Hydrocarbon gas separation, in conditions of low temperature

Geva Zis, process department, Ludan Engineering Israel, Yokneaam, Israel

In this presentation, we review a typical design of separation process of hydrocarbon gas stream, into its main components, in conditions of low temperature.

This article summarizes two projects we had last year. These projects share similar main principles.

The focus in this presentation will be on two main sub-systems:

1. A multi streams brazed aluminum plate heat exchanger (“cold-box”), in low temperature conditions.

2. Closed loop for circulation of coolant (usually propane-propylene according to the required temperature).

Main subjects for discussion:

1. Schematic block diagram of typical gas separation system:

• Pre-treatment section;
• Gas knock-out separation;
• Distillation section;
• Coolant cycle (compression, cooling and condensation, pressure reduction, cooling of process streams and re-compression).
  

2.   Simulation diagram of typical separation of ethane-ethylene from hydrocarbon gas stream.
3.   Simulation of a coolant closed loop, integrated in one master simulation of the separation and distillation process. This assures overall and coherent heat and mass balance of the process system with the coolant system.

The coolant system consists of the following main parts:

• A multi stage compressor;
• Coolant cooling and condensing heat exchangers;
• Control valves that reduce the coolant’s pressure.

In these conditions of reduced pressure, the coolant is partially evaporated and thus cools down. This coolant is now used to cool process streams in heat exchangers, where it is evaporated and returned to the compressors.

4.   Presentation of the structure of typical multi streams brazed aluminum plate heat exchanger (“cold-box”), and discussion of the limitations in simulating such operation unit.

5.   Examples of using process simulation in detail design e.g. calculating of process conditions for control valves based on piping detail design.

6.   Using process simulation for calculation of settle out pressure in the compressed coolant system, taking into account the volume of the system, location of isolation valves and operating process conditions.