PTQ Q1 2024 Issue

requirements for the process units, sub - sequently driving up the total nitrogen demand for the facility. Moreover, it could result in an overly conservative sizing of the nitrogen blanketing valve, which might introduce control challenges when the valve output is minimal, possibly triggering unnecessary nitrogen wastage. Refiners are adding gap control for the blanketing control valves, as described later in detail, and this practice results in near zero open - ing of the nitrogen blanket control valve during normal plant operation. Observations from actual plant data reveal a consistent level during normal operations. During emergency situations when the inflow to the surge drum is dimin - ished or completely lost, operator actions

Required blanketing nitrogen gas

Project

Project A

Project B

Equipment name

Lean amine surge drum

Lean amine drum

Vessel pressure (bara) Vessel temperature (°C)

Vessel head type Vessel ID (mm) Vessel T-T (mm) Out flow (m³/h)

2:1 Semi-ellipsoidal

3,000

2,500 9,000

10,000

180

Normal liquid level- NLL (mm) Lo Lo liquid level- LALL (mm) Nitrogen blanket gas flow (Nm³/h) 1. Method 1

7,000

6,000

750

500

358

139

2. Method 2

51 50

3. Design value

360

Table 1

pressure. This is done to account for slight degassing in the liquid and to prevent pump cavitation. Calculation of volume changes due to liquid outflow The volume changes due to liquid outflow can be estimated using two different methods: • Method 1: The flow of nitrogen blanketing is calculated to maintain the normal operating pressure in the surge drum when the outflow from the drum is continuous, but the inflow to the drum ceases. This estimation uses the API 2000 liquid outflow method. Licensor does not normally include thermal inbreathing. • Method 2: The nitrogen blanketing flow is calculated to maintain the vessel at a slightly positive pressure (1.1 bara) when the outflow from the drum is continuous, but the inflow to the drum fails. This is calculated based on the vapour volume change due to the decrease from the normal liquid level to the very low liquid level at which the con - nected pump is stopped. The operation of the surge drum is different from that of tanks. While tanks are either in receiving or dispatch mode, surge drums are always in both receiving and dispatch mode. They are frequently positioned between process units to help mitigate the impact of flow rate variations between intercon - nected process units. Unlike the typical control objective of maintaining a measurement at a set point, the goal of surge drum level control is to buffer the changes in controlled flow while keeping the liquid level in the vessel within limits. For surge drums, it is usually more important to allow levels to ‘float’ to minimise flow rate variations. Therefore, the level controller must permit this movement and try not to hold the level close to its set point. Instead, the controller should keep the surge vessel’s level between its upper and lower limits with the least possible change to its flow output. The estimation of nitrogen blanketing for the lean amine surge drum of two different licensors' diesel hydrotreater units using both methods is presented in the Table 1 . The calculation details can be found in Appendix 1. Employing Method 1 to determine the nitrogen blanket for the surge drum may lead to a high degree of conserv - ativism. This strategy could increase the normal nitrogen

ensure the feed surge drum level is maintained, preventing the connected pump from being tripped due to the activa - tion of the low liquid level trip. The surge drum’s hold-up time is generally set between 10-15 minutes, providing ample time for operator intervention. Considering the ample time available for operator inter - vention and the lack of benefits from designing the nitrogen blanket flow with high conservativism using Method 1, it is more pragmatic to adopt Method 2 for estimating the nitro - gen blanket gas flow. Pressure changes due to liquid contraction Volumetric contraction in the surge drum can happen due to the replacement of hot feed with cold feed, especially for hot hydrocarbon feed. Reduction of pressure can be estimated as follows: Liquid volume change (dV) = V NLL x (1- ϱ hot / ϱ cold ) Vapour volume above NLL (V vapour ) = V Total - V NLL P final = P normal x [V vapour /( V vapour + dV)]

Where: V NLL

= Normal liquid level volume (m³) = Vessel total volume (m³) = Hot liquid density (kg/m³) = Cold liquid density (kg/m³)

V Total ϱ hot ϱ cold P final

= Final vessel pressure (bara) P normal = Vessel normal pressure (bara) r = Density (Rho)

Example calculation: This example calculation is performed for a diesel hydro - treater feed surge drum (FSD) when hot feed is replaced with cold feed during hot feed failure. Assuming the FSD will continue to maintain a normal liquid level after replace - ment, cold feed with a higher density will have a lower liq - uid head. The reduction of the liquid head will result in a reduction in liquid volume at a normal liquid level, and this volume reduction is estimated considering liquid volumetric contraction.

PTQ Q1 2024