PTQ Q2 2024 Issue

Dew point estimation for recycle gas compressor-DHT Quoted liquid carryover from the various types of mist extraction devices are usually in the range of 0.1-1 gal/MMscf. Ref GPSA Chapter 7 Separator

PIPE-100

Sealgas

RGC Discharge

To Seal

RGS out

Amb Cool

Ex HP amine

RGC Suc

TEE-100

Comp Dis & Seal DE

Spread sheet-1

MIX-114

SAT-100

QRGC

Sat Watr

RGC Suc

RGC Discharge

HC dew point Water dew point

58.56 70.00

C C

HC dew point Water dew point

62.97 75.05

C C

Entrained Watr

RGComp

RT Dew Point

Ex HP Amine Entrained WTR

RGC Discharge

Sat Watr

CompDis& Seal DE

To Seal Sealgas

RGS Out

RGC Suc

1.0000 70.00

0.0000 70.00

0.9999 70.00

1.0000 95.40

0.0000 70.00

1.0000 95.40

0.9966 60.22

Vapour fraction Temperature Pressure Molecular weight

bar_g 49.00 3.788

49.00 3.877

49.00 18.02 10.00 10.00

49.00 3.878

60.00 3.878

49.00 18.02

60.00 3.878

57.40 3.878

Mass ow Phase comp mass ow (overall-HO)

kg/h kg/h

21 , 456.72 622.01

21 , 466.72 632.01

622.01 622.01

21 , 294 626.95

171.73 5.06

20 , 834.72 0.00 59 <empty>

HC dew point Water dew point

59 70

<empty> 264

59 70

63 75

<empty> 264

63 75

62 74

Figure 1 Hysys output for compressor.

However, these suggestions were perceived as exceed- ing the traditional scope of FEED, which typically deals with pipeline sizes of 4in and larger. The consideration of details for a DGS line, which is only 1in in diameter, was viewed as overly granular for the FEED stage. The argument was that detailing such specifics in the datasheet might unduly prolong FEED activities, with these aspects being more appropriately addressed during the EPC phase. While reviewing the EPC deliverables for a diesel hydro- treater (DHT) recycle gas (RG) compressor, the author, draw- ing upon extensive experience in project oversight, identified critical areas of improvement. This analysis aimed to ensure that the design and operational integrity of the compressor system met the highest standards of efficiency and relia - bility. The following sections detail specific observations made during the review, an analytical process undertaken to understand the implications of these findings, and mitigation measures adopted to address these gaps effectively. Dry gas seal line arrangement Findings: The review of EPC deliverables for a DHT recycle gas compressor revealed significant gaps in the DGS line arrangement: • The 1in seal gas line lacked tracing and insulation, a devi- ation from the vendor’s P&ID, which showed tracing and insulation post-seal gas filters. • EPC P&ID documentation failed to specify the need for sloping the 1in line back towards the tapping point. • Additionally, the EPC contractor’s approach to dew point temperature, set at 70°C based on compressor suction from

an RG gas scrubber treated with lean amine solution, was identified as a key area of concern. Analysis: The analysis commenced with the FEED contrac- tor’s recommendation, anchored in API-692 guidelines, which was added to the compressor mechanical datasheet. This recommendation emphasised the need for the seal gas line to be traced and insulated, potentially incorporating a heater to maintain the temperature at least 20°C above the dew point of the seal gas. The EPC contractor was then tasked with carrying out detailed dew point calculations at the compressor discharge, from where dry seal gas tapping is taken. The results from the Hysys simulation were pivotal in understanding the dew point dynamics of the DGS system. The Hysys simulation result shows the dew points of the dry gas, as seen in Table 1 . Simulation output (see Figure 1 ) coupled with consider- ations for entrained water, as per GPSA Chapter 7 recom- mendations, called for a comprehensive analysis. The RG compressor’s upstream knock-out drum (KOD) is equipped with a demister to enhance gas purity. A critical finding from the analysis was the gap between the compressor discharge temperature (95°C) and the water dew point (75°C). Heat loss calculation indicated a significant risk of temperature drop in the DGS line, poten - tially down to 60°C at the filter inlet. The analysis of heat loss, considering ambient temperatures ranging from 21°C to 30°C and wind speeds varying from 3 to 15 m/s, empha- sised the considerable effect of wind velocity on heat dissi- pation. This significant relationship is depicted in Figure 2 . Mitigated measure: In response to the comprehensive analysis, the EPC contractor agreed to trace and insulate the DGS line, aligning with the detailed dew point and heat loss calculations. Absence of gas conditioning system for dry gas seal Findings: The review identified that the current system

Compressor suction and discharge dew points

Compressor Compressor suction discharge

Hydrocarbon dew point (°C)

59 70

63 75

Water dew point (°C)

Table 1

PTQ Q2 2024