 and through a series of condensers 2015, 2020 and then to condenser 1030, which is a sub-atmospheric operating pressure flash cooler for C3. This third CTA crystallizer is typically operated at sub-atmospheric pressure and functions to flash cool the CTA solids and mother liquor to about 90 to 110 ℃ before recovering the solids. Heat is removed by flashing the vapor (acetic acid mother liquor vapor) and then condensed in a water-cooled heat exchange, followed by return to the slurry at typically 50 to 65 ℃. By filling the condenser with the vent from the methyl acetate stripper along with crystallizer flash vapor, two benefits are obtained: (1) the vent gas acts as a third CTA crystallizer pressure control gas, eliminating the need to add inert gas or backflush (kick back) vent gas from the crystallizer ejector pressure control device; and (2) it provides an efficient absorption solvent for methyl acetate in the methyl acetate vent gas, thereby reducing the loss of methyl acetate to the plant vent system and reducing the VOC load on the emission abatement system. The rest of the system is similar to that of the first embodiment.

Examples

By way of example, vessel C1 was operated at 9 to 11 bar a, when C3 was operated at 0.75 bar a and the effluent from discharger 1040 was 1.6 bar a, 0.5 to 1.5 mass% of flash vapor taken through line 1025 to drive the discharger was generally sufficient to maintain a steady pressure in the crystallizer.

As another example, vessel C1 was operated at 9 to 11 bar a, when C3 was operated at 0.75 bar a and the effluent from ejector 1040 was 1.6 bar a, when there was additional methyl acetate-rich vapor (typically 60 mass% methyl acetate and 40 mass% inerts, at ambient conditions) flowing at a rate typically 50% of the rate at which stream 1025 would drive gas, it was generally sufficient to take 1.0 to 2.0 mass% flash vapor through line 1025 to drive the ejector to maintain a stable pressure in the crystallizer. In this case, the total flow exiting condenser 1030 to ejector 1040 is less than 75% of the vapor inflow of the methyl acetate-rich stream, with typically less than 10 mass% methyl acetate.

Industrial applications

The systems and methods of the present disclosure are applicable to the chemical industry.

It is believed that the disclosure set forth above encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. The subject matter of the inventions includes all novel and non-obvious combinations and subcombinations of the various elements, features, functions and/or properties of the disclosure. Also, when the claims recite "a" or "an" element or the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certain combinations and subcombinations of the inventions directed to one of the inventions of the present disclosure and are novel and non-obvious. Inventions embodied in other combinations and subcombinations of features, functions, elements, and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to a different invention or directed to the same invention, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the inventions of the present disclosure.

Claims (5)

1. An apparatus in a Purified Terephthalic Acid (PTA) plant, comprising:

an evacuated crystallizer vessel for crystallizing Crude Terephthalic Acid (CTA) having a crystallizer discharger in fluid communication with a top discharge line of the evacuated crystallizer;

a condenser located in a top discharge line between the evacuated crystallizer vessel and the crystallizer ejector; and

a top fluid line that directs pressurized top fluid from another separation vessel in the PTA plant through the crystallizer ejector.

2. The apparatus of claim 1, further comprising a pressurized crystallizer vessel having a top discharge line upstream of the evacuated crystallizer vessel and having a top fluid line in communication with the top discharge line of the pressurized crystallizer vessel.

3. The apparatus of claim 1, further comprising a methyl acetate recovery column to separate methyl acetate from acetic acid having a top vent line for methyl acetate vapor in communication with the top fluid line and the crystallizer ejector.

4. The apparatus of claim 3, wherein said top fluid line communicates with a top discharge line of said evacuated crystallizer vessel between said condenser and said evacuated crystallizer vessel.

5. The apparatus of claim 1, further comprising any other low pressure process vent requiring a boost to normal vent header operating pressure in communication with said overhead fluid line and said crystallizer ejector.

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