Polymer Melt Flow Through Piping

A long part of my career was spent in polymer production, specifically Polyethylene Terephthalate (PET). In the initial years (circa. 1986) when I was working as an Operations Engineer, the technology in India was still pre-dominantly batch production. Some of the technologies for PET batch production were 'ICI' and 'Monsanto'. Today these technologies are practically obsolete.

In 1989, I got my first exposure to continuous PET polycondensation technology for a new 90 TPD plant being built with EMS Inventa technology. This was a grass-roots plant and the experience was amazing and rewarding. After spending almost six years I moved on to greener pastures. My family was expanding and I needed better financial returns on my profession. The next plant I worked was having Zimmer 5-reactor continuous polycondensation technology. Zimmer AG was and still is a leading supplier of PET technology. My duration at the Zimmer plant was not for long (15 months) due to reasons purely non-technical. Subsequent to that I worked for a period of 6 months in a small PET POY (Partially Oriented Yarn) plant with Teijin-Seiki Spinning lines where my basics about textile yarn spinning got strengthened.

The breakthrough for me to get into real engineering came in 1996 when I was selected by 'Chemtex' an engineering consultant having its Indian office in Mumbai and its corporate headquarters in Lexington Avenue, NY. 'Chemtex' was and still is a franchisee of DuPont Polyester Technologies located at Wilmington, Delaware. Chemtex global headquarters are now located at 1979 Eastwood Road, Wilmington, North Carolina. Chemtex is currently a part of the M&G group of Italy. In the early and late 90's and also in the early part of the 21^st century Chemtex and Zimmer AG were the leading technology and equipment suppliers for PET polyester technology.

Thus started a reasonably long career (8 years) in engineering of PET plants as a 'Chemtex' employee. Some of my co-workers felt that having no engineering consulting background would be to my disadvantage and I would prove to be a misfit. However, my practical experience of erection, commissioning, start-up and operations proved very useful and soon I could establish my credentials as a discipline lead within the organization.

During my stay at 'Chemtex' I also realized that the real technology part of the process that is the design of the main reactors and the flow of polymer through pipes to textile spinning and / or to the pellet making machine (pelletizer) was something which we were not privy to. The high-end technology part and believe me it is high-end technology was a closely guarded secret by the corporate office. We were mostly doing design work related to utilities and heating systems. The P&ID's were replications of earlier projects with hardly any scope of innovation and new engineering.

Nearly at the end of my career at 'Chemtex' we were asked to do some preliminary calculations for polymer pipe sizing. Initially I was involved in it. We were given a methodology to develop the pipe sizing equations. However, there were a lot of missing links in the methodology provided to us, for example the impact of degradation of the polymer due to longer residence times in the pipe for a given pipe size and polymer flow.

I was determine to do my own research on the subject and after a lot of searching for answers I came up with my own calculation set for calculating polymer line sizes. I cross-checked my results with some results provided from our own corporate office and was pleasantly surprised to find that the results of my calculations were in close tolerance with the results sent from the corporate office.

Polymer line sizing requires a few things to be addressed and which are inter-linked:

1. Polymer Throughput: This is the flow of polymer in the pipe expressed as kg/h. The polymer pipe needs to be capable of handling the complete range (turn-up or turn-down) of throughput of the downstream processing machinery (spinning machine and / or pelletizer). For a given pipe size the decrease in throughput increases the residence time and vice-versa. Again for a given pipe size the increase in throughput increases the pressure drop and vice-versa.

2. Residence Time: Minimum possible residence times in the pipe to prevent thermal degradation. Polymer melt flows at temperatures of 280-290 ˚C in jacketed pipes. Quantitatively polymer degradation is expressed as drop in "Intrinsic Viscosity" of the polymer from the start of the flow to the end of the flow. Most technology licensors give the maximum residence time from the polymer melt source (final reactor) to the end-user. A figure I remember for from the final polycondensation reactor to the spinning machine is 20 minutes.

3. Pressure Drop: Polymer melts are pumped using special gear pumps handling polymer dynamic viscosities of 250,000 to 300,000 cP. Dicharge pressures could range from 200-250 barg. Typical end user pressures such as textile spinning machines require a pressure of 30-40 barg at the inlet manifold at their nominal capacities

Polymer line sizing requires the optimization of the throughput, residence times and the pressure drop. Large residence times lead to thermal degradation of polymer which in turn leads to inferior quality end product, as an example low tensile strength and yellow color of the spun yarn. Too small pipe diameters although would provide lower residence times but cause excessive pressure drop which would hinder the operation of the downstream equipment which requires a minimum inlet pressure.

Selecting the correct line size involves:

a. Line size to be capable of covering the entire range of flow

b. Provide minimum possible residence times for the entire flow range

c. Provide an acceptable pressure drop range for the flow range to ensure uninterrupted and stable operation of the downstream machinery (spinning machine / pelletizer).

My endeavor to create a calculation procedure led to the development of a spreadsheet. This was my initial foray into using Microsoft Excel® for doing engineering sizing calculations and since then I have been churning out calculations in excel on a regular basis and have come to appreciate its amazing versatility in doing engineering calculations.

The spreadsheet which I have developed is attached for the benefit of the readers. Although I have put a disclaimer with the spreadsheet which is basically to protect myself from libel, I can assure the readers that this is an original development and unpublished anywhere else.

Hope chemical engineers who are involved in the polymer industry specially in the field of PET polymer would enjoy this blog and appreciate the spreadsheet.

Regards,
Ankur