SchuF is the leading global provider of valves and valve solutions to the PTA industry.
PTA Process Steps
Ingredients Para-xylene is mixed with solvent and catalyst and oxidised with Oxygen to give Terephtalic acid.
Most of the impurities are removed in the crystallisers and centrifuges. Drying yields a powder of TA about 99,8% pure. This is the TA or CTA process. SchuF valves are also used in this process.
To further purify TA from commercial (CTA) purity of 99,8 % to 99,99% another plant of the same complexity and cost as the TA plant, is required. It is worth producing PTA even at this additional cost as it commands a premium. This is because it significantly improves the quality of the Polyester for which PTA is the feedstock. High quality Polyester is especially important for PET (bottle grade) and film (carrier for all films from amateur to X-ray) polyester but also for fibres and bulk grade as the same quality can be achieved with less effort and less breakage.
The impurities in TA are converted to water solubles in a fixed bed catalytic convertor (preheater & reactor). The TA is then passed through crystallisers, crystallising PTA, and the soluble impurities removed through the centrifuge. The PTA is then obtained as a white crystalline powder through the dryers.
PTA Valve Portfolio
SchuF valves are primarily used in two stages. The first is in the booster stage where the PTA slurry is raised from 8 bar to 90 - 125 bar through 3 or more booster pumps. 3 way diverter valves split and reunite the flow before and after the pumps while drain valves are used to flush / drain the low pressure lines. Proprietary SchuF automatic recirculation valves allow trouble free start-up of the stand by pump.
A second important area in both CTA and PTA plants is around reactors, cristallisers, reslurry and mother liquor drums. In these stages the media can be very corrosive and the valves are mainly in 316 st.st, 317 st.st, Hastelloy or Titanium (with some interesting limits on Fe and H content in Titanium). Increasingly Duplex and Super Duplex are replacing 317 and in some cases Hastelloy or Titanium.
Ram and disc vessels are used at the entrance of transfer lines and to drain the vessels. Line valves are used to drain and to flush the pipelines with water. Our special angle control valves are used to control the pressure, level and flow.
Special angle control valves have long been used in preference to the traditional control valves, as they are flanged directly to the reactors, crystallizers, liquor drums etc. As the product expands due to the pressure drop across the seat without the traditional restriction of the downstream pipe diameter.
Patented PTA Valve Innovations
However the use of equal % and linear control contours has been subject more to the different licensor philosophies than to the demands of the actual duty. One of the reasons is the difficulty in determining the required Cv accurately, due to the high degree of flashing and solids contents, as well as the low quality of available data. For example pv is often assumed equal to p1. To counteract this, one user simply calculates required Cv using the ISA formulas for liquids and multiplying by 3.
This choice often leads to using equal % contours to give accurate start-up control even with oversized seat diameters. The high degree of controllability in the first 20% of valve capacity is offset by correspondingly bad control at operating conditions. Conversely, linear contours, while offering better operating condition controllability is vulnerable to oversizing.
The patented SchuF X3 control curve was specifically developed to address this shortcoming. It ensures good controlability in the start up and key operational phases and has led to higher PTA production efficientcy and end product quality.