Moisture Measurement in Hydrocarbon
The Problem
Hexane, C6H14, is very important in the production of plastics and synthetic materials. It acts as the solvent in the process. Together with ethylene, co-monomers and hydrogen, polyethylene can be generated with the help of aluminum-organic catalytic converters, the basic of all plastics and synthetic materials. Because of the costs, the Hexane will be centrifuged and distillated after the process-turn-around, and will be re-circulated. To protect the catalytic converters and thus the downstreamed process, the Hexane has to be kept on a trace level of water. The limit value for the humidity is between 0 to 20 ppm (it depends on the production process).
Many installations worldwide in HDPE- and LDPE-plants confirm HYGROPHIL F in the production of plastics and synthetic materials.
The solution
In general HYGROPHIL F 5673 takes reliable and accurate inline measurements within a range of dew point temperatures from -80°C to +20°C. Due to a customized special calibration in the liquid medium “Hexane”, a very high accuracy can be achieved in the requested range of 0…20 ppm. HYGROPHIL F 5673 controls and monitors the moisture in the medium, which is very important for the plant safety, and guarantees a trouble-free process.HYGROPHIL F is free of cross-sensitivity and resistant against aggressive ingredients. The sensor can be placed directly in the pressured pipe up to 200bar (with the BARTEC Retraction Tool you are able to take the sensor out under pressure at any time) or in a BARTEC Sample System.
The moisture sensor is long-term stable and accurate with experience indicating yearly inspections to be enough. The length of the fiber-optic cable to connect the sensor and the evaluation unit can be up to 800m.
The measurement principle
The sensor consists of a coated, very resistant and microporous substrate of quartz glass with a pore diameter in which only water molecules can enter. This penetration happens because of the equilibrium (all gases balance automatically).Because of the different refraction index (nair = 1,0; nwater = 1,33) the water molecules cause a shift in wavelength.
With increasing moisture, the refraction spectrum is shifted towards larger light wavelength and read out by a polychromator. This shift (no measurement of intensity => independence of fibreoptic cable length) is proportional to the dewpoint temperature of the gas.
The sensor can be used in hazardous areas (Zone 0, or Class I / Div. 1). The whole system is ATEX, CSA / UL and GOST approved.