Process and apparatus for removing vaporizable constituents from viscous solutions or melts of thermoplastics

• US 4,153,501 A
• Filed: 02/14/1977
• Issued: 05/08/1979
• Est. Priority Date: 02/19/1976
• Status: Expired due to Term

• Current Assignee: BASF AG (BASF SE)
• Original Assignee: BASF AG (BASF SE)
• Inventors: Fink, Peter, Wild, Hans, Thielen, Gunter, Reffert, Rudi W., Zizlsperger, Johann
• Primary Examiner(s): Yudkoff, Norman
• Application Number:
  US05/768,636
• Time in Patent Office: 813 Days
• Field of Search: 159/13 R, 159/13 A, 159/49, 159/DIG. 10, 159/27 D, 159/2 R, 520/501
• US Class Current: 159/49
• CPC Class Codes: B01D 1/22 by bringing a thin layer of the liquid into contact with a heated surface B01D1/065 takes precedence
• CPC Class Codes: B01D 19/0021 by bringing the liquid in a thin layer
• CPC Class Codes: B29B 13/02 by heating B29B13/06, B29B13/08 take precedence
• CPC Class Codes: C08F 6/10 Removal of volatile materials, e.g. solvents C08F6/001, C08F6/003, C08F6/005, C08F6/006, C08F6/008, C08F6/02, C08F6/04 take precedence
• CPC Class Codes: F28F 7/02 Blocks traversed by passages for heat-exchange media F28D7/0008 takes precedence
• CPC Class Codes: Y10S 159/10 Organic

Abstract

A process for removing vaporizable constituents from high-viscosity solutions or melts of thermoplastics by continuous evaporation along a heated devolatilization zone and separation of the vapor phase from the liquid phase in a downstream separating vessel. The plastic solution or melt is first heated stepwise in increments, in the devolatilization zone, while the product is in the form of thin layers from about 0.5 to 4 mm, so as to maintain a temperature difference of less than 50° C. between the heat transfer medium and the plastic solution or melt until the devolatilization temperature is reached, the distribution of the solution or melt to form thin layers of product taking place while the temperature conditions still correspond to those of the feed of solution or melt, after which the vaporizable constituents are evaporated so as to form a two-phase system of vapor and melt, and the heat of evaporation absorbed is substantially returned to the product during the evaporation along the devolatilization zone. The process permits the removal of vaporizable constituents under mild conditions.

Claims

1. In a process for removing vaporizable constituents from high-viscosity solutions or melts of thermoplastics by continuous evaporation along a heated devolatilization zone and separation of the vapor phase and the liquid phase in a downstream separating vessel, wherein the plastic solution or melt is first heated stepwise in increments to the devolatilization temperature by indirect heat exchange in the devolatilization zone under a pressure greater than the saturation pressure of the vaporizable constituents at the prevailing temperature, and the vaporizable constituents are then evaporated in the devolatilization zone under a pressure lower than the saturation pressure of the said constituents at the devolatilization temperature to form a two-phase system of vapor and melt, whereupon the vapor phase is removed in the downstream separating vessel and the plastic melt, which has been substantially freed from the vaporizable constituents, is discharged, the improvement which comprises:

forming the plastic solution or melt into a plurality of thin layers having a thickness of from about 0.5 to 4 mm at the temperature of said feed solution or melt;

heating the thin layers of solution or melt to the devolatization temperature by causing said thin layers to flow through the heated devolatization zone in product flow zones, said devolatization zone being heated by a transfer medium in a stepwise manner in the direction of product flow and containing at least two independent heating zones;

controlling the temperature throughout the devolatization zone such that the temperature difference between the heat transfer medium in the independent heating zones and the thin layers is always less than 50°

C.; and

substantially returning to the thin layer of solution or melt through the heated devolatization zone the heat of evaporation which is abstracted from said thin layer of evaporation of vaporizable components contained therein, whereby the drop in temperature of said thin layer due to evaporation of the vaporizable components is not more than 30°

C.; and

whereby vaporizable components are removed without exposing the product to excessive heat and without causing damage to the product.

2. The process of claim 1, wherein the temperature of the plastic melt does not fall by more than 30°

C. during the evaporation.

3. The process of claim 1, in which the temperature difference between the heat transfer medium and the solution or melt in the devolatilization zone is always less than 30°

C.

4. The process of claim 1, in which the thin layers of product which are heated are from 0.5 to 3 mm thick.

5. The process of claim 1, in which the vaporizable constituents are removed to a degree such that their residual content is less than 1 per cent by weight and preferably less than 0.1 per cent by weight.