September 2007

Consequence of Liquid Carry Over – Part 2: Fixed Heat Exchanger Area

 
Many facility operating problems are related to improperly designed or under-sized gas-liquid separators. Due to the importance of separators, in the July Tip of the Month (TOTM), we studied the effect of liquid carry over in a simple dew point control plant. In that study, assuming variable area of the heat exchangers, we found that one percent liquid carry over can cause considerable change in compressor power requirement and heat exchanger duty. This assumption is valid if we are designing a new plant and the equipment is being sized for construction. However, for an existing plant, the heat exchanger areas are fixed. In the continuation of our previous study, we will revisit the same case study and investigate the consequence of liquid carry over but we will use the same heat exchangers (i.e. keeping UA constant; U=overall heat transfer coefficient and A=area). 

Let’s consider the same process flow diagram as shown in Figure 1 for a simple gas plant. The feed composition and conditions are shown in Table 1. We have already shown the impact of liquid carry over on the “sales gas dew point” and we experienced that for even a small liquid carry over (1 %), the dew point offset was about 6°F [3.3 °C]. We will demonstrate the consequences of liquid carry over on the other equipment while maintaining the “spec dew point”.



Figure 1. Process flow diagram for simple dew point correction gas plant


It is desired to process this feed gas to produce a sales gas with a dew point of 20 °F [-6.7 °C] at 540 psig [3.723 MPa] The feed gas is mixed with recycle gas from a stabilizer, compressed and cooled to 110°F [43.3°C] and 555 psig [3.827 MPa], then cooled in the gas-gas exchanger, gas-liquid exchanger and finally in the chiller to 20°F [-6.7°C] before entering the separator at 540 psig [3.723 MPa]. In real equipment, there would be some liquid carry over. In order to show the impact of liquid carry over, in the simulation we withdraw a small portion of liquid stream from the separator and remixed it with the vapor stream.  The solid curve in Figure 2 shows how the dew point of sales gas goes off spec as a function of the liquid carry over.

In order to bring back the sales gas dew point to spec, we re-adjusted (lowered) the stream 7 temperature. The required degree of re-adjustment is shown by the dashed line in the same figure. As a consequence of re-adjusting of the stream 7 temperature, the conditions of other equipment and streams changed. Figure 3 shows the variation of compressor power and the heat exchanger duties as a function of liquid carry over. As can be seen in this figure, one percent liquid carry over can cause considerable change. For this case, the percent changes ranged from 0 to 27 percent. The changes in the reboiler duty, sales gas and LPG flow rates were negligible. Contrary to the findings of the July TOTM, here we see that the chiller duty increases as we expected.

Not included in this analysis is an examination of the affect of lowering the chiller outlet temperature on the refrigeration system.  In an existing plant, to lower the refrigerant temperature, the chiller would have to operate at a lower pressure so that the power required for the refrigerant compressor would increase. The overall effect of liquid carry over is the increase in the operating cost, as expected.


To learn more about similar cases and how to minimize operational problems such as liquid carry over, we suggest attending our G4 (Gas Conditioning and Processing), G5 (Gas Conditioning and Processing - Special), and G7 (Process Simulation in Gas Conditioning and Processing) courses.



By: Dr. Mahmood Moshfeghian