Impact on product purity
Process engineers often face the challenge of fixing operational problems resulting from process modifications such as adoption of a new catalyst, replacement of aging equipment, capacity expansion or retrofitting. The feed composition to the crystallizer may change as a result of those alterations, thus affecting the performance of the crystallization process. Understanding the SLE behavior is often the key to effectively solving such problems, as the following two examples highlight.
The first example mimics a real-life case study on the retrofit of an adipic acid plant, which has been simplified for illustration purposes to a problem involving only three components. As shown by the block diagram of the process (Figure 3a), the reactor outlet containing product A, an impurity B and solvent S (stream 1) goes to an evaporative crystallizer operating at 50°C, where sufficiently pure A is obtained as solids. The mother liquor (stream 2) is partially purged, with the remaining portion recycled to the reactor. At a purge ratio (defined as the ratio of the purge stream to the mother liquor) of 1.5%, the locations of the process streams on an isothermal cut at 50°C of the SLE phase diagram are as shown in Figure 3b. The retrofit project is aimed at increasing the overall yield of A. One seemingly obvious solution is to reduce the purge ratio. However, this action would actually increase the impurity content in all streams. Because the overall composition after solvent evaporation (point 1′) is already located at the edge of the blue shaded region where pure A can be obtained, an increase in B content would move it into the two-solid region. Therefore, if the purge rate is decreased, B would crystallize out and contaminate the product. Another possibility to minimize the loss of A is to reduce its concentration in the purge stream, which can be achieved by changing the crystallizer temperature. Comparison of the isothermal cuts at two different temperatures (Figure 3c) shows that, on a solvent-free basis, the double saturation point at 30°C contains less A compared to that at 50°C. Therefore, reducing the crystallizer temperature to 30°C would allow a higher per-pass yield of pure A. But because the recycle stream would now be richer in B, the purge ratio actually has to increase to ensure that point 1′ falls within the correct region. After material balance calculations, it was found that at 7% purge ratio pure A can be produced with a 0.5% improvement in the overall recovery, which represents millions of dollars in savings. The SLE analysis along with material balance calculations helped to avoid problems resulting from the retrofit.
The second example illustrates a purification problem that typifies many in the specialty chemical and pharmaceutical industries. Suppose that a liquid-phase reaction in solvent S yields the desired product P with 90% selectivity, the remaining 10% being byproduct Q. The output from the reactor is sent to a crystallizer to isolate product P. It has been decided to switch to a new catalyst that forms only a negligible amount of byproduct Q. Unfortunately, it produces a small amount of R due to polymerization. Because of recycle, the concentration of R in the crystallizer feed is about 5%. But, as the overall selectivity to P is 99%, which is significantly higher than for the original catalyst and there has never been any problem with product purity even when the crystallizer feed contains 10% Q, it was decided to go ahead with the switch. However, it was later found that with unaltered crystallization conditions, the solid product contains an intolerable amount of R. The polythermal projection of the SLE phase diagram (Figure 4a) shows why. Because R is a heavy component with high melting point, its solubility in the solvent S is considerably lower than P, leading to a large compartment of R. Therefore, even with a feed that contains a relatively small amount of R (point 1) crystallization of P would soon cause the liquid composition to reach the boundary between P and R compartments. If cooling then is continued beyond point 2, P and R would co-crystallize, making the product no longer pure. Based on this understanding, three solutions might make sense. First, increase the crystallizer temperature so that the liquid composition is still within compartment P (stop the cooling before reaching point 2). Second, add more solvent so that cooling to the same temperature still produces a liquid composition within compartment P (Figure 4b). With both these options, though, the per-pass yield decreases. The third alternative is, of course, to increase the purge ratio so that the crystallizer feed would contain less R.
Kinetic and mass transfer effects
Equally important in troubleshooting crystallization problems are kinetic and mass-transfer effects. Even when the SLE behavior indicates that the product should be pure, impurities can be incorporated into the solid product in various ways. One possibility is impurity inclusion, in which the mother liquor containing the solvent and other impurities gets trapped inside the crystal (especially when the crystal grows very fast), thus leading to reduced product purity. Because crystal growth rate depends on supersaturation, inclusion can be minimized by cooling more slowly or adding the right amount of seeds, so that the overall supersaturation level in the crystallizer is kept low. However, it is also important to ensure homogeneity inside the crystallizer, such as by providing better mixing, because stagnant or overcooled spots can produce high local supersaturation that contributes to nearly all impurity inclusion.