Iludest distillation plant defies thermodynamic limits

Of ASTM Standard D2892 and ASTM D5236

Fischer Autodest 800AC_HV

A brand new Iludest distillation plant has expanded limits of what’s thought possible and facilitated the intensly pure separation of bitumen fractions in a packed distillation column. As such, the process engineers st Waldbüttelbrunn have defied the thermodynamic limits of the ASTM Standards D2892 and D5236, and brought two seemingly incompatible requirements under one umbrella.

ASTM standards have been the benchmark for everything in crude oil distillation for decades. They make crude oil qualities comparable and distillation parameters transfer­able from laboratory standards to refineries. With the Standards D2892 and D5236, the operator can cover the distillation process for crude oil and heavy hydrocarbon fractions and design his plants optimally. In the last few years however, crude oil qualities have changed.

The proportion of heavy fractions is increasing. Refineries additionally develop new high-end mineral oil products which require a high degree of purity and therefore, more efficient working columns. The thermodynamic conditions specified by the ASTM standards are often no longer adequate to produce these required qualities.

Bitumen fractions with challenging boiling requirements and characterised by low overlapping fractions require a high separation capacity of the columns and high temperatures that can be achieved only under high vacuum conditions.

“These are conditions which cannot be satisfied so far with the parameters specified by the D2892 and D5236 standards”, says Dr.-Ing. Hans-Eckhard Koenen (Engineer), Team Leader “Thermodynamics” at IIudest/i-Fischer. For the delivery of this particular customer demand, the Waldbüttelbrunn Company who devised the solution has received the “PROCESS Innovation Award” at Achema, the second within the intervening period.

The seven strong team of IIudest/i-Fischer engineers has managed to reinvent distillation again and again. This time round, the Fischer Autodest 800AC/HV has put the competition in its place. The special features of the distillation plant are that this apparatus covers a temperature range which extends the range of the ASTM Standards D2892 and D5236 still further — and that too in a single packed column at a vacuum down to 10-3 Torr. The system therefore exceeds the boiling point of the bitumen fraction, i.e., achieves a temperature range of 560 to 600°C (equivalent to atmospheric pressure) This is clear if one examines the thermodynamic definitions of the two standards more closely, and this is what the team of engineers has actually done.

  • ASTM D2892 is based on a packed column with a theoretical separation capacity of 15 theoretical plates, operated at a vacuum of 2 Torr and a boiling point of 400°C (at atmospheric pressure).
  • In comparison ASTM D5236 is based on an open tubular column, a theoretical separation capacity of 2 theoretical plates, operated at 0.1 Torr, with a maximum boiling point range of 540 to 565°C (at atmospheric pressure).

    Dr.-Ing. Hans-Eckhard Koenen Iludest/i-Fischer

    Dr.-Ing. Hans-Eckhard Koenen Iludest/i-Fischer: (Photo credit: PROCESS Worldwide)

On the crucial point of getting both standards under one hat, Koenen says: “High separation capacities are possible only with packed columns, but high temperatures are possible only with open tubular columns, because a deeper lower vacuum is possible here.” In other words, if one wants to outsmart thermodynamics, one must develop distillation units that can withstand a high vacuum, because lower process temperatures are possible only at higher vacuum levels. Finally, the hydrocarbon chains crack at above 300°C, a process which one must avoid during crude oil distillation. The incentive for the development came from two customer requests, which followed one after the other in brief succession. “Based on our experiences from the previous project, we knew that this challenge could be solved.“

Meanwhile, the first prototype is already in use at a German petrochemicals company, whose request had initiated the development in the first place. The units installed there achieve a temperature of 560°C atmospheric equivalent, which corresponds to an operating vacuum of 0.1 Torr and achieve the theoretical separation capacity of 25 theoretical plates (also under vacuum). The winning Autodest 800AC/HV, which was commissioned for a company in South Africa, tops in performance once again with a maximum boiling point of 600°C atmospheric equivalent, and a separation capacity of 30 theoretical plates. Such temperatures can be achieved only with a vacuum of 10-3 Torr, i.e., under the operating conditions of a short-cut path evaporator. Fischer Autodest 800AC_HV from John Morris ScientificThe challenge is described by Koenen and CEO Stefan Opis as follows: “Thermodynamically, the requirements were mutually contradictory: The column design requires a vacuum of 10-3 Torr and the fraction volumes required for the separation capacity had to be bigger than the column hold-up volume.” If the preceding order, which had paved the way for this requirement, had not come through—who knows, perhaps the process engineers would have thrown in the towel. “According to the available experience values, we knew that a column design was possible”, says Koenen. The biggest challenge was the vacuum system. It became clear to the process engineers rather fast that a normal oil diffusion pump would have reached its limits. “We had to file the pipe sections and overcome turbulences which would have caused a pressure fall drop”, explains Koenen. But the calculations matched during the very first run, and the trial distillation was successful.

Read this article in full at: PROCESS Worldwide

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