Venting during operation is inevitable but controllable. It is also more than the limit sometimes, thus calling for special attention, as environmental authorities now-a-days days insist on practically zero venting, even during commissioning and start up.
Commissioning can be done in a controlled manner to restrict emission rate in conventional production plants, but commissioning in refrigerated unloading or storage terminal handling cargo like Vinyl chloride monomer (VCM) can be done only on ship arrival for gassing up and chill down. Considering heavy ship demurrage cost, there may not be much time for elongated operation. This article describes a novel approach to help minimise venting of VCM during commissioning of VCM storage facilities, and not only satisfies environmental needs, but also saves valuable material cost.
VCM storage facility generally includes:
- Marine terminal facility for receiving and transferring VCM from ships to tank.
- Refrigerated (at -13.6 deg C) storage tank with double wall, double integrity type and suspended deck.
- Pumps to transfer VCM from storage tanks to plant day tanks.
- Pre-cooling pump to inject cold VCM into the unloading line so as to keep it in cold condition approaching to tank temperature, prior to the ship unloading operation.
- Refrigeration package that includes compressors (i) holding compressor for handling normal boil off, (ii) unloading compressor for handling boil off during unloading and pre-cooling.
- Emergency vent handling system.
VCM storage facilities receive VCM through ships from international sources. Liquid VCM (at -13.6 deg C) is pumped from ship via ship pump through a double wall/insulated pipe to the double wall and double integrity type storage tanks, which is at atmospheric pressure.
From storage tank, the VCM is transferred via VCM transfer pumps to plant day tank. The boil off from the tanks during holding time and pre-cooling/unloading operation is taken to refrigeration compressors and liquefied using cooling water condensers. The liquefied VCM is then flashed back to tank.
Vapors from VCM storage facility should be handled by some means of vent handling system (refrigeration system, absorption system, emergency cold vent, incinerator etc.). This includes significant quantity of VCM venting during commissioning of the facility. However, during tank fire, the excess VCM is vented into the atmosphere.
Conventional commissioning procedure:
The following activities are performed as part of commissioning in general:
(i)Drying: to ensure that the tank meets the required moisture specification without excessive consumption of nitrogen (final drying is effected during the nitrogen purge). Refrigerated tanks need to be dried to avoid formation of ice in the tank, which might impair mechanical function of the in-tank equipment.
(ii)Purging: to ensure that a flammable atmosphere is not formed in the tank, when hydrocarbon is introduced.
The use of nitrogen also facilitates the final drying stage that occurs simultaneously with oxygen reduction.
(iii) Gassing-up: an operation during which inert gas in the storage tank is replaced with respective process
vapors. Ship VCM is used for gassing up, where no other source is available.
(iv) Cooling down: to establish the tank to its operating temperature in a controlled manner, without inducing excessive thermal stresses/thermal shock during normal unloading of VCM.
(v) Tank First Filling: to put sufficient liquid level in the tank so that the tank and associated equipment can be proven in process fluid service. Minimum liquid stock is required to keep the tank under operating temperature by latent heat of evaporation.
During gassing up, cooling down and tank first filling, VCM vapors are displaced/generated from storage tanks. Vapors that are displaced during final phase of gassing up, and generated during cooling down and tank first filling are recovered with refrigeration system, where the in-built inert purge system helps to get rid of residual inert.
Vapors generated during initial gassing up operation (mix of N2 and small amount of VCM) cannot be handled by refrigeration system as refrigeration compressors are started only if the vapor contains more than 90 per cent VCM during when condensation is effective. Also, capacity of the compressor will vary due to molecular weight difference.
Hence, during gassing up, purge gas with VCM vapors are sent to vent handling system.
It is proposed to use a modified technique of gassing up, utilizing the advantage of density induced stratification concept and conventional plug flow displacement of Nitrogen with VCM from bottom carried out alternatively. (Refer figure 1 below). The density of air is 1.18 kg/m3 and density of VCM even at a conservative room temp (25deg C) is 2.58kg/m3.
The facility has two tanks (A and B). The philosophy adopted is to first commission tank A alone, while keeping tank B as a buffer to receive VCM vapor vented from tank A, during gassing up operation. Later at convenience, tank B can be commissioned by alternate gassing up and stratification using the VCM vapor from tank A, without depending on ship. Tank B gassing can be done as slow as possible (as no demurrage cost is involved) utilizing available vent handling system and refrigeration compressor.
The dispersion of VCM gas in N2 depends on two factors:
(i) Turbulence caused at entry into the tank through the ring spargers, which promotes dispersion and eventual release of VCM into the atmosphere.
(ii) Dispersion by molecular diffusion from the rising VCM interface layer, which also leads to the leakage of VCM into N2 layer at top.
Studies carried out to mitigate constraints
CFD modeling can be used to find an effective hydraulic strategy for displacing nitrogen with VCM in gaseous form, while ensuring low discharges of allowable limit of VCM into the atmosphere. Density-induced stratification will result into layer rich in Nitrogen, while the tank bottom layer rich in VCM.
From CFD modeling, following are calculated:
(i) Calculation of settling time for VCM and N2 mixture to achieve stratification.
(ii) Estimation of diffusion time and height of medium interface.
(iii) Pressure difference to be maintained to have adequate flow of N2+VCM between two tanks without creating turbulence.
(iv) Flow rate of VCM vapor into the tank to minimize diffusion/turbulence.
(v)Flow ratio of VCM gassing up rate in main shell and annulus portion of the tank ring sprayers to ensure the stratification level is same to avoid spill of VCM from one zone to other zone. This is estimated to be 5:1.
The CFD model revealed that the density induced stratification of VCM/N2 layer has a more dominating effect than the diffusion of VCM into top N2 layer. The above advantage is well utilized for the objective of minimizing VCM venting.
Based on CFD results, by alternate purging and settling with suitable VCM gas flow and timing management, VCM venting is minimized during gassing up operation.
Details of modified gassing up operation are described in next section.
Modified gassing up operation
Slowly admit VCM vapor (10û20 deg C) from ship to tank A bottom shell and annulus spargers. Vent the nitrogen from top dome of tank A. Analyze VCM in vent gas till it reaches 100 ppm. At regular interval, VCM gassing is stopped to allow stratification based on CFD analysis. Stop purging and venting when VCM concentration reaches 0.5 per cent in vent at tank A top. At a stage, when interface layer approaches top of tank and controlling vent with diffused VCM concentration within 0.5 per cent is difficult, proceed to stage 2.
Make a provision to connect tank A top dome nozzles to tank B main shell and annulus spargers. Admit VCM vapor to tank A bottom and divert the vent gases mixture from tank A top to tank B. Finally vent from tank B top dome. Initial venting is pure N2 from tank B (already held with N2). Analyze tank B top vent vapor for VCM using gas chromatograph and if it reaches 0.5 per cent, close tank B venting. If required allow for periodical stratification in tank B. Gassing up of tank B is done subsequently at a later date with longer time.
It is expected that 90 per cent VCM concentration at tank A top is reached at end of stage 2. After reaching 90 per cent VCM in tank A top, commission the holding/unloading compressors with vapor from tank A top. Further balance small quantity of N2 is removed through refrigeration package inert gas/purge gas vent system.
This method benefits in expelling maximum nitrogen in tanks to atmosphere without VCM losses.
Conclusion and recommendation
By above method of gassing up, vented quantity is substantially reduced to less than 30 per cent of VCM loss when compared to conventional direct gassing up and venting operation.
This is recommended for VCM and other similar application of products with reasonable density difference and where more than one storage tank is involved, irrespective of statutory authority compulsion, since it saves environment and dollar value of product even with reasonable ship berth time.
The method is well suited for multi-tank installation; but can be tried for single tank installation also, by providing longer period of stratification /diffusion. The pipe hook up can be planned during engineering stage itself, based on CFD modeling and hydraulic demand.
Additional design criteria can be evolved for spargers distributor´s design based on CFD studies. It is important to adequately size the inert purge system in the refrigeration compressor package. Technip India Limited has experience in study, model, design and implementation of such VCM storage installation and successful commissioning with record of low venting.
Why not venting VCM to atmosphere?
VCM is considered one of the world´s most important commodity chemicals. Majority of VCM is used in production of poly vinyl chloride (PVC). It is extremely flammable, is a human carcinogen and large fires triggered by VCM are difficult to extinguish. Flammable limits (% by Vol.)
- Lower Explosive Limit (LEL): 3.6 per cent
- Upper Explosive Limit (UEL): 33 per cent
Authored by A Mohan, Vice-President and Head of Process and Technology Division, S Rajan, Chief Engineer, Process Design, and TNV Satynarayana, Head (Technology) Technip India Ltd in Chennai