Introduction
In 1995, BP Amoco commercialized a process developed to produce dimethyl-2,6-naphthalene dicarboxylate (NDC). NDC is used as the monomer to produce a new polyester: polyethylene naphthalate (PEN). PEN has superior properties compared to the conventional polyester PET (polyethylene terephthalate). The enhanced strength, barrier properties, and thermal stability offer significant opportunities for PEN based products in the areas of film, fiber, and packaging.
NDC is produced by the reaction of methanol with 2,6-naphthalenedicarboxylic acid (NDA). The NDA can be made by the oxidation of any one of several 2,6-dialkyl naphthalenes. The preferred starting material is 2,6-dimethylnaphthalene (2,6-DMN). Since there are ten isomers of dimethylnaphthalenes, separation of 2,6-DMN from the mixture at high recovery and high product purity is an important consideration. This paper is concerned with the development of a 2,6-DMN purification process based on melt crystallization using suspension, static, and dynamic falling film crystallizers.
Background
Many of the methods for purifying substituted naphthalene compounds have in common that they use suspension crystallization methods in which crystals that are formed remain suspended in the liquid phase (References 1-7). In all these processes it is desirable to minimize the formation of a crystal layer on the heat transfer surface so as not to reduce the ability to cool the feed mixture to the desired temperature (Reference 4). In addition, all these methods require the use of solid/liquid separation devices to recover the product solid crystals from the mother liquor. The ability to produce a high purity product from the solid/liquid separation devices depends on the filterability of the product cake and also on the morphology of the crystals. It has been noted (Reference 7) that alkyl substituted naphthalenes and dialkyl substituted naphthalene dicarboxylates tend to crystallize in small platelets (Reference 3) which make removal of impurities difficult by dewatering the product cake. For suspension crystallization it is also important to maintain the ratio of high to low melting point components in the feed mixture in a certain range so that the crystals produced do not cling to the scraping device in the crystallizer and make the crystallizer inoperable (Reference 5). In such suspension crystallization processes, mother liquor inclusion between the platelets and the adhering mother liquor on the product cake often prevents one from achieving the desired high product purities. For 2,6-dimethylnaphthalene purification from the melt a purity of 85 to 95 weight percent has been reported (Reference 7). It has also been reported (References 3, 4) that crystallization in a solvent is required to produce 2,6-DMN of 99 weight percent purity. Manufacturing cost is, of course, increased if a solvent is used in the purification process.
Alternative methods such as plate heat exchanger devices have been used to purify xylene, dimethylnaphthalenes, and aromatic acids (Reference 6). A major problem noted for this type of device was that the crystals deposited on the cold heat transfer surface greatly diminished the rate of heat transfer and required frequent shutdowns to clean the equipment.
From the preceding discussion, it is evident that there are limitations on attaining very high product purities using suspension crystallization methods, particularly if a solvent is not used in the process. The use of certain types of heat exchange devices lead to fouling of the heat transfer surfaces by crystals forming on them and rendering them ineffective for continuous operation for long periods of time. It is therefore desirable to develop a crystallization process that does not require the use of a solvent and that can produce a product purity of > 95 weight percent in equipment with high operating factor. In this paper we describe the development of such a process as applied to the purification of 2,6-DMN.
Description of the Purification Methods
Our process development work had several objectives:
- Develop a purification method that can produce on a commercial scale a product of atleast 95 weight percent purity
- Prove the method to be suitable to purify different dialkyl naphthalenes and dialkyl naphthalene dicarboxylates
- Demonstrate that the method does not require the use of solid/liquid separation equipment
- Demonstrate that the method can be reduced to commercial operation with a very high operating factor
- Demonstrate applicability of the method to produce high product purities from feed materials of different compositions ranging from less than 50 weight percent to greater than 80 weight percent of the product component
To meet these objectives, we considered the use of melt crystallization in two types of devices: Static Crystallizers and Falling Film Crystallizers. These devices were selected because they had shown the capability to produce high purity products. The envisioned process is continuous but based on semi-continuous crystallization steps which can deliver the desired heat transfer performance for crystal production. In addition, with these devices it is also possible to raise the temperature of the crystal layer grown to induce a partial melting of the cake which effectively removes impurities. This process is called sweating. After the sweated liquid is removed, the remaining crystal layer is purer than possible with conventional washing of the product cake. In a Falling Film crystallizer, most of the crystal layer that has been grown is not in contact with the circulating mother liquor in contrast to suspension crystallizers where all the crystal mass is suspended in the impure mother liquor.
The Static Crystallizer consists of a crystallization tank in which a number of parallel cooling plates are arranged side-by-side. A heat transfer medium can be circulated through the cooling plates. The product crystals grow on the plate surfaces. The Falling Film Crystallizer consists of a bundle of tubes arranged in a fashion similar to a shell and tube heat exchanger. A heat transfer medium on one side of each tube cools the falling film of the process fluid on the other side of the tube. The product crystals grow on the tube surface. Static and Falling Film Crystallizers can be produced by Sulzer Chemtech AG, Switzerland.
In both Static and Falling Film crystallizers, after the crystals are formed from the feed by layer growth on the heat transfer surface, the mother liquor is drained off. Sweating of the wet cake is induced by raising the temperature. The sweating process represents a very high efficiency cake wash and effectively removes impurities. After draining the sweated impurities, the residual purer crystal layer is melted and the resultant liquid collected. Several stages can be run in the same device to reach the desired product purity. The Static Crystallizers can be used to purify feeds containing 35 to 50 weight percent of the product component to a typical product purity of 70 to 80 weight percent. This material can then be purified to product of atleast 95 weight percent purity in Falling Film Crystallizers. Both of these devices have high operating factors and are being used in industrial processes to purify a variety of compounds.
The method developed here to purify feeds containing substantially less than 50 weight percent of the product component is described below.
- The feed melt is introduced into a Static Crystallizer to a level such that the crystallization surfaces are under the liquid.
- Cooling the crystallization surface according to a pre-determined temperature program to grow the layer of product crystals in a given amount of time.
- After the desired amount of crystals have been formed, the mother liquor is drained to a residue tank for further processing in another purification stage or for removal from the purification process.
- The crystallization surfaces are heated until the crystals start sweating to expel the impurities and the sweated liquid is collected for further processing.
- The residual crystals are melted by raising the crystallization surface temperature and the resultant liquid collected as feed to the next purification stage or as the final product from the last stage
The product purity from the Static Crystallization step is expected to be in the 70-80 weight percent purity range. It should be noted that we can use suspension crystallization and solid/liquid separation to achieve similar low product purities.
The method developed here to purify feeds containing 70 to 80 weight percent of the product component is described below.
- The dynamic Falling Film Crystallizer consists of a large number of tubes in the configuration of a typical shell and tube heat exchanger. The feed melt is introduced to the top of the crystallizer, such that it flows down as a falling film on one side of the tube.
- A heat transfer medium is introduced on the other side of the tube at a temperature that allows the product crystals to form on the tube surface.
- The feed melt that has collected at the bottom of the crystallizer is recirculated to the top to continue the crystallization process. This is repeated a number of times until the crystal layer of pre-determined thickness has formed on the tube wall.
- The mother liquor is then drained and collected for further processing or for removal from the purification process.
- The tube wall temperature is raised until the crystals begin to sweat. The sweated liquid is collected for further processing.
- The residual crystal layer is melted and collected as feed to the next stage or as product from the final stage.
Results
In contrast to the 85-95 weight percent purities reported previously (Reference 7), product purities of greater than 95 weight percent have been obtained by using one or more stages of the dynamic Falling Film Crystallization method. The Falling Film crystallization method provides the following benefits:
- Unlike suspension crystallization, this method does not require solid/liquid separation equipment.
- No solvents are required to produce products at very high purities.
- This method is suitable for the purification to very high purity of various dialkyl-substituted naphthalenes like 1,5-DMN,2,6-DMN,2,7-DMN, different isomers of diethylnaphthalenes or diisopropylnaphthalenes.
As an example, starting with a feed containing substantially less than 50 weight percent of the product component, the following two processing schemes were evaluated.
- The feed is first purified by Suspension Crystallization and the resultant product from the solid/liquid separation device melted and sent for purification by dynamic Falling Film Crystallization to produce >95 percent product.
- The feed is first purified by Static Crystallization and the resultant product melted and subjected to dynamic Falling Film Crystallization to produce the final product of >95 weight percent purity.
Description of the Purification Process
A schematic of the purification process is shown in Figure 1. The abbreviations shown in Figure 1 are defined below.
|
M1, M2, M3 |
|
Mother Liquor from purification stages 1, 2, and 3 |
| S1, S2, S3 |
|
Sweat phase from purification stages 1, 2, and 3 |
| C1, C2, C3 |
|
Melted product crystal phase from purification stages 1, 2, and 3 |
| T1, T2, T3 |
|
Tanks for storing the feed for purification stages 1, 2, and 3 |
| F |
|
Feed mixture to the Purification Process |
Figure 1
The individual stages correspond to different feed purities, with the lower numbered stage referring to a lower feed purity. The total number of purification stages depends on the feed concentration, reject stream concentration, and the desired product purity.
As shown in Figure 1, the feed F is fed into storage tank T1 which also receives the sweat phase S1 of stage 1 from a previous crystallization/melt cycle. Also routed to tank T1 is mother liquor M2 from stage 2 from another crystallization/melt cycle. The contents of tank T1 are purified in stage 1 by feeding into a crystallizer where the three fractions from stage 1 are produced: M1, S1, and C1. The mother liquor fraction M1 which contains high concentrations of impurities is rejected from the purification process as the Residue stream. The Residue stream contains such low concentrations of the product component that further fractional crystallization is either ineffective or uneconomical, for example, due to the formation of eutectics or unfavorable crystal shape. The sweat phase S1 is again routed to tank T1. The melted product cake C1 is retained in the collecting tank below the crystallizer. To this collecting tank material from tank T2 containing the sweat phase S2 from stage 2 from a previous crystallization/melt cycle and the mother liquor M3 from still another previous crystallization/melt cycle are added prior to commencing the next crystallization step. The combined stream consisting of C1, S2, and M3 is then subjected to another crystallization/melt cycle. This cycle produces the three phases: M2, S2, and C2. In tank T3 crystal phase C2 from stage 2, and the sweat phase S3 from a previous stage 3 crystallization/melt cycle are combined as feed to stage 3 crystallization. From the stage 3 crystallization/melt cycle three phases are produced: M3, S3, and C3. Mother liquor M3 is routed to tank T2 and sweat phase S3 is routed to tank T3 as feed for the next cycle. The crystal phase product is sufficiently pure to leave the Purification Process as the final product.
Summary of Experimental Data Demonstrating the Process
The viability of the process developed here has been demonstrated by recovering 2,6-DMN from a mixture of DMN isomers. This DMN mixture and other process streams were supplied by BP Amoco and contained varying amounts of 2,6-DMN, other DMNs, and lighter and heavier boiling components. Several tests were conducted at the Sulzer pilot facilities in Buchs, Switzerland. Feed compositions to the Static and the dynamic Falling Film Units were based on the two process schemes envisioned. In one, the process consists of a conventional suspension crystallization step followed by the dynamic Falling Film crystallization step. In the second scheme, a Static crystallization step is followed by a dynamic Falling Film crystallization step. Experiments were conducted over a wide range of feed, reject filtrate and the product concentrations covering the ranges expected for the two crystallization steps. Different feed purities were obtained by blending the available process streams. In all cases the desired product purity was met. Table 1 shows the range of data obtained in the pilot plant process development work conducted in this study.
Table 1
Results of Experiments Simulating the Suggested Process Schemes
| |
Static
Crystallizer
Residue |
Static
Crystallizer
Feed |
Static
Crystallizer
Product |
Falling Film
Crystallizer
Residue |
Falling Film
Crystallizer
Product |
2,6-DMN,
wt % |
17 - 25 |
38 - 50 |
70 - 80 |
55 - 65 |
> 95 |
Other DMNs,
Lights,
Heavies,
wt % |
75 - 83 |
50 - 62 |
20 - 30 |
35 - 45 |
< 5 |
Results of experiments with a blended feed containing 46.2 weight percent
2,6-DMN demonstrated the viability of the process. Stage 1 of the process was conducted in a Static Crystallizer and stages 2 to 4 were conducted in dynamic Falling Film Crystallizer. The starting temperature (Ts) and ending temperature (Te) for the crystallization and sweat cycles for the four stages are shown in Table2. Also shown in Table 2 are the batch times for each cycle for the four stages.
Static Crystallization produced in one stage a crystal product of 80 weight percent purity. The three stages of Falling Film Crystallization produced a final product of >95 weight percent purity. It should also be noted that products made by suspension crystallization were purified to final purities of greater than 95 weight percent in Falling Film units to demonstrate that processing scheme.
Table 2
Crystallization and sweat temperature and time profiles
| Stage |
Partial Melting (Sweating) |
Crystallization |
| |
Ts |
Te |
Cycle Time |
Ts |
Te |
Cycle Time |
| |
Deg C |
Deg C |
Mins |
Deg C |
Deg C |
Mins |
| 1 |
40 |
100 |
510 |
70 |
20 |
480 |
| 2 |
100 |
118 |
30 |
96 |
76 |
35 |
| 3 |
102 |
116 |
20 |
98 |
75 |
40 |
| 4 |
78 |
111 |
45 |
88 |
53 |
60 |
The purification of dimethyl-2,6-naphthalenedicarboxylate can take place essentially in the same way as the purification of 2,6-DMN. Crystallization is carried out at temperatures below 187oC. Typically, the heat transfer medium temperature is lowered from 187 to 175oC. Sweating of the crystal layer occurs when the heat transfer medium temperature is raised above 187oC. Melting occurs as the heat transfer medium is raised to 200oC or higher.
Conclusions and Summary
1. Purification methods have been identified to produce high purity 2,6-DMN product.
2. Two purification processes have been developed and demonstrated to produce greater than 95 weight percent pure 2,6-DMN product.
3. These two processes do not require the use of solvents.
References
1. J. K. Holzhauer et al, Amoco Corporation, Patent EP 0 444 132.
2. J. K. Holzhauer et al, Amoco Corporation, Patent EP 0 541 782.
3. M. Takagawa et al, Mitsubishi Gas Chemical Company, Patent EP 0795 529.
4. M. Takagawa et al, Mitsubishi Gas Chemical Company, Patent EP 0 792 858.
5. T. G. Smith, Y. Viswanath, J. M. Weis, Amoco Corporation, Patent WO/95/18086.
6. K. J. Abrams et al, Amoco Corporation, Patent WO 99/00167.
7. Teijin Limited, Patent GB 1 345 479.
back